Prop -v- RPM Mystery

[Geico266]

Nice lookin' bird!

Thanks Tim, "she" (no offense to female POA members ) is a hoot to fly.

 

[Geico266]

That is one of the advantages of experimental, you can pull the engine apart, measure and inspect it all out and put it all together at minimal expense if everything looks good. As for the prop, there's a lot more to a prop than simply pitch which can effect rpm. Tip design for one.

I think it is time to upgrade the prop to one of the new ground adjustable ones. You can set them for climb pitch for local flights and acro, and cruise pitch if you are traveling somewhere.

I might try taking the 1" tip off and resurfacing and reshaping the tip, but this is getting questionable on a 20 year old prop that has been repitched once already. Time to upgrade and hang this one over the fireplace on the mantel.

 

[Capt. Geoffrey Thorpe]

Dones anyone know how to figure Arc. tangent?​

In the olden days one would look it up in a table or use a sliderule.

Now we have computers to do that kind of thing.


Assuming you have a PeaSee running winders, the "calculator" will do it - put it in scientific (under view) type in the number, click the inverse check box then click on tan.

Or a spread sheet - purd near all of them should have arc-tan or inverse tangent...

I also wouldn't be surprised if your prop was carved with a Clark Y airfoil. A quick search should yield the coordinates if you want to make templates to check the airfoil. (I would post a link, 'cept my laptop is broke and I am using my kid's computer which has lots of stuff for instant message, but not links that I am likely to use - sorry)

 

[Capt. Geoffrey Thorpe]

That obviously changes the airfoil... I'm trying to figure out why they'd do it that way, as it doesn't seem like it'd be all that effective. In terms of wings, I've heard that Newtonian lift is responsible for a much larger percentage of the lift than Bernoulli lift. .

Newton accounts for 100% of the lift, while Bernoulli accounts for 100% of the lift. Just different calculations to get the same result.

If you are moving air to generate a force, then there is a reaction that causes lift/thrust. Newton. That reaction takes the form of pressure differences which are caused by velocity differences. Bernoulli.

 

[sba55]

Newton accounts for 100% of the lift, while Bernoulli accounts for 100% of the lift. Just different calculations to get the same result.

I'm not sure that's true. From what I remember, Bernoulli accounts only for a very small amount of the lift actually needed. With that theory alone, airplanes shouldn't be flying....

 

[gismo]

Newton accounts for 100% of the lift, while Bernoulli accounts for 100% of the lift. Just different calculations to get the same result.

If you are moving air to generate a force, then there is a reaction that causes lift/thrust. Newton. That reaction takes the form of pressure differences which are caused by velocity differences. Bernoulli.

I believe (from what I've read, haven't done the math) that while it is true that lift is both the result/consequence of over/under wing pressure differences and the reaction of air deflected downwards, only Newton's laws actually apply. Bernoulli's formulas relate to a fluid passing through a constrained space such as a venturi and don't apply (at least mathematically) to a fluid passing around an airfoil in free space. Specifically, the oft mistaken idea that the air over the wing "speeds up" by the amount necessary to make the longer trip so it can "meet up" with the air flowing slower under the wing is entirely incorrect.

 

[Capt. Geoffrey Thorpe]

I'm not sure that's true. From what I remember, Bernoulli accounts only for a very small amount of the lift actually needed. With that theory alone, airplanes shouldn't be flying....

By 'Bernoulli" I am referring to the equations that relate the velocity / pressure along a streamline in an ideal gas (air comes close at subsonic speeds except for in the boundry layer).

I am not referring to the myth about boy molecules and girl molecules that have to get to the trailing edge at the same time. Nor am I referring to anything related to "curved top, flat bottom" - Bernoulli is often blamed for these "explanations" but in fact, his equations have nothing to do with them.

If you can't explain lift using a flat plate, you can't explain lift. And, both Newton's and Bernoulli's equations apply to flow around a flat plate.

 

[Capt. Geoffrey Thorpe]

I believe (from what I've read, haven't done the math) that while it is true that lift is both the result/consequence of over/under wing pressure differences and the reaction of air deflected downwards, only Newton's laws actually apply. Bernoulli's formulas relate to a fluid passing through a constrained space such as a venturi and don't apply (at least mathematically) to a fluid passing around an airfoil in free space. Specifically, the oft mistaken idea that the air over the wing "speeds up" by the amount necessary to make the longer trip so it can "meet up" with the air flowing slower under the wing is entirely incorrect.

