Thrust isn't eveything

[MattPearson]

Yesterday I did a thrust test on the Super Fly with the 690 2si motor and three blade 64 inch powerfin. Static thrust was only only 260 lbs @ 5600 RPM 100% throttle. In flight ,however, I cruise at 80 mph open frame at 5300-5400 RPM (half throttle) My sustained climb at 50 mph is 900-1000 FPM. Can't beat that with a stick! :cool:

[CLS447]

What is your pitch setting?

[Chuck Irby]

Matt, did you incorrectly state that you did your static thrust test at 5600 r's?

[MattPearson]

My static RPM is 5600 revs at full throttle. From experience, I know that the prop unloads significantly in flight, so that's the reason for that setting. My powerfin is set at 15 degrees on the two blade and 13 degrees on the three blade.
It's also amazing that with an wooden 64-46 Arrowprop which is the exact same pitch and diameter as the powerfin, but very different airfoil, I can achieve 6000 revs in static and the prop allows the engine to overspeed in flight. I don't have the thrust numbers for that prop because it was a long time ago. The thrust would probably be higher simply because the prop was spinning faster.

[Chuck Irby]

Okay, I gotcha now Matt. Thanx for the explanation.

[raghu]

Just to reenforce, you want the prop to be fully loaded at some chosen speed. This speed could be your best climb speed or cruise etc. At all speeds below this speed you will not be able to red line the engine and so will miss out on some power. Similarly, above this speed the prop will overspeed and you will need to reduce throttle.

Maximizing static thrust means your chosen performance speed is zero mph, so at all speeds above zero you loose out on some performance. You can bragg about your static thrust but it does little to help climb, cruise, or top speed performance.

[Doug Riley]

Thrust IS everything. It's simply that STATIC thrust doesn't coincide precisely with REAL thrust (the kind you get in flight at a chosen airspeed). The lower-than-redline engine RPM's when running static are part of the picture, but so too is the difference in inflow airspeed between static and flying. A very fine-pitched prop (low blade incidence) will generate great static thrust, but at flight speed the slipstream speed may be so close to the aircraft's own airspeed that there's little thrust. In effect, the aircraft is running away from its own slipstream. From the point of view of an individual prop blade, the faster the gyro goes, the less angle of attack the blade has and the less lift (=thrust) the blade makes. If you go fast enough (in a dive), the blade may end up with a zero or even negative angle of attack. In that case, he prop begins acting as a dive brake, even though the engine is redlined.

Props set at higher cruising pitches do load the engine to below peak power when run up statically. Such coarse-pitch props may also be partly stalled when run static. That's a more severe problem with fast fixed-wing planes than with gyros.

A numerical measurement of in-flight thrust could be made by attaching strain guages to the engine mounts. In practice, though, it's easier to tune the prop by a series of flights and small adjustments.