Browsing articles in "Tech Info"

Why Fit Larger Wheels to your Model ?

Dec 14, 2012   //   by Nigel   //   Tech Info  //  No Comments

Why Fit Larger Wheels to your Model?

Most kits and ARTF models are manufactured down to a price. A price that will offer good value for money and be price competitive against other suppliers. An unfortunate result of these “competitive prices” is the wheels supplied have become much smaller over the years.

If you look at powered free flight and early radio controlled model aircraft in the 1930’s, 1940’s and 1950’s, they all had one thing in common. They all had large wheels mounted on undercarriages close to the front of the model.

Why was this??…. In those far off days, ready made propellers were rarely available and good ones made in wood, were very expensive. Most modellers had to make their own. So large rubber wheels were necessary to prevent the propellers from breaking when landing. And also, many flying competitions stipulated that the models had to “rise off ground”

Not many model flying clubs had access to tarmac runways or finely mowed grass. Most clubs flew from open heathland or farmer’s fields. This is often still the case today where many clubs, including our own, fly from mown grass areas in fields. Which can take many years to convert ordinary grazing pasture to the texture and flatness of a good lawn. So back to the problems of having small diameter wheels, as fitted to so many models.

Many wheels are also made from very hard foam rubber, which although is light in weight, has little “give” to absorb the shock of heavy landings. These landing shocks are then transferred to the airframe structure, which is a point of failure in many current ARTF models. The wooden plywood structures supporting the undercarriage is often so lightly built, that it shatters if any landing is “less than perfect”

Take Off’s

Small wheels have to rotate faster for any given ground speed than larger wheels. This increased rotational speed has the penalty of increased axle friction which will also slow the plane’s rate of acceleration. This increases the length of take off  run before flying speed, and a successful take-off, can be achieved. Small wheels cannot roll over small obstacles, whereas larger wheels will.

ie:        If a 2”(50mm) diameter wheel meets a 1” (25mm) high obstacle it will stop dead !

However, if a 4”(100mm) diameter wheel meets a 1”(25mm) obstacle, it will roll over it.

This is particularly true of large, soft rubber, hollow air filled wheels, which will also absorb most of the shock and prevent much of it being transferred to the airframe. There is also a phenomenon known as “prop-walk” where the tip turbulence of the propeller causes an increase in air friction, at ground level.

This also interacts with the torque required to turn the propeller, and makes a plane extremely difficult to steer straight on take-off. This usually results in the well known, “Ground Loop” as often experienced by pilots of full sized, tail wheel aircraft.

Hence the phrase: ”Real pilots fly tail-draggers….but….Real tail-draggers fly the pilots !!”

Fitting larger wheels also enables larger propellers to be fitted without making any structural modifications. So why fit a larger propeller ? 

Larger Propellers

Engine manufacturers always try to advertise the most powerful engines for their capacity. But the quoted maximum horsepower is often dependent upon achieving very high engine speeds. This is ok until you need to fit the best available “airscrew” to pull, (or push) your model through the air.

On full sized aircraft, the propeller is fitted to suit the horsepower required to give the best cruise performance and/or top speed. Then, a constant speed hub or a dual pitch propeller is fitted. This enables fine pitch to be selected for good acceleration on take-off and a good climb out, then provides a coarser pitch for improved cruise and top speed performance. This is because, at low speeds, a coarse pitch propeller is not efficient, as it will create a lot of turbulence and will absorb a lot of engine power without developing effective thrust until the plane is actually flying at cruising speed.

This can be seen in the very first pre-war squadrons of Spitfires and Hurricanes which were fitted with fixed pitch, wooden two blade propellers. When these were eventually fitted with constant speed, adjustable pitch propellers, take off runs were then much shorter and their rates of climb were significantly improved.

If a small diameter, high pitch propeller is fitted, the high speed airflow along the fuselage creates backwards drag during takeoff, so reducing acceleration and extending the take-off run. The larger the propeller diameter, the greater an area of air can be “pulled” by the “Airscrew”. which gives a more laminar airflow along the fuselage and  the wings and tail surfaces.

As a fundamental rule, the larger the fuselage cross sectional area, the larger is the propeller diameter necessary to achieve the most efficient thrust. Variable pitch propellers for model aircraft are extremely expensive, as well as being easily damaged!! So we have to make the best compromise, between achieving the best takeoff performance and the best flying performance.

Just like full sized aircraft, an engine must develop sufficient power to pull the model through the air fast enough to fly. A larger diameter propeller will pull, (or push) a larger diameter “tube” of air than a smaller propeller. The pitch (blade angle) of the propeller must then be selected to pull, (or push) the air fast enough for the plane to fly, AND for the engine to achieve it’s optimum RPM to do this.

Now this is when an engines maximum torque (Break mean effective pressure/BMEP) can be used to good effect. Rather than achieving maximum power revolutions per minute, on the ground, let the engine achieve this when flying. Thus, using a larger propeller with a finer pitch will give your model better ground acceleration with an improved rate of climb. The engine revs will rise after takeoff and the engine will then develop even more power and the plane will pull through large aerobatic manoevers even better.

As most of us do not race pylon racers, most of us are not concerned with achieving a model’s maximum speed when flying.

So which propeller to Use ?

If you have .20 to .60 sized, slower cabin models, trainers, and/or WWII type bi-planes, a 4” pitch propeller will give a good all round performance, with a larger diameter propeller.

For faster .20 to .60 sized trainers and aerobatic sports models, a 5”to 6” pitch propeller should suit most flying applications with a larger diameter propeller.

With very fast .20 to .60 sized aerobatic and very fast sports models, such as deltas, 6” to 8” pitch propellers are required to achieve the necessary high flying speed.

With larger engines, which normally achieve their best torque and power at much lower RPM, higher (coarser) pitch propellers are required to achieve the best flying speed with even larger diameter props to absorb the power of the engine.

