Props

His calculations, and his own assertions give no limits for his calculation. If you read his other reply to me, he also said that he tested the props to the limits of the engines used to spin them. He must be only using the props on four strokes and keeping the props oversized to only test below

10,000RPMS. This is certainly not very useful for most two stroke applications.

I still have that spreadsheet around here somewhere that was done for the great 46 engine shootout a few years back. These measurements were all done with APC props so are relateable between sizes.

Also, APC and Zinger MA Y have the same correction factor, but actual in-air performance is vastly different!

| His calculations, and his own assertions give no limits for his calculation.

His web page does tell where he's tested, to some degree ...

| This is certainly not very useful for most two stroke applications.

I can't argue with that.

| I still have that spreadsheet around here somewhere that was done for the | great 46 engine shootout a few years back. These measurements were all done | with APC props so are relateable between sizes.

Another problem is that you can't directly control RPM. You control the engine that you use, and the prop, and the fuel, and your throttle position, but none of these directly affects the RPM. (Well, throttle position does, but only up to a point.)

An engine shootout probably gives maximum static thrust readings for specific engines and specific props, which is likely to be more useful than knowing that this specific prop will give X lbs of force at this specific rotational speed (RPM.)

| Also, APC and Zinger MAY have the same correction factor, but | actual in-air performance is vastly different!

Well, static thrust isn't the entire story. In fact, the only things it really only directly affects are 1) the ability to hover (and pull out of a hover) and 2) how fast the plane accelerates from a complete stop (and as soon as it's moving, it's not at a complete stop anymore, so it's not that useful there either.)

The engines used in the testing were diesels.

Thanks for the input on the website, stating that the calculated thrust is static thrust. I should have thought of that because I always feel I have to mention that when I send the calculating spreadsheet even though it is stated on the spreadsheet itself. It will be changed today.

It really does one's heart good to see how everyone chooses to abuse the person who did the work.

Someone goes to a lot of trouble to test props and develop a "That'll get you close" STATIC THRUST formula, and just because it doesn't conform to other peoples theories as to what should happen, they choose to abuse the messenger.

In actual point of fact, I have used Brian's formula MANY times over the last few years, and it does a pretty good job of predicting STATIC THRUST.

As to actually FLYING, you have to "cut and try" till you are happy, but the formula gives a tool to pick a starting point.

If his work is not good enough for all the detractors, then I challenge them to develop a better formula, and post it here for FREE for all to use.

YMMV mine doesn't

There is no formula that can show what I have seen personally after testing hundreds of props on many engines as well as collating the data from several other tests. All I can say is that pitch DOES affect static thrust.

post your research so we can ALL see it Then maybe you can derive a formula that accounts for it.

Otherwise, I have just two words for you. Downwind turn

Brian,

I have NO instarting a war but experimental test involving several hundred props has shown that pitch is a factor in thrust and power.

Ray Shearer

I don't know about NASA Ames but back in the 30's and 40's work was done by NACA before it was formed into NASA. I found some of it in the archives during MIT school days in the 50's. I don;t know if NASA has much of the old NACA work available online.

Ray Shearer

| Saying pitch doesn't affect thrust of a prop is equivalent to saying | AOA doesn't matter to lift of a wing.

That sounds clever and all, but It's not quite so simple.

Your standard prop is an airfoil, but it's not an airfoil with a constant angle of attack. Near the root (or middle, if you will), the angle of attack is very high -- often around 45 degrees. This angle of attack decreases as you go outwards, and at the tip it's much smaller.

If you didn't do this, and instead had a constant angle of attack throughout the entire prop, the `pitch' for a certain radius would be porportional to that certain radius. Yes, your plane could fly like this, but it would be far less efficient than a standard prop in normal flight.

The variable angle of attack is an attempt to make for a somewhat constant pitch across the entire prop. But most plane props aren't designed for creating static thrust -- dynamic thrust is the thing they're trying to optimize. If the plane is stopped, most of the prop is likely to be stalled, and so it'll be creating a lot more drag and a lot less thrust. (In case it's not obvious, stalled wings do still create lift, but they create a lot less of it, and a whole lot more drag while doing so.)

