What angle are front rotors of tandem helicopters tilted and why this value?

Question

I've read that the front rotor of tandem helicopters are tilted forward so that you can get a forward velocity with no nose-down pitching of the fuselage, but what angle is used for this tilt and why this value?

Is this angular value chosen such that you can have no nose-down pitching at the design-point operation of the helicopter? ie, if the helicopter is designed to cruise at 150ktas, is the forward rotor's angle set such that your forward force cancels the total drag at this velocity and maintain no fuselage pitching? And if you wanted to go faster than 150ktas, you would just have to pitch the whole helicopter to get the rear rotor's upward thrust to join in to accelerate forward?

Answer 1

The forward tilt of the main rotors with respect to the fuselage is not related to the tandem rotor design, but to a combination of fuselage lift and the need to also provide thrust with the main rotor.

First off, a review of other tandem designs shows that those not related to the CH-47 do not always share the forward tilt:

Bell XHSL-1 prototype

Filper Research Beta 200A

McCulloch YH-30

Piasecki H-21B

Yak-24

And looking at non-tandem helicopters, we also find a similar tilt:

Sikorsky CH-53

NHIndustries NH90

Mil V-12

Mil Mi-26

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The reason is twofold:

A helicopter obtains its thrust from the same place as most of its lift: from the main rotor. This means that in forward flight the main rotor will always have its tip plane slightly tilted forward to counter the drag acting on the airframe.

Furthermore, large helicopters like the tandem designs you ask about experience significant aerodynamic forces on the fuselage at high speeds. To mitigate these, the main rotors are installed at an angle so that in cruising attitude the lift component from the fuselage will be positive. You can see it more clearly in flight:

NHIndustries NH90 in flight

Sikorsky CH-53 in flight

Boeing CH-47D in flight

I could find some corroboration of the fuselage lift effect in this Boeing report which includes a lift curve for the fuselage attained via wind tunnel tests. As you can see $\alpha_{zero-lift} \approx 4.5^o$

As to what is the exact angle, that depends on the design and is ultimately optimized after extensive testing, if deemed worthwhile.