Props

I just did an interesting experiment. I took a hunk of scrap hard maple I had in my shop and carved/sanded a zero pitch prop. I used the same profile that Master Airscrew uses and measured chords and thicknesses with a micrometer. I eyeballed the point of max thickness to be at 30% of the chord. It for sure is not to the high precision that a commercial prop for models would have. I am sure that my prop has significant errors in the airfoil shape. But I thought it would be close enough to do some testing. It is pretty much a Clark Y type airfoil just as MA.

I took this prop and put it on an FP40. I found I needed a prop length of 12 inches to load the engine so it would spin the prop at 11,000 rpm at full throttle. I chose 11,000 rpm as that is about the speed this well worn FP40 will spin a MA 9x6 prop on the ground at full throttle.

Then I took this setup and measured static thrust. With my fishing scale I found that it produced 3.3 pounds of static thrust. I also noted that it blew only a very small fraction of the air that the same mill will blow when it has a 9x6 on it. It blew so little air that standing right behind the plane I could not feel the air movement thru my pants. I had to put my hand in the air stream close to the tail feathers to feel the air it was blowing. In fact the amount of air blown was so small that it hardly put any oil on the tail feathers after a full 10 oz tank of fuel was burned. A 9x6 would have coated the tail with lots of oil to the point of it dripping off.

A couple of calculators on line predict a 9x6 would produce something like 2.3 to 2.9 pounds of static thrust at 11,000 rpm depending on what you use for a Cp for the prop. This sounds like the right range. The

2.3 seems a bit low to me. But at any rate we all know a FP40 is not exactly a high performance engine.

Now, I will be clear. I do not think this prop would have gotten the plane off the ground. With zero pitch as soon as the thing started moving forward at any speed at all the incidence angle of the prop would drop to zero stopping further acceleration. My point in doing this experiment was to simply show that you can make lots of thrust without blowing much air at all. Just like a wing can generate lots of lift with no downwash of air behind the trailing edge that is not exactly balanced by an upwash ahead of the leading edge or outboard of the tip.

If you do not believe my results go make your own prop and try it. It only took me an hour to make my prop with pretty standard woodworking tools. The only power tools I used were a planer, a jig saw and random orbital sander sander. I would think any modeler would have both of the later two. And the former is convenience not necessary.

Reply to
bm459
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  1. You may have had zero pitch, but if so your airfoil must have had some camber to it, or you would have gotten zero thrust.
  2. Induced drag in props works pretty much like induced drag on wings.

2a. This is why helicopters have really, really big propellers* that are geared way down -- they produce lots of thrust with relatively little power by moving lots of air slowly.

2b. This is why 3D planes have large, low-pitch propellers.

2c. This is why really fancy full-scale props have turned-down ends -- they act like little Horner tips & increase efficiency.

  • rotors.
Reply to
Tim Wescott

I said it was approximatly a Clark Y. A Clark Y has a postive Cl to negative 5 degs of angle of attack per the usual way to measure angle of attack on a flat bottom airfoil. Zero angle of attack = zero pitch = 5 degrees angle of incidence on a Clark Y. This is why, in theory, a prop can propel a plane at greater then its pitch speed.

So?? I never said otherwise did I?

No. If this were true the formula for thrust would be thrust = K x dia**2 x Pitch, when in fact it really is thrust = K x dia**4 x pitch at a constant rpm. Check any half decent text book. Simple F = MA leads to the incorrect former result.

Same as above.

Simply reduces the tip vortex and thus induced drag. It simply diverts wasted power from the tip vortex to thrust.

Reply to
bm459

F=MA leads to the only correct result. Your formula K x dia**4 x pitch is proportional to thrust, not equal to thrust. The error is that you have set K= Ct X rpm X rho. Ct, the thrust coefficient for the foil, and rho, the air density can be lumped to a K value, but not rpm. The " half decent text book" formula is derived directly from simple momentum theory, i.e., Thrust is by definition = axial rate of change of momentum.

Abel

Reply to
Abel Pranger

OOPs, I goofed too. You cannot lump air density into the K and arrive at where I was going, and that was to resolve units. Ct is unitless but of course density is not.

Back to keeping it simple, the thrust results from the prop ingesting a mass of air at the flight velocity and ejecting it at a higher slipstream velocity. No acceleration of the ingested air mass, then no thrust, period. F (thrust) =M(air density times X volume)A(rate of change in velocity).

Abel

Reply to
Abel Pranger

Ok so I should have said thrust = K x dia**4 x pitch at a given rpm and a given air density also compairing props with the same profiles. Mass in your formula would be proportional to dia**2 not dia**4. With all held constant except dia A would be a constant going from one prop to the next. Thus the F = MA formula suggests that thrust = K x dia**2 x pitch which is incorrect. Besides, I suggest you go and measure the volume of air blown by say a 9x6 prop installed frontwards and backwards and spun at the same rpm. Also measure thrust. You will find little difference in the volume of air blown but a large difference in thrust generated. I know as I did the experiment.