Landings

So now the takeoffs are quite good, and your plane climbs well and pulls through aerobatic manoevers without falling out of the sky…………So what about the landings.?

A large, fine pitch propeller will also act as an airbrake when the engine is slowed for landing. The propeller will then be rotating at less rpm than the airspeed requires, and so a steeper landing approach can be made without a significant increase in airspeed. This is when fitting larger wheels to give your propellers more ground clearance and less rolling resistance will also prevent your models nosing over, due to the wheels “digging–in“ on landing.

So how much do large pairs of rubber, lo-bounce, air wheels cost ?….£4.00 to £6.00 ??

AND, how many propellers will they save from damage, at £4.00 to £8.00 plus, per propeller??

It’s a “No Brainer” as well as preventing  damage to the airframe if any landing is less than perfect.

These guys can’t be wrong ? “ http://www.sportpilot.tv/video/bush-pilots-in-Alaska/  “

 Fitting larger wheels will help your models fly better and save you Money

Make Propellers Visible

Feb 26, 2012   //   by Nigel   //   Tech Info  //  No Comments

Make Your Propellers Visible !

We all know the safety rules and common sense about the safe handling of powered model aircraft, and of spinning propellers in particular.  Full size aircraft propellers can easily kill or maim the unwary.

But why consider full size aircraft ?… Because most full size aircraft have the tips of their propellers painted “Bright Yellow”, because this is the best colour to provide a good contrast against all other background colours. Many makes of model propellers are almost “invisible” when running, especially Grey, APC and Graupner propellers which seem to be the least visible, with the Black grp, “Master” and “JXF” propellers coming a close second. 

On many full size aircraft propellers, the rear face is often painted Black. This is so the pilot can easily “see” through the propeller arc without reflections. The tips are then painted “Bright Yellow”. The yellow arc of the spinning tips is then easily seen when the motor is running . 

It was proposed by Roger Perryman (E&DRCC Vice-Chairman), that all members paint the ends of their propellers “Bright Yellow”.  Some members are wisely, doing this already. There are many makes of  enamel paints for modelling, available in small tins, that are proof against petrol and glow fuels. It only takes a few minutes to paint the last half inch (12mm), or more, at the tip of each propeller.  In fact, an extra coat of paint can be easily added to any blade that is “light” to assist with balancing. The tips of  propellers can be gently abraded with medium 360 grade “wet and dry” emery paper to provide a “key” then cleaned off with cellulose thinners prior to painting the tips on both sides of each blade. 

In the 1980’s, on my Rowena 56cc glow motors, I always painted the tips of the 24” x 12” wooden propellers in this way. A propeller that size turning at up to 7,000 rpm could do anyone a lot of damage………….It’s high time I started painting the tips of my propellers “Bright Yellow “……again !!

Humbrol Enamels, “Chrome Yellow” or Flair Paints, “Cub Yellow” are both fuel proof , very visible, and will dry overnight.  

It recently took me 15 minutes to paint the tips of five propellers, whilst fitted to engines and installed in my models. Humbrol Enamels only take a few  hours to dry: Nigel Rollason   

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2.4 Ghz Equipment Installation and Use

Jan 24, 2012   //   by Nigel   //   Tech Info  //  No Comments

2.4 Ghz Antennas: Installation and Use 

The “polar diagram” of the signal radiation pattern of a monopole transmitter aerial is a large donut shape around the aerial with little radiation from the top and bottom. It is a similar three-dimensional shape for both 35 Mhz and 2.4 Ghz types of transmitters.

2.4 Ghz transmitters require a very small aerial, due to the much shorter length of the transmitted radio wave. Pointing the transmitter aerial UP is better than pointing the aerial DOWN. Which is ALWAYS better than folding an aerial sideways across the top of the transmitter!! This only for easy packing and transportation or for flying directly above you, or below you ! I always set my aerial vertical in the same way as recommended by Multiplex. This is the optimum transmitting, and receiving position for any 2.4 Ghz equipment, located at 90 degrees to the transmitter aerial.

With Spektrum and JR. receivers, the second “earth” aerial is the counterpoise part of the antenna system, and is just as important !!

On Spektrum 5 channel AR500 and AR600 receivers, there is one quarter-wave aerial and a lengthened aerial, which is coaxially sleeved to extend it away from the receiver and oriented at 90 degrees to “see” a different part of the transmitted radio wave. This extended antenna is an end fed, half wave antenna, which both transmits to & receives from the transmitter via it’s own donut shaped propagation envelope.  The sleeved part should be gently bent so the one aerial is at 90 degrees to the other aerial. As stated in the manufacturers instruction manuals, NEVER, cut or lengthen these antenna wires.

It is also most important NOT to mount the “receiver” aerial wires close to carbon fibre or any metallic surfaces. You should also keep them away from servo wires as well.  I always use a small pad of foam rubber glued to the fuselage side with contact adhesive, with a pad of velcro glued onto that. The side of the receiver then has a mating patch of Velcro.

It’s either this, or fully supporting the receiver and aerials within foam blocks exactly as 35 Mhz receivers ‘should’ be mounted.  NEVER, mount the velcro patches directly onto the fuselage or helicopter chassis, as we have to insulate any receiver from as much vibration as possible.

With my own 2.4 Ghz receivers, I often cut small cubes of foam plastic and burn a hole through in the correct place with a small, red-hot nail. I then glue the cubes of foam to the fuselage side with contact adhesive, on each side of the “receiver”, so the short aerial wires are then supported within the foam plastic. As everything is then flexibly mounted, removing and refitting a receiver unit is quite easy. On some models I also cut short lengths of silicone fuel tubing to fit over the antenna wire.  Silicone does not attenuate radio signals and the silicone grips onto the moulded plastic stub on the receiver casing. This supports the antennas and keeps them straight.  

