Building a thrust dyno - opinions please?

[rc_sjo]

Hi all,

I'm soon to be fitting out my new workshop and i'm intending to build myself a nice little thrust dyno for testing engines, props, fuel, plugs etc. (for my and potentially for the public if it's a success).

I have the design of the dyno pretty much sorted, but what i'm keen to find out are the implications of where I mount it with regards to airflow and clearance around the prop. One guy mentioned to me that the prop tip should be X distance away from the floor/wall etc. to avoid some kind of effect - it was just a passing comment though so I don't have any further detail.

The dyno will pretty much be mounted on a worksurface approx 1.5-2.0 inches thick and the prop will typically sit a couple of inches forward of this. The work surface will be approx 1 metre deep with a flat wall behind it.

Do any of you clever people see any problems with this and if so, any recommendations/pitfalls? I'm not a physics student :)


Many thanks,
Stu :cool:

[JetPlaneFlyer]

I'd guess 'as much clearance as possible' would be good advice... I don’t think that there is a 'magic' distance where the interference effects stop or starts.
A more serious issue is that without a wind tunnel to simulate the forward airspeed of the aircraft you can only measure static thrust, which could have very little relation to the actual thrust you will achieve at full flying speed. This is especially true when comparing different props where it's safe to say that the prop that gives maximum static thrust will NOT be the prop that gives best performance at actual flying speed.

Probably the best way to compare actual flying performance is 'go fly the model'... it's most fun too ;)

Steve

[Neil Stainton]

IMO a rig that measures thrust isn't a dynamometer, and it should't be called a dyno. Dynamometers work by measuring torque and RPM, which when multiplied together gives output power. Thrust rigs just give thrust, which is a very poor indication of output power or motor efficiency.

Sorry to be pedantic.

Neil.

[Tom Harper]

Same issue as Neil - do you want to measure thrust or build a dynamometer?

I have some torque reaction Dynamometers. They give good engineering data. If you want accurate data for fine comparisons, you have to be in it for the long haul - flywheels or calibrated loads; balanced forced air cooling; accurate torque measurement instruments. Not a casual project.

Static thrust is meaningful for low pitch props, but it's easier to measure with a scale tied to the tail wheel than on a fixture. Probably more accurate also.

What's your goal?

[rc_sjo]

Hm. Thanks for the replies folks.

It seems I may be wasting my time with this project, going by the comments above. Yes i'm familiar with engine dynamometers (be it inertia based or a brake dyno), and yes I could of-course make one of those for RC engines (some people do already), but I was trying to use some of my automotive/electronics knowledge in my new hobby, and apparently not really understanding the usefulness or what i'm trying to achieve!

Even though I do not understand the science (yet), I can appreciate the comments of how engine/prop performance will be completely different whilst in the air on a model, than sitting on a rig. It is this that makes me wonder if there is purpose in my idea!

I think the drive behind this was that in most engine reviews i've seen, they always show engine speeds on various prop sizes. I just thought that it might be useful to also add some (static) thrust figures as well. Apparently not!

Perhaps i'll go back to the drawing board and consider a proper engine dyno instead (seeing as i've bought a load cell now and everything!)..

Thanks fellas.. It's all a learning curve.. :cool:

[DT56]

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[JetPlaneFlyer]

Good point... Yes for 3D type flying where your interested in hanging on the prop at close to zero airspeed then a static thrust test is all you would need... Where a static test becomes less accurate is for coarser pitch props which come into their own at higher flying speeds but would tend to give poor static thrust.

Steve

[BMatthews]

Don't forget about powered gliders and slow flying old timer style stuff. Static thrust counts for a lot there as well.

But all in all I'd have to say that it's likely not worth setting up a dedicated area unless you're extremely curious about the whole thing.