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!

 

[turbomotor]

You may want to consider buying the dyno and applying your energies towards developing the engine.

http://www.magtrol.com/motortest/ed_dynamometers.html

IMHO, this is the way to go if you are serious about developing a small aircraft motor for use in a long endurance uav or other military/commercial application. It may not make sense if you are a hobbyist.

 

[roydm]

Have you ruled out an electrical method such as an automotive alternator generator?

Perhaps a squirrel cage motor with DC on the poles

Roy

 

[BigClive]

Roy - might be useful for starting the engine as well.

 

[FullSpool]

I have not ruled out anything for the load yet as im still working on the engine assembly. My work load is leaving me little time for projects so I am still up for suggestions. From my past conversations with mustang dyno they were suggesting a 7.5 hp AC motor with a constant torque from 0-6000 rpms and constant hp from 6000-8500rpms.

 

[KeithDyer]

I wish I had a nickel for every race engine "fresh off the dyno" that would not accelerate under load (off the corner or up through the gears) until we changed the tuneup, or fattened up the fuel or something.

Steady state dynos have their place and are the most widely used. But the inertia dyno will tell you more about how the vehicle will perform in your "real world" applications.

Just depends on what information you need to do what you need done.

Take care, K

 

[FullSpool]

LionelHutz

P=(inertia x N2^2/2 - inertia x N1^2/2)/dt
inertia = inertia of the flywheel.
N2 = final rotating speed
N1 = initial rotating speed
dt = time to go from initial speed to final speed.

I got looking at your formula again and I am interested to see how this might work for my application.
Can someone tell me if N1 and N2 are for the engines RPM or the flywheels rpm?

Thanks Guys for all the help, with out you this project would have never got to were it is now.

 

[imcjoek]

The frame of reference that you choose just needs to be consistent. Since the inertia of your flywheel should be much larger than the inertia of your engines (by design), use the inertia and rpm of the flywheel. If you have any gear reduction between engine and flywheel, that needs to be accounted for in your torque readings.

 

[LionelHutz]

First off, power at any point in the system is the power input by the engine minus the losses. So, you'd basically want to keep the losses in the flywheel assembly as low as possible (decent bearings, no "fan" blades on the flywheel etc) and you'll get decent power measurements regardless of which speed you use. However, if you do change speed you will likely have to compensate for the losses in the speed reduction device in some manner.

The inertia would have to be translated through any speed changing devices (ratio^2) if you want to use a different speed measurement than the flywheel speed.

 

[140Airpower]

Why do you have to calculate the energy content of the rotating flywheel? The normal power equal energy / time only gives an average power over an interval. We are talking about an inertial dyno here, right?

What you want is a rpm vs power curve. You only need the instantaneous torque reaction and rpm recorded together. It doesn't matter much about the flywheel mass except that the reading will be smoother with a heavier flywheel (geared or not doesn't matter) that will be slower with more readings per rpm. The torque reaction is against ALL inertial mass, flywheel plus internal rotational inertia.

What am I missing guys??

 

[patprimmer]

Airpower
I don't think you missed anything.
Torque reaction against the load is the best gauge. A snapshot of torque reaction vs rpm gives power at those rpm.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[LionelHutz]

You just pick a small interval....

You typically count pulses and calculate rpm, so your rpm number is already the average rpm over an interval. How can this be right but calculating the power over an interval be wrong.

The only difference is that measuring torque will account for losses in the dyno, assuming you measure torque accurately enough.

 

[140Airpower]

Lionel, you can calculate power from the torque times the rpm. You don't need to know anything whatsoever about the flywheel or anything else and there is no error due to not including the internal rotational inertia. The interval for rpm sensing will depend on how may notches there are in the sensor wheel. If only one, then that would be about 17 per second per 1000 rpm. The torque sensing could be triggered by the rpm signal or however you set it. If this is done by "hand" then you need a tach and an analog scale and a video camera.

 

[patprimmer]

It's simply a matter of practicality re accuracy. We don't really need power levels at better than 3 significant figures.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[imcjoek]

How are you arriving at a torque value, if not by T=Inertia*Alpha? Another sensor/loadcell/whatever? Why bother with the added complicated when RPM readings taken at regular time intervals can net you a good torque curve?

Even industrial-strength dynos are traditionally back-to-back comparative tools. Just needs to be consistent with itself.

 

[140Airpower]

imcjoek, You would need a load cell or a bathroom scale or whatever. This is a direct readout without having to calculate moments of inertia, gearing factors, friction losses, and still having to ignore internal rotational inertia. I don't think it can get simpler or more accurate. After all, this is essentially how brake dynos read torque.

 

[LionelHutz]

140Airpower - You shouldn't have to tell anyone here that you can use torque and rpm to calculate power and if you read my last response I already posted that measuring torque would account for losses.

Listen, you can do it either way.

Measure force and pulses and calculate rpm and torque and use that to calculate power.
or
Find the inertia of the flywheel and then count pulses and calculate rpm and power and use that to calculate torque.

Both require an accurate time reference to calculate the rpm from the pulses and either will work to give data.

 

[patprimmer]

A simple diaphragm or piston in a cylinder with O ring seal and a pressure gauge and a bit of basic maths to work out projected area of the piston or diaphragm will make a cheap easy load cell.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[LionelHutz]

Here's a link with some good info;

http://www.dtec.net.au/Inertia Dyno Design Guide.htm