4-blade propellor questions

The absolute best way (in my warped old mind), would be to acquire, or have made, a reduction unit to lower the rpm, while simultaneously increasing the pitch from six inches to something more appropriate for the amount of reduction employed.

OR, use a much larger engine, such as a YS .91. Now that I think of it, I'm not sure that the .91 would have enough extra power to maintain your 12,000 rpm objective, whereas, the YS .63 running a lightweight, low friction, reduction unit could probably swing a two blade 13" diameter, 12" pitch propeller at a respectable rpm.

Even if the final output rpm was only six to seven thousand rpm, the extra pitch should more than compensate. Not to mention that the wake turbulence of the preceding blade should be greatly reduced (because of the lower rpm), leading to more efficiency at producing thrust. This is usually the big problem with three and four bladed props running at very high rpm, resulting in much less thrust, much more drag and tons more noise.

I think the first thing I would do is to find the prop, then work with different amounts of reduction until I got the engine running in its power band.

Now that I think about it (again), you would probably need a YS 110 plus a reduction drive unit to swing a 13" four bladed prop with 12" of pitch at a usable rpm.

I have used multiblade props (meaning more than two blades in this discussion) in the past and the reduction in thrust is dramatic. I always wanted to revisit using multiblade props, but this time with reduced rpm and much more pitch.

It is late and my eyes are blurry, as is the mind. Sorry if this doesn't make a whole lot of sense.

Ed Cregger

Ed

Your comment about the speed changer is right on the mark - except the remark about a warped mind. You'r one of the best informed on this group.

As RPM changes, the Power required goes up with the cube of the RPM while Thrust generated goes up with the square of the RPM. Therefore, thrust per horsepower decreases with higher RPM and increases with lower RPM. A 2:1 speed reducer will allow twice the thrust of a non-reduced engine with same Power.

The general equations power and thrust are:

Power = Power Constant * Prop Factor * RPM^3 Thrust = Thrust Constant * Prop Factor * RPM^2

The Prop Factor in both equations is the same.

Prop Factor = Diameter^4 * Pitch * Number of Blades. I've tested this with

1, 2, 3, 4, 5, and 6 bladed props. Contrary to popular opinion, I have not any expermental evidence of flow from one blade interferring with another.

For the same model speed with the 2:1 reducer, the pitch should be increased by a factor of 2x.

If I can be of any assistance, let me know.

Ray Shearer

So you want static thrust or speed? It would take almost twice the power to add an extra blade and keep the speed the same. Thrust would not go up by the same amount.

I also notice more thrust with a broad faced prop ,2 blade(longer chord), than a narrower one. Test was done on an electric 3phase (brushless). Any comments about that??

Trouble is, there are simply too many variables.

I have a plastic 8x6 glo prop - would barely fly the same plane that flew brillantly on an electric 8x6 - its electric of course - the model.

Going from a 10x7 slofly prop to a non slo fly electric type of the same diameter and pitch and manufacturer (APC) dropped the current by 15% and increased RPM. Presumably dropped the thrust as well, ...

So blade area and profile obviously makes a lot of difference...

In-flight oomph :-) The sort that gets you out of trouble. I don't need a high top speed.

What would you expect it to go up by ?

As I said before, I can put in pretty much whetever engine it takes to drive the prop, so the horespower and efficiency don't really play much of a part.

My experiences with high pitch props is they make landing harder...

Thanks,

| If you are building an exact scale model where prop size is limited | because of the physical layout of the real model (like eg a twin or | a plane with a short u/c) then there seems to be no alternative but | to use a multi-blade prop. My understanding is this is the reason | why eg Spitfires started with 3-blade props and went up to 6-bladers | in the case of the last mk of Seafire.

I believe you are correct. The figher planes have so much power that they needed lots of blades, even though as you add blades your prop becomes less efficient.

| So if I have a YS63 for example, that might turn a 2-bladed 13x6 prop | at, say, 12000 rpm. What engine would it take to turn a 3-blade 13x6 | at 12000 rpm ? And what would be the thrust ? Also the same questions | regarding a 13x6 4-blade prop ? | | I'm obviously just looking for ball-park figures here, not expecting | precision to n decimal places.

The rule of thumb that I've often heard has been that a 2 bladed prop is roughly equivilent to a three bladed prop that is one inch shorter and has one inch less pitch -- that a 13x6 two blade is roughly equivilent to a 12x5 three blade.

Of course, if you're looking for `precision to n decimal places', I doubt `n' is even 1 :)

I don't know how it translates to four blades ...

