Why is the F16 fly by wire?
To enable super maneuverability. If you want to throw an aircraft around impossible corners, stability works against that. So they made it incredible unstable. Meaning it becomes impossible to handfly. I.e. a computer has to keep the aircraft stable by using lots of control inputs, and the pilot just tells the computer what he wants by using throttle and stick.
That’s why it has quadruple redundancy in the flight computers.
That’s why it has quadruple redundancy in the flight computers.
Maybe they got quadruple redundancy now, but the block 5 and block 15 I used to work with 30 years ago “only” had triple redundancy with a “voting” system. If all of them shuts down, all the moving surfaces go to neutral. This will keep the aircraft steady for a couple of seconds before it starts tumbling and disintegrate, if the speed is high enough (or so we were told, I don’t think it actually has happened, the disintegration).
That’s a comforting thought LeSving. I’m hoping Vans wasn’t counting on triple redundant synapses in the human body to maintain control of the RV… :)
Krister,
Because most of these are amateur build they can behave differently. As you suspect check by weighing the CG position. Then take the Vans construction and flight test manual and study and check the installation incidence of horizontal. Note and measure the trailing edge radius of elevator less than R 3/32 " . In the Final inspection and flight test section 15 is chapter Flight Test Procedure. There Vans tells “Factors which might counter pitch stability are aft CG loadings and elevator trailing edges with greater radius than the plans show”
The solutions for problems are first to be found from comprehensive material of Vans or to ask from Vans support. Neither my RV8 or CAFE foundation test aircrafts RV 6 or -8 have nothing like this behaviour although very sensitive on aft CG load but like my mind with normal loading. Contact the owner before anything goes wrong.
Thanks Matti,
The owner has done a weight check this evening so it is being looked into. I will mention the part about trailing edge radius because he did not build this one himself and it never hurts to check twice. He has also spoken with at least 6-7 different RV builders/pilots about the experience and it seems everyone agrees that the aft CG is one strong factor. The tailwheel versions apparently have a “problem” with CG and benefit from heavier engine installations, CS props etc. He owned a trigear RV previously and it had the opposite problem…
Someone within the EAA also mentioned that the center of lift moves as speed increases to the point that it causes some additional destabilizing effects… Sounds reassuring…
Adding all the known factors and behaviors together I feel confident that we’ve found the main problem and that it can be rectified.
Latest update on the RV6 handling issue.
We located the reason for the unpleasant behavior to the rigging of the flaps. After upgrading to electric flaps these were readjusted to a position slightly higher than before, at the trailing edge, in the clean configuration. This appears to have caused a slight negative flap setting and altered the wing’s behavior substantially.
During the test flight today we noticed a much more stable aircraft, like it was last year, and normal behavior in my point of view. Now it is light and responsive on the controls as it should be and no longer wants to throw the occupants through the canopy if you sneeze. Very delightful.
The main point here is that we actually found a reason for the behavior with the only drawback of the correct rigging being a loss of almost 10kts in cruise speed at a given rpm… :)
Thanks for the tips everyone!
After upgrading to electric flaps these were readjusted to a position slightly higher than before, at the trailing edge, in the clean configuration. This appears to have caused a slight negative flap setting and altered the wing’s behavior substantially.
This does sound a bit “far out” in my opinion. You (or someone) changed the neutral flap position, and forgot about it? no testing or anything?
Well, not quite. I don’t do anything with the aircraft except ride along.
The rigging change was introduced with the flap upgrade so it was known and discussed, however it was thought to be a correction for a previous misalignment..
I’ve flown some aircraft designed with negative flap and clearly the RV does not take well to that particular tweak whether intentional or not.
Testing? That’s what we did wouldn’t you say?
It sounded no one knew why the aircraft behaved strange, yet “someone” had purposely misaligned the flaps and discussed about it. The manual of my RV-4 includes a procedure for aligning flaps and aileron, and also includes a “foil” in wood for proper alignment.
LeSving,
I didn’t build the thing so I don’t know the details of the RV series. Let’s just say the flaps were not correct (apparently) although it wasn’t something you would catch visually unless you knew exactly what to look for and had the manuals at hand – perhaps the drawings as well. When I say discussed I mean to say that we talked about the upgrade and the fact that the flaps had been readjusted to be correct (or so it was believed).
We did not know why it behaved strange. Remember though that besides the flap there were some additional changes including a lighter starter and tailwheel that we first believed had caused a shift in CG to where it was too tail heavy. The flap adjustment was believed to be correct. After weighing the aircraft and realizing that it was indeed well within the CG range attention was turned to the only remaining change – the flap.
I would not have guessed that the slight change in neutral flap would have such a dramatic effect, but then I’m not a rocket scientist or aerodynamics expert as you may be.
