Propellers, Flutter and sundry

[L.E.D.]

I was going to start a thread on props and such and not impede very informative Reno-Stead reports -- I won't get there till friday night of the races and I need all the info I can get until then!

I figured CurtB would chime in and fix my mistakes and fill my gaps, and he sort of beat me to it - anyway, another thread still seems appropriate.

So first, propellers. I'm not an expert on them, but I am an interested user of them and one thing a propeller is *not* is a screw (and neither is a ship's screw, for that matter.)

A propeller, as has been pointed out elsewhere, is a rotating wing. It makes lift just like a wing does, and that lift is utilized as thrust. Wings make lift by pushing air down (with the equal an opposite reaction that the air pushing the wing up.) Propellers do the same. What makes a propeller interesting is the fact unlike a wing, it's constantly flying it its own wake, which makes the details of how the air gets pushed and how much more complicated.

However, the basic facts are the same for wings and propeller blades. Both have properties dominated by the angle the airfoil makes with the oncoming air. Both are affected by Reynolds' number and Mach number; both can stall.

Just like a wing, a propeller blade has an optimal angle of attack that determines how much lift (thrust) the blade makes, and how much drag (torque) the engine has to overcome (provide) to keep it moving. A propeller's angle of attack is a function of the RPM and forward speed, and since both can change, the angle of attack varies widely. Adjusting blade pitch angle allows some control over this angle of attack, but not entirely, because there's another factor: each part of propeller blade moves at a different speed from zero at the hub to fastest at the tip.

What the air does as it flows over the blades will be affected by the shape of the airfoil, the angle of attack, the Mach number, the Reynolds number, the surface condition of the blades.... To a large degree these things mostly affect the drag produced by the blade, and consequently the propeller's efficiency. Here's where CurtB can fill us in, I'm sure, especially as things go transonic.

[Curt_B]

Re: Propellers, Flutter and sundry

Excellent summary! With one small exception. It's a myth that the blades know the other blades are there aerodynamically. At race speeds, the shed tip vortex will be 5 or so feet behind the adjacent blade for a four bladed prop by the time it swings that 1/4 revolution. That's around 10 chord lengths and won't effect the other blades much.

Since the tips are transonic, you treat the tips like a transonic wing and sweep them. The prop manufacturers finally funded/ got funding, for this research years ago, but still decades after swept wings were invented. It's hard to get prop research funding since jets were introduced and all, and the manufacturers should be commended for advancing their state-of-the-art up to this point so far.

Minimal chord length tips, prop or wing, are the optimum, and I'm glad to see that concept being adopted in the biplane, Phantom's, prop. Don't understand why everybody's not using that prop, F1's and biplanes, unless it's not cost effective for other teams. It sounds like a tuning fork going by from the youtube videos and I'm gonna try to get out there early to watch the F1's and biplane's this year. If the prop is vibrating at, 'tuning fork', frequencies, then that's flutter and can become a fatigue problem.

Props can still be made to perform better, and the manufacturers will catch up to wing design technology eventually. Reno's so great in that a couple of the classes have moved beyond commercial propeller technology intentionally, or unintentionally in some cases.

[L.E.D.]

A ridiculously short course on flutter.

On to flutter.

Flutter is an oscillation of an airframe caused by the movement of the air, which causes a movement of the airframe, which causes a movement of the air.. you get the idea.

The propensity of a part of an airframe to flutter (and nearly anything that lifts can flutter - wings, tails, control surfaces, propeller and rotor blades) depends primarily on two things: The true airspeed, and the stiffness of the structure.

All airplanes can be thought of as being made up of springs. Most of the time the springs are so stiff that they don't bend very much when pushed on, but they do bend, and some parts bend a lot.

Anything springy that has a mass attached to it will vibrate (oscillate) when bumped. The stiffer the spring is in relation to the mass, the faster the oscillation. Think of a bobble-head doll. The one with the bigger, heavier head will bob more slowly. The freqency of this oscillation is called the natural frequency of the system.

If there's any damping in the system, the oscillations will tend to die out, which is why cars have shock absorbers. If you're driving on a rough road however, you'll still get a jolt every time you hit a bump. It just may be smaller than it would have been without the shocks.

Anything that oscillates can resonate - the frequency of the resonance is generally near the natural frequency, modified by the effect of the shock absorbers. If you hit the bumps with a low frequency and the resonant frequency is high, then not much bump will get through. But if you hit them near or at the resonant frequency, you'll feel them, and how.

If a lifting wing twists or flexes, the lift will change. In turn, this will change the airflow downstream of the wing which affects the load on the wing (subsonic.) This will change the lift - and on it goes. The amount the lift will change, and when it will change depends on the true airspeed; the amount the wing responds to the lift depends upon it's mechanical resonance frequency. If those two things occur at the same frequency - the wing will resonate, and flutter.

(I haven't done the math in a long time - the above summarizes about a page and a half of an 800 page textbook. )

[Leo]

Re: Propellers, Flutter and sundry

Resonance in a structure can be quite dramatic. In designing large structures (buildings) for seismic loads one of the considerations is to keep the structures natural resonance frequency from ever matching a potential quakes frequency. The results I've seen on models will tear a skyscraper apart. Most are built to sway and "float" on their foundations to absorb the loads. I imagine the problems are similar in any structure no matter what the source of the vibrations.
Many years ago I was asked to design a propeller for a friends boat. It was to be very low aspect, basically a twisted wide bar to be towed behind a sailboat and via a cable, turn a generator. I figured the math couldn't be that much different from the structural stuff I was doing. Wrong. several books later and a lot of calculations and we still ended up using trial and error for the final shape. What a black art!