Reply to
bm459

I am sure if you had bothered to pull out that sensitive instrument out of your pants, you would have measured a lot more airflow!

Reply to
Sport Pilot

Mass is not proportional to D**2. The area of the prop disc is. Mass is proportional to volume, not area. To get volume, you need to include the dimension along the axial line of flight, which is the integral over time of velocity. Seriously, the derivation of the canonical expression for Thrust from the simple expression for momemtum is pretty easy, and would be worth your while to follow through it in your text where you got the thrust formula. And stop listening to me! I said you could not lump fixed variables into a constant, but in fact you can IF you preserve the units. I think that's where your logic fell down. Ct is dimensionless so no problem there, but density is in units of mass/volume and rpm units are self explanitory, so your K value has units of (mass X revs)/(volume X time). See the problem?

Abel

Reply to
Abel Pranger

I hate to tell you this, but there is no pitch term in a good thrust equation.

Brian

Reply to
Brian Morris

Are you referring to Barry Hobson's Thrust Calculator?

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

Abel

Reply to
Abel Pranger

I know nothing of Barry Hobson's Thrust Calculator.

I have my own equation, and have done my own testing. The style of the prop. is the most important factor. You must have a coefficient for each style of prop. For instance, Zingers and APCs are almost identical in performance, but the common Master Airscrew, black with the white tip, is quite different. We went through a discussion like this a few years ago and a post by Ted Sanders pretty much ended it. It is as follows:

Subject: Re: Thrust as Fn(rpm, diameter, pitch) Date: Sun, 14 Apr 2002 13:38:25 GMT From: "Ted Sander" Newsgroups: rec.models.rc.air References: 1 , 2

Brian's numbers hold up better than anyones I've found so far as a predictor of thrust. I had been bugged by not being able to figure out the potential thrust of a given prop/rpm setting also. (And, at $50 bucks for fancy Bolly props on my gas engine, I can't afford to "buy and try" lots of props!) I compiled data on 69 thrust measurements as reported in R/C Report as part of their engine tests. Pitch, diameter, rpm, and measured thrust. Several other calculations found on the net gave answers very divorced from reality when compared to the actuals. Andy Lennon, in Model Airplane News published a formula that at least gets in the ball park. His uses pitch as a factor. Brian's does a better job. Here's the net error of each, as compared to real world tests: Lennon's Formula Brian's Formula Average Error -10% -5% Max 45% 20% Min -80% -36% StdDev 25% 13%

Reply to
Brian Morris

Brian-

You didn't cite your equation, so hard to analyze why it may be more accurate as a thrust predictor than what has been used by prop designers for many years. 'Has been' is probably worth noting, as the utility of the common expression is (was) in its scalability. Ct could be derived empirically from small-scale model tests, and then scaled up to practical prop dimensions. Except for Ct, it is purely mechanics. All of the aerodynamics is wrapped up in Ct, and likewise for Cq and Cp, the torque and power coefficients. I expect in this era of supercomputers, these coefficients are likely derived from math models based in aero theory rather than from physical model tests.

I presume your measured thrust values were for the static condition. The derivation of the thrust equation from simple laws of mechanics does have a problem with that condition, i.e., what is the velocity of the air mass being ingested by the prop, Vo? It certainly isn't zero, else there would not be any movement of air into the prop. Plays havoc with the starting equation in the derivation:

Thrust= axial rate of change of momentum = (rho)Vo**2 X A(Vo - Vs), where Vs is slipstream velocity

In that limit case of Vo= 0, the classical approach falls down and some other formula could well predict thrust more in line with observations. Fine, if predicting *static* thrust is your objective.

Abel

Reply to
Abel Pranger

Let me add to my last post. I have no secret formula, the rationale and equations are available on my web page:

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Reply to
Brian Morris

There HAS to be some accounting for pitch or the calculation will not work. With out pitch being accounted for, a 0 pitch prop will make the same thrust as a 10 pitch prop. We all know it ain't so!

Reply to
Paul McIntosh

Actual tests show that pitch has a GREAT effect on thrust. What you are doing is using other observed factors to account for the difference in pitch. The fact that it takes a lot more HP to achieve the same RPMs with a prop of higher pitch should be a clue.

Also, how much thryst would you generate with a prop having zero pitch?

Reply to
Paul McIntosh

I have no use for a zero pitch prop., so I didn't test there. Seems a stretch to expect the equation to reach to zero pitch, even though the airfoil shape would provide lift/thrust. If you read my earlier post you would have found that the equation itself was compared to other equations by Ted Sander, using test data from 69 thrust measurements as reported in R/C Report as part of their engine tests. They included pitch, diameter, rpm, and measured thrust in the data. The average error of my equation was -5%. That is as close as my fish scale will measure thrust.

