Could Transverse Rotors Replace Coaxial Systems for High Speed Flight?

Join the discussion
Ask a follow-up here, or get your own question answered by working scientists, mathematicians and engineers — people, not an autocomplete.
Real named experts · corrections over time · the nuance an AI answer skips
Sign up Log in
2 replies · 5K views
kjjohn
Messages
5
Reaction score
0
I was thinking about the design of the Sikorsky X2 and S-69 concepts today, and I thought of an interesting idea. I am by no means an expert, but I guess you could say I know more about this subject than the average person.

So anyway, if you know about the S-69 and the new Sikorsky X2 concepts, you know that they use something called the Advancing Blade Concept. Basically, the rotor head and blades are very stiff. This is not like a hingeless rotor system, in which the blades have elastomeric bearings or flexible materials so they can flap, lead and lag. The Advancing Blade system is designed so that the blades do not flap, or more realistically, flap minimally. There are no hinges in the rotorhead, aside from the pitch feathering hinges for cyclic control. Obviously in a typical helicopter, this design would be a complete failure. With blades that can't flap, or at least not flap very much, dissymetry of lift and retreating blade stall would greatly affect it. However, The X2 ignores retreating blade stall by using coaxial rotors, one on top of the other. This means that the retreating side of one rotor is the advancing side of the other, producing even lift (theoretically) at all times. This also allows the helicopter to travel at very high speeds, and simply "offload" the retreating blades.

My idea is to replace the two coaxial rotors with two transverse (side-by-side) rotors like the V-22 Osprey, which would be counterrotating. These would still work using the Advancing blade concept with stiff rotors. My idea is that instead of using a pusher propeller like the one on the X2 to achieve high speeds, could you actually vector the rotors themselves to achieve high speed? I'm not talking about a titltrotor where the rotors tilt all the way forward to turn into propellers, the rotors could only tilt maybe 15 degrees forward or backward, so it would still function more like a helicopter than an airplane. My question is, would this present any major problems, and would it go any faster than if you just used the cyclic (which does "vector" the rotor in a sense by increasing lift on one side)
 
Physics news on Phys.org
kjjohn said:
I was thinking about the design of the Sikorsky X2 and S-69 concepts today, and I thought of an interesting idea. I am by no means an expert, but I guess you could say I know more about this subject than the average person.

So anyway, if you know about the S-69 and the new Sikorsky X2 concepts, you know that they use something called the Advancing Blade Concept. Basically, the rotor head and blades are very stiff. This is not like a hingeless rotor system, in which the blades have elastomeric bearings or flexible materials so they can flap, lead and lag. The Advancing Blade system is designed so that the blades do not flap, or more realistically, flap minimally. There are no hinges in the rotorhead, aside from the pitch feathering hinges for cyclic control. Obviously in a typical helicopter, this design would be a complete failure. With blades that can't flap, or at least not flap very much, dissymetry of lift and retreating blade stall would greatly affect it. However, The X2 ignores retreating blade stall by using coaxial rotors, one on top of the other. This means that the retreating side of one rotor is the advancing side of the other, producing even lift (theoretically) at all times. This also allows the helicopter to travel at very high speeds, and simply "offload" the retreating blades.

My idea is to replace the two coaxial rotors with two transverse (side-by-side) rotors like the V-22 Osprey, which would be counterrotating. These would still work using the Advancing blade concept with stiff rotors. My idea is that instead of using a pusher propeller like the one on the X2 to achieve high speeds, could you actually vector the rotors themselves to achieve high speed? I'm not talking about a titltrotor where the rotors tilt all the way forward to turn into propellers, the rotors could only tilt maybe 15 degrees forward or backward, so it would still function more like a helicopter than an airplane. My question is, would this present any major problems, and would it go any faster than if you just used the cyclic (which does "vector" the rotor in a sense by increasing lift on one side)

Fast aircraft do not require large wings. The main reason for larger wings involves takeoff and landing. Aircraft with higher thrust to weight ratios , such as the T-38, the former USAF trainer, can achieve a higher takeoff velocity, particularly on longer runways (about two miles) common to many USAF runways.

Years ago, a wayward USAF pilot became disoriented with respect to his position (he got lost) and landed a T-38 on the 5,002' runway at North Little Rock (KORK), thinking it was Little Rock AFB. He skidded off the end. By comparison, using common short field landing techniques, I could land their Cessna 172, 150, and their Piper P-38 in less than 1,000'.

As for your idea, many, if not most counter-rotating helos use more heavily fixed blades (semi-rigid rotor system) than standard two-blade helos. From what I understand, there were even a couple of co-axial designs on the books, perhaps in testing. Turns out the balance issues were huge, and largely abandoned in favor of less rigid rotors.
 
Check out the Kaman K-1200 K-Max and the H-43 Huskie... sound something like your idea?

Both aircraft are great lifters, but less great for high rates of speed.

Njord