[1:1 Scale]

Re: Propellers, Flutter and sundry

Paul Lipps designed the propeller on Phantom, and used to be a great contributor to the RV forums before his passing. The guy was a genius in my mind, and a perpetual tinkerer. Here's an article that was published in 2009 explaining his propeller design, some myths, and some tweaks to his Lancair: http://rexresearch.com/lippsprop/lipps.htm

[L.E.D.]

Re: Propellers, Flutter and sundry

Just ask the Tacoma Narrows bridge about structural resonance:

https://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_%281940%29

That was a true case of aero-elastic flutter. Other bridge collapses have occurred when some other driver caused the structure to resonate, such as soldiers marching in lock-step or the pounding of a steam locomotive.

Airplanes can resonate at certain engine RPM; this is distinct from flutter although if the structure is close to fluttering it could push incipient flutter over the edge.

[Curt_B]

Re: Propellers, Flutter and sundry

Paul Lipps designed the propeller on Phantom, and used to be a great contributor to the RV forums before his passing. The guy was a genius in my mind, and a perpetual tinkerer. Here's an article that was published in 2009 explaining his propeller design, some myths, and some tweaks to his Lancair: http://rexresearch.com/lippsprop/lipps.htm

Thanks for the article! Would love to know the performance difference for his Lancair, and why his prop never caught on.

I realize this is heavily a photog site, and photos can provide plenty of information. Does anyone know if sound recordings of digital cameras are just as high quality. Shock waves produce sound and can be used to compare propellor technology between airplanes by measuring/recording the pressure pulses I'm speculating. Phantom has a unique sound from the youtube videos; www.youtube.com/watch?v=wLSKvKasMMY and it'd be good to quantify in some way. It'd also be useful if these frequencies can be picked up with a high quality microphone; www.youtube.com/watch?v=SPmW7GzxqdM

[440_Magnum]

Re: Propellers, Flutter and sundry

Just like a wing, a propeller blade has an optimal angle of attack that determines how much lift (thrust) the blade makes, and how much drag (torque) the engine has to overcome (provide) to keep it moving. A propeller's angle of attack is a function of the RPM and forward speed, and since both can change, the angle of attack varies widely. Adjusting blade pitch angle allows some control over this angle of attack, but not entirely, because there's another factor: each part of propeller blade moves at a different speed from zero at the hub to fastest at the tip.

I think you effectively said this, but my brain has to hear it in a specific format to make sense... correct me if I missed it:

The blade's angle of attack is controlled by the prop governor, and the governor only responds DIRECTLY to the inputs of 1) prop lever setting from the pilot, and 2) propeller RPM. But the blade angle that the governor settles on is directly a result of torque and airspeed. If you're flying a constant forward airspeed and don't move the prop lever, but you DO move the throttle forward, the governor will increase the angle of attack in order to prevent RPM from increasing due to the increased torque, and the net effect is more thrust. As airspeed increases as a result of the increased thrust, the governor will keep adjusting pitch to maintain the same angle of attack and the same torque at the same RPM.

[440_Magnum]

Re: Propellers, Flutter and sundry

Paul Lipps designed the propeller on Phantom, and used to be a great contributor to the RV forums before his passing. The guy was a genius in my mind, and a perpetual tinkerer. Here's an article that was published in 2009 explaining his propeller design, some myths, and some tweaks to his Lancair: http://rexresearch.com/lippsprop/lipps.htm

An interesting quote from that article:

" But on a propeller, as on a wing, there is no lift at the tip. The lift pressure differential on a wing or prop goes to zero at the tip—therefore, no lift. But there is drag, and lots of it, due to the high Mach. And since area is a product of span and chord, the wider the tip chord, the greater the area and the greater the drag. This is one of the main reasons why props with wide, rounded tips are so inefficient. A high-efficiency prop will have a pointed tip, zero chord. The high-Mach, wide, rounded-tip props are also the ones that generate so much noise. That noise is engine power being thrown away. "

That raises all sorts of thoughts about inefficiencies of C-130 and Electra blades, as well as the Bear's 3-blade prop based on one of those blade designs. And also makes Strega's prop modification (regardless of the fact that a runway ding precipitated it) look like a pretty good move.

[1:1 Scale]

Re: Propellers, Flutter and sundry

Exactly. Another consideration is the twist of the blade- it is really only dialed in for one specific pitch. That means on a constant speed prop, there is only one point in the hub's range of motion where the entire blade is "flying" at the same angle of attack. Any pitch angle above or below that point introduces inefficiencies.

[Curt_B]

Re: Propellers, Flutter and sundry

An interesting quote from that article:

" But on a propeller, as on a wing, there is no lift at the tip. The lift pressure differential on a wing or prop goes to zero at the tip—therefore, no lift. But there is drag, and lots of it, due to the high Mach. And since area is a product of span and chord, the wider the tip chord, the greater the area and the greater the drag. This is one of the main reasons why props with wide, rounded tips are so inefficient. A high-efficiency prop will have a pointed tip, zero chord. The high-Mach, wide, rounded-tip props are also the ones that generate so much noise. That noise is engine power being thrown away. "

That raises all sorts of thoughts about inefficiencies of C-130 and Electra blades, as well as the Bear's 3-blade prop based on one of those blade designs. And also makes Strega's prop modification (regardless of the fact that a runway ding precipitated it) look like a pretty good move.

The wide chord on the transports was too achieve the lowest airfoil t/c at the time to make the tip more Mach friendly. Tip sweep came along later. Combining tip sweep with minimal chord is the path forward.