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!

 

[FullSpool]

That 350 flywheel would probably snap cranks lol

 

[imcjoek]

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

Hardly "giant scale". So we're talking a ~16-18" prop. Where did 13hp come from? I'd take a stab at more like 3. Typical brushless electric motors designed for this size aircraft are rated at ~1500W (2hp).

That 350 flywheel would probably snap cranks lol

Well obviously you'd need to support the load (whatever it is) on its own bearings at these speeds. Being this is an engineering forum, I'd hope that much is obvious. Is this intended to be developed into a product? I'm assuming not. Have you tried RCGroups? Someone there has probably already done the leg work on this type project. Generally this forum is more aimed at pro-engineering type applications, although people here will cut slack if your topic keeps them entertained enough.

 

[FullSpool]

I don't think you understand the size of these air crafts ,104- 110" wing span swings a 27x10 to 28x10 prop.

25% 30cc 16"-18" prop
30% 50cc-60cc 22" -24" prop
38% 100cc-120cc 27"-29" prop
42% 150cc-170cc 30"-33" prop

 

[FullSpool]

And for the second part of your post I have a feeling you have trouble sensing sarcasm so I will leave it at that.

Anyways with the flywheel I am afraid that every time the cylinder fires there is an increase of acceleration of moving parts, with a heavy flywheel such as that 350 it will not accelerate that abruptly. The twisting stress on the crankshaft will eventually lead to failure on smaller engines. I know that this is a pro engineering site which is why I came to these forums, if I wanted information from people that don't have a clue what they are talking about I'd post on RCG. If You don't like the thread, don't post or have a MOD close it.

Thanks to the guys who offered comments that lead me in the right direction. I will update you guys on the build as my parts arrive, hopefully this week.

 

[imcjoek]

I don't think you understand the size of these air crafts ,104- 110" wing span swings a 27x10 to 28x10 prop.

Very well. I have a 72" Suhkoi I was basing my mental guestimates on. That runs a 1.2cu 2-stroke that swings a 16" prop and puts out a measly 2hp or so.

Still, don't discount an inertial setup. It can give you a nice smooth curve over the entire speed range and does away with the "what to do with all this heat?" problem, as well as steering away from expensive load-cells.

 

[patprimmer]

Whether the load on the engine is the propeller or a heavy flywheel, if it holds the revs constant at WOT, it's the same load on the crank.

An inertia dyno does not really measure steady state hp. it measures acceleration. Two similar powered engines can have considerably different acceleration due to different inertia within each engine. On engines with all identical weight rotating and reciprocating parts it will serve as a good ranking system.

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

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

for site rules

 

[BigClive]

The propeller used to absorb the engine's power does not have to be a genuine "flight" propeller. Test "club" propellers are used for ground testing of full-size aero engines - and I have heard of their use on model engines.

http://www.casa.gov.au/scripts/nc.dll?WCMS:OLDASSET::svPath=/airworth/awb/61/,svFileName=003.pdf

 

[FullSpool]

BigClive, the link doesn't work for me.

Thanks

 

[LionelHutz]

Are you serious about a flywheel weight breaking the crank?

Why not just accelerate a big flywheel and then measure the reaction torque and rpm which gives you all the data you need?

I'd watch the comments. You really don't have a soapbox to stand on, considering you're here with an obvious hobbiest question which is against the forum rules.

 

[FullSpool]

I understand.

 

[140Airpower]

Pat, Inertia dynos are known not to give the same result as steady-state dynos, but what are the causes? What is measured is only the torque reaction associated with each rpm as the engine accelerates. I suppose this is the same as a flash reading on a steady-state dyno? I know that an engine will see its power drop as it heats up under load. Is that the most important or the only difference?

 

[thruthefence]

Obtain a suitable test club prop.Typically short span, wide chord, and coarse pitch, compared to a "real" propeller. Shroud it. Obtain an electric motor, use the motor to drive the prop so some n rpm. Measure the electric load required to do so.Do the math, and you have the HP needed to drive the test prop to this benchmark rpm, at this pressure altitude, at this temperature. (Whatever ambient is on test day.)

Do several samples at progressively higher rpms. plot it on a graph, and you can interpolate rpm vs load (HP)

This is how power settings are determined on simple, fixed pitch prop recip powered airplanes. (Turbofans too, for that matter)

regarding torque indicating systems on propeller driven aircraft, A driving/driven helical gear set, driven from the prop gearbox, that moves a piston in response to the thrust loads the gears generate under power. This piston ports, or unports a chamber referenced to engine oil pressure, metering it into "torque pressure" This is read either on a transducer fed electric instrument, or a simple bourdon tube indicator, calibrated in ft/lbs, or % of torque.