Bernoulli's equations apply to an ideal gass flowing along a streamline (there are some other conditions) and work well when applied to the air flow past a wing.

The longer trip nonsense has nothing to do with anything written by Daniel Bernoulli - so stop blaming him.

 

[Dan Thomas]

Thanks for your input Skip. When I say back side I mean the side I see sitting in the plane.

The terminology is a bit counterintuitive with props. The blade face is the side you see from the cockpit. The blade back is the curved or cambered side.
See http://www.southendflyingclub.co.uk/lecture/propellers.htm
or http://www.allstar.fiu.edu/aero/flight63.htm

Dan

 

[Geico266]

The terminology is a bit counterintuitive with props. The blade face is the side you see from the cockpit. The blade back is the curved or cambered side.
See http://www.southendflyingclub.co.uk/lecture/propellers.htm
or http://www.allstar.fiu.edu/aero/flight63.htm

Dan

What if I'm in a pusher?

Sorry man. Just being a smart butt. That is interesting though. I would have thought the oposite, but that's what I get for thinking. On the other hand it does make sense when you think about how the prop its the air.

 

[docmirror]

I may have this figured out, but no guarantees. He took meat off the back side of the prop to repitch it. In doing so, he is supposed to take material off the trailing section of the blade - that is, the section of blade which tapers to a fine blade. In order to repitch to a higher number(coarser) he would take material off the leading edge near the front of the blade.

What it sounds like is that he took material from the front(leading section) of the prop blade thus moving it from 66" up to about 69" rather than taking material from the rear or retreating section of the blade.

Your tests confirm the results of this quite closely.

<edit - if it were me, I'd find a stock metal prop from a plane that had a O-235 on it and run that for comparison. >

 

[flyingcheesehead]

Newton accounts for 100% of the lift, while Bernoulli accounts for 100% of the lift. Just different calculations to get the same result.

When I say "Bernoulli" I mean the air being sucked down by the upper wing surface, and by "Newton" I mean the air smacking the bottom of the wing and being pushed down.

While the "Bernoulli" air going over the top of the wing is also lifting the plane due to Newton's laws, I'm not exactly sure how the air hitting the underside of the wing has anything to do with Bernoulli.

 

[flyingcheesehead]

Thanks for your input. When I say back side I mean the side I see sitting in the plane. That is the only side that was re-pitched. Ed Sterba said he took off only 1 degree, and he says he went from 66" of pitch to 62". But the same calculations Sterba says to use to calcjulate pitch says he took it the other way to 69" of pitch based on the increase of speed.

Aha, OK. I was thinking "backside" with respect to the air.

Can you tell where material was taken off? If it was more from the leading edge, that'd be an increase in pitch, more from the trailing edge would be a decrease. Probably really freakin' hard to tell by looking at it though!

 

[gismo]

Bernoulli's equations apply to an ideal gass flowing along a streamline (there are some other conditions) and work well when applied to the air flow past a wing.

The longer trip nonsense has nothing to do with anything written by Daniel Bernoulli - so stop blaming him.

OK, I can agree with all that. Both principles are sound and neither man ever actually attempted to predict the lift created by a gas flowing over an airfoil.

My "opposition" to applying Bernoulli to the problem really only stems from the (incorrect) but widely publicized notion of "equal transit time" for air flowing over and under a wing and since this concept was not Bernoulli's it is certainly wrong to "blame" him for it even though it has been so often tied to the combination of his principle and the generation of lift.

 

[Geico266]

I may have this figured out, but no guarantees. He took meat off the back side of the prop to repitch it. In doing so, he is supposed to take material off the trailing section of the blade - that is, the section of blade which tapers to a fine blade. In order to repitch to a higher number(coarser) he would take material off the leading edge near the front of the blade.

What it sounds like is that he took material from the front(leading section) of the prop blade thus moving it from 66" up to about 69" rather than taking material from the rear or retreating section of the blade.

Your tests confirm the results of this quite closely.

