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RC aircraft dyno

RC aircraft dyno

RC aircraft dyno

(OP)
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!

RE: RC aircraft dyno

how many rpm?

RE: RC aircraft dyno

(OP)
Smaller engines around 2-3hp see up to 8500 larger engines see 7k usually.

RE: RC aircraft dyno

hm. That's 13.2 N-m, right?

I have no idea how well a turbo would work as a water pump. It would definitely move water, but I would worry that it would erode or otherwise fail. My first thought was that you'd be better off with a hydraulic pump or (regular) water pump, but you probably won't find one big enough that you can direct-drive at that speed.

I imagine you'd want to measure torque (as force) at the corner of an engine platform while driving a bench-mounted pump of some sort. Maybe you could use a belt drive for speed reduction?

RE: RC aircraft dyno

(OP)
I would use Hydraulic fluid instead of water if I go the turbo route so that should take care of the corrosion issue. If I use the compressor wheel I can use water with a custom aluminium housing as the compressor wheel is AL however if I can use part of the cartridge and the exhaust wheel the assembly would be much easier to construct.

RE: RC aircraft dyno

I said erosion...I didn't mean corrosion. I was thinking about bits of metal being removed by localized cavitation or other processes (high velocity over the surface) . Water and oil have vastly different viscosity than air, so I'm suspicious that the geometry won't be appropriate.

RE: RC aircraft dyno

(OP)
I totally spazzed out on that I am sorry LoL.... Long day at work, I am looking for a different pump but having a hard time finding anything that I feel would be an acceptable to act as a load control valve. I had the old used t70 turbo laying around the shop so thats why I thought about using it.

RE: RC aircraft dyno

(OP)
I totally spazzed out on that I am sorry LoL.... Long day at work, I am looking for a different pump but having a hard time finding anything that I feel would be acceptable to act as a load control valve. I had the old used t70 turbo laying around the shop so thats why I thought about using it.

RE: RC aircraft dyno

Any load can work. It does not have to be pump that is pumping liquid. A variable fan pumping air will do it, a clutch pack from a small motor cycle or even moped could do it so long as the transmission fluid was circulated and kept at reasonable temperatures.

Regards
Pat
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RE: RC aircraft dyno

(OP)
ummmmmm....... I suppose I could measure the torque via load cell that prop exerts. Im just not sure how much torque can be measured with that method or if it would give a stable data.

RE: RC aircraft dyno

You would still measure at the engine support, I think

RE: RC aircraft dyno

Aeromodeller magazine in the UK., used to publish plans for an engine dyno, that used a beam balance reacting the engine against a known fan.
This was for much smaller engines than the one you are testing. But it may be an idea worth looking into.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

RE: RC aircraft dyno

Unless I am mistaken - I think a lot of propeller-driven 'planes do actually have a load cell in the engine mounts to give an indication whether the engine is producing full power.

RE: RC aircraft dyno

You could find an ~30hp 3-phase motor and feed DC into 2 of the leads to get a brake. It can't be run continuously because it would overheat but it would give you braking for long enough to do the tests you'd likely want.

You can also accelerate a flywheel and count pulses from teeth on the weight using an accurate clock. Figure out the inertia of the flywheel and you can calculate the torque and power curves.

RE: RC aircraft dyno

I'm failing to understand how an instrumented stationary engine mount, and a selection of propellers in the range of interest, could not give you all the data of practical significance.

Mike Halloran
Pembroke Pines, FL, USA

RE: RC aircraft dyno

I was thinking that perhaps he feels safer without spinning blades, but a guard could fix that.

RE: RC aircraft dyno

(OP)
Wouldn't the wind conditions affect the prop readings? How many props would it take to get reliable data? So for each size engine I would need X amount of props, that could get expensive quick.

Can someone explain to me how I would collect that data to usable HP ? Thanks for all the help thus far guys, I really do appreciate it.

RE: RC aircraft dyno

You get HP by measuring rpm and torque and doing a little math.
RPM can come from a photo tach aimed at the prop.
Torque can come from an engine mount that is free to rotate except for where it's restrained by a load cell or a scale.

I.e., you don't have to measure the torque between the engine and the prop, you just measure the torque that's required at the engine mount to keep the mount and the engine from rotating.

Mike Halloran
Pembroke Pines, FL, USA

RE: RC aircraft dyno

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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RE: RC aircraft dyno

(OP)
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.


RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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

RE: RC aircraft dyno

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

RE: RC aircraft dyno

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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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.

RE: RC aircraft dyno

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

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

(OP)
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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

(OP)
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!

RE: RC aircraft dyno

You could call the mfg??

RE: RC aircraft dyno

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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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.

RE: RC aircraft dyno

(OP)
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.

RE: RC aircraft dyno

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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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.

RE: RC aircraft dyno

(OP)
I am just looking for peak HP at the moment.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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

RE: RC aircraft dyno

(OP)
That 350 flywheel would probably snap cranks lol

RE: RC aircraft dyno

Quote:

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).

Quote:

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.

RE: RC aircraft dyno

(OP)
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

RE: RC aircraft dyno

(OP)
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. :)

RE: RC aircraft dyno

Quote:

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.



RE: RC aircraft dyno

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
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http://eng-tips.com/market.cfm
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RE: RC aircraft dyno


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:OLDASSE...

RE: RC aircraft dyno

(OP)
BigClive, the link doesn't work for me.

Thanks

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

(OP)
I understand.
  

RE: RC aircraft dyno

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?

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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
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RE: RC aircraft dyno

(OP)
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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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

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

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.

RE: RC aircraft dyno

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

Perhaps a squirrel cage motor with DC on the poles

Roy

RE: RC aircraft dyno

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

RE: RC aircraft dyno

(OP)
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.

RE: RC aircraft dyno

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

RE: RC aircraft dyno

(OP)
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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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??

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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.

RE: RC aircraft dyno

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
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