pitch moments, gee and tranducers
I welcome the "fresh blood" to the fray. Attaboy.
- Although the pitch moment chart I posted is for another aircraft, it illustrates two points ( one positive, one negative) that result in little or no pitch control authority for the existing HS/elevators/stabilators in either positive or negative direction.
My main point presenting that chart was to counter all the folks that think a deep stall is only a concern for the T-tail designs. I also wanted to show the c.g. of the Viper, and point out that under some instances the 'bus has a very aft c.g. The reduced trim drag of the main wing is reduced if we can fly with a more neutral longitudinal stability. Simple, really, as the HS doesn't have to induce a nose up moment, and can basically "float". The big savings are the reduced AoA of the main wing and associated reduced induced drag.
In my case, we had a great cruise drag reduction, but we also had the HS and the wing contributing to lift when in a turning fight. Yeah, I can just see the 'bus in a turning fight with a Sopwith Camel, heh heh. Except for the inital HS movement, our HS (stabilators) usually was "limiting" the nose up tendency. So when we got to the deep stall scenario, out HS were commanded full nose down. Turned out there was a small AoA/c.g. range that allowed the jet to settle into a classic deep stall.
- The gee and roll rate are two physical phenomena that pilots can sense instantly ( as opposed to the drone pilots at Creech AFB and other places). We tried pitch rate for the longitudinal axis, but seemed to most pilots that the gee was easier to sense and control. Rates are still involved to prevent overshoots of pitch commands and to "smooth things out" for the SLF's. No big deal.
'bus takes into account actual pitch attitude, so at a 30 deg climb angle you get a 0.87 gee command, not one gee. With a constant one gee command you would continue to pitch up once a few degrees greater than level. Because we weren't worried about the SLF's, we normally let the basic one gee trim setting alone and just manhandled the sucker. But many pilots would trim for zero gee and simply hold back pressure. Would be willing to bet that the Thunderbird slot pilot trims for slightly less than one gee, and prolly the wingies.
The Viper concept was that we flew around in something close to the 'bus "direct law", but we could never command actual physical movement of the control surfaces. Only exceptions were rolling down the runway and if we wound up in a deep stall with AoA above 30 degrees. Even then, we had no rudder control, as HAL cut us off, heh heh.
All of our "sub-laws" were due to things like gear down while airborne, special setting for carrying external ordnance and "standby gains" in case we lost air data. Interesting, that last feature, ya think?
- Our stick employed piezo-electric transducers - 4 of them for pitch and roll, Rudder pedals moved one half an inch and were linear voltage tranducers. After a few jets, we had 1/8 inch of movement and some springs or whatever could be felt. Initial jets had zero stick movement - all pressures.
As with every fighter built from the mid-fifties, there was no feedback from the control surface pressures. Some jets had bellows to provide a "stiff" stick at high speeds. Some had bobweights to help pulling too many gees too quickly. But zero actual physical feedback. The feedback was from your butt and inner ear.