<edit - if it were me, I'd find a stock metal prop from a plane that had a O-235 on it and run that for comparison. >

Aha, OK. I was thinking "backside" with respect to the air.

Can you tell where material was taken off? If it was more from the leading edge, that'd be an increase in pitch, more from the trailing edge would be a decrease. Probably really freakin' hard to tell by looking at it though!

Watson! I think we've got it!

The prop was spray painted flat blach where he sanded it, but if I look close at it I'm pretty sure he took meat off the trailing edge. That would help confirm the numbers I'm seeing.

IN the AM I'm gonna visit my ol buddy Jim Fix, Fix Prop Shop, Lincoln, NE. You would not believe what that guy can do with a bent prop. Anyway, he will put it on his station stand and find out what the pitch is. Pitch can vary from mfg. to mfg., but Jim thinks he can get close based on Sterba's published numbers for station checking.

I'll report the finding tomorrow.

 

[Capt. Geoffrey Thorpe]

When I say "Bernoulli" I mean the air being sucked down by the upper wing surface, and by "Newton" I mean the air smacking the bottom of the wing and being pushed down.

Why blame nonsense on Mr. Daniel Bernoulli and Sir Issac Newton?

While the "Bernoulli" air going over the top of the wing is also lifting the plane due to Newton's laws, I'm not exactly sure how the air hitting the underside of the wing has anything to do with Bernoulli.

The air flow along the bottom of the wing has a higher pressure due to the lower velocity according to the circulation that is required to satisfy the Kutta condition (velocity must be finite at the trailing edge). Given the velocity, one calculates the pressure using Bernoulli's equations. On both sides of the wing.

http://en.wikipedia.org/wiki/Bernoulli's_principle

The original form of Bernoulli's equation[5] is:

where:

is the fluid flow speed at a point on a streamline,

is the acceleration due to gravity,

is the elevation of the point above a reference plane, with the positive z-direction in the direction opposite to the gravitational acceleration,

is the pressure at the point, and

is the density of the fluid at all points in the fluid. The following assumptions must be met for the equation to apply:

• The fluid must be incompressible - even though pressure varies, the density must remain constant.
• The streamline must not enter a boundary layer. (Bernoulli's equation is not applicable where there are viscous forces, such as in a boundary layer.)

...

In several applications of Bernoulli's equation, the change in the

term along streamlines is zero or so small it can be ignored: for instance in the case of airfoils at low Mach number. This allows the above equation to be presented in the following simplified form:

where

is called total pressure, and

is dynamic pressure[9]. Many authors refer to the pressure

as static pressure to distinguish it from total pressure

and dynamic pressure

. In Aerodynamics, L.J. Clancy writes: "To distinguish it from the total and dynamic pressures, the actual pressure of the fluid, which is associated not with its motion but with its state, is often referred to as the static pressure, but where the term pressure alone is used it refers to this static pressure."[10]
The simplified form of Bernoulli's equation can be summarized in the following memorable word equation:
static pressure + dynamic pressure = total pressure[10] Every point in a steadily flowing fluid, regardless of the fluid speed at that point, has its own unique static pressure p, dynamic pressure q, and total pressure p0.
The significance of Bernoulli's principle can now be summarized as "total pressure is constant along a streamline." Furthermore, if the fluid flow originated in a reservoir, the total pressure on every streamline is the same and Bernoulli's principle can be summarized as "total pressure is constant everywhere in the fluid flow." However, it is important to remember that Bernoulli's principle does not apply in the boundary layer.

Sez nothing about air being "sucked down" by the upper surface of the wing.

And, air doesn't "get smacked down" by the bottom of the wing. Lift happens when the air flows off the trailing edge of the wing with some downward velocity. The air flowing off the top and bottom will have very close to the same velocity and pressure (unless you are stalled). The mass of air times the ammount it was accelerated downwards equals the force generated (Newton). Watch some wind tunnel stuff - you won't see much "smack down" going on.

I really wish people (FAA include) would teach real aerodynamics and not nonsense. It's really not that complicated - so why resort to bovine excretment???

 

[jesse]

And, air doesn't "get smacked down" by the bottom of the wing. Lift happens when the air flows off the trailing edge of the wing with some downward velocity. The air flowing off the top and bottom will have very close to the same velocity and pressure (unless you are stalled). The mass of air times the ammount it was accelerated downwards equals the force generated (Newton). Watch some wind tunnel stuff - you won't see much "smack down" going on.

