Hi all,
i am doing some dynamometer testing of model aircraft engines. I
am a bit surprised by the curves that i am getting since they dont
look like the usual power, torque curves that you could find in
books or online.
Another question i had is about how the standard curves are
obtained. Is the engine fully loaded at the start and the load is
gradually taken off or is it the other way round which is how i am
doing it.
I am uploading some curves into the files folder.
Thanks,
Shyam.

What are you using for a dyno? I have the AHP torque reaction dyno but haven't played with it for a couple of years.

Hi,
I am using a custom built dyno. Its a reaction type dyno where i measure the force developed by the engine on a moment arm. This gives torque and i measure speed using a speed sensor. I am attaching the curves too this time.
Thanks,
Shyam.

I'm assuming you're using different sized props to vary the load and that all readings are taken at full throttle with the optimum needle setting?

The curves don't look right to me either because even at the low revs you took your first reading they're both past what would seem to be their peaks. Even a mildly timed small 4 stroke would be giving a rise in HP (and most likely torque as well) from the initial (7000?) revs.

As for which direction to go when taking figures, it can be done either way but usually it'd be better to start with a high load/low rev situation because then you can see when the HP peak is reached and it's not necessary to go much beyond that.

Hi,
Thanks for the reply. I am using a kind of an electro-magnetic brake to apply the load.
The readings at the low revs and high load are actually the last readings that i took before i started unloading the engine. I start with no load on the engine and then start applying the load. However if i go beyond the highest loading pt.(lowest rpm point), the engine starts to stall, the rpms decrease real fast and the engine stops. So either i am unable to see the peak or the lowest rpm point is my peak and i can't take a reading below that.
The point is my highest loading point seems to be my peak and any rpm greater than that gives me lesser torque and power. But going by other power curves, this shouldnt be happening. There should be lower rpms where i can get some torque without the engine stopping on me.
All the data points were taken with the engine on full throttle and the mixture setting on the optimum point for the engine.
Another thing i want to ask is whether the engine is tested at full throttle to get the curves or is the throttle position adjusted at each load setting to get the maximum torque at each engine rpm.?
Thanks,
Shyam.

I think consideration must be given to the fact that when the rpm are pulled down at full throttle the airflow velocity through the carb is reduced which in turn reduces fuel draw causing the sudden fall off in power and subsequent quitting. A pump of some sort would cure this without altering the power characteristics.

I don't think your getting low enough rpm for a max torque reading. If I remember correctly, the formula is HP= torque X rpm / 5252 . I do remember that HP and torque always cross at 5252 rpm's with torque on the declining line and HP on the increasing line.

This is based on automotive type dyno's that load from low rpm to high. Engine dyno's are, or used to be, the water brake type. I've witnessed actual pulls on my own engines but cannot comment on the workings of the dyno itself. Chassis dyno's use a rolling road type of drum to time drum acceleration by the drive wheels of a car, and calculate torque and HP off of the time to spin of the drum. This is my preferred method for 2 reasons. It tells you what you have, HP and torque, at the ground with all the mechanical losses in the drivetrain removed, and, more importantly, you don't have to take the engine out to do it!

Think about this:I saw an episode of "Celebrity Wings" on Discovery's Wings channel about racer and aviator Bobby Allison. They mentioned how he fashioned an automotive engine dyno out of a propeller. The car engine spins the prop for resistance. There has to be a formula for this somewhere. And since we're already using props, it should be adaptable

i think someone used to sell a set of load beams (or whatever), just mount on the engine, record rpm and a simple calculation would give hp and torque. i guess a set of flywheels would also be useable.

dave

HP=FxDX2PIxN/33,000 where T=F(lb)xD(ft)
N=rpm (rev.=2PIr
HP= 33,000 FT-LBS/min.
Simplified to: HP= TXN/1,008,000, Plagiarized from the instructions for the AHP Torque reaction Dyno and Dave Gierke..
The slashes mean over.

I think the problem is with the type of brake you're using. It's either a fixed load or it increases too slowly as torque builds up from the lower revs. Working backwards as you're doing from high revs and gradually increasing the load is fine until you get close to the point of max torque. Then with a fixed load the engine simply runs out of available torque to overcome the load so it slows down. This reduces the torque even more and it ends up stalling. If you started from lower revs and used a load that would suit the torque at lower revs the engine would reach this point then continue to gain revs and torque and then stabilise on the downward slope of the torque curve.

So what you need for a brake is one that increases the load very quickly compared to any RPM rise. This is what a prop does. It's load increases by the cube power of rpm so fitting a very large prop holds the revs well below the max torque point of the curve and reaches a balance.

You must use full throttle even for low revs. Closing the throttle restricts the amount of air that can be taken in and effectively makes the engine think it's smaller than it really is. In other words, a .60 at half throttle runs about the same as a .30 at full throttle on the same prop.

Hi!
Kavan (of Germany) earlier had a set of different polished aluminum prop blanks that could be mounted to the engine and gave correct hp and tork figures depending on rews..perhaps this set is still available?!


Regards!
Jan K
Sweden