Airfield Models - Model Aircraft Engines

Selecting and Using Propellers for Model Airplane Engines

January 21, 2009



Home
About
What's New
History
Models Gallery
Model Building Safety
Articles
Mail & FAQ
Site Map
Site Feedback
Contact
Register
Add to Favorites
Tell a Friend
Comments
Design and Build Contest
Items For Sale
Search Airfield Models

Back to Model Aircraft Engines

 



 

Airfield Models (http://www.airfieldmodels.com/)Model Aircraft Propellers

Caution!  Some propellers have extremely sharp edges especially plastic and fiberglass props.  For your own safety, scrape and sand this edge down.  If you try to flip the propeller by hand, you will get a nasty razor cut as your fingers slide along the back of the blade.

Propellers come in a variety of sizes and styles and are made from five materials that I know of:

  • Wood (Maple, Beech or balsa wood on lightweight rubber powered models)
  • Nylon
  • Fiberglass-reinforced Nylon
  • Fiberglass
  • Carbon Fiber

Wood, Fiberglass and Carbon Fiber props give the best performance.  Nylon props are the least expensive, but they are flexible, cause vibration and rob power.

Do not use nylon propellers!

With the exception of nylon propellers (not to be confused with glass-filled nylon which are fine), all commercially available propellers work well.  If you choose a propeller that the engine can swing in its comfort zone then it is a matter of personal preference after that.

Also see

 
 
A variety of commercial propellers

Typical Propellers

From top to bottom:

  • Master Airscrew fiberglass-filled nylon
  • Zinger
  • Top Flight Super M (out of production)
  • APC fiberglass
  • Top Flight Power Point (current style)
  • Graupner fiberglass-filled nylon
  • A different style Graupner fiberglass/nylon
  • Top Flight Super M

All of these propellers are fixed pitch meaning the pitch is not adjustable.

Propellers are designated by two numbers: Diameter and Pitch.  Thus a 12-6 propeller is 12" in diameter and has 6" of pitch.  Pitch is the distance a propeller will move forward in one revolution in a perfect fluid (which air is not).  Therefore, a 6" pitch will move forward 6" with each 360 revolution of the propeller.

There are also a variety of styles as you can see in the photo above.  For example, there are wide and narrow blade propellers.  Additionally there are scimitar propellers such as the APC gray propeller fourth from the top.

The last time I came across numbers regarding propeller efficiency was over ten years ago.  At that time the best propellers were about 80% efficient.  Note that propeller efficiency is not the determining factor of overall flight efficiency.

The propeller should be chosen to match the aircraft not the engine.  For example, mounting a racing propeller to a WWI aircraft will severely limit the model.  An early warbird has so much airframe drag that the propeller will never come close to living up to it's potential and the model will be a sluggish flyer at best.

By the same token, using too slow of a prop (one with low pitch) on a model intended to go fast may prevent the plane from gaining enough speed to fly at all.

A lot of pilots make the mistake of finding a propeller that works great on a certain engine in a certain airplane.  From then on they swear by that propeller on the same engine regardless of the aircraft.  If the pilot were to experiment with other propellers when the engine is in a different aircraft he would probably find that the current "best" propeller isn't best any more.

 
 

Propeller Properties

Pitch

  • High Pitch Propeller properties:

    • High speed flight

    • Poor Acceleration

    • Poor Climb

    • Can be difficult to slow down for landing

  • Low Pitch Propeller properties:

    • Low speed flight

    • Good Acceleration

    • Good Climb

    • Finer speed control throughout throttle range particularly at low throttle settings

The easiest way grasp the concept of propeller pitch is to draw a parallel to the gearing in your car.

  • Low pitch propellers = low gear in your car.

    It will get you up hills well but will not take you any where fast.

  • High pitch propellers = Beginning your drive in fifth gear

    It will take forever to accelerate to speed but the plane is cruising when it gets there.

Propeller Material

Wood propellers are the lightest and present the smallest load to an engine assuming all else is equal (diameter, pitch and shape).  They are capable of turning higher RPM than a heavier propeller.

Wooden propellers are also the most easily broken.  My opinion is that wood propellers are the most efficient and best performing in the air.  Others disagree and they may be right because I'm not a good enough pilot to really have an opinion.

In any case, if you nose over your planes often enough then wood propellers are probably a poor choice for you.

Fiberglass-filled nylon propellers are the heaviest propellers and also the most durable.  These are a good choice for beginners because they hold up better than other types to propeller strikes.  They are less efficient than wood or carbon fiber propellers, however.

Carbon Fiber Propellers are very rigid, but extremely expensive.  I have seen carbon fiber propellers only for large engines.  They may be available in smaller sizes in the future.

