I have read some of the comments about the R22 helicopter printed in this forum. Most were favorable and I appreciated that. However, some were obviously misinformed, and I will comment on several of those.
SIDESLIP WITH THE R22
Concerning the caution against excessive sideslips in the R22 flight manual, this was in part due to a misunderstanding by the FAA. In the Army training film on mast bumping, it showed excessive side slipping as one cause of mast bumping. This was true for the Army Bell Cobras and Hueys, because both aircraft have high centers-of-gravity and relatively low side silhouettes due to their high-mounted powerplants and low-mounted tailcones. During a severe sideslip, the resultant fuselage drag could be below the center-of-gravity and cause the helicopter to roll out of a turn, instead of into the turn, i.e. a negative dihedral or adverse roll characteristic. Airplanes prevent this by having wings with positive dihedral.
The basic R22s and R44s have low-mounted engines, high tailcones, and aerodynamic mast fairings. Consequently, neither the basic R22 or R44 had any tendency toward adverse roll during FAA certification. However, all helicopters (including the R22 and R44) tend to have an adverse roll characteristic when they are equipped with inflated floats, because the floats move the side silhouette area down considerably. For that reason, I did not object to the caution in the R22 flight manual against extreme sideslips during forward flight.
R22 FAA TYPE CERTIFICATION
During the R22 certification, both the FAA test pilots and our own company test pilots flew the R22 through all required maneuvers and flight regimes, and it met all of the FAA regulations. No exemptions were issued for the R22 by the FAA during its certification. Also, I was not a DER (designated engineering representative) during the FAA certification of the R22. No DERs were used during its original certification. After it was certified, the FAA appointed me as a DER with limited authority, so I could approve some minor design changes which commonly occur during production of a new aircraft.
R22 ROTOR SYSTEM
I have read various explanations in this forum attempting to explain the dynamic and aerodynamic characteristics of the R22 rotor system, especially the 18-degree delta-three angle designed into the R22 swashplate and rotor hub. This is a highly technical subject which can only be fully explained using very technical engineering terms. However, since there appear to be a number of misconceptions and a great deal of interest by some pilots and mechanics, the following is a physical explanation of the reasons for the 18 degree delta-three phase angle.
First, keep in mind that the 18 degrees is only in the upper rotating half of the swashplate. The lower non-rotating swashplate is aligned with the aircraft centerline and always tilts in the same direction as the cyclic stick.
Many helicopter engineers have difficulty understanding how delta-three (pitch-flap coupling) affects the phase relationship between the rotor disc and the swashplate. Delta-three only affects the phasing when the rotor disc is not parallel to the swashplate and there is one-per-rev aerodynamic feathering of the blades. For instance, feathering occurs while the rotor disc is being tilted, because an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor. But once the rotor disc stops tilting, the rotor disc and swashplate again become parallel and the delta-three has no effect on the phasing. Aerodynamic feathering also occurs in forward flight, because it is necessary to compensate for the difference in airspeed between the advancing and retreating blades. Otherwise the advancing blade would climb, the retreating blade would dive, and the rotor disc would tilt aft.
The R22 rotor system was designed with 18 degrees of delta-three to eliminate two minor undesirable characteristics of rotor systems having 90-degree pitch links. In a steady no-wind hover, when forward cyclic pitch is applied, the 90-degree rotor disc will end up tilted in the forward direction, but if no lateral cyclic is applied, the rotor disc will have some lateral tilt while the rotor disc is tilting forward, sometimes referred to as “wee-wa.” This occurs because while the rotor disc is tilting, the forward blade has a downward velocity and the aft blade has an upward velocity. This increases the angle-of-attack of the forward blade causing it to climb, and reduces the angle-of-attack of the aft blade causing it to dive. If no lateral cyclic was applied, this would result in a rotor disc tilt to the right while the rotor plane was tilting forward. Pilots subconsciously learn to compensate for this by applying some lateral cyclic as the cyclic is being moved forward. The amount of delta-three required to eliminate “wee-wa” in the R22 rotor system was calculated to be 19 degrees.
The other undesirable characteristic in rotor systems having 90-degree pitch links is the lateral stick travel required with airspeed changes during forward flight at higher airspeeds. The ideal rotor control system would require only longitudinal stick travel to increase or decrease the airspeed. This is not possible with a 90-degree pitch link system, because the rotor coning angle causes the rotor disc to roll right as the airspeed increases. This occurs because the up-coning angle of the forward blade increases that blade’s angle-of-attack with increased airspeed, while the up-coning angle of the aft blade reduces its angle-of-attack. Consequently, the forward blade then climbs while the aft blade dives, thus causing the rotor disc to roll right with increased airspeed. To compensate for this with a 90-degree pitch link rotor, the pilot must apply some left lateral cyclic as the airspeed increases. The amount of delta-three required to compensate for this effect in the R22 rotor system was calculated to be 17 degrees.
A delta three angle of 18 degrees was selected as the best compromise angle to reduce or eliminate the two undesirable characteristics described above, which would have been present in the R22 had a 90-degree pitch link design been used. Subsequent instrumented flight test data confirmed the choice of the 18-degree delta-three angle.
Hopefully, this will help clarify a few of the misconceptions concerning the design of the R22.
Frank Robinson