The Master Airscrew K-series propellers require a 1.33 multiplier to get good results. The APC and Zinger propellers use the basic equation and thrust coefficient.

The relationship of the variables in the equation came from a Mark's Mechanical Engineers Handbook, in a discussion of full sized aircraft propellers.

I don't really care whether you believe any of this. I shouldn't have brought it up.

Brian

Reply to
Brian Morris

And therein lies the problem. If the calculation can't account for ALL possibilities, it is invalid.

A 12X4 prop at 10,000 RPMS provides quite a different amount of thrust than a 12X8 prop at 10,000 RPMs. The 12X8 also requires a LOT more power to reach 10,000 RPMs. That power is being converted to thrust.

I won't get into pissing contests over math. I have been flying far too long with too many different engines to fall into that trap.

Reply to
Paul McIntosh

It is not the math, it is static testing reality. The 12x8 prop is stirring up a lot of air and requiring a lot of power, a great deal of which is wasted because the prop. blade is twisted and the lift/thrust vector is not directed back as close to the plane centerline. Likewise, the airflow vector is offline. The extra pitch doesn't deliver more thrust than the

12x4 until the plane gets up to speed. I didn't do testing at constant rpm. I tested to the limit of the engine's ability to turn the prop., measured the ambient temp., the thrust, and rpm. My thrust coefficient is empirical, based on those tests. Now, I read the rpm, substitute the parameters into the equation and calculate thrust. A plot of rpm vs. prop. load for a range of prop's can be used to select the prop. for my next airplane.

If you haven't looked at the web page yet, take a look. Many folks, students and modelers alike, in the U.S. and other places, have sent for my spreadsheet with the equations and a calculator, and I have had no negative reactions. I have no axe to grind or money involved, just want to be helpful. I remind them that the thrust is static only and that the airspeed is approximate and can vary widely depending on the drag of the aircraft.

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Brian

Reply to
Brian Morris

Hmmm.... Speaking of supercomputers and the implications of wind tunnels, I wonder if any of the folks at Ames ever did any propeller work.

Reply to
Six_O'Clock_High

| I have no use for a zero pitch prop., so I didn't test there. Seems a | stretch to expect the equation to reach to zero pitch, even though the | airfoil shape would provide lift/thrust.

It depends on how you define pitch. For the definition of prop pitch that I've seen used most often, a zero pitch prop would create zero thrust, as that's pretty much the definition. Assuming a cambered airfoil, the angle of attack would then be somewhat negative. Not a very useful prop for most applications (but it might be useful in a blender/mixer!), but it could be created.

| They included pitch, diameter, rpm, and measured thrust | in the data. The average error of my equation was -5%. That is as close as | my fish scale will measure thrust. | | The Master Airscrew K-series propellers require a 1.33 multiplier to get | good results. The APC and Zinger propellers use the basic equation and | thrust coefficient.

Sort of a `fudge factor' I guess. But then again, there's lots more variables than just diameter, pitch and rotational speed, so you'll need to account for them somehow.

You did take measurements, and that's more than most people do. And if your formula fits your data, then that's about all anybody could expect from it.

| I don't really care whether you believe any of this.

Then why bother to defend it? Obviously you care somewhat.

Looking at your web site, you did your tests with a fish scale. Seems that you could be a lot more accurate than that, especially if you test with electric motors rather than glow engines.

I've seen magazine reviewers measuring electric static motor thrust with a little rig they set up. Basically, they set up a lever where the motor/prop is connected to one end, and the other end pushes down on a scale that's normally accurate to around one gram. It would be relatively easy to calibrate by adding a few known weights to the motor and seeing how that affected the force down on the scale (assuming a 1:1 ratio of sides of the lever, the weights added and measured should be equal.)

In any event, the motor is hooked up to a variable power supply, and RPMs are measured with a standard R/C tachometer. The voltage/current is adjusted until you get the desired speed, and then measurements are taken. Rinse, lather, repeat.

You'd want to make sure that the motor/prop was hanging out over the edge of your table so as to minimize any ground effects (and that's why the lever is needed -- the prop shouldn't be directly above the scale, though it might be good enough if it's way above the scale.)

I may have to set up my own rig like this and do some testing, as I'm sort of curious now. I suspect that the pitch does have an effect (certainly, common sense tells me that it must) but it may be that the effects of the pitch are mostly counteracted by other differences in the commercially made props of various pitches.

You could get one of the new electric variable pitch props which generally have just flat blades and that would probably work very nicely for determining just how pitch affects the static thrust, but then again these props are known to be a good deal less efficient than the standard R/C props, and the results wouldn't necessarily apply to other props.

Your web site ought to explicitly say `static thrust'. Just saying `thrust' isn't very precise, and somebody might very well misinterpet it.

| I shouldn't have brought it up.

Why not? Certainly, I think that people are often overly concerned with static thrust, but it's still worth measuring, modelling and discussing.

Reply to
Doug McLaren

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