Q for a helicopter aerodynamacist.

I understand the reasons for high aspect ratio's in the wing planforms of sailplanes.
As well as low aspect ratio's for very high speed aircraft.

The wings of mid-speed airliners and similar aircraft being somewhat high aspect ratio doesn't enter into this discussion.


What I am curious about is why heli's have such high aspect ratio rotor blades?

Seems a low aspect ratio rotor blade would have the advantages of lower tip speeds which would translate to a higher aircraft top speed.
(Tip speeds exceeding supersonic creates problems and limits the top speeds of heli's.)

If the high aspect ratio rotor blade has X amount of area and generates Z amount of lift at a particular rotor rpm and collective setting, couldn't you end up with similar X and Z ratings with a low aspect ratio rotor blade?
The low aspect rotor blade being half the length and twice the chord would have the same area should generate about the same amount of lift at similar collective settings.

The smaller diameter rotor could be spun at double the usual rotor rpms for additional lift and speed before excessive tip speed came into the picture.

Rotor airfoil shape would have to be juggled to reach an optimum.

An additional advantage and an important one would be the lesser ground area covered by the high aspect ratio rotor blade.

The way heli blades are hinged it looks to me like the low aspect ratio rotor blades wouldn't have to be flexy and would work just fine.

I'm guessing here, but the flexy nature of the high aspect ratio rotor blades are just the nature of the brute so to speak.
I do know they have a very limited life.
1100 hours is what they are rated for on a Hughes 500 if I remember right.

I would guess at it being a question of wake management - if the blade aerofoil section was scaled up there would be a larger boundary layer and greater downwash downstream the blade, which would negatively impact on the following blade's aerodynamic loadings.

While my focus is more on fixed wing a/c, lift is lift. Lift is directly proportional to the SQUARE of the velocity over the airfoil, but only proportional to the area of the blade. Thus a longer blade gives more velocity (and more subsequent lift) for a set rpm, which means less load on the engine for a required lift. Secondly, as they gain altitude the air thins out and they either need a lot more area or a little more velocity. Velocity can be throttled by the engine. Area, short of some fancy morphing rotor (experimental at best currently), is fixed. You want to get by with as little area (translates directly into weight) as possible, not only for pure a/c weight but for limiting the weight the engine needs to spin at high velocity. Like I said I'm a fixed wing guy so any helo experts please chime in here.



This is my first guess, but like I said I don't work with helos too often.

This really is a well informed thread if we could get some pics up of the boundry layer of a helo blade it may help to visualize the problem that i think is being talked about. I got basic knowledge of this stuff but these type of threads would help expand all of it.

On topic I think the idea of halfing the length of the blades makes sence but as nipples said you would still be dealing with more weight for the engine right? what if you halfed the the lenght and doubled the chord of the blade.

Originally posted by Canada_EH
I think the idea of halfing the length of the blades makes sense but as nipples said you would still be dealing with more weight for the engine right? what if you halved the the length and doubled the chord of the blade.

If you halved the length, and doubled the chord, the rotor weight should stay approximately the same, but the moment of inertia would be greatly reduced, which would mean lower centrifucal loads at the blade root, and it would allow for quicker throttle reaction from the engine.


As nipples says, the further out you go, the higher the effective airspeed of the blade at that particular point is - so it will generate more lift, until the blade tips begin to go supersonic, and efficiency drops off dramatically. This is the reason a propellor aircraft cannot get into high subsonic mach number flight easily. (Neglecting the fancy prop-fan designs for a moment, they are quite different in design)


But, if you compare the size of a propellor on an aircraft to a helicopter, you can see it is much smaller, so you may say - why is a helicopter's blade not much smaller. Well, if you consider the stream-tube of a helicopter as well as propulsive efficiency it becomes more clear - it is always better to accelerate a larger mass flow by a lower amount, than a small mass flow by a large amount. This is the reason for high bypass ratio turbofan engines being so efficient compared to old turbojets - it is also the reason for turboprops being even more efficient than turbofans at low mach speeds.


When you think about the flow above and below a helicopter blade, you can see it is operating at almost stationary speeds (extemely low mach), therefore it is most efficient to use as large a rotor swept area as feasible (thus needing longer blades) before blades go supersonic (some blade tip designs are swept to offset this to allow greater rotor blade spans).


Thinking about what I said earlier, the wakes probably wouldn't be a large issue unless the camber of each blade profile became extreme and there was a large number of blades on the rotor design.

Originally posted by kilcoo316

Originally posted by Canada_EH
I think the idea of halfing the length of the blades makes sense but as nipples said you would still be dealing with more weight for the engine right? what if you halved the the length and doubled the chord of the blade.

If you halved the length, and doubled the chord, the rotor weight should stay approximately the same, but the moment of inertia would be greatly reduced, which would mean lower centrifucal loads at the blade root, and it would allow for quicker throttle reaction from the engine.


As nipples says, the further out you go, the higher the effective airspeed of the blade at that particular point is - so it will generate more lift, until the blade tips begin to go supersonic, and efficiency drops off dramatically. This is the reason a propellor aircraft cannot get into high subsonic mach number flight easily. (Neglecting the fancy prop-fan designs for a moment, they are quite different in design)

So i understand for myself your saying that the short the blade the faster it has to go to produce the equal amount of lift of a longer blade? Also with a shorter balde your would get quicker reactions from when the engine is throttled up or down?
So with this thinking a short thicker blade is good for a more agile helo?

