Barry Cazier

My Feedback: (17)

Question...

I measure the thrust of my engines while it is mounted to the airplane. I use a digital fish scale. I just hook it up and give her the throttle and read the the thrust in lbs.

My question is: "Does the weight of the plane affect the thrust readings?" For instance: "Would a DA50 on a 15lbs plane measure the same (everything else being equal, day, humidity, temp, prop, etc) thrust as it would on a 12lbs plane?" Are there any other variables that might affect the thrust rating? Having the plane on asphalt or grass?

I've also wondered about pulling the plane backwards while taking the thrust ratings? I always get higher ratings when I do this but I've always posted just static numbers without pulling the airplane.

Just wondering.

Thanks
Barry

Spacey

I don't think that method of measuring thrust would yield very accurate numbers at all no. Way too many variables involved there. Only way I can see you really getting an accurate reading is to hang the plane hanging nose down from the scale somehow, you would also want the scale to be exactly in line with the thrust line vertically. Then of course run up and quickly cut the motor when done measuring thrust there. After motor's cut then subtract the full weight of the airframe (Remember you don't want to run out more fuel before measuring the actual weight then). This whole method is obviously going to be very dangerous regardless how you do it and yes very unpractical. Unless you can come up with a contraption to do this. So ya in the end to sum it all up your method of doing it already will give you a ballpark figure to work with but not the most accurate.

For your method I would suggest a really smooth surface to do it on to begin with so the wheels can really rotate freely, and also still try to get the scale to be exactly inline with the thrust line! Another thing to avoid while using this method is any other forces on the scale like picking the plane up slightly etc. Very difficult.

To answer your other question, the weight of the model will have no effect in the perfect enviroment. But yes like I said above if it's for instance a tail tragger and you're picking the tail up by the scale somehow or whatever you could get some different end readings on the scale.

Another point you should bare in mind as that static thrust measurements is not going to reflect what is actually happening upstairs! You can maybe measure your thrust readings when hovering sure (Yes it's the 3D forum I know so it is applicable there). But we have to remember that there are other factors involved like wind even when hovering which will make the prop tend to become a little more efficient (This theory I base on my heli flying experience and it's just a theory). Also you have other factors like prop cavitation etc which will change as the model gains forward speed depending obviously on the prop. Example: it's impossible to to do thrust and amp draw readings on my little brushless pylon racer because it is swinging a 5X5 prop and it just cavitates on the ground, upstairs when the model's moving it draws MUCH more current as the prop gets enough air supply to bite into.

I'm by no means an engineer. And yes I'm sure someone will chime in sometime that has a good method of measuring thrust statically. Your method of doing it is really what I would do also to just give me an idea of what the motor is capable of when the model is hovering perfectly still.

Cheers
Spacey

bodywerks

My Feedback: (4)

As long as you are measuring the thrust in calm wind and on a level surface, you will get the same static readings on a 12 pound plane as you would a 20 pound plane. Just remember that static thrust is an entirely different animal than dynamic thrust, so don't be suprised if your, say, 12 pound plane doesn't have very good pull out with, say, 20 pounds of static thrust. Once the plane starts moving, that thrust is now called dynamic thrust. The prop has unloaded and the resistance to movement has been substantially reduced (you are no longer holding the plane back). This is why a plane will hover at half throttle but still not have great pull-out even if you have a 2:1 static thrust-to-weight ratio.

Barry Cazier

My Feedback: (17)

Interesting info...Thanks guys.

But for comparisons (although not perfectly accurate) It wouldn't matter if the plane was heavier. Is that right? I also hadn't thought about the surface being a factor but I'm sure it is. Grass would reduce the thrust a bit because the wheels would have to overcome that friction as well, true? I do lift the tail and hold on trying to keep the fuse level.

What I'm trying to do is compare engines and props to establish which is the best for me at my elevation. Right now I'm comparing the DA50 to my EVO45GX. They are on planes that weigh about 2.5lbs difference I think. I was just wondering if I have to take the weight of the plane into the theory. Sounds like I don't.

Thrust readings are fun. When I take my scale to the flying field everybody there wants to see what thrust they are makeing. Also a good tuning tool and helpful for prop selection. Most interesting to me is on my mighty 110s (I have 5 of them) the 16x6 prop gives me 1 full pound extra thrust than the 17x4W. I would have never guessed that. Yet, the 17x4W is easier to hover with. Go figure.

Anyways, thanks for the info. If any others want to offer an opinion, I'm listening.

thanks
Barry

bodywerks

My Feedback: (4)

That's right, so long as you are on a level surface, the weight of the plane doesn't matter.

Sethhunter

To answer your question, yes friction affects the measurement, weight of the plane affects friction, as does the quality of your wheels, wheel size, etc. Whether the plane is moving (rolling) while you take the measurement affects the contribution of friction. Grass will give you pretty unreliable measurements. Pavement is better. For kicks, you might compare the load while pulling the plane slowly back, with and without power on. The difference, theoretically, is thrust net of friction. You might find in your case the friction is really small - or not. A more accurate approach would be to suspend the plane in a craddle, which would offer less friction than the ground. Probably the best experiment would be to mount the plane on a plate with high quality slides, or better yet, do that with an engine test stand.