Originally Posted by burkefj

Bob, it looks like your up trim is just an artifact of offset thrust line above the wing..do you think it was due to something else?

Frank

Thats DOWN elevator trim on the stab. Thrust line is about through the 3d CG, or maybe a bit above it, so thats a pitch down from thrust also. The wing is a bit positive incidence on the fuselage, but the stab is even more positive.

I never tried to figure out why it needed so much down, but both of mine did.

Bob

Originally Posted by X30PILOT

I’m sorry to hear it had an untimely demise.

Yeah, but it was a total adrenaline rush to fly it, so I am not complaining!

The snap/spin incident happend on downwind leg for landing and was so fast that nobody watching had any idea what happened. The model was flying fine and then it seemed like it was instantly a hole in the ground. But, someone was shooting a movie of it (i.e. photographic film type movie) and going through that frame by frame showed a classic stall and spin. It was something like the 10th flight on the true scale model. I had about 70 flights on my boxy boilerplate model.

I thought about fixing it, but the fuselage was molded Kevlar, and had totally shredded when it hit.

One of the issues we had was trying to find good documentation.

I have some pretty good stuff. LOTS of photos. No good drawings. Jay Millers X Planes book is pretty good, but scarce and $$ now

http://www.amazon.com/The-X-Planes-X.../dp/1857801091

The other issue with documentation is that the planes got modified a LOT over time, and I never got enough to pin down the configuration on a given flight. (which makes it kind of rough for spacemodeling scale contests, where most of the full scale birds flew exactly once!). In 1978, some of the X-2 stuff was still classified, and I managed to get it declassified (it was past due). I am not sure where my stuff went, but its all in one binder still. There are quite a few black and white 8x10 photographic prints. Ok, I just found it. Its a binder about an inch and a half thick, mostly the photos, but also a rather long magazine article by Jay Miller. I even found my original plans.

I ended up doing all my own drawings, and also did all my own markings.

Where are you located? I have loaned out scale data in the past and had it never come back, so I am a bit paranoid about loaning it out. Photocopies would be easy to do, or maybe I could convince my son to scan it all.

You can see from the CAD wing drawings that Mike Oser produced; the airfoil we selected is not scale.

That will help. The scale biconvex airfoil is horrible subsonic at full scale and even worse at low Reynolds numbers on a model. I would still do the scale wing fences. Washout would also help on that wing, but you are past that point already.

Unlike the full sized aircraft I am going to use elevators with a fixed stab. Getting a full flying tail working well on this model considering its size and speed looked problematic.

I agree.. given the scale and speed of what you are doing, full flying tail is risky. My concern would be having enough elevator throw to trim the boost. I was really surprised at how much down mine needed. Like REALLY REALLY surprised the first time the first one left the launcher!

I did test using some up aileron for glide trim and this worked well. It also creates a little washout at the tip to help prevent tip stall issues.

Reflexing ailerons does not help anywhere near as much as actually twisting the wing. The reflex cuts the lift at the tip, but the negative flap deflection cuts the lift the airfoil can make, and the stall angle actually does not change that much. Sometimes it even gets worse (depends on planform and airfoil etc).

Anyway, please, please put the fences on it! It may not help much with that airfoil, but it might help a LOT.

Bob

Bob,
Consider tossing the binder in your bag if you make it to the flyoffs. I'd love to take it to kinko's.

kj

Ah, ok, it was hard to tell from the photos what the stab angle was.....That's even stranger that it needed down trim.....especially with the thrustline of the motors. Wing incidence must be a major contributor..

Frank

Originally Posted by parky

Thats DOWN elevator trim on the stab. Thrust line is about through the 3d CG, or maybe a bit above it, so thats a pitch down from thrust also. The wing is a bit positive incidence on the fuselage, but the stab is even more positive.

I never tried to figure out why it needed so much down, but both of mine did.

Bob

Originally Posted by burkefj

Ah, ok, it was hard to tell from the photos what the stab angle was.....That's even stranger that it needed down trim.....especially with the thrustline of the motors. Wing incidence must be a major contributor..

Frank

For a straight vertical boost, you want to get the wing trimmed at zero lift while glide you want trim at some positive lift. So boost will always be a bit of down compared to glide. With fully symmetric airfoils like the scale X-2, I would expect that it would get zero lift trim with the stab incidence aligned with the wing incidence. My X-2 models both needed several degrees more down trim than that.