Originally posted by Canada_EH

So i understand for myself your saying that the short the blade the faster it has to go to produce the equal amount of lift of a longer blade? Also with a shorter balde your would get quicker reactions from when the engine is throttled up or down?
So with this thinking a short thicker blade is good for a more agile helo?

- Yes

- Yes

- Not necessarily, an agile helo needs alot of excess thrust, the thrust can be varied by changing the angle of attack of the blades, without having to change the engine speed, so its not quite a straight trade-off.

Oh, another thing, I reckon the fuselage of the helicopter will affect the efficiency of the inner-most sections of the blade, reducing their performance slightly.



I'm in the same boat as nipples, I'm not into helicopter aerodynamics, I'm just making (slightly educated) guesses here - they could well be wrong.

[edit on 29-10-2005 by kilcoo316]

Ok as it was explained to me when I was in helicopter mechanic's school the amount of lift is a function of the area of the rotor disc not the sum of the blade areas. Therefore you need the longer blade to have enough disc area for the desired performance. Just using shorted blades and increasing their speed isn't going to cut it.

Originally posted by JIMC5499
Just using shorted blades and increasing their speed isn't going to cut it.

We said shorten the baldes by half and increase the thinkness by half so as not to loss the total area. We understand that it would me slighty less lift prodused so we would have a higher throttle or speed setting but would also have a more responsive helo. That to me is of interest if someone was to want to design a new attack helo to replace the AH-64.

[edit on 29-10-2005 by Canada_EH]

You beat me to it JIMC5499, I also think the lifting area is the circular area above the rotor not just the sum of the blade areas...and circular area goes as the square of the radius (blade length). If this thread is still running on Monday I'll ask a helo-guy I work with for more specifics.

kilcoo316 is right as far as the difficulty of getting propellers to produce thrust at high flight speeds. It may 6-of-one-1/2 dozen-of another, but I think that has more to do with the velocity triangles coming into the propeller. Crossing the prop tip vector with the flight vector eventually starts building negative thrust. I can't remember how much of this has to do with the prop tips going supersonic though. But I do know NASA, and maybe Honeywell, were doing do work on the propfans someone mentioned, and I am sure some quick scouting on that topic would lead you to the limiting factors regarding propeller performance. Later.

To add a couple more things to the mix . . . and maybe the second one should be in a different thread, but I'll toss it out here and see how it works.

One of my interests in having a smaller rotor swept area comes from watching fire fighting heli's in SoCal fly into a couple of river ponds that had banks close to the rotor on both sides.

The heli would come in, dip the bucket in the pond and wait for it to sink.
All the while holding a stationary hover while the bucket filled.
30-40 seconds or so.
It was interesting being up on the bank and a little way up or downstream from the heli in case things came unglued and viewing the heli from above . . . which isn't too common for observers on the ground.

A smaller rotor swept area would be a real advantage there.


The second thought is, if we're going to be stuck with longer rotor blades why not install winglets on the tips - or build the rotors that way to start with and block the span-wise flow of air thereby making the rotor more efficient with it's now increased lift and speed capabilities . . . disregarding the tips hitting supersonic.

Most know that airflow over a fixed wing is fore to aft - until separation anyway - and there is also span-wise flow that drops off the end of the wing at the tip thereby losing lift.
The winglet stops span-wise flow at the tip - most of it anyway - which generates extra lift which translates to more speed and better (albeit small) fuel economy.

Winglets are an aerodynamic freebie by giving back more than they take in terms of drag and weight.
Seems like they would be quite viable on heli rotors.

Even a propeller is designed and limited by the same factors. Maximum cruise RPM of the engine, and tip speed of the blades, just below supersonic, limit the overall diameter of the propeller. Same is true for the rotor blades of a helicopter.

Which is a point to consider, the helicopter does more than just lift, it also has forward flight. Lifting a helicopter is easier, and can be do various ways. But the forward flight results in even more restrictions and changes to the design and shape of the rotor assembly.

It's like a big constant speed variable pitch prop. Which is good, because in a helicopter you don't want to reduce RPM's. With no wings to glide anywhere, the rotational energy from the higher RPM's and big diameter is what will allow an auto-rotation to a hard landing. In helicopters low and slow is bad.

www.verticalreference.com...

At the instant of an engine failure, the helicopter pilot must change the main rotor blades' angle so that it can take advantage of the up flowing air. If the pilot waits too long, the up flowing air will "stall" the rotor blades and autorotation will not occur (in which case, the pilot and the helicopter become victims of gravity.) To change the angle of the main rotor blades, the pilot immediately lowers the collective when the engine quits. By lowering the angle of the blade, the main rotor blade will be able to use the air flowing over the wing to create lift that will "pull" the main rotor blade and keep the rotor blades spinning.

A short blade would not have a good moment arm of the longer blade. That force of physics, is the wind pushing on the blade to keep it rotating. It develops the energy to keep the blade turning, create some lift, and "float" down to earth.

Helicopters are complicated beasts. Here's something else to consider; ground effect hover. The larger diameter of the rotor assembly allows a nice cushion of air below the helicopter. That column of air is still in effect even at fairly high altitudes. With a smaller diameter and higher speed, I don't think it would be there.

I hope this helped understand how a helicopter flys by beating the air into submission.

Anybody ever see a Huey with a 5 blade rotor? I hear some have been changed from the two rotor to a five blade whisper set-up, but I've not yet seen one.

Edited spelling...

[edit on 29-10-2005 by ZPE StarPilot]