1

Golden Rules Known and Not Known

• Every fixed-wing pilot knows that it takes more power to go slower (safely) when flying “behind the power curve”, i.e., slower than some critical speed.

• What every rotorcraft pilot does NOT know is a similar rule for rotorcraft that states it takes more power to descend safely faster when flying “ahead the thrust curve”, i.e., flying with a rotor disc angle of attack greater than some critical angle (safely is the key word here). (see slide 6)

2

What Happened to A/C 06-0031 and Why

1. Due to being late, high, and fast, the pilot of A/C 06-0031 elected to execute a tactical approach with a quick stop at the bottom. This is a common maneuver to avoid hostile fire (actual or anticipated) (Remember, this was the, or one of the, first combat mission)

2. The quick-stop maneuver, if performed poorly will end up with too high airspeed and too high rate of descent at the critical point just before landing

3. In this situation, the rotor disc angle of attack can be very high

4. The high rotor disc angle of attack translates to rotor conditions mimicking a much slower forward speed and much higher vertical speed. This places the helicopter rotor(s) into, or much closer to, the VRS domain envelope. (see slide 5)

5. Prior to landing, the crew would have computed the maximum power available and the power required for the landing conditions. However, at the conditions resulting from a poorly executed quick-stop, maximum available rotor thrust (as distinct from power) is significantly reduced (by as much as 30%) and the aircraft response to collective pitch is significantly reduced.

6. In a conventional helicopter, at this point the pilot will typically “droop” the rotor(s) (i.e., decrease the rotor RPM due to too asking for more power than the engines can deliver). This allows energy stored in the rotor to cushion the landing. Typically, a hard, but safe landing results.

7. In V-22 all the above is the same, but the rotor cannot droop as the flight control computer will remove collective pitch in order to maintain rotor RPM. This will result in a very hard, and usually fatal touchdown.

3

Figure 1:Tactical Approach with Quick-Stop Maneuver

Relative Wind

Critical phase of maneuver:At this point, rotor disc angle of attack is high with airspeed still high. In this flight condition, rotor trust is decreased by back-flow through the rotor. Typical pilot error is to end up at this point with too much airspeed and too much rate of descent. In most helicopters, the pilot will droop the rotor speed in attempting to make a safe landing. In V-22, this is much more dangerous as the rotors have no energy stored, and the flight control computer will remove engine power in order to maintain rotor speed.

4

Figure 2:A/C 06-0031 Impact Point Rotor-Flow Geometry

58 kts 1800 FPM

VT = 47 kts

14º

35º21º

VA = 3800 FPM

35º

Pilot (and accident board) Saw This

Rotor Saw This

5

Figure 3:Test Data and A/C 06-0031 Position on VRS Plane

NON-DIMENSIONAL HORIZONTAL VELOCITY

NO

N-D

IME

NS

ION

AL

VE

RT

ICA

L V

EL

OC

ITY

×

×

Accident investigation board states the aircraft was here at impact.

Actually, the aircraft was here at impact

6

Available Thrust Reduction Due to Rotor Backflow

Rotor Disc Angle of Attack

Ro

tor

Th

rust

Collective Pitch

Rotor Thrust vs. Rotor Angle of Attack at Fixed Power – NASA data

Notice rotor thrust decreases rapidly past 30 rotor disk AoA.

7

Some Comments for the Record

1. As stated in the Mishap Report, only a dual engine failure in the V-22 can result in low RPM of the prop-rotors.

2. The speculation that the mishap aircraft had a dual engine failure on short final is nothing short of idiotic.

3. Destroying the aircraft, and the data recorder, to protect the technology was stupidity of the highest order. What technology were we protecting? From whom? What Taliban scientists were ready to exploit this technology. Stupid is the only word applicable!

4. The estimates of rotor PRM and ground speed are suspect. This analysis was not released. I suspect there are problems with it and rotor RPM was normal and ground speed significantly lower. This analysis needs to be redone by independent, competent authority.

5. The level of ignorance on the subject of VRS, in both the USAF and USMC, is astonishing!

6. Pilots have no clue to what is going on, aerodynamically, in non-equilibrium situations such as the quick-stop maneuver. And, they are precluded from experiencing them by restrictive training rules.

7. Everything stated in these slides is elementary aerodynamics for rotorcraft and should be common knowledge for all rotorcraft pilots – it is not.

8. Until this becomes common knowledge, this accident will recur many times in a combat environment, as I have pointed out repeatedly for the past 10 years (to deaf ears). The alternative is to take the USMC road and use the V-22 as a “Combat Circulator” for which it is well suited.