How do you know which propellor to use when? Here's some tips.

First is matching the porp to the engine. Each engine has a range of propellers appropriate for it. Some the range is narrow due to specialized purpose of the engine, but most have a fairly wide selection of props.

A .40 Nelson racing engine... you basicly use one specific size propellor to get the mximum speed from the airplane. This propeller would be inapropriate for any common .40 to .46 powered sport-aerobatic airplane.

A typical .40 engine such as the O.S. LA.40 can take from a 8X8 to a 12X3. You note that the pitch decreased and the diameter increased... that keeps the propeller load relatively constant.

The 8X8 is something you would never use as a beginner. It is for SPEED. The high pitch and short diameter prop would try to pull the airplane very fast. This would sacrifice "static thrust" (the pull whent he plane is stationary on the runway) for thrust at high airspeed. Takeoffs would be very difficult if you tried to use this on your trainer. The plane would have a hard time slowing down to land when the engine was idling too...

The most commonly used prop for a .40 is a 10X6. This is a compromise propellor... having decent static thrust, and reasonable speed capability. At 10,000 rpm it wants to pull the plane close to 80 mph. The drag of the plane resists that... and the trainer typically cruises closer to 50 mph. Actually... this propeller is more appropriate for use with the .40 on a plane such as a Sig Kougar... a relatively speedy airplane. when the engine is running at maximum rpm on the trainer... you are "spinning your wheels"... the plane just has too much drag to go anywhere near as fast as the propellor is trying to pull... and if it did go that fast, you would easilly rip the wings off doing a loop!

The 11X4 or 12X3 are propellors which give great static thrust, at the cost of top speed of the airplane. On a trainer... this is NOT a sacrifice! The better static thrust will improve take-off performance. The top speed of the plane will actually increase because of the the propeller working more efficiently. Aerobatics become easier, and its easier to slow the airplane down for a smooth landing. But put one of these propeller on the Sig Kougar and you will lose a significant amount of top airspeed vs the 10X6. (you'd still gain some improvement in some aerobatics though...)

The basics outlined here can be applied to any propellor range appropriate for any given engine. If the plane is designed to fly slowly... use a long, low pitch propeller to see the best possible performance. If the plane is designed for speed, use a shorter, higher pitch propellor, but realize you are sacrificeing some low speed performance (and will lose some of the pull needed to get through those long vertical legs of the square loop...)

Match the prop to the engine AND the airplane. (and you will find that the designer was correct about the engine to put on the airplane...)

Heh heh! Well said Huber-man!

You da beginner's man!

Well said, Mr. Huber. In fact, on the plans I'm drawing now, I'm recommending and engine range and the propeller pitch, but not the diameter. That part will depend on the engine, but the pitch should be what I recommended and the plane will fly well.

Good stuff!! [sm=thumbup.gif][sm=thumbup.gif]

Cheers!

Jim

Thanks for the info on props

Bumping it up..... I'm getting more propellor questions.

This is what its all about!.. guys like you who make this site worth visiting, helpful hints for us newbbies thanks FHHuber

Good basic tutorial! Well written, clear and succinct.
Dennis-

I have incorporated the post in my site
http://www.geistware.com/rcmodeling/articles/index.htm

Mr. Huber--The answer to my prayers--I'm brand new to this hobby and was always confused about prop selection- Your column has helped me a lot..Thank you Mike Burnett

what about high altitude locations. if I'm at 5k above sea level is their a thought on adjustments in pitch or would it be addressed just with a diameter selection? or is it both ... how much affect could barimetric pressure have on scale anyway ....my head is starting to hurt....

