Pitch speed and stalling speed questions - I wish Sparky Paul could provide his input

[S55]

An article by Keith Shaw indicates the stalling speed (mph) to be 3.7 x square root of wing loading (oz/sq ft). He says it pretty well matches reality for 15 to 22 oz/sq ft loading. Then he says that in order to have good flying behavior the prop pitch speed needs to be at least twice that value with three or four times being needed for aerobatics. How come the number of blades does not come into picture here? How about the number of props?

Have any of you gone deeper into this?

Tank you,
S55

[Joe Yap]

Different number of props and blades well only change the static thrust. The prop pitch speed will still be the same assuming all props turns at the same rpm and having the same pitch. The prop pitch speed is pitch(inches)/12 x rpm/60 [ft/sec].

[steve lewin]

These are general rules of thumb not mathematical certainties. They're just a way of getting somewhere close and there are lots of other things to take into account before you get a flyable system.

Anyway if you think about it "pitch speed" is not affected by either the number of blades or the number of motors. An 8x8 prop will have the same pitch speed as a 14 x8 at the same rpm but it won't fly the same model in the same way.

Steve

[Sparky Paul]

My input?
I'm flattered you ask.
I look at what other people use for similar airplanes, noting that sometimes the reported performance might be "enhanced".
Then I'll try the suggestion. If it works, fine. If it doesn't, I have a bucket full of other props to try. And batteries and motors and gear boxes...
I have yet to find what "pitch speed" for any of my airplanes is.
Since we really don't know the airspeeds, I don't worry too much about this technical stuff.
There's way too many variables involved to be able to pinpoint the exact effect of any one of them.
I try it. If it works, I keep using it.
As Steve mentions, pitch -and- diameter give significant different results.

[S55]

Sparky Paul, thank you. Based on previous comments of yours, I was expecting a less empiric approach though. Not that empiric does not work, but for a beginner without a good feel, the iterations may start far from a working combination.

Of course the 14x8 and the 8x8 spinning at the same RPM will have the same pitch speed. And of course the pitch speed does not care about the number of blades.

What I wanted to say is this: let’s assume you build a plane and the above rule says stall speed is 20 mph. The other rule says you need at least 40 mph pitch speed to fly nicely. But you can get there with a pitch and RPM combination or double the pitch and half the RPM. Both cases fit the rule, but I would expect quite different results. Basically I think I am intrigued that thrust does not come into equation.

By the way do you know of any formula for thrust? I have seen the graphs in Bob Boucher’s book about electrics, but those ones only help to show how thrust of a given prop varies vs speed.
I hope there is one clean thrust formula out there, like the one giving the power absorbed by propeller, which I attach bellow in case some of you do not know about it. It should be a surprise though: if I know about it, everybody else on e-zone should.

Thanks.
S55

[balsaman]

Watts per pound also play. All these things work together. You need 50 watts per pound minimum to fly well. 100 watts per lb is better for aerobatics.

A speed 400 with a 5" x 4" prop will have a pitch speed in the 40's, but will not fly a 3 lb plane with a stall speed of 20 mph, or 10 mph for that matter.

With e-flight, there are a few minimums we should shoot for:

50 w/lb
pitch speed = stall speed x 2
low wing loadings (<20 oz.square foot)

Eric

www.e-zflight.com

[Sparky Paul]

It's too much like work to do all that math! :D
Basically watts/pound and a light wing loading are the most important criteria. Fit the prop/battery/gearbox to the airplane's performance.
Fly it, see if it's deficient.
Determine what fixes the deficiency.
Try that.
I suspect some of the constants in the various calc programs are chosen to make the answer look right, rather than having been experimentally determined.
I have measured airspeeds for a lot of planes,
here's the results..
http://www.angelfire.com/indie/aerostuff/inflights.htm
.
The e-Spirit in particular -glided- faster than the maximum speed one of the calc programs predicted predicted for it under power! And the power-on speed was way higher than the max prediction.
Until there's a reasonably thorough test of calculation vs reality, I prefer the TLAR school of design. If it looks good, it'll fly. How well depends on how much effort the thing might be worth investing.

[Sail 'n Soar]

Being new to the electric branch of the hobby, I try to follow the rules of thumb faithfully. But unless I'm missing something, the one that doesn't ring true is stall speed ~3.7*wing loading in oz/ft squared. At sea level that corresponds to an average wing coefficient of lift of ~1.8. An NACA 2412 at airliner scale can't get there from here without flaps (~1.7 max at Reynolds numbers in the millions!)

