I also helped a friend design a turbine F-20 years ago, and after many complex calculations and napking drawings we both decided to get rid of the flying stab and use a conventional elevator.. LOL. His plane is a rocket.


Back to the Mig, is also interesting to think how differently the mass balance will work with a non angled pivot.. maybe it would be better to not balance the stab at the scale location!

Wow! Thank you guys. In just one day we have more design ideas than the factory can come up with in a few years.

I would summarize this thread at the end of this week and come up with a couple of best configurations for the stabs based on your input.

Mike

If airex is used in CARF wings, then it is probably the R70.55 in roughly 1/8 to 3/16 thickness. I never knew what it was but I think that is it. The skin construction of the CARF Flash wing is very similar to the Bandit ARF wing. For the skin, basically structural foam sandwiched between 2 layers of glass. Weight of glass used I would guess totals 5oz maybe. The spar and rib structured is glued to this with a epoxy/micro balloons type mixture lined with carbon tow. The use of the epoxy/micro balloons adhesive is quite liberal. The Flash wing is so strong it will hold up to moderate/aggressive flying even if the adhesive is compromised (fractured) over, I would say 25% of the wing....

I don't know how the pivot angle affects things. I would balance it on the pivot axis. That way, any excitation vertically (like a gust) will not generate a torque on the pivot axis.

With the stabilator CG aft of the pivot (“unbalanced�), if a gust pushes plane up, this acceleration generates a torqueing moment that pulls the aft part of the stabilator down, and this adds to the “tail up� force of the gust. Trouble.

I think you are right.. balancing in the rotation should help (or at least it does´t harm) , presuming the pivot is in the right place.

As long as the moment of inertia does not get crazy high, then combined with some cheezy servo, it beats itself to death taxing around (I have seen that)

In my opinion the greatest killers of full flying stabs on jets or any aircraft, are flutter and poor control geometry. The large majority of stabs are engineered adequately, though occasionally lack a little in fabrication.

Having a wildly tail-heavy stab and relying purely on the servo and linkage to provide flutter protection is madness. The general rule for a full flying stab is that the surface must be mass balanced with the CG ahead of the pivot, and the pivot ahead of the aerodynamic center. This greatly reduces the load on everything; from bouncing around during transport, to flight loads.

-Airex is fine (or Herex, Divinycell etc.)
-Titanium also fine, though perhaps overkill.
-Anti-rotation pins would benefit from a little greater diameter than 2.5mm.
-Carbon fiber will add stiffness and cost, depending on the glass fabric schedule may not be needed.
-Balsa or foam spars are fine if correctly sized and installed.

CRG

How light is this stuff you guys use? Is it 3-4 lb density or less? I am interested in 1 mm or 1.5 mm only

Yes if I you can increase your order by a 1/3, we can split the shipping accordingly and both us can save. What's the price on each sheet?

Its as light or lighter as a similar sized piece of balsa.

The sheet cost depends in which strength version you want. For c70.75 1.2mm thick 24"x40" is around $40

Thomas, is this stuff similar to Rohacell 71?

That stuff was always murder to get in thin sheets.

Yea, that's one thing you have to keep in mind - mass balancing does reduce some factors in initiating flutter, but it also increases the rotational moment of inertia, which can increase the propensity to flutter if there is any give in the linkage...

Bob

Matt,
Yep very similar to rohacell. Rohacell is available here in the us through CST, but the pricing is insane!

You can also order it through ClaraSonics (in taiwan i believe) for slightly less, but shipping kills you. Especially if you need large sheets

There is also the pink PVC foam that B1-Bob uses in his F105's, i believe he gets that from associated industries. Its much cheaper ($60-80 for a 1/8"x4x8 sheet i beleve) and they are willing to cut its size down to work for cheap shipping. 1/8" is pretty thick stuff though and typically overkill, unless its a BIG model.

Harald Huff's Su-26 utilizes 1.2mm airex for the fuse and 2mm for the wings.

I have built the stabs for my F14 using 3mm rohacell 71 with only 2 layers of glass, a carbon tow spar cap, 1/2" titanium pivot shaft w/ 1/8" titanium antipivot rod 6" long sandwhiched in a 1/8" aircraft ply rib and a 1/4" balsa sheer webbin running from the end of the pivot rod to the stab tip.

The stab is so strong, i have literally clampedthe pivot shaft to a chair with the composite section hanging off the chair and sat ON the stab without it breaking, im 185lb's.

Point being, 3mm is overkill for most everything, but i still plan to use 3mm on my Tomcat wings as i dont mind the extra weight for the increased strength