George,
Full disclosure, I am not an engineer. If it is pointed out that my
generalized comments are incorrect, I'll gladly take them back. My whole intent in this thread is simply to caution against thinking that our aircraft can simply be made to go faster, and faster without careful consideration of the margins of safety that were engineered into them.
As Paul Romano mentioned in the post above yours, we are gathering info related to the operation of our aircraft with the end goal of learning how/what/where/when/why are we occasionally seeing cracks, or other problems. We welcome discussion and feedback and invite F1, F4, and other Rocketeers to visit our display at Oshkosh this year to chat with us. Space 645, just East of the forum buildings, and just SW of the Van's display.
So, your question was:
I am curious about this. Should not the downforce load on the tail be the same regardless of airspeed at the same G? If you pull 6 G’s at 160 knots is that less load than 6 G’s at 200 knots?
The pilot will feel the same load, but various parts of your aircraft may experience differing amounts of those Gs. This thread began with a question about VNe and tailfeathers. Since there have been few, if any, wings or engines falling off, we'll concentrate on the tails. There have been incidents related to the tails, but thankfully, no accidents... that I'm aware of (for the F1s or F4s).
Large transport, and other, aircraft typically have a moveable HS. The HS is adjusted to trim the aircraft at various speeds, loading, etc. (or they can move fuel to adjust CG, or other magic tricks, to maximize efficiency). In theory, the loads on their properly trimmed HS should remain relatively low, i.e. only as much load as is required to maintain whatever flight condition they are in. Generally speaking.
We are limited in what we can do here, as the incidence of our HS is fixed. Aerodynamic loads react on it depending on airspeed, although AoA of the main wing, CG, aircraft weight, etc. all have an effect. When we trim our aircraft, it is by moving the elevator, not by changing the HS incidence. Therefore, in general, the HS sees the aerodynamic lowest loads at low speed/high AoA, i.e. landing, and sees the highest loads at high airspeed, although elevator position certainly does transfer load into the HS rear spar.
Generally speaking:
- no one is breaking anything at low airspeed, but we have seen cracks, and a few incidents at higher speeds.*
- Flutter, while a bad thing, isn't necessary to break something.
- Since there is already down force on the HS, at higher speeds pulling on the stick just adds to that downforce. This is why we're discussing that safety margin! At some point, there is no margin left to pull on the stick.
As I said earlier, We all want to go fast(er), but give that need for speed the respect it demands.
YMMV
*Consider the various service bulletins related to cracking that have been issued by Vans, and others.