Determine by iterative flight testing
Determine by iterative flight testing
It would be fairly difficult to compute the HS parameters for any gyro. As mentioned, many gyro configuration and HS factors go into the HS configuration.
However, it is fairly easy to tune a HS through iterative flight testing. The first step is to flight test the "Static Power Stability". Do this by flying level at best rate of climb airsped (or about 55 mph). Add power and, without cyclic pitch input, the airspeed (in the climb) should remain within about 10 mph of the original airspeed. Verify the airspeed remains within 10 mph of the original airspeed with full power applied. Then, do this same thing - starting at higher airspeed - 60 mph? Then a higher airspeed. It is better to have no airspeed change with power. You certainly want to avoid a trimmed airspeed INCREASE with INCREASED POWER - this indicates the CG is moving aft relative to the RTV at higher airspeeds - more buntover prone!
What this does is verifies the HS is properly balancing two things:
- Propeller thrustline offset
- Airframe pitch changes from airframe aerodynamic effects at different airspeeds
Then, you should verify that this configuration is both statically airspeed and G-Load stable:
Static Airspeed stability: Trimmed at any and all different airspeeds, verify that it requires a forward stick position AND pressure to increase airspeed. Verify that it requires aft stick position AND pressure to decrease airspeed. do this at all allowable airspeeds - not too fast though, you don't know yet if it is DYNAMICALY stable (PIO resistant). This test verifies that the HS is properly downloaded to move the CG forward of the (Rotor Thrust Vector) RTV and balances it.
Static G-Load stability: Trimmed at any and all different airspeeds, verify it requires aft stick pressure AND position to maintain the original airspeed in a banking spiral - about 30 degrees. Aft stick position and pressure to maintain the original airspeed in a bank - higher than 1G - verifies that the CG is properly forward of the RTV. This G-Load static stability is what minimizes the potential for a buntover.
Once you have configured the HS for Static Power Stability, especially if your propeller thrustline is a bit higher than the CG, the other two static stabilities are probably proper - but test them, it doesn't take but a couple of minutes.
What can you adjust on the HS? You can change it's size, it's location aft, it's location in the propstream, it's airfoil shape (airfoil is almost twice as effective as a flat plate!), and it's mounted angle of incidence. Generally, to mostly ensure very good results on a moderately high prop thrustline, start with as large HS as you can with a clean symetrical airfoil shape - more effective and less drag. Mount it as far back as possible on the tail. Mount it on the keel to start - probably will not have to change that vertical position. Allow for adjustments to the mounted angle of incidence. (Keel mounted HSs are bst to have a little dihedral to avoid scraping the ground in a crosswind landing.)
For most Rotax Bensen configurations with 60 inch props - a bit high prop thrustline, a HS stab similar to the Air Command 2-place tandem HS will reqire only level angle of incidence mounting. Start with that, then see how trimmed airspeed responds to changes in power. You should be able to fine tune the angle of incidence to minimize airspeed changes with power.
Standard Mac Bensens with smaller props and lower prop thrustlines and no enclosures have very little prop offset or airframe aerodynamic pitching moments to require a large HS for the static Power stability. However, you should still install a large effective HS - statically tuned for most prop offsets on Bensen similar configurations - providing both static stability and the all-important DYNAMIC stability. Without a HS, it would be very difficult for any aircraft to be dynamically stable enough to be strongly resistant to PIO.
- Greg Gremminger