Aileron + Elevator + Rudder or Flap Mixing for F117?
 

I would like to know if it is possible to mix roll, pitch and yaw and how that might be done using on board mixers.

I have an F-117, built from a kit offered about 8 years ago by a fellow from a western state. I don't recall the name of his company. It has a 48 inch wingspread and power is a Super Tiger 51, in front, with tractor propeller. It's the plane the kids want to see fly at fun flys.

The problem I have is the design's "failure to rotate" on takeoff. The design relies on the ailerons for pitch control. They are in fact elevons, responding to both aileron and elevator input, via a v-tail mixer on board. However, the elevons are located only slightly behind the center of gravity, and even with full "up", both stick and trim, it takes at least 250 feet for liftoff, and the climb out is very shallow. This presents issues at some flying sites, for example, when a tall corn crop is growing at the end of the runway. In the air the elevons perform well both for roll and pitch.

I recalled that F-18s turn BOTH rudders inward during takeoff, to help them rotate. Because the fins are slanted, there is a pitch component to rudder deflection. (Link below)

I wonder then if I could also mix the elevator input to the rudder servos on my F117, and have the benefit of the pitch component from the slanted rudders? Or perhaps a better solution: Could I mix the flap channel to the rudder servos? The mixing would only be used on takeoff (or also landing if necessary), and that way I wouldn't affect the present good in flight performance.

I presume I would need the ability to reverse the flap input to one of the two rudder servos which currently move the rudders in the same direction. (Thinking about it, I presume that would also have to be done in mixing elevator to rudder.)

I guess this raises the question of whether or not it is possible to mix three inputs, and if so, what would the wiring arrangement be?

I would prefer to use and on board mixer for this purpose, though, by changing transmitter and receiver, I could have flap elevator mixing via the radio.

F-18s rudders inward:
http://theaviationist.com/2009/10/25...ring-take-off/

Edit: The link isn't working: Here is the comment from a post on Airliners.net:

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[TD]Quoting Brendows (Reply 3):
the rudders are put in this position to help the elevators with pushing the nose up on takeoff.[/TD]
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Brendows is correct. They create a lifting moment on the nose for takeoff. It you were to see the hornet in the picture above a split second later, you would have noticed that the rudders went back to their normal position. They only toe-in when you are on the ground and have the flaps down (1/2 or Full).

Most modern radios would have mixes that allow you to select the separate channels and mix volumes. That being said, I'm not sure it would be the correct thing to do. Moving both rudders in is going to act like a speed break and may do more harm then good. I would suggest taking a good look at the CG and see if you can move it back any farther. The 117 is basically a flying wing anyways. The next thing that could help is to lengthen the nose strut so the airplane sits at a positive AOA.

As speed said most modern Tx's should be able to mix it. If it were me I would set it up on a toggle switch to operate similar to a flap function. If you think the rudders need to function the same on landing as take-off then you could literally set them up with the flaps.

I agree on all three points. I have already played with the C/G and gear length. My thots re the extra drag are this: This plane has plenty of power during takeoff, but the speed is still slow compared to speed in flight. If I turn off the "ruddervator" mixing shortly after take off, I should be okay and the current good in flight performance would not be affected.

I think I could limit the ruddervator mixing by use of the flap channel instead of elevator. Once airborne I would "retract the flaps", that is, the rudders would be driven only by the rudder channel. Wiring would look like this:
From the receiver, rudder output to Y; one branch of Y input to first mixer, second branch to second mixer
flap output to a Y: one branch of Y input to first mixer, second branch to servo reverser; output from reverser to second mixer

first mixer output to left rudder servo, second mixer output to right rudder servo

While I think that would give me the desired ruddervator control when wanted, I wonder if the effect would significantly affect rotation. To evaluate that I measured the size of the control surfaces and their distance back of the C/G. (The elevons are, on average, 18 inches behind C/G, the rudders 31 inches). Then I multiplied the area in square inches by the distance in inches. The total "moment" (area x distance) for the elevons was 1197. The same figure for the ruddervators was 1023, but that had to be reduced because the rudders are at a slant, approximately 42 degrees above horizontal. The cosine of a 42 degree angle is .743, so the total "up/down" moment would be 760 (1023 x .743).

I reason then that pitching effect of the ruddervators would be (760/1197) 63% of the pitching effect of the elevons for the same deflection.

