measuring Engine RPM in the air

I know that this hobby is all about applying various scientific principles to toy machines for the purpose of having fun, but this discussion about airspeed and RPM data is getting a little bit ridiculous. If you want to know which prop works the best, try 4 or 5 of them on the plane in flight and see which one works the best. If your data disagrees with your flight performance you would disregard the data anyway. The good news is that there aren't more than a half dozen propellers to choose from in the first place for a given application.

AFAIK, yes. The average would give you RPM, the difference would give you airspeed (after much calculation).

All this assuming a straight-line flight path & no RPM changes during the run.

Trouble is large props are bloody expensive. I got two 16x8 props for =A3 10 each special offer (about $19 each). 4 large props would be more than the brand new petrol strimmer engine cost me.

| I know that this hobby is all about applying various scientific | principles to toy machines for the purpose of having fun,

For some people, yes ...

| but this discussion about airspeed and RPM data is getting a little | bit ridiculous.

Why is it ridiculous? I don't see anything ridiculous about it.

| If you want to know which prop works the best, try 4 or 5 | of them on the plane in flight and see which one works the best. If | your data disagrees with your flight performance you would disregard the | data anyway.

Maybe. You might also see if you can track down the source of the discrepancy.

| The good news is that there aren't more than a half dozen propellers | to choose from in the first place for a given application.

Seems to me like there's usually more than a half dozen *brands* of propellers, let alone different possible sizes ...

On Mon, 06 Nov 2006 22:00:32 GMT, "Doug McLaren" wrote in :

I see it as fun--IF I can persuade someone else to invest in the $500-$600 setup. :o)

If I were into air racing, I'd want to have the data on RPM and true airspeed for each prop, along with some sense of the air temperature and humidity at the time of the test run.

I would think that would give me more satisfaction than trying to evaluate performance solely on the testimony of eye and ear.

There are amazing prop gurus out there. We may have been visited by one from Australia a few days back. Some of the pattern guys heat their APCs and repitch them. Gives me the shivers just to think about it. I think some of the props are set by the racing classes, so that wouldn't be an option.

Marty

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Look at this page and scroll about 19 pages down and you will see a reference to an audio tach from 1951 by some engineers from GE.

A flying friend of mine tried this in the 1960's and it was not sucessfull. After he passed I got some of his junk box stuff and found the audio tach. So being an individual that wanted to experiment and having technology to use that did exist in the 60's I applied microprocessors to the job. This was about 2002 and it did not work any better now than it did way back then.

This is a technique that works well in theory but not in practice. The narrow band pass (zero beating) of the human ear is too narrow to compensate for the doppler effect and other noise made while a plane is in flight. With the plane held on the ground you could make it work.

My $.02

On 7 Nov 2006 03:05:25 -0800, "IFLYJ3" wrote in :

Seems to me that trying to use it in the air would be incredibly hard (I don't know nothin' 'bout narrow band pass).

Doesn't the frequency heard vary continuously as the plane approaches (rising note) and departs (falling note)?

Maybe it'd work if you were flying CL and the plane was at a constant distance from the pilot, or if you flew the RC plane in a circle around the pilot. We try to discourage that at our field, but some of the oldtimers did fly that way when I started at the field in 1995.

Marty

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OK, I have managed to get the 28cc strimmer engine running on an 18x8 prop (stationary mount). Now comes the running in and tweaking.

This is where it would be nice to know the equivalent prop size to put the engine under the same load stationary as it would be moving in the air.

Any suggestions?

It doesn't work that way. The airframe has influence over how much the engine will unload in the air. This is why we usually have to take a few props to the field with us in order to find out which one loads the engine properly on a given airframe. There is still a bit of "art" involved in model airplanes.

Ed Cregger

I am talking about maximum revs, not power at various throttle settings. If the load is X and the plane doing Y mph horizontally, then that is the approximate load I want to simulate on the ground. Obviously the engine would have virtually no load on it in a dive, and the same as stationary when it is prop hanging. If I build an electronic ignition unit with timing advance, it would take some trial and horror guesswork out of it if I tested the various advance timings at simulated air loading. So, If I am swinging an 18x8 prop stationary, what prop would present the same load as 30mph ?

Well, you can sure get *some* better idea of max air rpm by using a smaller prop in stationary testing. It would also be better for running the engine in.

Big props are expensive here and to buy a range of them would cost more than the strimmer engine did. In fact I already bought two props at a reduced price, if I buy a third, they will have cost the same as the engine.

| > It doesn't work that way. The airframe has influence over how much the | > engine will unload in the air. | | I am talking about maximum revs, not power at various throttle | settings. If the load is X and the plane doing Y mph horizontally, then | that is the approximate load I want to simulate on the ground.

