Airspeed

Incalculable without knowing power and airframe drag as well.

Like how fast will a cox teedee 049 capable of doing 20,000 RPM propel a ferarri with a 4.2 ratio back axle and 21 inch tyres?

Gerald,

Here's the site for Bob Leserve's Model Aircraft Performance Calculator... It has a JavaScript app that will do the calculation... I'm sure other variables would affect accuracy; but it's a good ballpark figure...

| Dr1Driver wrote: | | > What's the formula for approximate airspeed calculation given RPM, prop | > diameter, and pitch? TIA!

It's simple enough to figure out.

A 6" pitch prop `moves' 6" of air in one revolution. At 10,000 rpm, that's 60,000 inches/minute, or 57 mph.

The formula you're looking for, given prop pitch in inches, rotational speed in RPM and speed in RPM is probably this --

speed = rpm * pitch / 5280 / 12 * 60

The figures come from: 5280 = feet/mile, 12 = inches/foot, 60 = minutes/hour. You can of course just replace `/ 5280 / 12 * 60' with `/ 1056' if you wish --

speed = rpm * pitch / 1056

| Incalculable without knowing power and airframe drag as well.

Not entirely true. For a draggy plane, you're certainly right, but for a sleek plane, you can get right up there. Perhaps a little faster than you might think, as the prop will unwind in flight.

This formula won't always tell how fast your plane will go, but it will give you a top limit on your speed.

| Like how fast will a cox teedee 049 capable of doing 20,000 RPM | propel a ferarri with a 4.2 ratio back axle and 21 inch tyres?

Not quite ...

And its actual pitch - in terms of what speed it generates zero thrust - may be considerably above what is stamped on the prop times the RPM.

Exactly like. If you are spinning a 1/2" diameter prop at a pitch speed of 100mph, you will get nowhere fast...

Try this link:

... | > The formula you're looking for, given prop pitch in inches, | > rotational speed in RPM and speed in RPM is probably this -- | > | > speed = rpm * pitch / 1056 | > | > | Incalculable without knowing power and airframe drag as well. | > | > Not entirely true. For a draggy plane, you're certainly right, but | > for a sleek plane, you can get right up there. Perhaps a little | > faster than you might think, as the prop will unwind in flight. | | And its actual pitch - in terms of what speed it generates zero thrust - | may be considerably above what is stamped on the prop times the RPM.

This formula is not a `this is as fast as your plane will go' magic formula. But under certain conditions, it will give you an approximate idea of your maximum speed.

| > This formula won't always tell how fast your plane will go, but it | > will give you a top limit on your speed. | > | > | Like how fast will a cox teedee 049 capable of doing 20,000 RPM | > | propel a ferarri with a 4.2 ratio back axle and 21 inch tyres? | > | > Not quite ... | | Exactly like.

No, not even close. First of all, you're not even talking about a prop at all. Second of all, we know that this formula is only good under certain conditions, which I stated right up front, and I don't think a 2000 lb car and a 0.049 engine qualify. Do you?

| If you are spinning a 1/2" diameter prop at a pitch speed | of 100mph, you will get nowhere fast...

If your plane has a 2" wingspan, you just might get there at about 100 mph.

How many extreme examples are you going to give us? Sure, it's not hard to find cases where this formula is way off. And it's not hard to find examples where it's really close. It's an approximation, and everybody knows it. Quit trying to say that if it doesn't work under this condition, it's useless -- because it's not.

And that's just what I wanted, Doug. Thanks! :) Dr.1 Driver "There's a Hun in the sun!"

Mike,

Don't worry about DH. He's just trying to impress himself and see how annally annoying he can really be.

No, pointing out the danger or presenting a very approximate rule of thumb as gospel truth.

You may feel justified in asssuming there are no people stupider than you on this newsgroup, but I assure you, you are not alone.

This is a nice post except for this one line. If you would change 'moves' to 'moves through' you would be a lot more accurate. There are simply no circumstances under which a 6" pitch prop 'moves' air 6" in one revolution. Do not believe me? It is simple enough to measure with a variety of readily available stuff. If you really think that this prop moves 6" of air I suggest you go make the measurements for yourself. Props do not generate thrust by simply blowing air. If they did the formula for energy absorbtion would be dia squared not dia to the fourth as it actually is.

At 10,000 rpm,

It's amazing how much controversy this one post has generated ...

`moving air' and `moving through air' is just a matter of perspective. In one case, the air moves and the prop doesn't go forward, and in the other the air stands still and the prop goes forward. It's all relative.

I did put the word `moves' in quotes because I knew it was oversimplified. Really, what I was doing was explaining how the formula was derived, so that if somebody forgot it they might remember what it came from and could figure it out themselves. If you want to provide more details, feel free.

| There are simply no circumstances under which a 6" pitch prop | 'moves' air 6" in one revolution. Do not believe me?

Sure. Certainly, a 12"x6" prop does not nicely move a 6" radius cylinder of air back 6" inches.

| It is simple enough to measure with a variety of readily available | stuff. If you really think that this prop moves 6" of air I suggest | you go make the measurements for yourself. Props do not generate | thrust by simply blowing air.

At the very simplest level, they do exactly that -- they push air (generally) in one direction, pushing the prop in the other direction. Newton's second law.

(And no, I'm not interested in getting into a `Bernoulli vs. Newton' discussion here, so if one is started, I'm staying out of it.)

| If they did the formula for energy absorbtion would be dia squared | not dia to the fourth as it actually is.

And now you're oversimplifying things :)

Ultlimately, every formula we deal with that deals with the real world is an approximation at some level. Some are just more accurate than others ...

Not to worry DH! I've been convinced for a very long time that you are far more stupid than I could ever be!

It is not Bernoulli vs. Newton. They are the same exact thing. The Bernoulli equations are straightforward derivations of Newton's laws. There is no contradiction whatsoever between the two.

The full statement, if you wish, is that at a constant RPM the power absorbed by a prop (and thus the thrust generated) is directly proportional to the prop diameter to the fourth power times the prop pitch. This, of course, assumes the same number of blades, the same chord, the same taper and the same airfoil. Sorry if I made it too simple the first time. Any theory based on blowing air would say that power absorbed, with the above provisos, would be proportional to diameter squared. As Pe put it so aptly some months ago, the skipping rock theory of props and wings is simply not correct.

Sure. The above formula is often stated as power absorbed is proportional to prop diameter to the fifth power. No reference at all to pitch. The reason is simple. As prop dia goes up pitch also goes up pretty much in direct proportion, at least if you want to keep flying. So the approximation is good enough for government work. But that does not make it correct. We all know there is a pretty big difference between a 10X4 and a 10X7 in the way they perform. So getting sloppy in your thought process is not always appropriate.