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Old Mar 16, 2004, 09:37 PM
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FlyingW's Avatar
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Why Are Heli Rotors Different Than Fixed Wing Props?

A propeller for a fixed wing airplane is typically formed to have constant pitch from its hub out to its tip ("efficient" design").

A helicopter rotor is a slab with a constant pitch angle.

Why are heli rotors not shaped like fixed wing props?

I appreciate your opinions.

Thanks,

Paul
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Old Mar 16, 2004, 09:59 PM
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I don't know for sure, but most helicopters are collective pitch, so you couldn't give the blades a pitch at one angle that wouldn't mess up at other pitch angles. Now for a fixed pitch heli, I'm looking right now at a Corona blade, and it is "twisted" from root to tip like a plane prop. And a Piccolo F.P. blade is undercambered, and tapered, the combination giving more angle of attack (twist) at the root.
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Old Mar 16, 2004, 11:44 PM
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The difference between a propeller and a rotor blade is quite a lot.

A propeller is designed to pull forward through the air and hence is designed like a screw. It generates it's thrust by displacing air. It spins VERY fast compared to a rotor. In a model case, 18000 rpm vs 1800 rpm.

A rotor is actually a long thin wing that is swung through the air to produce lift rather than being towed through it (hence the name 'rotary wing aircraft').

It does displace air downwards (thrust) due to the angle of attack of the blades, but this accounts for only part of the lift produced by the rotor.

The longer and thinner you make the wing, the more efficient it is at making lift (look at a glider vs a fighter jets wings). A propeller has short stubby 'wings' compared to a rotor.

On smaller (micro) helicopters, blades are often wider / more undercambered near root (like a propeller) to improve the thrust component, but as you increase rotor span, width of the blade doesn't increase in 1 : 1 proportion to the increase in length, so the efficiency of the aerofoil and hence the lift generated increases exponentially.

Past a certain point, the thust that could be generated by twisting and widening the rotor towards the root like a propeller is less than the loss of efficiency (and lift) caused by the extra drag, so it is not worth it.

Hence rotorblades (on real thing and bigger models more than about 70-80cm rotor span) are fixed chord and constant pitch along their length.

Hope that made sense.

-Tim
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Old Mar 17, 2004, 03:22 PM
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Thanks Googleplex,

That makes sense. Given that the ends of the rotor blades are passing through the air the fastest, they are also producing the most lift - like a "ring" of lift.

Does the combination of gyroscopic action and the ring of lift provide some natural stability?

Paul
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Old Mar 17, 2004, 11:35 PM
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Yes and no .

The increase in lift at the tips vs root caused rotor coning - the bending of rotors upwards - to give a form of all round dihedral angle and hence stability in hover.

The gyroscope effect is used, but not in the flywheel sense.

On a rotor head with a sub-rotor / flybar / stabilizer bar or whatever, there is actually reduced (in some cases VERY reduced) gyroscopic tendancies, as the rotor head can freely roll along it's blade axis, feeding this motion into the flybar, which in turn causes a cyclic pitch correction.

For example, a gust tries to push the nose of the heli up, the gyroscopic effect on the rotor disk causes it to twist 90 deg out of phase with the direction of the force, which goes straight into the flybar (90 deg to rotors). Now the gyroscope of the flybar deflects at 90 to the force applied to it by the rotors (now 180 out of phase with input wind force), causing a cyclic pitch input to be fed into the rotor head that makes the heli want to drop the nose the same amount (in an ideal world - in reality, there is friction in the rotor head joints, so you need to do some small input too).