Ok then. Then determine your RPMs in the air, and then try props on the ground until you reach the same RPMs.

| Obviously the engine would have virtually no load on it in a dive

That is not so obvious -- that will depend a lot on the prop and the airframe. Some high drag planes won't go signfigantly faster in a dive, for example, and just because the pitch speed of the prop matches the airspeed of the plane, that doesn't mean there's no load on the engine from the prop.

| and the same as stationary when it is prop hanging.

That much is approximately true. (But not exactly, as the conditions around the prop will affect it's performance to some small degree.)

| So, If I am swinging an 18x8 prop stationary, what prop would present | the same load as 30mph ?

There's way more variables involved than just that.

| Big props are expensive here and to buy a range of them would cost more | than the strimmer engine did. In fact I already bought two props at a | reduced price, if I buy a third, they will have cost the same as the | engine.

If you want it cheap(er), try smaller engines. Or go electric -- for engines, this stuff isn't so easy to calculate, but for motors it can be calculated to a high degree of accuracy.

The whole point is not to buy and try a bunch of expensive big props and then test them all.

What are they? The only thing the prop sees is air stationary, or air moving at 30mph.

OK different props have their own slightly different drags and efficiencies but I'm only looking for an approximate equivalent so I can test my converted strimmer engine at roughly "air" loading. So If I fly a 16x8 prop at 30mph, the rough equivalent stationary might be 16x5 or 15x6, yes?

I have a 13x8 but that might be a tad too small even if I use it back to front.

I have plenty of smaller planes and engines, I am building a camera plane/s for video and want to tune a converted strimmer engine for the job. I also want to build some electronic ignition ignition sytems, including timing advance, so I want to have the same load on the engine as it would (approximately) have in the air. All I'm looking for is a guesstimate of how much to reduce the Diameter/pitch for stationary testing.

| Doug McLaren wrote: | > In article , | > wrote: ... | > Ok then. Then determine your RPMs in the air, and then try props on | > the ground until you reach the same RPMs. | | The whole point is not to buy and try a bunch of expensive big props | and then test them all.

Yes, and my point is that this is a pipe dream.

| > | So, If I am swinging a 16x8 prop stationary, what prop would present | > | the same load as 30mph ? | >

| > There's way more variables involved than just that. | | What are they? The only thing the prop sees is air stationary, or air | moving at 30mph.

Ok, I'll start by listing a few of them ...

-- at what RPM? (this is the biggest one.)

-- (ok, you've picked diameter and pitch)

-- what shape exactly is the prop?

-- what is behind/around the prop? (the cowling/engine/etc will affect this, as will your test bench)

-- how about the cooling of your engine? Your engine will cool a lot better in flight than it will on the ground or hovering, and this will affect how it performs.

-- at what altitude above sea level?

-- where did this 30 mph figure come from?

| OK different props have their own slightly different drags and | efficiencies but I'm only looking for an approximate equivalent so I | can test my converted strimmer engine at roughly "air" loading. So | If I fly a 16x8 prop at 30mph, the rough equivalent stationary might | be 16x5 or 15x6, yes?

Maybe if you can pick a specific RPM speed target ...

That is what will be determined from tuning the engine and spark timing. The expected *maximum* rpm is about 7-8000.

Master airscrew classic, most likely.

Exactly the same as it would be in the air. The test bench won't make any discernable difference

Well not significantly. It is a strimmer engine which is normally cooled by the flywheel fins and used to running hot. The difference of extra cooling from a prop running stationary or in the air is barely going to affect it. I could even trim the fins and loose some weight.

500m max.

That's the approximate cruising speed I intend my plane to fly at and optimum tune my engine to. As you might have gathered, *any* prop a

*bit* less powerful than the one actualy used for flight, would better simulate flight conditions in stationary testing. The only question is how much less.

About 7000rpm.

over!

If I did the math correctly and you want to fly at 30mph air speed at

7000 rpm engine speed you need a 4.5 inch pitch. So lets say ground testing gives you the 7000 rpm you want with a 15X5 prop. In the air you will need a 16 or 17 X 5 prop to get the same engine loading. A lot depends on airframe drag which can only be determined by testing in the air. Such a combo does give an acceptable prop tip speed.

This seems to me like an awful lot of low gear and I think you would be a lot happier with the way the plane handled with a bit more pitch and a bit less diameter. For instance if you ever try to fly in any wind it is going to be tough to get the plane up or down safely with such a low pitch. Even if wind is only 10 mph gusts of 20 are common. At 30 mph airspeed you are not going to be much above stall speed even with a floater. Set up to land in a gust and when the gust passes your airspeed could be below stall speed and no altitude to recover. You do have a throttle control for a reason. But throttle only works if you have enough pitch to use it. Your specs are going to limit flying to calm, overcast days I think.

Back in the 70's a friend of mine used a simple AF generator and ear phone. He had the dial calibrated and just read the rpm's directly off the dial.

Phil

snipped-for-privacy@digiverse.net wrote:

It certainly seems the easiest method, and the cheapest for ground use too. Nice one. I'll check if I've got something lying around. I've got loads of NE555 ic's to put one together.