aerodynamics of ka 50

[quo vadis]

Can anybody tell me why ka50 has two sets of three rotor blades (and not two sets of two or four) ? As far as I understand, more rotors mean they can rotate slower so the tip of the rotor is less likely to exceed the speed of light (eh, sound). Downside is, the more rotor blades, the more likely they are to enter the unstable air left by the previous rotor. Still, some heli's have two, and some have five rotor blades. Is there a rule of thumb for that?

+ does the lower rotor not suffer from the passing of an upper rotor blade ? (upper blade pushes air down, lower blade suddenly has denser air to go through, doesn't that create vibrations/stutters in the aircraft?)

 

The more I think about the aerodynamics of a helicopter, the more confused I get, Even before taking a co-axial rotor into account :blink::crazy:

[sobek]

Three blades is the minimum for a fully articulated rotor system IIRC. Don't know why not more blades, probably too much drag.

[quo vadis]

Three blades is the minimum for a fully articulated rotor system IIRC.

 

But the Bell UH-1 Huey has only two blades....

[sobek]

But the Bell UH-1 Huey has only two blades....

 

and does not have a fully articulated rotor, it has a semi rigid design if i'm not mistaken. My guess is that the restrictions of a semi rigid design (no negative g maneuvers) are not feasible for the flight envelope of a combat helicopter.

Edited September 11, 2008 by sobek

[AlphaOneSix]

Pretty much all helicopters with two main rotor blades have two sets of three (CH-46, CH-46, all Kamov aircraft), but I know of one for sure (Kaman) that has only two blades per hub. Of course, the Kaman hubs are semi-rigid, as are all two bladed helicopters that I can think of off the top of my head.

[sobek]

The new sikorsky x2 design has 2 four bladed main rotors, for whatsoever reason.

[AlphaOneSix]

The new sikorsky x2 design has 2 four bladed main rotors, for whatsoever reason.

 

 

I knew that would come up. True, of course.

[DeCreator]

I guess, number of blades depends of take-off weight. So, highest workload of each blade is slightly equal to physical deformation limit.

Mi-8, Mi-24, they are ~10 Tons, have the 5-blades main rotor. Mi-26 is fitted with 8-blades MR, and can easy fly with itself weight+20 Tons. AH-64 feel himself better with ~6 Tons and 4 blades.

 

So, three blades for Ka-50 isn`t enough, but set of 2x3 is very well.

[-Skipper-]

I guess it depends on a lot of things.

 

heavier Payload => more blades => more drag => stronger engines => heavier weight => more fuel consumption...

 

 

Besides that at least AFAIK the 2-bladed rotors have a problem at higher speeds. They have to turn faster anyway, so the difference when flying at higher speeds increases on the blade moving forward and backwards. That's the big problem with maximum speed on helos anyway. So it would mean with a 3-blade-system you have better aerodynamics at speed and you can maneuver better.

 

For lifters like the Kaman Huskie, for example, the main goal is not maneuverability or speed, but stability in flight combined with a minimum of weight and a maximum of payload. These systems are also easier to maintain than the coax-rotor of the Ka-50, I guess.

[arneh]

Pretty much all helicopters with two main rotor blades have two sets of three (CH-46, CH-46, all Kamov aircraft), but I know of one for sure (Kaman) that has only two blades per hub.

 

The Yak-24 and Bristol Belvedere were both tandem rotor helicopters with four-blade rotors:

 

 

As was the Boeing 347 demonstrator:

 

and the 360 demonstrator:

[Peyoteros]

[quo vadis]

Don't worry, before you have come to a definitive answer, some other questions popped up (wasn't it: a wise man can't answer in a lifetime the amount of questions a fool can ask in an hour ;)?):

 

With an increase of the number of blades, can they become shorter ? (seems like an advantage to me: I once heard the biggest force on a heli is of the blades pulling outwards. shorter blades are lighter=> less force pulling the rotor head apart...)

 

Does a co-axial heli have shorter blades than a (comparable) single rotor heli ?

 

Do the jet engines deliver any thrust to heli's like the apache or the ka50?

 

Ehm, can anyone advise me on a good book on aerodynamics on heli's :book:, cause ehm, I'm afraid more questions might pop up :cry:.

[GGTharos]

They do not - all the generated power is used for turning that big fan above your head ;)

 

Do the jet engines deliver any thrust to heli's like the apache or the ka50?

[Yellonet]

Don't worry, before you have come to a definitive answer, some other questions popped up (wasn't it: a wise man can't answer in a lifetime the amount of questions a fool can ask in an hour ;)?):

 

With an increase of the number of blades, can they become shorter ? (seems like an advantage to me: I once heard the biggest force on a heli is of the blades pulling outwards. shorter blades are lighter=> less force pulling the rotor head apart...)

I'm guessing that the rotor would then have to spin faster as well as having more blades, thus creating more pulling force.. I'm guessing there's some golden rule for the ratio of spinning speed, rotor blade lenght and blade numbers.

 

Do the jet engines deliver any thrust to heli's like the apache or the ka50?