Now with metal and carbon structures, and especially large petrol engines with metal spinners, there can be a lot of reflective material for the 2.4 Ghz radio signals to get through, especially when your plane is flying straight towards you !!!

On my own ¼ scale Spinks Acromaster and Spitfire, I will be mounting an extra satellite receiver, offset within the wing structure on an extension cable. These longer extension cables are recommended for large models and supplied by Spektrum !!

IF, you use a carbon fibre propeller as well, this may attenuate your 2.4 Ghz signals. 

Please Remember that ALL 2.4 Ghz radio control equipment uses continuous two way data links……….it is not only when telemetary units are fitted………The receiver is transmitting data back to the transmitter all the time, because how else can the transmitter know when to shift frequency? or the next frequency to use with frequency hopping systems? 

Many modellers have both 35 Mhz and 2.4 Ghz radio control systems for their models. Never Forget to fully extend the telescopic aerial of your 35 Mhz tranmitter, before taking off !! You might get up to a hundred yards of air range with the aerial down………If you’re lucky?  Many pilots have done exactly this, and re-kitted their plane ?

Quite recently, I saw this almost happen again, but I was able to quickly extend the telescopic aerial of another member’s 35 Mhz transmitter, just in time to save his model……….and NO, I’m not saying who………..but he was an experienced pilot, and thankful for my actions. I do like using my  “Classic” 35 Mhz radio control gear. I suppose I am being sentimental, (some say tight-fisted !), But I’m also quite careful too……….

If any members would like to have their 35 Mhz equipment checked on a spectrum analyser, tested for FM deviation and relative transmitter output, as well as having their crystals checked for accuracy, please let me know……….We had a 35 Mhz and 2.4 Ghz equipment testing night at the Ten Tors Inn where many makes of radio control of equipment were tested. How  many perfectly good sets of “redundant” 35 Mhz radio gear are there? 

I also have a special receiver and computer program and that displays the transmitted radio spectrum of 2.4 Ghz transmitters and receivers.

Once you have verified that your transmitters are working properly, the best way to test your any receiver installation is by range testing in the usual way, and comparing the actual range achieved using different aerial orientations…………end on, and sideways to. You will quickly realise the best position for your transmitter aerial…….and it is NOT with the end, or the base of the antenna pointed towards your plane !!.

Please look at the following Video supported by Spektrum / Horizon Hobby about 2.4 Ghz radio gear installation . The two presenters seem a bit “goofy” but their facts are good and describe how radio signals radiate and receive via the antennas. Please forgive these “Young American” presenters.

”  http://www.youtube.com/watch?v=AOI1kWuT5FA&feature=player_embedded “ 

NO SIGNAL = NO CONTROL ! 

Voltage Losses in Wiring Systems

Jan 1, 2012   //   by Nigel   //   Tech Info  //  No Comments
  • Voltage Losses : A Salutary Tale

 A re-print of a previous article submitted by Brian Smith

Essential wiring system safety checks: Especially at the start of the flying season

 I always charge my plane up prior to flying, using the normal transmitter and receiver charger as supplied with the set. On arriving at the flying site, I checked the receiver battery with a meter and noticed that the battery was not fully charged. I then fully charged the battery with a “Pro-Peak” 12 volt charger and obtained a reading of 90% voltage.

I then flew the model.  After two flights, I noticed that the battery voltage seemed to be going down quickly. I naturally thought that as the battery was two to three years old, that this was the problem and it needed replacement.

Further investigations revealed that this was not the problem. Taking the battery out of the ‘plane and checking the battery voltage directly on the terminals, revealed that it was still indicating 90% voltage capacity. However, taking a voltage reading through the normal charging lead on the “off” side of the on/off switch, showed a reading of only 60% battery voltage. I then disconnected the battery feed wire to the receiver, switched the switch to “on” and connected the battery to the receiver feed wire directly to my meter. This caused the current to flow through the switch harness. In this mode, I only managed a reaing of 40% battery voltage.

 I guessed that I either had “ Black Wire Corrosion” or that the switch was worn out. This was a JR switch that had been used a lot since 2001.  I replaced the entire switch harness, after which I immediately achieved a battery voltage reading of 90% of full voltage. Incindently, I have taken voltage drop readings along a two foot servo extension lead which gave me only 60% of the battery voltage.

Can anyone confirm that this is a normal voltage drop to expect ?  I also checked a “Y” lead, which indicated a 15% voltage drop. So I tried a different make of “Y” lead. This was equipped with a double socket and this gave me only a 10% voltage drop. 

The moral of this, is that from now on , I shall be testing all my wiring leads before installing them in my plane. 

Brian Smith.

In models with long extension leads, you will need to fit a 6 volt battery pack to compensate for the voltage drop. Also, as you note. It is possible to buy thicker leads made up from more strands of wire.  Which, although more expensive,  are well worth the money.

Nigel Rollason

Radio Propogation of Transmitters

Jan 1, 2012   //   by Nigel   //   Tech Info  //  No Comments
  • Radio Propagation (35 Mhz and 2.4 Ghz)

 The 20 degree cones from the ends of the transmitter aerial are the areas of poorest radio signal radiation. (and reception: 2.4Ghz).

The ratio of signal reduction compared with the maximum signal off the sides of the aerial is up to 10 to 1………….with more, or less  depending upon local conditions which are affected by damp ground and reflections from nearby vehicles and structures.

This is a tested and proven FACT…………..whatever frequency you use !

So even with 2.4 Ghz transmitters fitted with stubby antennas, NEVER point the end of the antenna directly at your plane ………..especially at a distance. It also has to receive the return data signals from the plane to maintain it’s bind  

I have seen many flyers using the transmitter with the antenna folded flat along the top…………This will work fine as long as the transmitter is pointed directly at the plane, BUT, when the plane is oriented to one side or the other at a distance, you’ll have the TOP, or worse, the folded BASE of the antenna pointed at your plane………..with a significant reduction in radio range………..and the loss of receiver binding, and, your model !!

