Thrust vs. Airspeed

[PW_Plack]

I know many of us measure static thrust as part of initial testing of a new machine, or new engine, or at least set the prop pitch for takeoff RPM while the gyro is tied down. This thrust reading is then used to calculate the pitching moment in a non-CLT gyro, and determine the downward force needed from the horizontal stabilizer.

Is there any way to estimate how much thrust will fall at cruising speed or top speed? I've always heard, for example, that small, fast-spinning props are much less efficient than large, slow props in a static thrust test, but close the gap somewhat as airspeed increases. KB2s with VWs and Macs seem to manage decent top speeds with small props turning at 3400 RPM or higher.

It would be nice to be able to figure this out without a wind tunnel, but I'd imagine the aerodynamics of the fuselage have an effect. Has anyone made a serious attempt to measure this?

[Friendly]

Paul , I have a 2.2 soob on a custom gyro. It has a 72 in warp drive with the Kurshur redrive, (not sure if I spelt his name correctly). I am still setting this up. It is 3 blade configuration. I had to order a face plate from warp drive and while discussing my set up with Keith at Warp Drive he said he thought I might be happier with a 4 blade smaller prop on the pusher gyro to allow the engine to rev more during climbing rather than idle back during cruise. I know very little about props but it does seem to make sense. I used to drag race in my youth. We always said you can't believe what you read in Hot Rod magazines because the Manf. are the advertisers and pay big bucks. If you bolt something on and run the 1/4 and it drops your time then it works. I guess I may have to buy another prop at some point and find out.

[Chuck Irby]

Mark, come up with the blades and let me know, and I'll bring my Soob 4 blade hup for you to try at the Bastrop fly-in. What length were you wanting to try?

[Friendly]

Chuck' thanks for the offer, I won't be bringing the custom gyro to Bastop as I have not built a trailer for it yet. The wheel base is 79 inches and my tandom trailer is 74. I don't want to cut the side rails off so I will build a new trailer. I will try to bring my KB-2. But it is very interesting ideal that the 4 blade with less pitch may be a better performer than the 3 blade with longer blades and more pitch. I had always heard the longer the prop the better, just keep the tip speed right. What did you have the 4 blade on and why did you take it off?

[Chuck Irby]

Hello Mark,

It's a two place tandem, that needs to be rebuilt. I've been having too much fun with my single to mess with it. Maybe this winter.

I'm looking forward to meeting and flying with you at Bastrop.

[bones]

[COLOR=DarkSlateBlue]Mark you said above; "I had always heard the longer the prop the better, just keep the tip speed right."
There is alway variasations but yourself or Chuck care to comment on this please, what do you consider the best tip speed to run for performance. :confused:

[Chuck Irby]

Mark, I am the wrong Chuck to be answering this question. But I always thought anything between 600 and 700 MPH was about optimum. Then too, I realize that there are variables that need to be considered.

[Friendly]

Bones, I am not qualified to say either, I suggest you talk with people who are flying the same type machine and what works well for them. I have seen so many soobs with 3 blade prop that I did not even consisder one until someone at Warp Drive said I might be happier with the 4 blade, shorter length less pitch. He need more information to make a firm recommendation and I not ready for more changes at this time. I just have too many irons in the fire and need to concentrate on flying what I have already before makeing improvements

[mceagle]

Info re prop diameter - basically, within desireable constraints, the old saying "bigger is better" is true.

[rfi]

Mark,
I am a Warp Drive dealer and it has been my experience that a three blade prop is more than adequate for an EJ22. You would need a four blade for an EJ25 or equal power.

[quadrirotor]

Don, what about HOMEBUILT ROTOCRAFT?

[pwendell]

How about SPORT ROTORCRAFT?