I really wish people (FAA include) would teach real aerodynamics and not nonsense. It's really not that complicated - so why resort to bovine excretment???

I have always been *so frustrated* with the way aerodynamics were taught and simply wish that they wouldn't even bother trying. It doesn't translate to flying (the crap they teach) and instructors teach two concepts, apply them incorrectly, and that continues.

It is almost as if some people think Bernouli invented the wing by programming the physics of this world....and therefore the man is responsible for your lift.

I wish people would realize that such concepts help *explain* why the wing produces lift and with an understanding of them you can design a wing..not that the world revolves around these principles.. They are nothing more than an explanation that helps you predict what something will do in our world.

 

[gismo]

I have always been *so frustrated* with the way aerodynamics were taught and simply wish that they wouldn't even bother trying. It doesn't translate to flying (the crap they teach) and instructors teach two concepts, apply them incorrectly, and that continues.

It is almost as if some people think Bernouli invented the wing by programming the physics of this world....and therefore the man is responsible for your lift.

I wish people would realize that such concepts help *explain* why the wing produces lift and with an understanding of them you can design a wing..not that the world revolves around these principles.. They are nothing more than an explanation that helps you predict what something will do in our world.

Heck, we can't even get a decent explanation of a real concern like wing stalls. I can't believe how many time's I've read or been told that when a wing stalls it quits producing lift. If that were the case your airplane would plummet to the ground at 32 ft/s^2 and would reach a VSI of 9600 FPM in 5 seconds if you held it in a stall.

 

[docmirror]

Watson! I think we've got it!

The prop was spray painted flat blach where he sanded it, but if I look close at it I'm pretty sure he took meat off the trailing edge. That would help confirm the numbers I'm seeing.

IN the AM I'm gonna visit my ol buddy Jim Fix, Fix Prop Shop, Lincoln, NE. You would not believe what that guy can do with a bent prop. Anyway, he will put it on his station stand and find out what the pitch is. Pitch can vary from mfg. to mfg., but Jim thinks he can get close based on Sterba's published numbers for station checking.

I'll report the finding tomorrow.

Uh, I think you've got it backwards. If he took meat off the trailing edge that would move the prop flatter, or indeed work toward a lower numerical pitch (toward 62"). If he took meat off the leading edge area that would make it steeper into the wind and move it to a higher pitch.

Let us know what you find.

 

[Capt. Geoffrey Thorpe]

Heck, we can't even get a decent explanation of a real concern like wing stalls. I can't believe how many time's I've read or been told that when a wing stalls it quits producing lift. If that were the case your airplane would plummet to the ground at 32 ft/s^2 and would reach a VSI of 9600 FPM in 5 seconds if you held it in a stall.

Stall just means that increasing the angle of attack results in less lift - not more. You have gone over the peak of the lift vs angle of attack curve - attached is a scan of some data from Abbot and Von Doenhoff. Across the bottom is the angle of attack - the line going up at an angle is the lift at each test point. (different symbols are at different Reynolds numbers and surface condition). You can see the lift drop off once you get past the peak, but it doesn't instantly go away...

 

[gismo]

Stall just means that increasing the angle of attack results in less lift - not more. You have gone over the peak of the lift vs angle of attack curve - attached is a scan of some data from Abbot and Von Doenhoff. Across the bottom is the angle of attack - the line going up at an angle is the lift at each test point. (different symbols are at different Reynolds numbers and surface condition). You can see the lift drop off once you get past the peak, but it doesn't instantly go away...

Yeah I know that, but I've heard plenty of CFI's (and some aviation texts) say otherwise. Like you said, at the stalling AoA lift doesn't cease, it just goes down with further increases in AoA and at the critical AoA the slightest loss of lift will increase the AoA further creating the "bottom falling out" feeling.

 

[Geico266]

Uh, I think you've got it backwards. If he took meat off the trailing edge that would move the prop flatter, or indeed work toward a lower numerical pitch (toward 62"). If he took meat off the leading edge area that would make it steeper into the wind and move it to a higher pitch.