Try a range check with the transmitter aerial pointed directly at the plane.

(One section of aerial: 35 Mhz, or reduced power: 2.4 Ghz  )

At about 100 yards (95 metres) distance, slowly lower the transmitter to within one foot (300mm) of the ground…………with the aerial still pointed directly at the plane …………..this will attenuate the transmitted signal and save you walking too far.

How far do you get before the receiver loses it’s signal ?………Do you get an immediate return of control when the aerial is re-oriented at 90 degrees to the plane ? Just by moving the transmitter very slightly, you will gain and lose the bind. Even with the latest equipment, it can take a least a few seconds to recover and re-bind the data link.  

Please look at out the following Video, supported by Spektrum / Horizon Hobby. The two presenters appear to be a bit “Goofy” but their advice as regards radio installation and use is good information, although, they do forget to mention that the “receivers” are also transmitting data back to the “transmitter” all of the time in order to maintain the “bind” or  “Lock-On” .

”    http://www.youtube.com/watch?v=AOI1kWuT5FA&feature=player_embedded “

No Signal = No Control

Quieter Engines

Jan 1, 2012   //   by Nigel   //   Tech Info  //  No Comments
  • Quieter Engines (Exhaust & Induction Noise)

Modern “super-quiet” types of silencer have been designed to achieve greater reductions in noise level without too much reduction in power output. In general, the larger the internal volume of the silencer, the greater is the reduction in noise output without loss of power.

 With reference to the diagram:
Baffle position (A) can give you the lowest noise output.
Baffle position (B) can give you a better performance, (200-300 rpm+) but at a higher noise level.

Some silencers have aluminium baffles at (A) and (B) to create a silencer with three chambers. These are normally alloy washers that can be drilled out or sleeved to suit.
Steel ‘penny’ washers can be often fitted into single chamber silencers by reducing their diameter to fit using a lathe. This simple modification will reduce the loud, hollow sound of older types of silencer.

On some, you can open up the baffle holes by using a pointed pair of pliers, for more performance, or close them using a hammer and a piece of dowel on a flat surface, to reduce noise. But be careful, closing the baffle holes too much can cause an engine to overheat and create running problems. If this happens, just open up the baffles until satisfactory running is achieved.

Note C:  The noise output can be further reduced by cross drilling and tapping the outlet pipe to fit an M3, or M4 grub-screw. This will reduce the outlet area and reduce the noise output. Some care and attention is needed to do this without damaging the silencer.

Firstly, only drill thro’ and tap at 90 degrees to the die line. Never drill through the die line or it can split. Secondly, this will increase ‘back pressure’ to the engine if the screw is too big and reduces the outlet area by too much, the engine will lose power and overheat. (The needle adjustment may also become sensitive). Once you are happy with the noise reduction and performance of the engine, remove the grub-screw and re-fit with permanent thread lock (e.g. Loctite) then fit a silicone extension pipe.

An alternative is to tap the outlet bore to a suitable thread size then screw in a threaded insert with a smaller hole through the middle. And again fit a silicone extension pipe to direct exhaust residue away from the airframe.
These super quiet silencers normally assemble with a long through bolt that often screws into the rear outlet piece.

Please note: The nut at the outlet end is normally the lock nut for the assembly. If you try and tighten the silencer by only using this nut you will strip the threads!! To dis-assemble, or re-position the outlet to suit the model, undo the lock nut anti-clockwise, then loosen the assembly bolt from the front of the silencer.

Re-assembly is the opposite. Use the correct screwdriver to tighten the bolt into the outlet piece, then fit the lock nut for added security, using a small spanner.
Should any part of the silencer leak, simply take it apart, (as above) clean up the components with cellulose thinners, and re-assemble with high temperature silicone sealant around the joints. Then clean off any excess silicone.

Note: The actual silencer joint to the engine exhaust outlet is best sealed with a very small amount of epoxy adhesive. This ensures a secure, gas tight seal. How many times have you seen loose silencers with black oily residue coming from the joint ?  Please ensure the mounting bolt threads are lightly oiled to ensure they are not epoxied in solid !

The silencer outlet stub is always best extended with a short length of straight, or cranked, silicone tube. Fitting one of these these will reduce the sound output by about 1 dB(A), as well as directing exhaust residues away from the plane.

JP products make small add-on silicone expansion chamber silencers for .30 to .50 size motors. They can be fitted to both two stroke and four stroke engines. The outlet is moulded internally to accept an outlet reducer sleeve. But you will have to make your own sleeve from alloy or silicone tube. These are easy to fit and cost about £4.00 each.

Induction Silencers & Aircleaners  Many carburettors are noisy due to “induction roar”.  On engines fitted with effective silencer systems, this “induction roar” can be quite loud at full throttle. On most carburettors, there is a “bell mouth” to direct air to the inlet venturi. It is this bell mouth, acting like a megaphone, that amplifies the sucking noise of each induction stroke.

This can be reduced by fitting induction silencers or air filters to the carburettor inlet.

JP products make a range air filters to suit model engines. They come in various sizes to suit the inlet spigot diameter. A metal ring secures a disc of fine metal gauze at the top and this in turn is connected to the carburettor spigot by a short length of silicone tube and held in place with a small cable tie. I find it is best to shorten the silicone tube by about 5 mm to create more clearance from the propeller.

These cost about £2.00 each, and as they won’t break the bank, they are worth fitting, as they will also prevent dirt from getting into the engine. My own OS 20, Thunder Tiger 40, RMX 40, MDS 48 and JEN 56 engines, all run well with air filters fitted, although the mixture needle may need adjusting slightly to prevent richness at full throttle.

