bob27s
Posts: 4730
Joined: 4/9/2002
From: Cleveland,
OH, USA
Status: offline
|
Dont try to over think this one on the prop too much  Many people do. Also worth while to forget a bit about the math aspect of things (rpm x pitch is not always indicative of forward velocity). And forget static thrust. This means absolutely nothing (very little at best) unless you are hovering or starting a take-off roll. Dynamic thrust is a different animal. Some of those concepts have meritt and are useful, but for what we are doing here.... go with what is suggested.
General oversimplified explaination ....
small diameterxhigher pitch = speed, small frontal disk area is less drag - this is for speed performance. Sometimes at the expense of a longer take-off run (slower aircraft acceleration), but not always if engine rpm increases enough to overcome the reduced blade area. (example - This is why a QM40 turnign a 7x8" diameter prop at 25,000 is quicker than a Q500 turning a 9x8" diamter prop at 20,000 - and by reason that the airframe can make use of this prop)
big diameterxless pitch = slow,low speed thrust - larger disk area is drag - slows plane down, but you get better take off performance, sometimes better vertical. Yes more efficient, but the added drag of the larger prop disk sometimes eliminates that advantage. If the plane is going to be slow to start with, sometimes a bigger diameter prop is indeed the way to go. (works sorta like gearing in a truck or car)
The key with any engine, especially one with a tuned exhaust, is selecting a prop that allows the engine to run at its best power rpm. With a tuned exhaust, you have to prop to match the tuned rpm of the system.
And, something to remember for all 2c engines - more rpm represents more fuel burned per minute, which represents more power being produced. Clearly, and engine turning a 12x6 at 15,000 rpm is producing more power than another engine turnign that same prop at 13,000 rpm. just an example. (engine must by design be capable of achieving the performance).
For the Jett 90L engine, you want to target between 13,000 and 15,000 peak ground rpm. A 12x6 will hit or exceed 14,000 rpm easily. So it is the baseline prop. It can turn a variety of props. Anything from an 11x6 to a 13x6 is fair game. Can turn up to a 13x8 or 14x6 (for scale or aerobatics) by using the torque version jett-stream or a turbo-jett muffler which is tuned for lower rpm use (12-13K rpm or so).
For the 90LX, the target ground rpm is 14,500 or higher. Higher is better. A 10x10, 11x7 or 11x8 is ideal. Also works great with a 12x6, and 11x9. On this engine we baseline an APC 11x8 at around 15,000 rpm.
With either engine, you can expect the engine to unload nicely in the air, upward of another 1000-2000 rpm in level flight.
Prop selection also has to be based on the application and desired flight characteristics, and requires some experimenting depending on your aircraft and performance desires. A super clean 60 size, 7 lb pattern ship with retracts may require a different prop selection than say a fixed gear 9 lb F-14 Tomcat.
What you will find described here by myself and many in the form are prop suggestions representing some of that real world trial-and-error testing.
_____________________________
Bob Brassell
Jett Engineering - Engine Mfg Support Forum Host
|
Printable Version
All Forums >> RC Airplanes >> Extreme Speed Prop Planes >> RE: AKM SU-27 Flanker-Does Look Good 
RE: AKM SU-27 Flanker-Does Look Good - 
