Steve Sampson said:
What I dont understand is why the handling does not change massively when the aircraft enters a gust depending on the incidence of the HS.
Let's imagine two identical aircraft, except aircraft #1 has no spacer under the HS front spar, and aircraft #2 has a 1/16" thick spacer under the HS front spar. Both have the same weight and CG. These two aircraft are flying side-by-side in formation. Each pilot uses the trim to zero out the stick forces.
Both aircraft have the same wing design, and they are going the same speed, so the nose down moment from the wing is the same on each aircraft. The CG is ahead of the centre of lift of the wing, so the weight is also producing a nose down pitching moment that must be countered by the tail. Both aircraft have the same weight and CG, so the nose down moment from the weight is the same. The tail on each aircraft must generate the same amount of nose up pitching moment to counter the nose down moments from the wing and the weight.
The HS on the two aircraft are at different angles, but the pilot puts the elevator at the angle it needs to keep the aircraft steady, and he then uses trim to zero out the stick forces. The result is that while the elevator on aircraft #1 has its trailing edge further down than the one on aircraft #2, the nose up pitching moment generated by the HS + elevator on the two aircraft are identical.
Now, imagine that the two aircraft fly into a vertical gust. This changes the angle of attack of both the wing and the tail. The amount of lift change on the wing and the tail is proportional to the amount of the change in angle of attack. Both aircraft see the same change in angle of attack, so the change in lift on the wing and tail of the two aircraft are identical. Thus the two aircraft have identical response to the gust.
Steve Sampson said:
I take your point that in fact the empenage incidence is controlled by the hand holding the stick. So in that case why does the incidence matter?
The HS incidence is fixed, but the elevator angle is controlled by the pilot.
If the HS incidence is within a reasonable range small changes probably don't matter, in the big picture. The elevator deflection needed for any given flight condition will depend on the HS incidence. For each flight condition, there will be a "magic" HS incidence that will result in zero elevator angle, and thus the minimum drag.
If we did a series of flight tests with different HS incidence settings, we would see the following results:
If we put spacers under the HS front spar, for any given flight condition the elevator would be more trailing edge up. This will require the trim tab to be more trailing edge down. There will be some minimum speed at which the trim will reach full travel. This minimum speed will increase as the HS incidence increases. We can still fly slower than this, but we'll have to hold aft stick force. If we keep putting thicker and thicker spacers under the HS front spar, eventually we'll reach a point where we don't have enough up elevator left to properly flare the aircraft at forward CG.
If we could lower the HS front spar, we would find that at high speed the elevator would need to be more trailing edge down, and the trim tab would need to be trailing edge up. There would be some maximum speed at which we would run out of trim. We could still fly faster, but we would need to hold forward stick. The maximum speed at which we could trim would decrease as we lowered the HS front spar. We could address this by reworking the trim system, to change the range of angles that the tab could move through. If we did stall and spin flight tests at aft CG we would find that the ability to push the nose down would be degraded as we lowered the HS front spar. At some point, if the HS front spar was lowered far enough, we would have problems with stall and/or spin recovery.
I have described handling problems that could occur if the HS incidence was changed too much. But, there should be a fairly large range between those two extreme HS incidence angles. Any HS setting in between those limits should result in acceptable handling, but it may be necessary to change the range of motion of the trim tab to optimize the range of conditions where the aircraft can be trimmed.