On larger engines fitted with Tillotson or Walbro carburettors, the induction roar can be extremely loud when the exhaust noise has been subdued by an effective silencer. I will be fitting a large, lightweight air filter to my 35cc Webra Bully glow motor that is fitted this type of carburettor.

The BMFA / Department of the Environment recommended maximum noise level, is 82 dB(A) at 7 metres, measured at every 90 degrees from the model. Even if your model does pass the noise test, it is no reason NOT to attempt to make it quieter, if possible.  These days, it is extremely rare for a model to be underpowered. Model noise is a technical challenge where we can all help each other to achieve satisfactory results.

Richard and I, hope these notes will help members to make their engines quieter. It is only by everyone ensuring that their models are as silenced as “effectively” as possible, that any noise complaints can be managed in a proper manner.

This is an article based on notes and ideas submitted by Richard Winter. (Club Safety Officer).

Nigel Rollason. Hon Secretary.

35Mhz Interference Tests

Dec 30, 2011   //   by Nigel   //   Tech Info  //  No Comments
  • 35Mhz. Interference Tests

The new Exeter & District Radio Control Club flying site at Wrayford Field, near Kingsteignton, is only 2.1 miles from the  flying site at Little Haldon and only 2.3 miles from the Teign Valley flying site at Newton Abbot Racecourse. It was considered necessary to conduct actual flying tests to see if co-channel interference on 35 Mhz would be a problem, or not. Our concerns were due to both sites being each side of a large valley and easily visible each way with the naked eye.

 
 

 

Wrayford Field by Moto-Cross Track

On Saturday afternoon, 12th February 2011, I prepared my old Junior 60 fitted with a Futaba FP-R115F PPM receiver and servos, to operate from two different Futaba 35 Mhz transmitters. A “J” Series six channel transmitter, and a Gold “F” series seven channel transmitter. Both of early 1980’s vintage. Both transmitters had equal power outputs and RF bandwidth as verified on a spectrum analyser. The Gold “F” series transmitter had full alignment tests five years ago at the Ripmax service centre, along with a written technical report that verified full compliance with all EC regulations for 35 Mhz radio control equipment. Both transmitters were operating on Channel 70, which is in the middle of the 35 Mhz band.

Richard Ray picked up the Futaba Gold transmitter and the club “Mainlink” 35 Mhz frequency scanner from my house and then drove to the new club site with his wife. I drove to Little Haldon with my wife, assembled the Junior 60 and we walked to the runway intersection. Roger Perryman ( E&DRCC Vice Chairman) met us there to assist and observe the tests. Telephone contact was established between our repective wives and Richard confirmed that no 35 Mhz signals were being received on the scanner, now located on his car roof. My transmitted signals from Little Haldon were not being received by the scanner. So far so good. 

I started the OS FP20 fitted with a wooden 11” X 4” propeller, and with a short takeoff run, it climbed briskly into a 10 mph south-westery breeze to an estimated altitude of over 500 ft, at about 500 yards distance towards Richard’s location. The height and range was at the limit of my “comfort zone”. Richard was instructed to switch on and hold the transmitter high and rotate the aerial to the horizontal plane and back again. Nothing happened…..Not a single glitch…..Full control was maintained. By this time, I had fed in full down trim and shut the engine down to tick over. The Junior 60 as still climbing in the slope lift like a homesick angel.!!

I then switched my transmitter off, the engine immediately went to full throttle and the plane commenced a left hand spiral dive…….I switched on again quickly, and full command was resumed immediately. This was exactly the control inputs that Richard was giving…..with 50% rates ON.

I switched my transmitter off again. As the plane commenced a full power left hand spiral dive again, I requested, via our telephone operators, that full right rudder be given. The Junior 60 immediately banked to the right. I then called for half throttle, but this was misunderstood and the plane reverted to a left hand spiral dive. I then switched on again and full control was immediately resumed. This was as Roger voiced his concerns, just a moment before the wings might have been ripped off !

 I could have stayed in the slope lift but did a long traverse with the aerial pointed straight at the plane giving minimum signal propogation and glided down with a dead stick engine for a landing just 25 yards away.

 Results:

:         With the plane at over 500ft height in distant visual range and at an estimated horizontal distance in excess of  500 yards towards Richards location, the main transmitter was in full control at all times with no observed glitches.

 2:         When the main transmitter was switched off, control was immediately established by the “interfering” transmitter.

 3:         Each time the main transmitter was switched on again, full control was immediately re-established.

 Conclusions: 

1:         The known FM radio receiver phenomenon known as “capture effect” whereby the receiver “locks-on” to the strongest signal was apparent during this test.

 2:         The observation that control was established by the distant transmitter when the main transmitter was temporarily switched off, showed that the PPM receiver was sensitive enough to lock-on to the other transmitter at an estimated one and a half miles range.

3:         Full control of the model could be maintained at all times when the main transmitter was switched on.

 4:         The COMPULSORY use of ODD or EVEN channels on 35 Mhz at our local flying sites need NOT be imposed.

 Recommendations:

1:         These test results and recommendations shall be distributed amongst the local flying clubs.

2:         It is a recommendation that EVEN channels are used at Little Haldon for all model aircraft.

 ( This due to prior use of EVEN channels on a national basis by model gliders)

3:         It is a recommendation that ODD channels are used at the E&D RCC flying site at Wrayford Field.

4:         It is a recommendation that EVEN channels are used at the Teign Valley Flyers site at Newton Abbot Racecourse.

 These are only recommendations because the testing of all makes of transmitter, receiver and model combinations would be an impossible task. The growing popularity and use of 2.4 Ghz equipment avoids frequency clashes.

