Model Forum / General / Rockets / September 2003

And that brought up a question... has anyone built a
boost-glider with cruciform wings?

I can see some of the problems... rocket-boosted antiship
missiles, for example, can afford the extra weight of
four smaller wings where their speed and deployment
environments forbid two larger wings. And the wings are
metal... :)

Still... has anyone worked with any version of this idea?
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A major inherent problem with making a Rocket Boosted Glider glide stably with
"X" type wing is lack of dihedral. OK, sure, the top "V" set has positive
dihedral, but the bottom “V” set cancels that out. Or worse, in addition, the
bottom "V" partly blanks out the upper "V", so there can be a net negative
dihedral at certain angles of attack.

Now, having said that, I made a gliding Star Wars X-wing in the late 1970's. It
was a very crude semi-scale model. I cheated it a bit by giving the upper wing
"V" pair a bit more wing area. Also, I gave a net overall dihedral to it of
about 5 degrees or so, the upper wings were higher by 5 degrees, and the lower
wings were 5 degrees less downward than they should have been.

I didn't attempt to deal with trying to make that fly like a "X" flying wing,
with tons of upwards trailing edge reflex. What I did was to rig up a
scissor-wing type of clear plastic canard, mounted under the main fuselage near
the nose. At ejection, the scissor portion deployed, with a lot of positive
angle of attack. The model was trimmed to make a very  fast glide.

It had a unique dual stability mode. If it didn't have enough noseweight, so it
stalled, it would go into a very deep stall and then stabilize in that deep
stall. It would fall straight down, but with the fuselage horizontal (Pancake,
belly-flop, etc.). Much like a "dethermalized" Free Flight model plane with a
pop-up tail DT. Reason it did that was due to the lower wing starting to blank
out the wing area of the upper wing, as it reached a high angle of attack. If
it got too far, then the lower wing would blank out the upper wing too much,
and it would stabilize like that because the canard only had a single surface,
so it didn't get blanked out.
Another way of looking at it would be to imagine if suddenly the canard's area
had doubled, therefore shifting the center of pressure significantly forward,
to the point it could not glide but actually stabilized in a flat vertical
descent. Same sort of thing is occuring when the upper wing gets blanked out by
the lower wing, the CP shifts forward.

The solution was much like the classic joke:
"Doctor, it hurts when I do this"
"Then don't DO that!".

So, I made it so nose-heavy that it would drop its nose and go into a high
speed glide, before it could pitch up enough to risk going into a deep stall.

Note this in theory might have happened with the Wright Brothers, but
fortunately on their Flyer they had a biplane canard. I saw a show recently, on
Discovery I think, where a team is practicing to fly their own replica Wright
Flyer, by using a replica Wright Glider (1902?) towed by an SUV. They practiced
with a single canard, which could get into this very same problem if they ever
let it reach a significant stall. Fortunately they later switched to a biplane
canard for more practice with what they figured was a less stable combination.
At least the biplane canard version doesn't have the potential for that deep
stall when using a single (non-biplane) canard.

I had to remember that blanking out lesson with my shuttle models. The ET and
SRB's can tend to blank out the wing area of the orbiter at only a few degrees
of angle of attack. So if a person tries to calculate the stability of a
shuttle stack by including the orbiter wings/fuselage  "behind" the ET and
SRB's, they are going to have a more forward CP in real life unless it is flown
on a dead calm day and has no inherent problems that would make it try to pitch
significantly. I try to make sure it's plenty nose-heavy, to the extent
possible without being too much (extra effort to keep things light, especially
aft of the CG, and to shift as many internal components as far forward as
possible..  

To get a bit back to your original question, if someone wanted to make a
gliding model of a missile with cruciform wings, that would glide, they could
solve the dihedral problem this way. Get a horizon-reference stabilizing
"Gyro", and mount it so it would keep the cruciform wings oriented in the
scale-like "X" pattern, assuming the model was R/C or at least used a servo to
maintain roll control. FMA makes a really good one for model planes.

Landings would be rough though, with the lower set of wings. If I made a model
like that, as a somewhat serious scale model that had a lot of time in it, I'd
probably rig it up to deploy a chute (or chutes) by R/C when it had glided over
a desired landing area. I gave that some thought for a potential model of the
A-4b, the winged version of the V-2. Glide around awhile then pop chutes for
landing.

- George Gassaway

Yup.

Built several cruciform CG shift RG's circa '91 - '93. Cruciform tail and
wing surfaces. They flew well, but trim was critical. I found that by
driving the CG progressively rearward I could get stable gliding flight with
0-0 trim ("super-critical" trim I suppose a la HLG's). They had a funny
"hunting" glide profile, much like a Sidewinder that can't make up where it
wants to go - quite amusing actually.

There's no benefit for duration, but for fun they are neat.

Mike Dennett
CTI

For cuise type missiles, wings increase range.  For air-to-air
missiles, wings increase maneuverability, usualy at the expense of
range.

I have not, but you might try a canard configuration with X main wing
and canards with dihedral.

Well, I did some work on the Harpoon missile.  The Harpoon is usualy
launched out of a canister, which means that the wings must be folded,
and four wings perform better than two, in this case.  The metal wings
have nothing to do with it, they are just cheaper than composit
construction.

Alan