With a reasonable symmetric foil, a better figure would be ~4.9*wing loading in oz/ft sq, which equates to CL, max~1, to about 4.5 for a high lift foil (CL, maz ~1.5) Things just get worse at altitude.

My guess is that a factor of 2 was dropped in the original derivation. (W/A = (rho/2)*CL*U^2, where U is in fps.) In that case, the 3.7 should have been 3.7*sqrt(2), or ~5.2, which corresponds to a very believable wing average CL of .9.

These figures don't consider prop wash or hanging on the prop. If that's the answer or I'm missing something else, please let me know.

Of course, assuming a too low stall speed doesn't invalidate the other rules of thumb. That model just isn't flying as slowly as you might think. :D

[S55]

Now this is more than I can chew. I guess this what I deserve. I also am not sure if I explained the formula right, so here it is bellow.
Anyway I was not questioning the accuracy of the speed as a function of wing loading formula, I was only wondering how come other factors (like thrust for example) do not count.

S55

[steve lewin]

The answer is that you have only quoted 2 of the many "rules of thumb". To be confident of a plane flying properly you want to meet ALL of them not just the odd one or two you choose on the day. E.g. watts/pound, cells/sq ft wing area / prop diameter to pitch ratio. They all tell you something about the way a plane might fly.

It is a basic characteristics of these sorts of "rule of thumb" that they assume a fairly average setup. Yes you might in theory get a high pitch speed by running a 20 x 2 prop at 120,000 rpm or a 5 x 200 prop at almost no rpm but as the props don't exist and nobody does, if you've found a way to get those numbers you're on your own.

The reason why the *calc programs are so popular is that they integrate all of various rules and give you a simple and fairly accurate answer out the end.

People spend a good few years at university learning the detail of how planes fly. You're not going to get it condensed to one or two simple calculations, sorry.

Steve

BTW SailnSoar - did you miss the "square root of wing loading" in the original formula ? Square root not squared. I'm still not sure if I believe it but it certainly makes a difference ;).

[joao]

Hello S55,

If you get a Varioprop you can change the pitch at will

http://www.aircraft-world.com/prod_datasheets/varioprop.htm

You can also get 2,3 and 4 bladed props.

My approach to the prop "dilemma":D is to look at what other people say that works, compare to my own guess and try it.
In the pre-Ezone era I used to look at Graupner´s catalogue and search for similar planes to the one I wanted to power.
Cheers, João

[Sail 'n Soar]

S55,

You quoted the often published formula correctly. My point is that the factor in the formula is wrong. The factor should be something like 5 plus or minus to reflect model size lift capability.
Enough spent on that.

To your point about why other factors, e.g., thrust, don't count, as Steve Lewin stated quite well, those other factors do count. Steve mentioned a few of the other rules of thumb. The Max Cim site published a list of guidelines, including thrust, very similar to others I've seen.

http://www.maxcim.com/guide.html

Their bottom line is, "If you find that you cannot satisfy all of the above rules with some particular model, BEWARE! That first flight could be quite interesting."

Without belaboring the too much point further, they quote the 3.7 factor you mentioned. You can still use it to determine a pitch to stall speed ratio. The rule of thumb pitch to stall speeds based on that factor are still good guidelines, reflecting the experience of others. It really doesn't matter that the actual stall speed will be higher.

Steve,

If you look at my equation, W/A = (rho/2)*CL*U^2, to get U you take the square root of W/A*2/(rho*CL), hence the root 2 factor. Sorry for not writing more clearly.

SNS

[S55]

SNS,

Thank you for the maxcim link. Nice short summary.

S55

[steve lewin]

Originally posted by Sail 'n Soar
If you look at my equation, W/A = (rho/2)*CL*U^2, to get U you take the square root of W/A*2/(rho*CL), hence the root 2 factor. Sorry for not writing more clearly.

Sorry, I tend to completely glaze over when someone throws an equation at me without saying what any of the variables are intended to mean.

Anyway what I was mainly reacting to was your statement "the one that doesn't ring true is stall speed ~3.7*wing loading in oz/ft squared".

Glad we've got that cleared up now ;).

Steve