 Nigel Rollason  (Hon.Secretary Exeter & District Radio Control Club)

13th February 2011

Adverse Yaw

Dec 15, 2011   //   by Nigel   //   Tech Info  //  No Comments
  • ADVERSE YAW

 What it is, and how to cure it

 Often, when I mention adverse yaw to other aeromodellers, I get a blank look as if I’m off another planet. It is a common aerodynamic condition that is rarely mentioned in the modelling press, almost as if it’s a dark secret. Pilots  full size aircraft rarely get a second chance to cure the problem, as the usual outcome is for the plane to just “ fall out of the sky” with teminal results for the plane and the pilot.. This is a very serious condition which can happen in an instant, usually just after after takeoff, or on final landing approach when flying low and slow. When flying slowly, just above normal stalling speed, it is always necessary to use “up” elevator to increase the angle of attack of the wing in order to generate suffivcient lift to keep flying. So far so good, nothing amiss so far.

On many models and full size aircraft, the ailerons move up and down by equal amounts. This is fine when flying at normal flying speeds……BUT, when the wing is at a greater angle of attack to the airflow when flying slowly, the downgoing aileron will then have more drag than the upgoing aileron. The result being, when right aileron is given to roll the plane to the right, the plane will YAW to the left, and vice versa. This is why many full size aircraft always require simultaneous rudder AND aileron input to roll to the left or right.  (Most First World War aircraft required this) This is why coupled aileron and rudder is recommended on many high wing models that have a flat bottom wings and little dihedral. So why do planes just “fall out of the sky” ?  As the plane yaws, the fuselage is then angled sideways to the airflow, creating a lot more drag and also, the air is angled across the wing section which raises the speed at which the airflow over the wing will stall. AND,  because of the increase in drag, the air speed then drops very quickly to below the now raised stall speed and gravity wins again !

This is also the principle of a “ sideslip approach “ whereby an aircraft can approach a runway with a very steep approach and keeping the runway in full view, without building up excessive speed. The increase in drag caused by the plane flying slightly sideways prevents the plane from speeding up. Full rudder is used in conjunction with the ailerons to keep the wings level. In this case, the steeper approach, assisted by gravity, keeps the aircraft well above the minimum stall speed.

How many full sized aircraft prevent adverse yaw.  The “Cure“ is to ensure that the upgoing aileron always has the same, or more DRAG than the downgoing aileron. Sounds simple doesn’t it ? In practice, it is……..usually by mechanical adjustment of the aileron linkages as in the full size, or by transmitter adjustment, if you have that facility. For example:

1: The Pitts S2A Special has 60% up aileron and 40% down aileron movement.

2: The Tiger Moth has significantly more up aileron movement than down aileron movement. (I’ve been told, as much as 75% up and only 25% down) 

I used the above settings on a 1/3 scale Pitts S2A and it was absolutely spot–on first go and never needed changing. Why did I take the trouble to find out the full size settings from a friend at the CAA ??………….because years earlier, I had built a quarter scale Pitts S1 Special that had ailerons only on the lower wing, and powered by a Webra 60. I thought it was a good idea to droop both ailerons like flaps to give more lift at take off and enable it to fly more slowly.

WRONG !…….It taxied and took off ok but the first aileron input saw it cartwheeling through the grass on each of three attempts.  I didn’t know what was happening, I could fly, but my plane didn’t ? So I asked a more experienced model pilot to try the next takeoff………yes, he got it up, and higher than me, and fought two or three vicious adverse yaw flicks on the climb before the airspeed dropped, gravity won, and re-kitted the plane. Then another modeller knowingly took me aside and slowly explained the cause……………He was so right. 

My next semi-scale biplane also had ailerons on the lower wing so I made sure they were both slightly “UP” when in the neutral position and adjusted to give 66% up movement and 33% down movement. The first flight was absolutely fine with the ability to steer the plane using ailerons only. The upgoing aileron having the most drag so the plane started turning in that direction without any rudder input. Rolls were a bit corkscrewey and lost height unless the plane was climbing at about 15 degrees before the roll. So tuning out SOME of the aileron differential was easy to do over the next few flights until rolls were more axial and, at height, full aileron just before the stall did NOT yaw the plane wrong way. 

I am currently building a YT Spitfire Mk XIV to a reasonable flying scale standard and will be incorporating 60% up and 40% down aileron movements for my first flights and NOT the recommended 25mm up and 25mm down. Especially from reading the YT Spitfire blogs, some have spun in on landing approach especially when built with many scale additions. So if my rolls don’t please me and it aileron steers too readily, I can always fine tune the aileron movement for later  flights………..I should get more than one chance to get it exactly right …………….     

Follow this link to the very similar KMP Spitfire build with it’s first and final flight…………….What do you think happened ?? 

http://www.hawkertempest.se/uploads/spitfire.htm

Noisy Plane ?

Oct 19, 2011   //   by admin   //   Tech Info  //  No Comments
  • Noisy Plane ?

Not all the noise emitted from an aeroplane is exhaust noise. Although properly designed and fitted silencers work very well, the final choice of propeller can make a considerable difference. Scimitar type blades with curved ends will reduce most of the noise created at the tips, especially if the blade tips go supersonic. Wooden propeller blades tend to self damp with regards to natural resonances. Zinger and Master propellers with square ended tips are noisier than most others. As with any  propeller, they must ALL be carefully balanced before use, with no visible damage, and have a smooth finish.


New propellers are seldom in perfect balance. In my experience, less than 10 % I have purchased over many years have NOT required any re-balancing. An unbalanced propeller will cause vibration that will be transmitted to the airframe of the model..  Excessive vibration will reduce engine and airframe life, by loosening the engine and silencer mountings and also damaging the servos and control mechanismst. Most model aircraft engines have just a single cylinder. Only a percentage of the reciprocating piston and conrod mass can be balanced by the rotating crankshaft mass. Thus, some vibration will be present at all times. Although this will vary in frequency with engine rpm and amplitude according to the crankshaft “ balance factor “ and any damping effect of the engine mountings.

