SECRET AGENT

My Feedback: (18)

Does anyone out there know how many pounds of thrust a Kress RK-720 (3 and a half inch diameter) fan puts out??? I also need to know what the maximum RPM is that I can put into the fan before it stops pulling and starts chopping air. I am looking for the correct motor (.25 size) for the most thrust and was contemplating the Dubb Jett .35 (built on a .25 case). I am afraid the Dubb Jett motor will have too much power for the fan, or that the fan is too small for that much power and over-rev the motor. Any insight from the model engineers out there will be greatly appreciated.

Rahlberg13-delete

I have this fan right now, and I can get about 3.5lb of thrust out of it. I'm using a .21 MVVS with 30% nitro.

Kress claims just under 4lb with a K&B 3.5 at approx 23K RPM (all with 25% nitro).

As far as max RPM's, I hear that Danny Starkey (the master of .21 fans) can get 29K with a top end Nova Rossi .21 engine.

The .21 Rossi's put out more thrust than the K&B 3.5 and the OS .25 VRDF.

What are you putting the fan into? I'm always interested in hearing from other guys flying the .21 fans. Right now, I'm preping a Mirage and an F86 for test flights, with a dual .21 SR 71 on the drawing board.

Email me at [email protected].

Hope this helped. Good luck.

Russ

bentbone

I'm getting 22000 with os car 21 on 10% nitro been side tracted and not had a chance to test thrust on higher nitro.

SECRET AGENT

My Feedback: (18)

Have you gotten any thrust measurements at all? How did you rig your thrust meter? I am in the process of fabricating one and could use ideas. Thanks!

Rahlberg13-delete

To measure the thrust, I actually use a force gauge we have here at work. They're like $500, but can measure them to the .01lb. But I hear you can get mechanical force gauges (which is nothing more than a spring and a face plate with the tick marks to show you how much you're pulling). I think people have called them fish scales. They're similar to the scales in super markets. You might be able to find some at www.mcmaster.com. Do a search for force gauge.

Although, you could make one of your own. Using a spring with a known spring rate (which is usually given when you buy them-units are force/distance), you could calculate how far it needs to be stretched for a given load. Calculate this out for a handful of different loads and distances, and you can make up your own gage. It's really not as hard as it seems. You'll know the rate, and know the force you want: then figure the distance it needs to be pulled.

I set the plane on the ground, attach the back of it to the gage, and fire it up and give it full throttle. I know the wheels and ground add some friction, but that's negligible.

So there you have it. That's how I do it.

Hope this helps.

Russ

SECRET AGENT

My Feedback: (18)

Thanks Russ for the ideas. I think I will make a test stand on wheels with a fish scale attached. Who knows, it might work. I just want to make sure I can get at least 4 pounds of thrust before I put the fan in the F-22. If the F-22 weighs 5 pounds and I have 4 in thrust, it should fly OK. We will see. Thanks again.

Rahlberg13-delete

I've heard that with a thrust ratio of 1:2, you'll get pretty good performance. With 1:3, you get moderate performance, and with 1:4, you get sluggish (but still fliable) performance. Of course with a 1:1 or better, you should be able to get vertical performance!

Can someone else back me up on this one?

If you can, post some picts of your plane!

Talk to you later

Russ