-Tim
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Old Jun 19, 2004, 08:19 PM
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Hi Ya guys, New member here. Just got my first heli. In answer to the rotor blade question, I refurbish rotor blades for various aircraft for a living and can tell you all that in full size applications rotor blades are washed out to equalise the dissymetry in the lift produced along the blade length due to increasing relative velocity towards the tip (7 degrees root to tip for a bell jet ranger) This is also the same for props. airfoils used are also not standard symmetrical ones. At the very least they have a drooped leading edge to assist with stall characteristics at high angles of attack.
At Colledge I was told that using a non symmetrical airfoil section on helis was not feasible due to the relatively unstable centre of pressure above the blade moving in response to all other variables and causing the blades to twist in pitch. The trouble at the time I was told was in the current materials not being suitable.I now know that a variety of assymetrical foils are in commercial use and I cant see why I couldn`t manufacture a set of assymetrical blades with Twist for testing on a model. Just need accurate cores cut!!
Love to get touch with someone in the Brisbane northside area who can get me started on this contraption!!
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Old Jun 19, 2004, 11:12 PM
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Another consideration is the speed of rotation... propellors designed to produce efficient thrust at lower RPM usually look much different than those designed for high RPM. In the case of model aircraft vs. model helis, the RPM difference is significant. I can't comment on full scale machines.. I have no idea what the nominal main rotor RPM is for a full-scale heli, but I'd bet it's considerably lower than that of most fixed-wing aircraft engines.
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Old Jun 20, 2004, 09:03 AM
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Boywonder
Wish I was close to ya,,, I have made twisted (47 deg root to tip) for my V-22 osprey project. They are just modified wood blades and not to fancy or acurate ,,but they work.
If you make some carbon ones that are 510 mm let me know.
Larry



http://www.rcgroups.com/forums/attac...d=231086&stc=1


Here is a string on the project.
http://www.rcgroups.com/forums/showt...=232044&page=1
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Old Jun 22, 2004, 07:45 PM
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Fixed pitch helis like the Corona do have rotor blades with a twist in them like an airplane prop.

For helis with collective pitch there is a simple reason for the flat "slabs"... negative pitch. The blades on a collective pitch heli move through a range from negative to positive pitch, not just for aerobatics (inverted flight) but also in normal upright flying.

Tim
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Old Jun 23, 2004, 04:16 AM
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Boywonder is exactly right. I had a real heli lesson the other day and the instructor explained that as the outer part of the rotor blades is going so much faster than the inside, it has to have less pitch at the outside, increasing as it works its way in to the middle. Otherwise the blade would have very uneven lift and cone up at the outside to the extreme!
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Old Jun 23, 2004, 05:20 PM
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model heli's don't have to worry as much about uneven lift distribution across the blades, since our blades tend to have a much higher strength to weight ratio than full size blades. having all the lift out at the tips of the blades is no problem for composite blades. however, it is apparently a problem with the plastic blades on micros and the corona, so i guess they use blades with washout to get more pitch on the inboard part of the blade.
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Old Jun 23, 2004, 07:57 PM
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Isn't there also the lift/drag ratio to be considered? A blade with no washout would develop more lift at the tip (because of higher relative speed) but would also tend to create more drag, and more important, would tend to 'stall' (stop generating lift) before the inner portion. That's calle a 'tip stall', usually followed by complete loss of lift and (in fixed-wing terms) an unplanned snap roll.
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Old Jun 24, 2004, 06:49 AM
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If you look at wing plan form theory a rectangular wing will stall at the root first so tip stall is not going to be a problem with out blade design.
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Old Jun 24, 2004, 07:36 AM
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Propeller blades are designed with constant pitch (in inches not degrees).
The angular pitch usually varies linearly along the blade with the maximum pitch being at the root.

This also implies that depending on the pitch, part or all of the blade will be stalled at low speeds. Airfoils with pitches exceeding approx. 17 degrees are stalled.

Propellers with pitch/diameter ratios exceeding 0.6 have most of the blade stalled.

Not very good for helicopters.

As the aircraft accelerates, the blade will become unstalled and at some aircraft speed and some blade RPM, the entire blade will be unstalled, the relative pitch angle will be constant (the vector sum of the angle of the incoming air and the angle of the spinning prop will be constant along the blade) and the blade will be operating at maximum efficiency.

In real aircraft (not model aircraft) the excercise is to determiine this operating point.

Helicopters on the other hand, have their optimum lift/drag ratio at approximately 5 degrees pitch (the lift/drag ratio being improved by the high aspect ratio) and except for some of the valid reasons given in the preceding posts, propeller blades and helicopter blades are different basically because of efficiency requirements demanded by the difference in velocities of the air flowing through them.
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