 Electric motors are easily damaged by out of balance propellers. The motor bearings will have a much shorter life and the internal magnets have been known to come loose. A badly out of balance propeller can even damage the output shaft of the motor.  

Rubber mounting any engine will reduce the vibration transmitted to the airframe. Thus, less noise will be radiated from the model. Foam cored wings tend to damp airframe noise quite well. I have “heard” many modern models with strongly constructed lightweight airframes and solidly mounted engines . Although the design and construction has been of excellent quality, the airframes tend to be very noisy and resonate like a drum at certain engine speeds. These really are a case for rubber mounting the engine, as airframe vibration at these levels will cause a significant reduction in servo life and control fittings. This is assuming the receiver is properly secured and isolated from vibration. Velcro mountings on their own do not properly isolate receivers from vibration.

Ocasionally, a plane that seems reasonably quiet when on the ground, can become noisier when being flown. This is often due to increases in engine speed the airframe resonance. Two of my own planes have exhibited this phenomenon. On both occasions it has been due to undercarriage vibration. One plane, a “Limbo Dancer” once the exhaust and propeller noise had been reduced to acceptable levels, I could then hear the wheels vibrating on the axles in flight. So I bushed the wheels with plastic sleeving from some electrical wire and lightly oiled the axles. Job Done!…..Vibration cured……………. On a 1970’s vintage aerobatic design with a fixed tricycle undecarriage, the torsion bars of the main undercarriage vibrated in sympathy with the exhaust note to produce a loud vibration noise. So again, I fitted plastic sleeving from electrical wire to prevent the torsion rods vibrating in the wing mountings.

 So, having a quiet plane is not just about keeping the neighbours happy. It is also about good engineering practice and reducing electronic and airframe failures. And, saving a lot of your hard earned money in the long run! I clearly remember the radio controlled model aircraft of the 1960’s. Nobody ever used any silencers at all !!  And boy!!!………….did those McCoy 60’s, Merco 49’s and similar engines …………

                                        CRACKLE!!

             *****************************************

Hints & Tips

Oct 19, 2011   //   by admin   //   Tech Info  //  No Comments

I prefer to fit larger diameter propellers than is recommended by most engine manufacturers.
Over the years, largely by trial and error, rather than trying to get maximum rpm on the ground, I try to get maximum static thrust for good acceleration and climb out.
( Maximum Engine Torque or BMEP = Brake Mean Effective Pressure)

This is how engines were tested and propellers were selected for most early aircraft. ….There is a tree preserved at Farnborough Aerodrome where many early aircraft were tethered for engine tests.

In this way, the acceleration, take-off  and climb out of most models can be significantly improved.

As a general rule of thumb if a 12″ x 6″ propeller is recommended for a 10cc motor, then this can be simply transposed to a larger 13″ x 5″ or even a 14″ x 4″ propeller which will load up the engine in a similar way within it’s useable RPM range.
Bearing in mind that the engine revs will increase as the model achieves it’s normal flying speed.

Also, the engine revs will rise during a dive, but the actual air speed will be not so high when compared with a smaller propeller with a greater pitch.

And, on a landing approach, when the engine revs are reduced, a large fine pitch propeller will also slow the model up when the airspeed is greater than the engine rpm and propeller pitch will allow, so a few more revs are often needed untill final flare out for landing or a slightly steeper approach can be made without any increase in airspeed.

Transversly, if you want a plane to fly faster, then you would normally fit a propeller with a greater pitch. But, if the engine is loaded up too much, and the engine rpm will be lower and the plane may be be sluggish at low speeds, then for every inch you increase the pitch, select a propeller with an inch smaller diameter.
So if a plane is initially flying with a 15″ x 6″ propeller, then fit a 14″ x 7″ propeller to fly faster or a 16″x 5″ to fly slower, but climb better.

I have used this principle on many different engines and aircraft over the last 50 years, from Cox .049’s, and various 40’s, 48’s, 60’s, and 90’s, up to 56cc glow motors from first flights with a 20″ x 16″ propeller to an eventual 24″ X 12″ propeller which gave an improved flight performance.

Successful motor and propeller combinations, have been as follows:

Cox .049’s = Tornado 6″ x 4″…..or…….7″ x 4″

Enya .19 = APC 10″ x 5″ ( Small trainer)

OS FP.20 = Smart Prop 11″ x 4″ (Junior 60)

OS .26FS = Smart Prop 11″ x 4″ (Junior 60)

OS.25 = APC 10″ x 5″ (Funfly) 

SC .25 = APC 10″ x 5″ APC (Funfly)

MDS .40 = APC 12″ x 4″ ( Limbo Dancer)

RMX .40 = APC 12″ x 5″ (Arising Star Trainer)

Irvine .46 = APC 12″ x 6″ (Irvine Tutor Trainer)

SC .40 = APC 12″ x 5″ (Boomerang Trainer)  

MDS .48 = APC 13″ x 4″W (Wildcard)

Webra .60 = Robbe “quiet prop”11″ x 8″ (Tuned pipe)(Bulldog FAI Aerobatic)

Webra 10cc = APC 12″ x 7” (Crossflow engine: Harry Brooks “Rebel” )

OS .70 ( 4T) = APC 13″ x 6″ ….or …..14″ x 5″ (1/6 scale Harvard)

Webra .60 = APC 13″ x 6″…..or……14″ x 5″ (1/5 scale Spinks Acromaster, Sukhoi 31, 1/4 scale Cosmic Wind)

Webra .90 = JXF 16″ x 6″…..or……17″ x 5″ (1/4 scale Spinks Acromaster)

RCV 120 = Graupner 20″ x 10″…..or …..three blade 18″ x 12″ (1/6 scale Mk XIV Spitfire)

Webra Bully 32cc = JXF 18″ x 10″…..or……20″ x 8″ (1/4 scale Spinks Acromaster)

Rowena 56cc = Airflow 24″ x 12″ (1/3 scale Pitts S2A)

Comparative Propellor Loading Chart[1] This chart clearly hows the relative power required to turn various sizes of propellers at their optimum rpm. It clearly shows that adding or reducing the diameter of a propeller by one inch will also require the pitch reducing or increasing by one inch to achieve a similar loading.  

