Originally Posted by
JohnDixson
004, I think the point SAS was making was that the FVL ( and FARA ) speed requirement puts them way out in front of the logistical support helicopters that provide the necessary logistical support for combat. I.e., air assault vs a special ops mission.
One other thing that has me puzzled is a subject I’m a rank amateur at: to whatever standards were set up, the Army tested the Comanche re radar and IR detectability and as far as I know, it passed. Comanche main rotor is oriented edge on looking forward. Now the tilt rotor 280 has its blades ( propellers ) oriented with flat areas lookiing forward. Then again so does the V-22 so does radar cross section not matter?
But then again, Bell initially presented a smaller FARA sized tilt rotor idea, but since have reverted to the Comanche look-alike Invictus. Curious.
It is the FARA requirement of a 40’ width that kills a tiltrotor with the desired payload. With a 5’ separation between the rotors, that only gives you 481 sq ft of disk area. At 16 lb/sq ft that’s a GW of 7700 lb and at a V-22 like 27 lb/sq ft that’s a GW of 13,000 lb. I deeply suspect that with the desired payload, crew, fuel, 20 mm cannon and ammo, a tiltrotor would have weighed well over 13,000 lb and the design doesn’t close. If you push disk loading even higher, your lift per hp curve goes the wrong way and now you’re a twin engine aircraft, making the empty weight higher as well as required fuel. The Army was firm on a 40’ width and desired a single ITE and the way to get that is with a single main rotor (whether coaxial or not).
I can’t speak to RCS specifics, but the rotor blades on a tiltrotor shouldn’t be thought of as “flat” to an onlooker ahead of the aircraft. The airfoils still need a modest angle of attack with the airstream to produce thrust and the higher cruise speed and lower rotational speed at cruise means the collective is very high to achieve those conditions. So the blades are still largely edgewise to the viewer ahead of the aircraft. To a Doppler radar, the blades have a low change in speed since they’re rotating around an axis pointed to the viewer/radar ahead of the aircraft. A conventional helicopter has a large Doppler return off the rotor from all azimuths. I suspect that a tiltrotor and edgewise rotor helicopter are just “different” with respect to RCS and the specific radar system thread being evaded.