1.Provided, that the surface area is the same and the distance from CG to the vertical stab MAC same: Is the effectivness of outboard placed vertical stabs on the flying wing same, worse or better than one single stab placed in the midline ?

2. If the material for the stabs is the coroplast, and the material is placed in such way that the flutes are parallel(horizontal) with flight - does one piece actually serve as two surfaces, because it has double surface joined by the flutes ? (Coroplast is hollow double walled polypropylene corrugated plastic - like cardboard paper).


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Thanks for help

Petr

1:

Twin vertical stabs MUST be more effective. Otherwise the F-18 wouldn't have them. They wouldn't waste the cost and weight of twin stabs if one would do.

2:

No because the double surfaces are too close together.

The reason the F-18, F-15, F-14 and various MiGs and Su's have twin fins is so they will have yaw stability at high angles of attack. The wide fuselages on these twin engine planes can blank out a single, center mounted vertical stab. At high AOA the outside sees most of the airflow.

The single engine F-16 has a single vertical stab, but you'll notice, it is a very tall one so the tip is in good air at high AOA.

The F-111, which I flew, with its short, single tail, could get into a position of high AOA such that the vertical tail was completely blanked out and it would swap ends. These "wide" fighters needed the twin tails to cure this problem.

I had an RC biplane several years ago that could get the nose up very high on landings if you tried. It would get into a position where the fin was blanked and any gust or disturbance would cause it to swap ends about a foot in the air. A taller tail cured it.

Ed,

Good info. I suppose also, that the short moment arms benefitted somewhat from the apparent huge amount of vertical area. Question remains though. On the delta in question, will two vertical fins with the same total area be as effective as a single fin? Any advantage one over the other in any case?

This is a little off topic, but interesting. I was watching the Wings channel the other day and they were showing aviation in Germany before WWII. They were banned from most powered planes so they did mostly gliders. You could tell that there was not much aerodynamic theory known when many where built. The documnetary showed many crashing and many flying. Some actually flew very well.

One in particular had no vertical stab whatsoever. The glider made it maybe a hundred feet before yawing uncontollably and spinning in. OUCH!

If the fuselage is not a factor as it is in the F-15, F-18, etc. then the single vertical fin will be a little more effective than two small ones of the same total area.

This is because big wings basically work better than small ones for a lot of reasons. However just add a little more area to the twin fins and you have about the same thing. I don't know what the ratio of single to twin would be though.

The main question though is how big should be vertical fins be to start with. On the F-15 project we performed wind tunnel tests using a slab flat surface and we would add or subtract area until we got the directional stability that our analysis had indicated we needed. Then we would make a normal airfoiled surface and verify that we had what we thought we had!

In a model it is a different case as it is mostly cut and try.

Thanks for your replies.

The model in question is a scale combat Ho-9, which is a flying wing.
So far on my conventional models with fuse I used the method of Center of Lateral Area, which is old, it has been critised a lot, but for me it worked well.(Andy Lennon explains it in his book).
For the flying wing Ho 9, I made it first with one central fin attached to the tail . It seems to fly well, but of course it is not scale.
So instead, what I came up with was to make immitations of the drag rudders - they would be triangular vertical fins 4 on each side (2 top and 2 bottom) and one clear lexan tail skid. That should provide the plane with enough lateral stability for the combat , even for the drag imposed by having a streamer cought on one wing for example.
This is a link to pohoto of one of my prototypes.
This one is actually the SSC (Slow Survivable Combat) and not the scale, but the dimensions are the same, engine is smaller 0.15 instead of 0.25.



click for larger pics:

http://www.msnusers.com/Compositemol...oto&PhotoID=75
http://www.msnusers.com/Compositemol...oto&PhotoID=76


Petr

It sounds like a reasonable approach since you want to look scale. You did have a really big vertical tail to start with so it will be interesting to see what flight test brings. Post the results.

The engineering mockup of the North American A3J Vigilante had two verticals. It went into production with the single center fin.
Presumably the two verticals were intended to ease below-deck storage, which the single fin adapted to by folding down.
.
It interesting to note that almost all high-speed planes since the A3J of its mockup configuration... large rectangular inlets, lots of wing, twin verticals... F-15, F-14, MiG-25,MiG -31, Sukhois... all follow that lead.

Agreed Paul, on the F-15 project while doing the directional study we tried the center line location and it was really bad over the flight range we needed. You could get the needed stability but it looked like a third wing panel. The verticals we ended up with are much bigger than those originally proposed.

It could be North American had the same problem. That twins they ended up with still violated the storage height so if you had to do it just fold one big one. I guess a weight tradeoff would select the winner of that configuration and apparently the single vertical won. Interesting stuff indeed.

That basic twin everything configuration you mentioned is hard to beat. We once tried a low wing, variable sweep, 2 engine, configuration but it still retained the design elements you mentioned.