However, after conducting over 225 BMFA/DoE model noise tests comprising many different propeller, engine and airframe combinations since October 2011, the E&DRCC noise meter operators have realised that more consideration must be given to the “Noise” generated by the propeller.  Often, fitting a larger diameter propeller will reduce the engine rpm and make the exhaust note quieter.  But, the noise created by the propeller tips has then been even noisier!  To reduce the tip noise, it has sometimes been necessary to fit a smaller diameter propeller with a greater pitch to achieve an overall reduction in model noise.  Or, by changing both the make and type of propeller.  Three or four  bladed propellers are normally quieter than an equivalent two bladed propeller due to the smaller diameter and reduced speed of the blade tips.  When the propeller blade tips go supersonic, a lot of extra noise is generated by the continuous shock waves created at each blade tip. 

Most propeller manufacturers have introduced new designs onto the market in recent years.  The modern trend is for the propeller tips to be slimmer and curved to a “scimitar” shaped tip. These are now much quieter and more efficient than their older products.  The turbulence created by blunt  propeller tips  creates lots of noise and drag that also robs the engine of  power.

I will soon be testing some modified propellers.  By carefully re-shaping the blade profiles of various sizes of Zinger and Master propellers, then measuring the reduction in noise created by the tips.  I will be creating tip profiles similar to the “Robbe” quiet props which is almost identical to the “Smart” quiet props and most APC propellers. The latest types of “Master”, “Graupner”, and other popular propellers all have similar shapes.  I will be publishing my test results later this year.

 I hope this article helps you to choose a propeller which is better suited to your model and style of flying.

FITTING THE PROPELLER

ALWAYS fit any two blade propeller in the horizontal position with it just rotated gently against compression.

WHY?…..Because IF your model has a dead-stick landing, the motor will stop against compression with the propeller in a horizontal position. You are then less likely to damage the propeller if the landing is often less than perfect.

This is ALSO, the optimum position for hand starting ANY motor (If you’re right handed) with the hand pulling horizontally into the body when flicking the prop.

WHEEL SELECTION

 Most ARTF models are fitted with very small wheels and often, wheel spats as well……….which are perfectly fine when flown from smooth tarmac runways.
As we fly off grass, as do many other model flying clubs, models will take off and land more easily if larger wheels are fitted and the wheel spats removed !
On our flying strip, we find that the smallest useable wheel size is about 3″ dia ( 75mm ) with 3.5″dia, or 4″ dia wheels giving a lower rolling resistance which make for easier take-offs and smoother landings.

Buying a larger pair of wheels won’t break the bank, and easier, shorter takeoffs will be welcomed by most as well as giving more clearance for larger propellers.
However, if your model is fitted with retracts, please ensure that the wheels won’t jam in the wings. This is because a stalled retract servo will quickly pull down the receiver battery voltage, often with disasterous results.

This is all common knowledge for experienced model flyers, but we need to help the newcomers to our hobby enjoy safe, sucessful flying. It is most important that their “budget holders” are not put off by damaged models.

ENGINE MOUNTINGS

When mounting engine mountings to the firewall, bed them onto epoxy resin.
This will ensure that fuel and oil will not seep behind the metal or plastic mounting bracket and soak into the airframe structure. I always paint the bulkhead area and inside the cowling with a good fuelproof paint.
I prefer to use a matt black, fuel proof paint.

LEAKPROOF SILENCER to EXHAUST JOINTS

When finally fitting the silencer to the exhaust stub on the motor, ensure both faces are flat and perfectly clean, then use a tiny smear of mixed, epoxy resin on both faces.
Please make sure the mounting screws and silencer threads are lightly oiled to prevent the screw threads from being epoxied as well.  When the silencer needs to be taken off , remove the screws or clamp, then a light tap with a screwdriver handle will easily open the joint.  

VIBRATION PROOF METAL QUICKLINKS

Seal the metal threads with a blob of flexible “PVA Canopy Glue”.
Locknuts are not required, and adjustment is still easy. It works like a plastic locknut, yet does not immobilise the thread like locktite !

IMPROVE THE RUNNING OF MOST ENGINES

Always shake up your fuel to mix it thoroughly before using it for a days flying. Some mixes of glow fuel separate out slightly to give an uneven mix from top to bottom of your fuel container. Some petroil mixes can also de-nature the petrol and lubricant mix over short period of time.

Motor Cycle race teams always use old two stroke fuel mixes just for cleaning engine parts.

Glow fuels sometimes develop small pieces of organic matter which can form a light covering at the bottom of the fuel container. It is seen in suspension once the fuel has been shaken up and easily seen when the container is held up to sunlight. But all is not lost, as this fuel can be easily filtered using a paper coffee filter positioned in a plastic funnel. It normally takes about half an hour to filter one gallon of fuel and it is sometimes necessary to change the filter paper after each three or four pints filtered. As Methanol is hygroscopic (absorbs water), Filtering your fuel in this way can also remove water from your fuel.

It also pays to remove the fuel tank from your model every year or two, to see if any debris or oil residues have collected in the tank. These residues can often go un-noticed for a number of flights the next flying  season, until a blob of congealed oil blocks the carburettor needle or causes irregular running.

Finally, Always fit an inline fuel filter between the tank and the carburettor, as well as using a filter in the pump outlet of the filler pipe.  Who really wants a deadstick landing anyway ??

NOTE:  All fuels are highly inflammable. Please ensure that no naked lights, or any heaters, are in the room. It is always best to handle fuels in the open air, on a warm, dry day.