Well as the old saw has it - "if a rule of thumb works once, apply it until you run out of thumbs" :-)

That is because there is no wake interferance. If for example the prop is a 6 inch pitch and there are 4 blades then the wake is 1 1/2 inches behind the next blade when it comes to the same point. The reason multiblade props are less efficient is that their are more tip and hub intersection loss. Although multi blade propellers are usually used in full scale airplanes for ground clearance, there is some benifit in high speed thrust with additional blades. The Merlin powered Mustang went from a three to four blade prop because of increased thrust and thus higher top speed. Many high performance GA planes also go to multi blade props for increased speed, even though there are two bladed props that have enough ground clearance.

You forget: a propeller blade is an airfoil, and any airfoil generates tip vortices. At high angles of attack these vortices can disturb the air as far out as one-half the wingspan, or one-half the prop diameter. In a fixed-wing airplane, the effect is seen in ground effect causing floating during landing as the presence of the surface limits the vortex formation and both drag and lift loss over the tips is decreased. On a prop, the effect is noticed at low forward speed and high RPM, putting the blades at high AOA, and efficiency suffers. At high forward speed the AOA is much lower and vortices far smaller, and the extra thrust created by the higher horsepower (which mandated the extra blades in the first place) outweighs the tip losses. Moreover, since multiblade props are often shorter than two-blade props in any give application, the tip speeds are lower and drag is somewhat less. One more: full-scale multiblade props are almost always constant-speed (variable blade pitch) and for takeoff and climb the pitch is reduced to lower the AOA to allow the egine to reach redline, and the effect also reduces tip vortices at those low forward speeds. The modeller is operating in a different world, a fixed-pitch world, and can't expect the same benefits. Some will argue that blades don't interfere with each other; just look at the turbofan jet engine! Well, their similarity to free-air props is as about as close as a bicycle is to a truck. They run in a very close duct that eliminates tip vortices; they have larger hubs that eliminate hub interferences; they are well back inside divergent inlets that slow and raise the pressure of incoming air, and they often have inlet guide vanes ahead of them to further control airflow. Not the same thing at all.

Dan

It is the tip vortices that causes the loss of efficiency, not the blades penatrating the vortices. The vortices are behind the next blade, the next blade does not go through them. They do spread out and probably as much as you say, but not soon enough for the prop to go through them. Just as the wing of the airplane does not penatrate them the next blade does not penetrate the votices. Even when stationary the air is moving through and behind the prop.

Keep in mind that at high speeds it is better to move a smaller column of air at a higher speed than a larger column at a lower speed, this is to maintain a thrust at high speeds. Not a large factor for our models.

I wish you were here. I'm a fullscale flight instructor and aircraft mechanic, and we often have occasion to watch aircraft running up while sitting stationary on the ramp. If there's water on the surface, the vortices become very visible and powerful waterspouts form under the prop. I have seen stones sucked FORWARD from more than a foot BEHIND the prop, along the surface and up into the front side of the prop. There's a very strong vortex there, and at anything less than full forward speed they will still be there and interfering with the following blades. You can't assume that each blade of a three-blade prop having a nine-inch pitch is a full three inches of air from the previous blade unless the airplane is travelling at full geometric pitch speed.

Of couse an airplane's wing doesn't penetrate its own vortices; it doesn't travel in a circle. Except, of course, in a steep turn, when we often encounter our own wake turbulence when completing a 360. In my training I have seen videos taken of wings, props and helicopter rotors operating in smoke streams. The average Joe has no idea of the degree of disturbance his prop or wing makes as it passes through the air.

Dan

Well I am a long time private pilot and engineer, and I can say that what you saw was the vortices bouncing off of the ground and has nothing to do with a plane in flight. The vortice rolls off the tip and goes back (even on the ground) and out, they do not interfere with the prop blade, except possibly any vortice bouncing off the ground or off of fuse sides when stationary..

Sport Pilot opined

What do you call high speed, 400kts or supersonic?

-ash Cthulhu in 2005! Why wait for nature?

Maybe you need to study a bit more. Do you know what the terms HIGE and HOGE are in helicopter terminology, and what the differences are and why? Do you think propellers are different somehow from wings or rotors, that they don't generate significant vortices at anything less than cruise speed? Have you ever seen any smoke tunnel testing?

Dan

. So thats the problem, you are trying to adapt rotor tech for propellors. Helecoptor propellers tilt towards the diriction of motion, the vortices are blown up by the ground and back by the foward motion, so yes they can cut through their own vortices. The airplane propellors almost never do.

Picture of C - 130 propellor vortices.

Notice that they spiral back behind the prop?