a reply to: anzha
The two questions here are:
Why contrarotating props? and 2) Why a flying wing?
As a UAV designer, I would say both design choices come from the stated mission requirement of very long range, very long endurance, combined with the
requirement for vertical takeoff and landing (VTOL).
For long range and endurance, the design has to be all about aerodynamic efficiency (high Lift-to-Drag ratio, or L/D) and thermodynamic efficiency
(low Specific Fuel Consumption, or SFC) For efficiency in horizontal flight, a fixed wing beats a rotor system (helicopter, gyrocopter, quadcopter,
etc.) every time. This is why the V-22, for instance, approximately doubles the range of a comparably sized helicopter. In order to meet the
requirement for long range and endurance, the cruise portion of the mission HAS to be carried by wings, not by rotors.
The stated mission requirement from DARPA also specifies “medium altitude” (typically about 20,000 ft or so) so there is no particular need for
high altitude or high speed. However, the payload/range requirement objective of 270 kg over 1670 km implies a relatively large airframe by UAV
standards (about the size of a Cessna Centurion). A vehicle of this weight category puts it outside the capacity of battery or fuel cell propulsion,
so you are pretty much stuck with a heat engine of some sort. A well-designed turboprop system is a lot more efficient than a turbojet or turbofan
generating the same amount of thrust (at least at medium altitudes and speeds) so again, if you want long range and endurance, you go with a
turboprop. This answer hasn’t changed in 30 years because the laws of physics haven’t changed. (A reciprocating engine could theoretically have a
lower fuel burn than a turboshaft engine of the same power, but the turboshaft will win on weight and reliability.) I wouldn’t be surprised if they
end up with something like a PW PT-6 turboshaft and a cruise Mach number of about 0.5 (around 330 kts.) In any case, the wing airfoil section, aspect
ratio, and sweep angle all indicate a decidedly subsonic cruise speed.
Once you’re committed to a turboprop-fixed-wing combination to satisfy the range and endurance requirements, then it seems logical to accommodate
the VTOL requirement by using the same turboprop-fixed-wing combination for hover, if you can. You always want to minimize the number of moving
parts. Accommodating VTOL requires two additional factors above and beyond the basics required for horizontal flight. First, the engine has to be
sized bigger for hover rather than for forward flight; this amounts to approximately doubling the shaft power that would otherwise be required.
Second, you have to provide some additional actuator to counteract the propeller shaft torque when hovering—so you don’t spin in place.
The requirement for an overpowered engine also favors a turboshaft over a reciprocating engine, considering the weight difference.
The requirement for counteracting the propeller shaft torque in hover favors a propeller of some sort since that will be much more efficient than a
jet. You could mount an additional propeller like the tail rotor on a helicopter, but that would then contribute aerodynamic drag in forward flight.
Both Aerovironment and Northrop opted to use contrarrotating propellers, instead. Aerovironment opted to place one each of the contrarrotating
propellers on each wing, and Northrop opted to place one in front of the other. The Northrop design has the advantage that a well designed
counterrotating prop pair can have better propulsive efficiency than a similar sized single prop. So again, every design choice contributes to range
Finally, there is the choice of conventional fuselage-and-wing planform vs an all-wing planform. Contrary to Northrop’s propaganda, an all-wing
design is NOT universally superior to a wing-and-fuselage design. An all-wing design CAN BE superior in some circumstances and inferior in others.
One problem with all-wing designs is that they can’t—as a general rule—sustain very high lift coefficients without including a tail and
elevator. The requirement for high lift coefficients is greatest during conventional, rolling takeoffs and landings. Since this design is strictly
VTOL, it doesn’t do rolling takeoffs and landings and therefore doesn’t necessarily NEED high lift coefficients. So, I will give Northrop the
benefit of the doubt on this one. Overall, a pretty intelligent design.