There is another system, that measures the angular difference between a driving power shaft, and the driven prop gearbox. Measures the "twist", if you will. It also works by the variable orifice method, ending up with a fluid pressure that varies in a linear fashion with load on the prop.

 

[patprimmer]

In my experience, correlation between inertia and brake type dynos is mediocre at best.

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

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

for site rules

 

[dgallup]

Not to mention the infamous Dynojet "correction factors" that will give you any result you want.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[LionelHutz]

There is no reason an inertia dyno can't be accurate, assuming a large enough inertia is used.

The basic formula is as follows;

P=(inertia x N2^2/2 - inertia x N1^2/2)/dt
inertia = inertia of the flywheel. Should include inertia of the motor if the flywheel isn't 20 or 30 times larger.
N2 = final rotating speed
N1 = initial rotating speed
dt = time to go from initial speed to final speed.

As you can see, something to measure the rpm and an accurate clock will give you the power.

Personally, I'm thinking that a hybrid dyno would be easiest in your case. Use your mount to measure torque and the pickup to measure rpm. Connect the engine to a big flywheel of some sort as a load. Get it running just off idle and then open the throttle and start measuring rpm and torque points until you hit max rpm. Of course, some kind of PC board logging the data would make it very simple.


The manufacturers of inertia dyno's to measure the RWHP of a vehicle don't provide any details on their systems so only they know what causes the errors. However, I would suspect an honest owner/operator who uses all the correct info could get accurate results.

 

[140Airpower]

Lionel, I also see no problem with the theory. However, there is still the problem of flash readings not agreeing with steady-state readings. Precise, repeatable results usually require equilibrium conditions. An inertia run from idle or an unloaded condition would be far from that.

 

[LionelHutz]

Since no-one knows any details of the inertia dyno you've found that didn't match steady state readings it's rather hard to comment on why.

No having any real clue as to the measuring capability of the equipment the OP is building means it's hard to comment on that as well. If he's rigging up some rather difficult to read gauges then using a number of props is likely the best. If he's rigging up some equipment with better and quicker measuring capabilities then using a flywheel load might be the easiest.

I haven't seen a commercial load dyno that normally tests at steady loads either. Most engine dynos sweep through the test rpm range during the testing.

 

[patprimmer]

Most engine dynos I have used are water brake. You pull the throttle as you apply the brake via water supply and juggle the water supply until you get a stable rpm at the number you wanted to test at.

On these dynos, light weight internals in the engine make no difference.

On an inertia dyno, I have seen no suitable explanation as to how using light weight components does not change readings and show as more power.

I do accept that the higher the load, the lower the %age difference until a point where it becomes insignificant.

Correction factors are always a risk for fudging the figures no matter what type.

You can also fudge a brake by sudden application of load to bounce the load by inertia effect however this is obvious if you are present and watching.

Unfortunately for dyno operators, a happy customer means a customer with high numbers to brag about rather than accurate numbers. I know some many make "commercial decisions" re the corrections to the reported numbers.

In this case a flywheel should rank engines accurate with a very heavy flywheel. Yes I guess that's a back-flip on the issue.

Gearing can also be used to match flywheel weight to required load and SAFE rpm limit for the flywheel. I would much prefer to see steel rather than cast iron if the wheel is to be spun at high rpm. It could be something as simple as a large truck or tractor wheel mounted on a front hub with a chain drive.

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

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

for site rules

 

[FullSpool]

I have a pickup sensor mounted to the engines case and a magnet on the prop hub, as it spins I capture RPM on a digital screen for read out. As far as capturing torque I have a shaft that mounts to a plate (engine mounts to this) on one end and the other goes through 2 pillow block bearings. Keeping in mind the center of the crank is in line with the center of the 5/8" shaft that has an arm that hits a load cell as it pivots. The arm measures 12" from center of shaft to point of contact on the load cell. The load cell has a digital read out along with an usb connection for my lap top.

This is my assembly thus far in a nutshell, I am up for suggestions if anything sounds problematic . As far as applying the load I think using propellers would be the easiest method however it would be nice to adjust the load on the fly.

 

[BigClive]

FS - Your arrangement seems pretty good to me.
I am not sure you really need automotive-type dyno graphs of torque and HP for a model aero-engine.
I think you have to keep in mind that for aero-engines (both full-size and model) things like good mid-range or low RPM torque (and a smooth idle) which are important in a car don't matter as much in a 'plane. Maximum power output and fuel consumption at cruise power (in a full-size engine) are the important features.