The lower the overall pitch of the prop, the less of the prop will be stalled if the plane isn't moving. So it may be that given standard airplane props, the pitch of the prop doesn't have a large effect on the static thrust due to this (but it probably does have a large effect on the power absorbed.)

Taking a prop where the angle of attack does not vary across the length of the blade, it's very obvious that the angle of attack (pitch, if you will, but on such a prop pitch is not anywhere remotely close to a constant) *does* affect your static thrust.

If there's any doubt, just whip out your helicopter (but make sure it's one that has collective control) and hit the throttle hold control and then adjust the collective and see what happens to your static thrust. In that case, the static thrust is probably roughly porportional to the angle of attack (assuming an uncambered airfoil. If it's cambered, then that'll be sort of like having a little bit of built-in angle of attack), at least for small angles.

In any event, I'm a bit surprised that the pitch ratings of standard airplane props have been found to have so little effect on the static thrust measured, but after thinking about it, it's not so unexpected. Our props are optimized for certain airspeed/rpm ratios, and outside of that they lose efficiency, and having zero airspeed (static thrust) is an extreme example of getting outside of what they're designed to be good at.

And as mentioned before, static thrust should only really matter to you if you're hovering. So helicopters -- they care. 3D planes -- they care. Everybody else, not so much.

Sure, you can come up with some extreme cases where you have a plane that has so little static thrust that it can't even fly, or a lot of static thrust and yet it still can't fly (because it has a super low pitch speed) but these cases are not the norm.

Of course it will. The prop is most efficient at an AOA of around 2 to

4 degrees, and so a rather low pitch will pull well in the static condition, allowing for the inflowing air speed (it's not zero). A high pitch will produce thrust but not efficiently, since much of the prop blade may be stalled, especially in the inboard sections where the blade angles are too high. The high pitch prop will work best at high cruise speeds, where the inflow is high and AOA drops to the 2-4 degree range. This whole thing is the reason most complex aircraft use constant-speed props, whose blades rotate in the hub to give a low pitch for takeoff and allow the engine to come to full rated RPM and therefore full rated power, and the blades then rotate to progressively higher pitches as the forward speed increases to keep the load up and the RPM at redline and the AOA in the right place. Depending on operational procedures, RPM may be reduced in the climb or upon reaching cruise altitude.

Dan

Excuse me, Doug, but the reason the prop blade is twisted is to provide a for a (near) constant AOA relative to the influx of the air mass. Obviously the blade angle must decrease with diameter to have a constant pitch. Blade angle at an arbitrary radius from the hum does not equal AOA.

Abel

| On Tue, 17 Jan 2006 16:55:12 GMT, snipped-for-privacy@frenzy.com (Doug McLaren) | wrote: | | >In article , | >Abel Pranger wrote: | >

| >| Saying pitch doesn't affect thrust of a prop is equivalent to saying | >| AOA doesn't matter to lift of a wing. | >

| >That sounds clever and all, but It's not quite so simple. | >

| >Your standard prop is an airfoil, but it's not an airfoil with a | >constant angle of attack. Near the root (or middle, if you will), the | >angle of attack is very high -- often around 45 degrees. This angle | >of attack decreases as you go outwards, and at the tip it's much | >smaller. | | | Excuse me, Doug, but the reason the prop blade is twisted is to | provide a for a (near) constant AOA relative to the influx of the air | mass.

Yes -- that's part of how props are designed to work best at a specific airspeed/rpm ratio. I already mentioned that, though perhaps not in much detail.

If you're going slower than that, the AoA increases, and if you're standing still, the AoA over much of your prop will be high enough that much of your prop (because the twist usually varies by radius) will probably be stalled.

I'm not sure why you used the word `constant' in there, however. You aren't claiming that the air comes in much faster to the middle of a prop (of a plane in flight -- not hovering) than it does to the tips of the prop, are you?

| Obviously the blade angle must decrease with diameter to have a | constant pitch.

