RC aircraft dyno

[FullSpool]

I was looking at building a hydraulic brake dyno for rc gas aircraft, the engines are 2 stroke with a HP not exceeding 12-13. I am trying to figure out
what I want to use as my brake and was exploring the possibilities of using turbo parts as the exhaust wheel/shaft and housing would make a decent starting point.
Do you think I would be able to exert enough load on the engine to take torque measurements with this configuration or should I look into a mass produced toroidal?

Thanks!

 

[GregLocock]

You can vary the power absorbed by a prop by using a backing plate to stall it. I'm really not a huge fan (geddit) of using a prop to load the engine, I think a nice little waterbrake is going to be much less anguish if you want a power vs rpm curve, but you'll need to do a bit of development on it first.

Cheers

Greg Locock


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[FullSpool]

RPM is easily measured as I already have that device, a hall sensor is mounted to the engine case and an earth magnet on the prop hub, as far as the assembly goes I have a shaft going through 2 pillow block bearings. 1 end of the shaft is connected to the engine mount and the other end of the shaft that is connected to an arm that measures 12" long from center of the shaft to the load cell.

What I am failing to understand if using the prop as the actual break, maybe you guys can clear it up for me.
Obviously each type of prop will vary the torque readings so how do I know which one is suitable to start the conversions?

Take 3 different props and 3 different rpm readings per engine. This will give me "9 points" of collective data, now I analysed
the data to see which prop offered the highest torque reading from the load cell. Take the highest torque reading and the RPM that it made it at and plug it into the BHP formula? Do I understand this correctly?

Thanks guys.

 

[btrueblood]

I'd think you would need the prop to provide cooling airflow for the engine.

I don't think you will be able to achieve much rpm variation, as there is no throttle I can recall on a 2-stroke model engine? All you will be able to do is vary the mixture.

 

[FullSpool]

I have full throttle control through out the whole rpm range.

 

[MikeHalloran]

Btrue, R/C engines have throttles and R/C models have servos to drive them.
Agreed that you need to cool the engine during dyno runs. ... and a typical propeller is the simplest way to do that and provide a load.

Full, fix the throttle in one position, e.g. WOT, and use 3 different props to give you 3 different torque/rpm pairs. Fit that curve. Compute the HP points from that.

The HP peak occurs just a bit above the torque peak because HP = factor x rpm x torque.



Mike Halloran
Pembroke Pines, FL, USA

 

[dgallup]

I'm with Btrueblood, you need a prop to cool the engine. Then you need a way to vary the load so you can hit different rpm points. You don't want to throttle the engine because you are trying to measure the full power curve. Switching props is the easiest but some kind of air flow damper might work. I suppose if you shrouded the prop you could restrict the airflow coming into it.

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[FullSpool]

The prop cools the engine, it also allows an easy way for me to start it.

 

[btrueblood]

You could do it with one prop, if you don't mind ruining it. Start with the largest wooden prop you propose to use, and keep trimming the prop o.d. after each run.

 

[FullSpool]

what I would do is use the same manufacture for each prop but different sizes, 20"x8, 19"x8, and 18"x8 for a 30cc size 2 stroke.
That would be easier if you think it would work.

 

[btrueblood]

Any of those ideas could work. You're measuring engine reaction torque and rpm, which gives horsepower.

Add a second load cell to measure thrust? This will give prop efficiency. And add a blower on the duct, to simulate at-speed performance...

I'd say you might still want to run part-throttle tests, just for the sake of completeness. Next you need a way to measure fuel consumption, too, if you want to say, find the best settings for maximum range or maximum duration flights.

But maybe all those are not big concerns for RC models.

 

[FullSpool]

The thing with propellers is that depending on the type the torque measurements will change, so do I need to find which prop provides the most torque and use it for my HP calculations?

Thrust can easily be captured as im not concerned about that nor fuel consumption as that isn't an interest for the moment.

Thanks Guys!

 

[MiketheEngineer]

You could call the mfg??

 

[dgallup]

You just want different loads, as long as you have a range each one will run up to the RPM point that the engine output balances the power absorbed by the prop. You will measure the torque reaction on the engine mount. You may need some pretty big ones if you want to hold the RPM down very low.

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[FullSpool]

For HP? There readings are theoretical with a known test prop and no actual real world data to back up there claims. Well 90% of the manufactures are guilty of this.

 

[dgallup]

Are you trying to just find/measure the peak power or do you want to measure the power curve over a wide range of engine speeds? Peak power is always going to be up near the top of the RPM range. That's a lot less testing than trying to measure across the entire RPM range.

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[FullSpool]

I am just looking for peak HP at the moment.

 

[patprimmer]

No matter what you use as the load, you need some way to adjust it to control rpm at WOT.

Regards
Pat
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[imcjoek]

13hp sounds on the "rather large" side for R/C aircraft. Unless we're talking 1/4 scale or bigger.

I would suggest the KISS approach and use some kind of (wide) leather strap over a drum. Add a spring so you can adjust the braking force so you get multiple RPM test points at WOT. With a big enough drum and/or fan, it should stay cool given the test should not take more than a minute or two once you are good at it.

For your torque measurement, you can either incorporate a load-cell, or characterize the brake's torque vs knob turn-age beforehand and just do a sort of chart look-up. Depends on what kind of accuracy you need.

 

[imcjoek]

Or... alternately,

Get yourself a flywheel off something like a 350ci chevy. Direct drive it from your engine. (Put a nice thick scattershield over it too).

Set WOT until max RPM is reached. Record the RPM every couple ms with a microcontroller reading from some kind of shaft encoder during the ramp-up. Do some math. Instant "inertial" dyno.

 

[FullSpool]

These aircraft are giant scale 71" to 120" wing span. 30cc to 120cc 2stroke
Gas engines.