Topgun2mo

My Feedback: (45)

Can anyone point me to some resources to assist me in designing a flying stab, especially the pivot point relation to the MAC? Any other helpful tidbits? I am designing a sport T-38 with a stabilator and wish to fasten it to a vertical fuse bulkhead.

Thanks

Mike

dribbe

Here is my 2 cents..... I am sure someone will disagree!

I think for modeling apps, 25% MAC and statically balanced is a great place to start.

(You could probably go back a couple percent. I think the servo loads will get high if you go forward very far)

David

Topgun2mo

My Feedback: (45)

Thanks David...anyone with any ideas on homebuilt stabilator mechanisms or anyone have some experience with an off of the shelf version that could handle a 12 lb plane @ 150 mph? I do want to bolt it to one of the rear fuse bulkheads.

Thanks

Mike

Kevin_W

My Feedback: (1)

I redesigned my Interceptor to a full flying stab. If I remember correctly I put the pivot at about 22% MAC, and they were statically balanced.
I came up with the 22% number because it was an average between what the stabs on my Yellow F-16C and F/A-18 were using, not rocket sience but it worked out well.

I built the mechanics myself, it consisted of 7 basic parts;
Each stab had a 1/4" aluminum (2024 T-6) pivot rod
A Delrin bushing on each side (1/4"id x 3/8" od, with a flange to the inside).
The bushings are each held in an aluminum bearing block in the fuse, these are actually just very short and thick tubes (3/8" id x about 3/4" od x about 1/2" wide).
A control horn the fit tightly between the flanges of the two bushings. The pivot rods each fit halfway into this horn and met in the middle. There are set screws that tighten down onto a flat spot on each of the pivot rods, this holds them in place and keeps them aligned.
I cut holes slightly oversize in the fuse just behind the rear former (which had to be relocated just slightly to be in the correct location), then I made a jig that would keep the bushings (along with the bearing blocks) aligned on the pivot rods and hold everything that way until the glue cured.
The pivot blocks are glued to the fiberglass fuselage and the rear former using a thick mixture of laminating resin, chopped carbon fiber and cabosil.

This worked out very well, the stabs are easily removed for transport by simply loosening the two set screws, and the only parts that cannot be replaced if damaged are the two bearing block (but if you damage those I doubt there will be much left of the rest of the airplane).
If you have some basic machining skills (mine are VERY basic) you can make the parts yourself, or you might be able to find part from another manufacturer and design your system around those.
If I remember correctly Air Majic has some nice stab hardware that they sell seperately.

Topgun2mo

My Feedback: (45)

Thanks Kevin,

Is there any chance at getting a picture of your setup?

Mike

Kevin_W

My Feedback: (1)

Not really, unless you can find the one of the guys who now own either of the Interceptors that I made these mods to.
I flew the first one for about a year and a half, then sold it in 1998 when I upgraded from the FD3-67 to the P-80. I flew the second one from 1998 until just a few months ago, and it was recently shipped to a buyer in Michigan.

Cowdog

My Feedback: (13)

Kevin (or others),

I know when you had your F-18 you probably saw the note from yellow not to statically balance the stabilitor or severe flutter would be induced. I'm curious as to why that stab shouldn't be balanced while your interceptor and others are balanced..

Does it have to do with the placement of the pivot point?

Regards

Hank

1Eye

My Feedback: (2)

Kevin wrote ...
A Delrin bushing on each side (1/4"id x 3/8" od, with a flange to the inside).
>>>

I've used the setup Kevin described but stepped up for precision RC car bearings instead of bushings. The ones I used have a 1/4" ID with a flanged 3/8" OD. The ID fit with the 1/4" OD al aly rod was about as slop free as you can get. A search of the Tower Hobbies web site (LX3284) will turn up exactly what you need.

Mike

Kevin_W

My Feedback: (1)

Mike,

I actually used those same bearings in my first Interceptor.
You are correct the bearings will give a good slop free setup, but I found that they are very intollerant of any mis-alignment or loose tolerances in the bearing blocks.

After about 75 flights on my first interceptor I started to notice slop in the stabs . Upon closer inspection I could see that the bearings were moving around inside the bearing blocks, either the tolerances were not tight enough to begin with (my poor machining skills), or there was some strange harmonic during flight that was causing the outer race of the bearing to wallow out the ID of the bearing block.
When I changed to the Delrin bushings it eliminated this problem.

mr_matt

My Feedback: (10)

My guess is that marginally aerodynamically stable stab would be made worse by balancing, as the polar moment of inertia would be increased.

Topgun2mo

My Feedback: (45)

Kevin,

It appears that you have flown the same aircraft, the interceptor, with both a stab and elevator and again with the stabilator. How do the control throws relate to each other? Is the stabilator more sensitive?

Mike

Topgun2mo

My Feedback: (45)

So I take it that the control horn actually acts as a joiner of the two torque rods? What have you made that out of?

Thanks

Mike

Kevin_W

My Feedback: (1)

I have actually built a total of three Interceptors The first one was ducted fan and originally had fixed stabs and elevators. I retrofitted the full flying stabs later because I knew I would be installing a turbine in the next one and the elevator linkages were in the flow of the exhaust (HOT), and wanted to test it on a proven airplane before adding my first turbine to the equation.
The full flying stab proved to be much smoother and more predictable than the elevators were. With standard elevators the airplane did not want to stay trimmed in pitch, it was always diverging from straight and level flight (somewhat airspeed dependant). With the full flying stab it was much smoother and controllable. The full flying stab required a little more throw than the elevators.

You are correct, the horn joins the pivot rods (and stabs).
I made it from aluminum (either 2024 or 6061, whatever I had handy at the time).

Topgun2mo

My Feedback: (45)

Kevin,

Did you drill the outboard portions of the torque rod and insert a pin to serve as an anti rotation device to keep the stab from rotating on the torque rod?

Thanks

Mike

Kevin_W

My Feedback: (1)

The pivot rods go into hard balsa blocks in the stabs and are not pinned, I used a cut-off wheel to cut a series of diagonal grooves all around the part of the pivot rod that is embedded in the stab, then I simply glued it into the block using 30 minute epoxy and microballoons (Aeropoxy would be even better).