Bob

Originally Posted by kmj73us

Bob,
Consider tossing the binder in your bag if you make it to the flyoffs. I'd love to take it to kinko's.

kj

Sorry, no intent at all to go to flyoffs or NARAM. I am doing a bit of tube model sport flying, but most of my hobby time is going into RC airplanes now. (current project is welding up a stainless steel tailpipe for a turbine powered F-117)

Bob

Quote:
Originally Posted by burkefj
Ah, ok, it was hard to tell from the photos what the stab angle was.....That's even stranger that it needed down trim.....especially with the thrustline of the motors. Wing incidence must be a major contributor..

Frank
For a straight vertical boost, you want to get the wing trimmed at zero lift while glide you want trim at some positive lift. So boost will always be a bit of down compared to glide. With fully symmetric airfoils like the scale X-2, I would expect that it would get zero lift trim with the stab incidence aligned with the wing incidence. My X-2 models both needed several degrees more down trim than that.

Bob

With the wing tips far aft of the C/G, any washout would also make it pitch up.

X30

I completed glassing the fuselage and that was quite a project.
The main skid installation has been completed as well.
My next task is to cut all of the control surfaces free and hinge them.
The current weight for the entire aircraft with only the fuselage glassed, the main skid / nose wheel retracts and nose cone is 22.7 pounds.

X30

The build continues.
I’ve completed the front canopy fairing and have separated the ailerons from the wing.
Now I’ll start the process of building the leading edges of the ailerons and the trailing edges of the wing to form a good seal to prevent flutter.

X30

"Now I’ll start the process of building the leading edges of the ailerons and the trailing edges of the wing to form a good seal to prevent flutter."

Please document this, I'm interested in how you go about it.


Richard

Originally Posted by X30PILOT

The build continues.
Now I’ll start the process of building the leading edges of the ailerons and the trailing edges of the wing to form a good seal to prevent flutter.

X30

My day job is as an aeronautical engineer. I am by no means a specialist on flutter, but in my experience, NONE of the professional literature discusses sealing the hinge line. I think thats really a modeling urban legend that just keeps propagating.

Stiffness is critical, both torsion and bending. And you need slop free linkages, and good servos (good digitals are a big help, since they deflect less for a given load). And beyond that, mass balancing the surface (but there are good ways to do that and very very bad ways to do that).

So, seal the hinges if you want, but dont scrimp on the things that really matter.

Bob Parks

I took two more flights on the small X2 today.
Now I have about a dozen flights on this thing and it flies very well.
The model is set up 0-0 with no thrust or wing / stabilizer offsets.
Today I got to use all of the features that I have installed. I’m using the ailerons for pitch trim and “pitch boost”. This was the first time I got to try out the Eagletree Guardian system. In addition to providing control dampening the Guardian has the ability to self-level the X2 from an unusual attitude. With this system on, every time I rolled into a bank and let go of the stick the model would roll wings level.
Even though this is a three-axis system I’m currently using it in the roll axis only.
A short video of the flight control system can be seen here. I’ll take a few more flights with the electric power plant and then remove it to make some rocket powered flights.

X30

X2 control demo (2 min 21 sec)

Be very careful with something like the Guardian on boost. The higher the airspeed, the higher the effective gain, and it goes up as the square of the airspeed.

The gain you want for peak boost speed probably will have almost no effect at glide speed.

Bob

Be very careful with something like the Guardian on boost.

Hi Bob,

Good advice, that’s why I’m doing all this testing. In addition to the three modes I can change in flight. (Off, 2D and 3D) I can also change the gain settings on the fly as well. A friend of mine will be flying this system in a good-sized rocket.
He will record how it reacts on boost. I’m not just concerned how the Guardian handles the boost but want to make sure that the sudden sustained acceleration doesn’t have any residual effects on 2D flight.
I’m running duel aileron and elevator servos. While the Guardian supports duel ailerons, it does not support duel elevator servos. Right now I’m using it in roll only and potentially roll and yaw in the big X2.
From the testing I’ll decide if and how to employ this system during a flight.
Right now the little overweight X2 flies very well. I‘m considering leaving the Guardian off during the flight unless I run into an issue but I do feel this little device may have some great applications in R/Gs

Thanks

X30

I finally got someone to capture the little X-2 in flight with my camcorder.
The auto focus has an issue keeping track of this thing so I edited out the worst parts and made a short video you can see here. This will give you an idea how the little X-2 flies.
I’ll take just a few more flights with the electric power system, then remove it and replace it with a rocket motor.

X30

Small X-2 flight 4-1-13 (1 min 40 sec)