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.

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