All of the above propeller materials maintain their shape well under load.  Wood and carbon fiber are best.

Fiberglass-filled nylon propellers are the most flexible of propellers that I recommend, but not enough to cause significant problems.

Pure Nylon propellers are always a poor choice unless you crash every time you fly.  If that's the case, then nylon propellers aren't the answer for you.  Stamp collecting is.

Nylon propellers are so flexible that they twist in use which means they are constantly changing pitch.  This flexing also creates a lot of vibration.  The end result is akin to spinning the wheels of your car a lot of energy is going to waste.  Nylon propellers are just bad.  Don't use them.

Weight

Heavier propellers have the advantage of flywheel action.  Flywheel action will allow a lower, more reliable idle.  Note that all properly designed engines idle reliably anyway, so this is really a moot point.

Another consideration is balancing the aircraft.  Personally, I think the best propeller should be chosen for flight qualities, not for weight, but if the aircraft is close to being in balance then changing to a propeller of different weight may put the CG on the money.

Shape

Low RPM engines, such as four-strokes, use wide blade propellers because four-strokes turn fewer RPM.  The lower RPM means the air that the trailing blade is entering is less disturbed.  Additionally, four-strokes have more torque than two-strokes.  Because of this, a four-stroke can swing a wider propeller efficiently.

High RPM engines, such as two-strokes are more efficient with narrow blade propellers due to the more disturbed air caused by the higher RPM.

I can't tell the difference between a scimitar blade and any other shape.  I don't even know what the difference in performance is supposed to be so I can't help you here.

 
 

Determining the Range of Suitable Propellers for an Engine

Engines like to do work.  They are happiest running with a load.  An engine can over-rev and be destroyed in short time if it has too little of a propeller load.  If the engine has too much of a load it will be sluggish and tend to over-heat.

Any engine can swing a wide range of propellers provided the propeller load is within the engine's comfort zone

As a rule of thumb each inch subtracted from one property (pitch or diameter) allows one inch to be added to the other.  This theory permits any of the following propellers to be used on the same generic .40 size engine:

  • 8 x 8
  • 9 x 7
  • 10 x 6 (propeller recommended by manufacturer)
  • 11 x 5
  • 12 x 4

This rule breaks down as the propeller size moves farther from the center of the range.

For example the engine above may not be able to swing a 12" propeller of any pitch without over-heating.  Or it may have to drop additional pitch.  The 12 x 4 propeller may not work, but a 12 x 3 might if such a thing is actually available.

Also note that the load presented by any given propeller varies by manufacturer and material the propeller is made from.  One manufacturer's 6" pitch propeller may be the same as a 7" pitch from another manufacturer.

 
 

Variable Pitch Propellers

Wermacht is the Airfield Models Shop Kitty

Wermacht - Master Modeler, Elite Infanty KittySpeed Vs. Pitch

In this example two different propellers will be mounted to the same engine.  One propeller has 4" of pitch and the other has 8" of pitch.

The propellers used present an identical load to the engine at all settings.  This is accomplished by lowering the diameter of the high pitch prop.

We have a perfect model aircraft having zero drag in a perfect world where propellers are always 100% efficient and there is no such thing as drag.

The lowest reliable idle is 2,000 RPM.  Each additional click of throttle adds 500 RPM.

At 2,000 RPM we calculate the following airspeeds:

4" Pitch = 7.8 MPH

8" Pitch = 15.2 MPH

Add one click of throttle (+500 RPM) and calculate the new airspeeds:

4" Pitch = 9.5 MPH

8" Pitch = 18.9 MPH

Speed increase:

4" Pitch = 1.7 MPH

8" Pitch = 3.7 MPH

Conclusion:

The lower pitch propeller provides finer speed control.

As you can see, there is no single ideal propeller for the entire flight envelope.  When landing, for example, a finer pitch propeller will allow minute control over the speed of the aircraft because small changes in RPM using the throttle translate to small changes in speed.

See sidebar to the right for a specific example.

A high pitch propeller is desirable if you want an aircraft to fly very fast.  It is easy to see that any propeller selected will exclude desirable (and undesirable) properties available with a prop having a different pitch.

This is why most sport pilots choose a middle of the road propeller.  The airplane won't achieve the highest possible speed that the high pitch would allow, nor will it have the best climb that a low pitch would provide.  Instead the pilot is teased with a little of both.

This is the reason variable pitch propellers were invented.  A variable pitch propeller allows the pilot (or an onboard flight system) to change the pitch of the propeller to give the best thrust characteristics for any given flight condition.

