I've recently experimented with measuring the actual thrust of an engine/prop combination as it is mounted on the airplane. I would like to run this by all the engine gurus out there to see if this seems like a valid way to do so.

The way I've been doing this is to use a digital fish scale which is hooked to a 10' leather dog leash (it seemed to have less elasticity to it than some of the different types of rope I looked at...plus I know it will hold at least 40 lbs, the weight of my stupid mutt pulling her heart out). The other end of the leash is placed around the tail of the plane that I'm testing and the digital fish scale is attached to a metal spike driven deep into the ground. I start the engine and let it idle, take the slack out of the leash, and zero out the scale. I then throttle up and read the "weight" of the pull generated in lbs and oz.

It seems like it has been a fairly reliable test, especially when comparing different propeller sizes/pitches/brands to determine which one can generate more thrust. Does anyone see any flaws in this?

Thanks,

Dave

That is the way it is done for a static thrust test.

The actual thrust produced while the engine is moving through the air may be different?

Very good. So if I get a higher number on the ground, would there be any way to relate that to speed or vertical climb? In other words, would a propeller that pulled harder in a static situation like this test also pull the plane upward at a higher rate of climb?

Dave

Yes, the more static thrust you have, the greater the vertical capability you would have.

But in most cases, a combination producing the highest static thrust will not produce the greatest speed.

Thanks. I've been trying to decide on the best props for a GMS .76 mounted in an Extra 330 and a G90 mounted in a H9 P-47. In both planes I'm not interested in screaming top speeds, but want to be able to pull vertical easily. I should be able to figure that out fairly well now.

Dave

The proof is in the fish scale but most of us think that the GMS 76 is better at higher rpms and that the G90 is better at lower rpms with a larger prop.

Your scale will tell you but don't expect the same prop to work best on both engines.

Also, the P47 may require a higher airspeed to fly properly than the other plane you have?

The only thing I see wrong with your test is that the model needs to be on a paved surface. You need to totally eliminate the rolling resistance from the setup. Uneven ground and grass will lower your thrust measurements. How much? Who knows.

As for prop selection and static thrust, the prop that pulls the hardest in a thrust test like this may still not be the prop you want to fly with. It will, however, give the best punch out of a hover, or otherwise the most thrust at very low airspeed. As you bring forward speed into the equation, the prop's efficiency changes. If you generally fly at a moderate speed, the prop that pulls the hardest will be maxed out, while one with more pitch and less diameter will be in it's element. I have proven this to myself with my Sukhoi. The 18-6 prop gives the most static thrust, but the airplane flies much better with a 16-10. Both props turn 9000 rpm, and the 18-6 definitely pulls harder on the ground.

Static thrust tests are good for figuring the thrust to weight ratio of your model, but not much more than that.

So far the P-47 has been a great plane to fly and if slowed down it gets mushy, but won't tip stall. In fact, it's hard to slow the danged thing down enough to land sometimes. I've had a Graupner 13.5 x 8 prop on it, but I have a Graupner 14x7, MA 14x6, and an APC 15x6 that I want to test. I think the APC might be a good size, but too heavy to turn enough RPM's. I might look for a MA 15x6 or an APC 14x4 just to see what they do. The P-47 (dry with Robart retracts, inner gear doors, etc.) weighs 9 lbs 6 oz.

As far as the GMS goes, I originally had an APC 13x6 on it, but it would barely do a good loop. It's still a fairly new engine with very little time on it, so maybe it will perk up with a few more gallons through it. The Extra 330 weighs 7 lbs 8 oz.

Dave

The MA 15-6 really buzzes on a fast fly by.

I always liked the APC 14-6 on the G90 but due to variations, you should try several props and decide for yourself which one is best for your combination of plane and flying style.

Good points.

The test is conducted on a flat, paved surface (I'd already thought about that).

Based on what you're saying, a 14x6 to 14x8 prop might be what I need for the P-47, even though a 15x6 might pull harder. I want it to have OK speed, but lots of power and vertical climb. There's nothing like bringing it in for a strafing run at a good clip (the dive helps build up speed) and then pulling up and watching her climb like she's on rails.

As far as the Extra, I keep the speeds low on it and just want to be able to do aerobatics, so the larger diameter lower pitched props would be what I'm looking for there and the best puller on my scale will probably be what I mount up.

Dave

Have a look at "Aero design propeller selector" on google; safe download. See how your test compares to theory; I have found the fish scale test to be very close to the chart. Also shows how thrust drops off with speed.

As far as i am concerned, the 4" pitch props are for pure 3D or funfly type flying only. I tried one on my F90 and didn't like it. As far as thrust goes, the 16-4W pulled exactly the same as a 16-6. The difference being the 16-6 had a bigger speed envelope and the thrust started coming on at a much lower rpm. The 4" pitch props only work well when the engine is really turning up. A friend of mine tried a 14-4 on his Saito 65 and nearly crashed his Martha. That was because the throttle needed to be at half or more before it had any pull. He wasn't expecting that and caught it just in time from a spin recovery.

Also, pitch numbers are only relative within a certain range of diameters. As you go with bigger engines and larger props, the pitch numbers increase also to give you the same effective pitch or blade angle. In the 16 and under range, 10" pitch is a lot. In 24 to 32 inch diameter, a 10" pitch would be very low and maybe not available.

I would like to add at this point . . . .

The reason the prop pitch goes up with the "Big engines" is that they turn less rpm. A 150cc twin gas engine may only turn around 6200 rpm. So in order to get the same forward speed, you will need to increase the prop pitch accordingly.

Thanks for all the informative posts guys...

Dave

Well, I used the scales to narrow my prop choices down to a 14x6 MA, a 14x7 Graupner, and a 15x6 MA. I took the bird out to the field on Saturday and decided to keep the 14x7 on her. It really let the engine rev up to get some speed and still had loads of power for climbing. Thanks for all the help.