But, I have a lot of "up" in the elevons, with the trim also thrown. I doubt I'll get as much deflection on the ruddervators, but if I only got another 25% pitching effect, I think that would be very significant.

I am sure that the mix of ruddrer that you propose would definitely add some pitch authority. But just for giggles there is one other approach that will work that has not been mentioned provided you have fixed gear and that you did not mention.

Changing the aircrafts CG is not a reasonable option in my mind and that's the last thing you want to do. On the other hand moving the main gears footprint forward is a reasonable thing to do if they are non retractible. Anyway I have in some airplanes I have fabricated a simple leading link or trailing link to move the footprint to where it was needed. I suspect an one inch forward move would offer you an airplane of an entirely different character with good rotation and the same good in flight chacteristics perhaps without the need to remember which switch you needed or which flight condition or whatever.

Just a thought dunno

John

Good point, John. I like that idea. It currently has fixed gear, but I am considering changing to retracts..
Then again, there is still the matter of the very shallow climb out.
Regarding C/G; I favor stability, but after the first few flights I added some tail weight and liked the way it handled. Perhaps I should try a little more.
I see that I have almost a 45 degree deflection of the elevons for takeoff. My guess would be that increasing that will probably increase drag but not improve pitch effect.

[QUOTE=

Changing the aircrafts CG is not a reasonable option in my mind and that's the last thing you want to do.

Just a thought dunno

John[/QUOTE]

John, the reason I suggested the GC shift is that it is fairly clear to me the airplane is severely nose heavy. Navy's comments on post #6 support this even more.

Not trying to be a smart-butt or anything but has there been any thought to the idea that maybe it just does not have enough speed for take-off when you take-off?

I have watched many a jet look really bad at Top Gunn, Florida Jets, and monster planes simply for lack of speed at the moment of becoming airborne.
No matter the amount of deflection, airflow is still required. And you did say it flies well once up to speed.

Ace: I think that's a fair question. Without a speed gun, I can only say the plane weighs about 5 pounds. I fly off mowed grass. I increased tire diameter to 2.5 inch. The plans call for a .46 c.i.. I run the ST 51 on 15% nitro with a 10/6 master airscrew 2 blade. I don't recall a tach reading. It seems to be moving right along at lift off. It's not near stall or hanging on the prop.

I found the construction manual. The kit maker was Moto-tek. After stating the throws for the elevons (both high and low rates) he adds: "NOTE: Set the trim on elevons up 1/8" to 3/16" from center position to achieve a level flight"

I have the throws and trim set as recommended.

The plane is undergoing "maintenance" (okay....some repairs) so it will be next May before we fly again in northern Wisconsin.

+I've been thinking about this and indeed the climb out reminds me of a plane I flew years ago that was nose heavy. While I recall checking, and later rechecking the C/G on the 117, and that I had it at the suggested rearward limit, maybe I was wrong.
On the other hand, on landing, it comes in slow; It's a real floater, and loves to go nose up in a big way in ground effect. That was contrary to my experience with other tricycle gear aircraft, which were so easy to "plant" on the runway on landing.
I just did quick measurements to determine wing area. It's about 600 sq.in. excluding any part of the fuselage (tho I suspect there may be some lift generated even there). Assuming, with fuel, we are at 5 pounds, the wing loading is about 19-20 oz/sf.
Next spring I'll try again, and probably try adding some tail weight gradually to see if that solves the problem. I once flew a tail heavy Sukhoj 26 and surprised myself by getting it safely back to Earth, tho my heart rate was elevated to say the least.
I'll build in another hatch for easier access to the electronics in case I want to try the ruddervators.

A sequence of photos of an F-117 takeoff appear here: http://richard-seaman.com/Aircraft/A...117/index.html

It looks to me like the ruddervators are used during the takeoff while the elevons are at neutral.

How much up elevon deflection trim do you have to maintain level flight? If its much more than 1/8-3/16" your nose heavy.

Try lengthening your nose gear strut 1/4". If your plane sits on the ground with the nose down slightly, it will be hard to rotate.

Once in the air, trim for full power. Fly by in front of yourself and cut power to idle quickly. If the plane enters a gentle glide, your CG is good. If it noses down excessively, you are nose heavy.

Thanks everyone for the help! My enthusiasm for the Stealth has been renewed.