Even a variable pitch propeller is not perfect because the blade airfoil and the shape of the blade will be ideal at only one RPM and one pitch.  Therefore these shapes are a compromise.  Nevertheless, a variable pitch prop is a marked improvement over fixed pitch versions.

The bad news is that only a handful of in-flight variable pitch propellers have been marketed for model aircraft.  They are always expensive and few people purchase them.  Therefore the propellers do not get developed and the range of sizes from any given manufacturer is usually limited to approximately one.

I don't know if there's an in-flight variable pitch propeller available now or not.  The last one I was aware of was made by Kavan, a German company.

My point for including this information is so you can join me in brooding over our lack of nearly perfect propellers which you may not have done if you didn't know that such a thing could be made.

Variable Pitch Propellers that are Non-Adjustable in Flight

Note that some propellers are marketed as being variable pitch which is true, but misleading.  The type of propeller I am talking about allow the blades to be mounted in the hub at a variety of pitch angles.  In practice the propeller is still a fixed pitch propeller because the pitch can not be changed in flight.

 
 

Number of Blades

Propellers have one, two, three or four blades.  Single blade propellers are counter-balanced and used on extremely high rpm racing motors.  I've never seen these sold anywhere and suspect they are built by the user.  You don't need to even think about these props unless you're building control line speed aircraft.  In fact, they may not even be used any more.  I haven't been paying attention.

In our realm, the most efficient propellers are two bladed.  Because the diameter of our propellers is so small, multiple blade propellers disturb the air that the trailing blade is entering.  Therefore, 3 and 4 blade propellers are less efficient.

In general, the only time a 3 or 4 blade propeller should be used is for a more scale appearance or when a smaller propeller disk is necessary.

For example, a scale, twin-engine aircraft may not be able to swing a 2-blade propeller of a small enough diameter to clear the sides of the fuselage.  A three and four blade prop can be used here because it can be a smaller diameter and present the same load on the engine.

For best performance with sport aircraft, stick to 2-blade propellers.

 
 

Selecting and Flight Testing Propellers

Everyone has an opinion regarding what material is best to make a propeller from as well as their pet manufacturer.  As I mentioned at the beginning of this article, most manufacturer's propeller's work well and unless you're an expert pilot, you probably won't be able to tell the difference between comparable propellers from different manufacturers.

As far as I'm concerned the determining factor when selecting a propeller is pitch.  The diameter is selected after the pitch to put the proper load on the engine.  Often kit and plan manufacturers do not recommend a pitch for their models.  I've never understood this.  I know what pitch propeller is best for every plane I've designed and if I could ever complete a set of plans the number would be on them.

When I am performing test flights of a new plane (first several weeks) I take every prop the engine can swing and try each of them.  Sometimes I find that none of the props I have give the plane the performance I want, but I will have an idea of what prop to purchase.

A 6" pitch prop is a generally good starting point for any engine .15 to .60 cubic inch.  Smaller engines should start with a 4 or 5 pitch propeller.

I will perform the first test flight with a higher pitch propeller than what I think is needed just to ensure the aircraft will have enough airspeed.  If I want the airplane to fly slower or climb better then I will switch to lower pitch propellers when I am satisfied that the original propeller provides more than enough airspeed.

If you want to get the most from your models, it is important to learn how to prop your plane correctly and to learn to use the throttle.  There are too many pilots who think the engine is a three-position switch:  Off, Idle and Full-throttle with no settings in between.

 
 

Balancing a Propeller

An out of balance propeller or spinner can cause a lot of problems.  Here's a short, albeit incomplete list:

  • Robs engine of power.
  • Causes fuel foaming which can lean the engine causing it sag, quit or just run inconsistently.
  • Causes excessive vibration through the airframe potentially damaging it.
  • Ditto for the onboard radio.

I do not bother balancing spinners.  So far I have not had any real problems due to this, but as I mentioned earlier, I do not wring out my engines.  If I did have a spinner that was noticeably out of balance I would probably make a couple attempts at balancing it and if that did not work I would just discard it unless it was particularly expensive.

Typically a spinner is balanced by drilling small indentations into the backplate on the heavy side.  It is a lot of trial and error and if your results are anything like my attempts, then you will have pits all over the backplate with no significant improvement in the balance.  Obviously I am doing something wrong here.

The Robart Balancer is an excellent tool and highly recommendedThere are a variety of propeller balancers on the market.  The worst of these are the type that wedge a propeller between two cones on a shaft that is held between your fingers.  This is not real accurate, but it is better than nothing at all and it is inexpensive.