An excellent summary. I’d like to make just one more point regarding pitch. The pitch varies from very steep near the hub to quite flat toward the tips. This is so that when moving forward, the angle of attack at each section along the blade is close to the same. For this reason manufacturers specify their propeller pitch at a particular place on the blade (for instance 75% out). To say that pitch is the distance the propeller would move forward in one revolution if there were no slip is only true at that particular place on the blade. Manufacturers are fairly consistent in specifying location of the pitch but not completely so. There is also some difference in the twist distribution from hub to tip between manufacturers. For these reasons comparing propellers from different manufacturers can be sometimes be misleading. Also some blades are more flexible than others and tend to flatten pitch under load while more rigid blades don’t. Using the general guidelines given in the original post, it is still expedient to try blades of different manufacturers to find the one that gives the best results on your airplane.

High altitude would need more power to swing a bigger propeller.

Actually... High altitude... you just want a little less pitch. Lower pirch gives better low speed POWER. the engine is mre efficiently using the propeller at high rpm with low airspeed. This gets you off the ground better.

In general, RC aircraft have MUCH higher power:weight ratios than full scale.

They don't put a larger engine into a Piper Cub when it takes off from sea level and then flys up to its operating cieling of appx 12,000 ft (because the pilot need oxygen above there... the engine does not mind at all)

With the Piper Cherokee-6 (6 passenger 300 hp airplane) that my parents used to own... at high altitude they did a more careful weight and balance calculation... and verified the "density altitude"... then they cofirmed that the runway was long enough. NO OTHER CHANGES to the normal takeoff. (generally the airports they went to... it would have had to be 190 deg F and have the lowest barometer reading in history for the airport for the runway not to be long enough for full load take-off...)

Normal takeoff proceedure on the Cherokee was to set a specific ( in the low range of the variable ptich prop's capabilities) pitch and run the engine up to a specific rpm. No change for altitude.... just use more runway.

I guess I don't understand. If the air density is less, then you will have less resistance to the air, and have a little more RPM. So are you saying that to maintain the same speed, you need less pitch? This will convert to more thrust at the lower density.

No... its not quite that simple.... its actually so complcated I don't know how to explain it.[&:]

Airplanes and aerodynamics get EXTREMELY complicated explaining what happens with different density altitudes. In order to avoid each pilot getting a PHD in aeronautical engineering... the aircraft manufaturers give an operating manual wiht each aircraft sold... that manual lists specific procedures and tables of required runway length at different weghts and density altitudes. These proceedures and tables are obviously different for each airframe {aircraft design} and engine {rated design... they don't specificly test the individual engine except to see that it MEETS specs.} combination...

Simplisticly.. all you need is more runway to gain the required airspeed to fly. You already have a MUCH higher powe-weight ratio than any common 4-passenger private aircraft. (higher than many aerobatic demonstration or competition aircraft...)

Our typical at sea level powe:weight for a R/C trainer approaches 1:1. Typical 4 pasenger private aircraft would be VERY happy with 1:2 (or 0.5:1... double the weight compared to available thrust.) A full scale Piper J-3 Cub is down near 0.3:1. (try that power:weight on an RC model... you've just about got it with a 0.10 on a Dynaflite Butterfly.)

Its not POPULAR to point this out... but its the way things are. R/C modelers are spoiled about having excess power. if the model doesn't have "unlimited vertical" climb capability they think something is wrong. (at sea level ANY .40 will take a 5 lb trainer unlimted veritcal with the correct propellor...)

Hi, a question please from anyone looking at this post ( who knows for sure)
I have a thunder tiger .61 Pro engine. The manufacturer recomends a 11x7 prop for
break in, and 11x7 for both sport and aerobatic applications,
I understand a longer prop and correct pitch will deliver more thrust at lower rpm.
Could I use a 12, 13, or 14 inch prop on this engine?
The main reason im in wonder is, the cowl on this plane is unusually wide, and the 11x7 prop I have mounted presently
only extends past it by One inch on each side, about four inches top and bottom.
Im worried this prop dosnt extend past the cowl enough to provide the correct amount of thrust.

This engine is for a 60 size Extra 330L airplane, and I looking for maximum Aerobatic performance.
Help on this would be greatly appreciated!