Obviously. (Though I prefer not to use the term obviously, because what's obvious to me often isn't obvious to others and vice versa. And sometimes what's obvious to one person is ... wrong.)

| Blade angle at an arbitrary radius from the hum does not equal AOA.

Not in normal flight. However, if you're looking at static thrust (which we just happen to be doing) then the velocity of the air coming in is relatively small, and so the blade angle at an arbitrary radius IS pretty close to the effective angle of attack.

My point is that most airplane props are designed to be efficient in crusing flight, not hovering flight, and that this can easily explain at least _some_ of why one might see similar static thrusts from a

12x8 vs as 12x4 prop at a certain rotational velocity. I'm not saying it's the entire story, but it's probably some of it.

No, actually quite the opposite is true. The blade is traveling much faster at the tip than at the root. Looking at the velocity vector of the inrushing air mass as seen by a given prop section, the AoA is constant for a true helical pitch prop planform.

I think we were looking at the general case rather than static conditions at the time I wrote what you replied to. I have no idea, really, what the velocity distribution of the air mass is in front of a prop under static conditions, but I doubt that it is anywhere near uniform.

I think you are right about this. As I said in another post, the velocity of the air ingested by the prop is presumed to be the axial flight velocity of the aircraft (vo). The derivation of the classical expression for thrust from the very definition of thrust (the axial rate of change of momentum) breaks down and yields the absurd answer that thrust is zero when Vo is zero. The Vo clearly is not zero in the static case, but I have never seen any theory to predict what it actually is (not to say it has not been developed).

Abel.

My tests showed that pitch has an effect, but not a large one, and I didn't know how to account for it in an equation. The following was written back when I was working on my static thrust coefficient.

Below is data resulting from thrust measurements compared with calculated data using: Thrust = 2.83E-12 x RPM^2 x D^4 x (In. Hg)/29.92 x

528/(460+deg F)

Prop Actual Calc. Type Dia. Pitch RPM Thrust Thrust Err%

---------------------------------------------------- Zinger 14 6 6500 4.5 4.5 0 Zinger 14 8 6000 3.9 3.7 -5 Zinger 15 6 5700 4.5 4.6 2 Zinger 15 8 5200 4.0 3.8 -5 Zinger 15 10 5200 4.0 3.8 -5

My explanation for "why" pitch effect is small is just an attempt to rationalize what I have found to be apparently true over a limited range.

A prop is a flat bottom airfoil and will have lift at zero pitch. At higher pitches you get extra thrust (at a given rpm) resulting from the angle of attack. But, the displaced air at high pitch angles is not all directed rearward, and neither is the airfoil lift, thus it is consuming power but is not all useful for thrust. This results in dragging the engine to a lower rpm, producing less thrust at that new rpm.

Anyhow, I can't explain it, and Mark's Engineering Handbook says that in practice static-thrust predictions are based on correlations of test experience. They have a Ct in their equation which I found to be 2.83E-12 for the Zinger props in a narrow range of pitch. Certainly not for all props or pitches.

The test results were posted for several years on my web site. I have no idea where it all is now and frankly, I really don't care any more.

Watch out! They will attack your observations like starving dogs! You couldn't possibly have seen what you saw. they have the formulas to prove it!

Looking at the lift charts for some common prop airfoils shows that they are able to maintain flow at AOAs far greater than average prop pitches. In other words, most of the prop blade is not stalled. Add to that the fact that the prop is not actually working in totally still air and the AOA is actually less even when the plane is still.

| Looking at the lift charts for some common prop airfoils shows that | they are able to maintain flow at AOAs far greater than average prop | pitches. In other words, most of the prop blade is not stalled.

Even a stalled airfoil still generates lift -- it just generates 1) less of it, and 2) a lot more drag than one that's not stalled.

Very true Paul,

Buy the way, thanks for the Zinger thrust table data you sent me several years ago. The data was very helpful!

I've just about stopped working on the equations which now can hit within

10% of the thrust and power about 90% of the time. That's good enough for me. In addition to the standard 2-blade, they also cover 1, 3, 4, 5, and 6 blades. As you can imagine, construction of the multiblade props was a dog!

Good luck for now, Ray Shearer