Let us know what you find.

Doc, That can't be right, is it? All work has been done on the "flat" side of the prop. (side you see sitting in the plane looking forward.) If you take material off the trailing edge you increase the pitch. The prop is stationary, mounted on the engine flange. Remove material from the leading edge and you are flatting out the angle. Did I lead you to think they were taking material off the curved side?

 

[gismo]

Doc, That can't be right, is it? All work has been done on the "flat" side of the prop. (side you see sitting in the plane looking forward.) If you take material off the trailing edge you increase the pitch. The prop is stationary, mounted on the engine flange. Remove material from the leading edge and you are flatting out the angle. Did I lead you to think they were taking material off the curved side?

Consider a prop that has no camber with blades having parallel surfaces to start with. If you take material from the leading edge on the side that air exits from (the pilot's side in a tractor arrangement) or conversely add material to the trailing edge of that side you will be increasing the blade pitch which on that side of the blade is somewhat proportional to the axial distance from the leading edge back to the trailing edge. And since the leading edge is already closer to the entry side of the airflow than the trailing edge, removing material from the leading edge moves it further forward and increases the axial distance between those edges. The reverse would be true if we were talking about the side of the blades that the air comes into i.e. the curved side and if you combine some of both (e.g. remove leading edge material from the exit side and trailing edge material from the entry side it should become very clear that this increases pitch.

 

[docmirror]

^^^ What he said ^^^

 

[Geico266]

Well, I took the Sterba prop to a local prop shop and took readings from each station 3" from the hub to the tip. It was interesting to see this done.

I then talked to Ed Sterba about the findings, but he still says it decreased the pitch.

He then said the problem might be that he "rounded the back side and it is "cancelling: some of the lift generated from the front side.

New Catto prop ordered, Craig Catto says I should see a 15 MPH increase in top end, and 1,000' a minute increase on climb out.

I'm pumped!

 

[docmirror]

Well, I took the Sterba prop to a local prop shop and took readings from each station 3" from the hub to the tip. It was interesting to see this done.

I then talked to Ed Sterba about the findings, but he still says it decreased the pitch.

He then said the problem might be that he "rounded the back side and it is "cancelling: some of the lift generated from the front side.

New Catto prop ordered, Craig Catto says I should see a 15 MPH increase in top end, and 1,000' a minute increase on climb out.

I'm pumped!

Arrggghhh! What was the result of the prop shop measurement?

 

[Geico266]

Arrggghhh! What was the result of the prop shop measurement?

Sorry Doc,

I have the results, but they are in degrees. They are meaningless unless we know the OEM's "station calculations" to see if he actually did them back-wards, but Sterba says he double checked and his measurements have the angle decreasing, not increasing. He wants me to send the prop back to him, but I'm gonna put this one on the mantel and order a Catto.

Once bitten, twice shy.

Craig Catto guarantees 15 MPH to my top end, that's 180 MPH! Plus the Catto's look cool.

 

[wby0nder]

How much do you have to fork over for a Catto? That sounds really interesting. I'm going to check out Catto for the Fly Baby!

 

[Geico266]

How much do you have to fork over for a Catto? That sounds really interesting. I'm going to check out Catto for the Fly Baby!

I think it will be around $1,650. The quote will be here tomorrow. There are several RV drivers that I know have Cattos. Pretty much the standard for fixed pitch. The 3 blade Catto is smooth as silk.

 

[Dan Thomas]

New Catto prop ordered, Craig Catto says I should see a 15 MPH increase in top end, and 1,000' a minute increase on climb out.

An increase of 1000 FPM? What's your climb rate now? 5000 FPM? This is an O-235, right? 108 hp?

Sounds like snake oil to me. Or maybe it comes with an IO-540 attached.

Dan

 

[Geico266]

Uh, I think you've got it backwards. If he took meat off the trailing edge that would move the prop flatter, or indeed work toward a lower numerical pitch (toward 62"). If he took meat off the leading edge area that would make it steeper into the wind and move it to a higher pitch.

Let us know what you find.

Ahhh! The light just came on. You are right. In any event it ain't right, the Catto is on the way.

 

[Geico266]

An increase of 1000 FPM? What's your climb rate now? 5000 FPM? This is an O-235, right? 108 hp?