Robart makes an excellent propeller balancer called the High-Point balancer.  The High-Point has been very popular since it was first released because it is accurate and easy to use.  It also uses two cones and a shaft, but the shaft rides on four large, free turning wheels.  The idea is that the out of balance condition is magnified due to the smaller shaft having a leverage advantage like a small gear turning a large gear.

The High-Point is not limited to aircraft propellers.  Anything that is has a hole in it can be balanced statically.  This includes spinners, helicopter rotor heads, flywheels, boat propellers, etc.

Before you attempt to balance a propeller, be sure to clean it.  I wipe the propeller down with alcohol or wash it in warm, soapy water.

Most propellers are close to being in balance when you buy them so they should only need a small amount of work to bring them into perfect balance.  If the propeller is severely out of balance I return it because too much material would have to be removed which would significantly change the shape of the blade.

If one blade is heavier than the other, then the usual method to bring the propeller into balance is to remove material from the heavy blade using sandpaper.

Do not trim the tip of the heavy blade!

The blade may balance statically, but it will not be balanced when the engine is running do to unequal mass distribution.  Some people say to remove material from the face (front) of the prop and others say to remove it from the back.  I have done both and have never noticed a difference.  Usually I sand the face a little.

If you use wooden propellers and you sand them to bring them in balance, you may also remove the fuel-proof coating.  Obviously you don't want your propellers to become fuel soaked.

Another way to balance a propeller is to spray a very light coat of clear fuel-proof paint on the light blade.  This is an exceptionally good method because if you spray the paint evenly it distributes the added weight of the paint evenly over the blade.  It also will not change the shape as much as sanding will.

The only drawback to this method is that you can't check the balance until the paint has dried because you have to wait for the solvents in the paint to evaporate.

 
 

Mounting the Propeller

Back in the days when I flew control line combat ships I learned that the best way to mount a propeller was in the dead stick position.  That simply means the propeller is in the horizontal position (parallel to the ground) when the engine is turned to where you can feel compression.

The airflow will push the propeller into this position minimizing the chance of the propeller contacting the ground and breaking if the engine dies in flight.  Additionally, this is the most convenient position for hand starting the engine.

(By the way, I never flew control line combat, but I did like the ukie combat planes because they are wickedly maneuverable.)

Do not use a 4-way wrench to tighten a propeller nut!

Use an adjustable wrench (Crescent), open-end wrench or socket to tighten propeller nuts.  Four-way wrenches do not allow enough leverage to torque the nut really tight.  The propeller nut provided with the engine and the crankshaft are both hardened steel.  I have never managed to strip either of these items and I am not even sure if I could if I tried.

Check the propeller and spinner for tightness and damage as part of your pre-flight inspection

I have seen a lot of props come loose as well as some parting company with the engine on the ground or in the air.

In fact, somebody recently gave me a 91 four-stroke engine.  I mounted the engine to my test stand and started it up at idle.  I then moved behind the engine and waited about 30 seconds for it to warm up.

When I advanced the throttle, the engine backfired and threw the propeller and spinner.  The propeller flew a good 75 feet at a high rate of speed.  I would have been diced had I been in front of it.

The $25.00 aluminum spinner spread out and will not fit back in the base.  It's ruined.

 
 

Tracking the Propeller

Every once in a while you will buy a propeller that does not track properly.  Either the mounting hole is off-center or the hub is not square to the plane of rotation.  In either case, if the propeller is noticeably out of track you should not use it.

For a propeller to track properly, both faces of the hub must be absolutely parallel with the plane of rotation and the mounting hole must be centered and perpendicular to the rotation plane.

It is easy to see if the prop is tracking correctly.  Stand back for safety and look at the propeller from the side and from the rear.

From the side, both tips should be clearly visible in the same line.  If you see two lines, then the hub is not square to the plane of rotation.  You can attempt to sand the hub square, but I would simply return the propeller because it is defective.

When looking at the propeller from the back or the front, both tips should be in the same circle.  If you see two circles, then the mounting hole is off center.  Assuming you did not modify the propeller, then this too is a defect and the propeller should be returned.

Videos of a propeller that tracks properly

(There's nothing exciting here.  99% of all propellers you buy will track correctly)

A properly tracking propeller on a model aircraft engine
Side Tracking
(0.6 Mb)

A properly tracking propeller on a model aircraft engine
Front Tracking
(0.6 Mb)

The Bolly web site that was recently brought to my attention.  This site has a great deal of information about propellers and additional theory.  It is worth taking a look.

 
 

Previous
Next

Setting up a Model Aircraft Engine Throttle Linkage
Spinners for Model Airplane Engines

Comments about this article

 
 

Back to Model Aircraft Engines
Airfield Models Home

 
 

Copyright 2003 Paul K. Johnson