You mean like a jet engine on a plane? Then no, the energy from the turbines goes to the rotors. Some Helos might use turboprop engines, but I'm not sure there are any like that. Obviously most helo engines are technically turboprop engines but I mean where the exhaust is contributing somewhat to the forward movement. Edited September 12, 2008 by Yellonet

[arneh]

Does a co-axial heli have shorter blades than a (comparable) single rotor heli ?

 

Yes, indeed they do.

 

The Ka-50 and Mi-28 weigh about the same (around 7800 kg empty for both). The Ka-50s rotor diameter is about 3 meters less, and the combined disc area of the Ka-50's two rotors is under 50% bigger than the Mi-28's single rotor (i.e. each rotor's disc area is only about 75% as big).

[arneh]

Some Helos might use turboprop engines, but I'm not sure there are any like that.

 

There have been some experimental helicopters with pusher propellers at the rear for increases speed. Like the AH-56 Cheyenne or the new Sikorsky X2. The Sikorsky S-69 even had turbojets for forward propulsion:

[quo vadis]

I'm guessing that the rotor would then have to spin faster as well as having more blades, thus creating more pulling force.. I'm guessing there's some golden rule for the ratio of spinning speed, rotor blade lenght and blade numbers.

 

Say you have four blades in stead of two (same length). With the same spinning speed, you create double the amount of thrust (?) => So you can reduce the length of the rotor blade to get the same amount of thrust or am a making a mistake here ??

[sobek]

The lift won't be exact double, but roughly if i'm not mistaken, but you also need twice the torque to turn it at the same speed.

[RvETito]

I'll try to put it without the scarry mathematics that stand behind the answer of your question.

 

For every helicopter rotor there's one very important parameter which determines it's efficiency. Not sure about the exact english term but the closest translation is 'fill ratio'- it is the proportion between the sum of all blades' surfaces (projection of the blade in the rotor disk) and the rotor disk area. The optimal value is 7-9%. This range provides the best aerodynamic and power efficency of the rotor.

 

Every gas-turbine helicopter has a powerplant governed at Nr=const. This is another initial parameter that determines the number of blades, their shape (lenght), airfoil(s) etc. The basic rule while designing the rotor is to predict the velocity field during blade's azimuthal motion- blade's tip must not reach critical Mach numbers where local shock waves might occur- those phenomenas will highly decrease rotor's performance, increase the noise and structural loads of the whole construction. If you design a helicopter that should hit 300km/h you must pay attanetion on rotor's diameter and rotor governed RPM. By choosing a fill ratio will give you the number of blades and their chord.

 

Reducing the aspect ratio (the length) of the blade is a bad decision- like a fixed wing this causes increased induced drag.

 

Simple illustration of the advantages of the coaxial rotor is the experiments made by Kamov design bureau with two rotors- one single rotor with 6 blades and one coaxial in 2x3 (same blades- diameter and shape) set of counter rotating rotors. Both rotors (considering the coaxial rotor as one rotor composed of two counter rotating rotors) have the same RPM, collective pitch and fill ratio. The results have shown that the coaxial rotor needs in total 22% less power to create the same lift.

[sobek]

Is there some consensus why the coaxial system is so much more efficient? Is the downwash more linear (rotating less around the rotor axis) than the single rotor? I know of the effect, that the second rotor moves more "fresh" air (that does not come from the downwash) into the rotor system, but does this account for all the additional lift?

[Draco]

I would guess greater induced air flow through the lower rotor disk. like a permanent ETL.

but now I'm curious and I'm gonna have to look that up. too bad none of my training books deal with co-ax rotor systems.

[-Skipper-]

Is there some consensus why the coaxial system is so much more efficient? Is the downwash more linear (rotating less around the rotor axis) than the single rotor? I know of the effect, that the second rotor moves more "fresh" air (that does not come from the downwash) into the rotor system, but does this account for all the additional lift?

 

Good question. I always thought they were more efficient, because they don't have 2 seperate rotors, one for lift, the other one primarily to counteract the torque. In a coax-system, both rotors produce lift and more than that, the negative effect from overspeeding is slightly reduced and in theory maneuverability at speeds should be better with a coax-system.

 

But that's rather my personal conclusions. :music_whistling:

[Draco]

I was actually just about to post about that with out the tail rotor leaching power from the engine there's more power available for lift. must be nice to without the changes in power demand from using the tail rotor

[nscode]

I'd say it's both actually. The coax rotor is more efficient, and having all the power is a bonus.

 

But what is more efficient - coax or tandem?

[RvETito]

Is there some consensus why the coaxial system is so much more efficient? Is the downwash more linear (rotating less around the rotor axis) than the single rotor? I know of the effect, that the second rotor moves more "fresh" air (that does not come from the downwash) into the rotor system, but does this account for all the additional lift?

 

It does. The upper rotor downwash is narrowed and intersects the lower rotor disk in about 0.8-0.85 of the radius. Thus the lower rotor interacts with additional air from radius 0.85-1.0 which adds lift.