Sounds like snake oil to me. Or maybe it comes with an IO-540 attached.

Dan

Current climb rate is 2-300 FPM. The engine is lugging and struggling at 1970 RPM until I can get the airspeed up. Currently, static RPM is 2030 RPM, it should be 22-2300 RPM. Climb rate should be 1200 - 1400 FPM with the new prop.

Before you call Craig Catto a snake oil salesman I strongly suggest you talk to people who fly his props.

http://www.cattoprops.com/

 

[gismo]

An increase of 1000 FPM? What's your climb rate now? 5000 FPM? This is an O-235, right? 108 hp?

Sounds like snake oil to me. Or maybe it comes with an IO-540 attached.

Dan

That 1000 FPM must be a typo, 100 FPM is plausible. 1000 FPM with a 2000 lb GW would require an additional 60 HP from somewhere. Maybe if the original prop was on backwards.

 

[Geico266]

That 1000 FPM must be a typo, 100 FPM is plausible. 1000 FPM with a 2000 lb GW would require an additional 60 HP from somewhere. Maybe if the original prop was on backwards.

An RV-3 that weights 2,000 pounds gross? They weight right at 1,075 pounds "acro" weight.

I guess I've not flown in a certified plane very much, but RV's typically climb out at 1,200 - 1,500 FPM routinely. I had an RV-9 220 HP that would climb out at 2,400 FPM sustained to 8,000' MSL.

On the RV-3 if the prop is only turning 1970 - 2000 RPM on take off now and climbing 200-300 FPM, then adding 300 - 400 RPM to climb out would easily get the climb rate to 1,100 - 1,300 FPM or close. It will be a heck of alot better than what I have now.

 

[jesse]

200-300 fpm does sound really low even at that 108 hp. I'll be interested to hear how the new prop works out.

 

[Geico266]

200-300 fpm does sound really low even at that 108 hp. I'll be interested to hear how the new prop works out.

It used to spin at 2075 before the "re-pitching". Even that was low. Lycoming says the 0-235 should be spinning 2200 -2300 RPM static, 2100 -2200 RPM take off.

 

[jesse]

It used to spin at 2075 before the "re-pitching". Even that was low. Lycoming says the 0-235 should be spinning 2200 -2300 RPM static, 2100 -2200 RPM take off.

Any idea what the power curve is supposed to look like? Maybe you should just spend even more money and throw 180 hp on it

 

[gismo]

An RV-3 that weights 2,000 pounds gross? They weight right at 1,075 pounds "acro" weight.

As is probably obvious I was guessing at the MGW and assumed that it was similar to my brother in law's RV-9.

At 1100 lbs the 1000 fpm increase would only require about 35 more HP, still too much to expect from a different prop when you're talking about a 100-125 HP engine.

On the RV-3 if the prop is only turning 1970 - 2000 RPM on take off now and climbing 200-300 FPM, then adding 300 - 400 RPM to climb out would easily get the climb rate to 1,100 - 1,300 FPM or close. It will be a heck of alot better than what I have now.[/quote]

Sure, I'd believe you could get 1100-1300 fpm but I'm having trouble believing you only get 200 fpm now because the prop pitch is a few percent too high.

 

[Geico266]

Sure, I'd believe you could get 1100-1300 fpm but I'm having trouble believing you only get 200 fpm now because the prop pitch is a few percent too high.

That is the problem, I was getting 400-500 FPM climb after lift off. After lift off, if I stay on the deck and build up speed it will climb more, but I like to use the after lift off climb out as the standard.

The RV-9 has a great wing on it. Very easy to land, and very responsive, not as quick as the other models, but the 9 has a different mission. Still a very sporty wing. Probably my favorite plane to land of all the RV's.

 

[Geico266]

Okay, Here are the station readings we got after the re-pitch. Can anyone out there in POA land extrapilate these readings and give a pitch? Blade 1 & 2 were pretty close.

The formula Sterba uses is;

Pitch/Circ. = the arc. tang = angle (Degrees)

Blade #1 Station Readings

15" = 35.5 degrees
18" = 29.4
21" = 25.25
24" = 22.5
27" = 20.25
30" = 18.75
33" = 16.20

Any calculations you can do to determine pitch?