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8.2.15 My glider glides like the space shuttle (or worse). What is wrong?




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This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.

8.2.15 My glider glides like the space shuttle (or worse). What is wrong?

Unless you are very good and very lucky, your glider will need
several adjustments before it glides well. The process of
making these adjustments is called trimming. The goal is to
get a glider that transitions quickly and flies smoothly,
gently circling overhead. If you are right-handed, you will
probably have best luck trimming your glider to circle to the
LEFT. If you are left handed, reverse all the following
references to left and right.

All trimming is done with the model in glide
configuration. For a BG, this means without the pod, For an
RG, it means with a spent motor casing installed, and wing,
pod, or whatever deployed as it will be in gliding flight.

The first step in trimming is to locate the CG at the proper
position. If you are lucky, the instructions or plans will
tell you where to locate the CG. If not, you will need to
compute the Neutral Point (CP), or use a typical location like
1/3 of the wing chord from the leading edge. Gliders are often
tail heavy. Add weight to the nose if necessary to get the
glider to balance 10-20% of the wing chord in front of the NP.

All the rest of the trimming should be done by controlled
warping of the flying surfaces. Start by getting the model to
glide straight, which is much easier if it was built without
any warps. In an open area gently toss the glider forward,
releasing it with both the wings and fuselage level. Note its
action. If the model dives (drops its nose), warp the stab
trailing edge UP a bit. If the model stalls (noses up, then
suddenly drops, often straight into the ground) warp the
trailing edge of the stab DOWN a bit. The best glide us
usually right on the edge of a stall.

I like to warp both wing tip trailing edges up to prevent tip
stalls, and the center portion of each wing down to increase
the wing lift.

Then add a left turn until the model has a slow flat circular
glide. Some turn is often added during construction by tilting
the wing in the direction of the desired turn, or tilting the
stab in the OPPOSITE direction. Turn can be increased by
warping the trailing edge of the OPPOSITE wing down a bit. I
try to avoid warping the inner wing panel trailing edges up at
all, as this can lead to spiral dives. Turn can also be
adjusted with the rudder.

For a left roll on boost, warp the left tip of the stab
trailing edge up, and the right tip down. This works at high
speed, but has little effect at glide speeds. Use wing warp,
stab tilt, and a bit of rudder to increase or decrease the
turn as needed.

Try a few harder throws. The glider should quickly settle down
into a flat gentle circle. Continue adjusting the surfaces
until you get this result.

Now you are ready for a serious hand launch. This is an art
form in itself. Throw the model up as hard as you can, at a
45 degree angle up and to your right, and with the wing banked
at the same 45 degree angle. The model should slowly roll to
the left, changing from a right turn to a left turn. If you
are lucky, the model will be gently circling 30 or more feet
overhead. If not, it probably smacked the ground, so pick it
up and try again. Go back and check the trim with a gentle
toss, and if all is OK, try again. You may want to vary the
angles between 30-60 degrees each, until you find what works
best for you and your model.

Now you are ready for the first launch. Pick a reduced power
motor, just enough to get the glider to a reasonable altitude,
and launch it. Use a power tower as described
previously. Carefully observe the boost, transition, and
glide. Watch out for a "death dive" where the glider never
transitions and comes straight down. This can be fixed with
increased stab incidence or warping the trailing edge of the
stab up. Also watch for "spiral dive" where the model turns
very tightly and crashes into the ground. This is caused by
too much turn, or a wing that isn't producing enough lift. Try
reducing the turn or warping down the inside edge of the
inboard wing.

Continue to adjust the flying surfaces until you get the
flight you want. Now move up to the desired motor size, and
fly again. Soon you'll need to read the answer to the next
question.

Trimming can be very difficult in humid climates! At NARAM-30
in Huntsville I had a hard time trimming my gliders. I'd toss
one, then recover it and make an adjustment. Toss it again,
and it was better. Pick it up and toss it a third time and it
was just like the first toss. ALWAYS made sure that models
were well doped before trying to trim them, and do so before
going to a humid climate.

Of course, there's my NARAM-42 BGs. After building them, took
the pair out to the back yard, and gave each one a couple
tosses, just to see how they did. DEAD ON in trim. Spend some
time in Canon City futzing around with one, and accomplished
nothing but making it worse! Put one in a little thermal
(nothing like my ELD model) and easily did 2 minutes on an
A8-3. The same model won B BG at NARAM-44.

Kevin McKiou's trimming advice:

1. Make the dihedral 15 degrees!!!! Last year I had prototype
which just
would not trim out. It did death dives. It would go inverted
at the top of the hand launch (HL) and go inverted into the
ground. It did all kinds of crap I could not figure out. The
dihedral was set at 12 degrees, which was plenty for spiral
stability, but not enough to make it really stable in the
transition on a HL. All I did was increase the dihedral to 15
degrees. After that, it was easy to trim. It would roll out
like a champ at the top of the HL.

2. Make the distance between the wing 1/4 chord and the tail
1/4 chord about
50% of the wing span. This will be adequate for spiral
stability, but not excessive.

3. Make the horizontal stab area between 15% and 20% of the
wing area.
When I have gone below 15% I have had problems with excessive
elevator deflection required. Anything above 20% is
unnecessarily draggy.

4. Make the vertical fin area half the horizontal stab area.

5. If you can calculate the neutral point, set up the CG so
the glider has a
15% static margin as a starting point. If this works out,
fine. It is probably close the the safe minimum. If you can't
calculate the static margin, start with the CG at about 40%
back from the leading edge at the wing root.

6. Trim the elevator so the glider *just* will not fly in a
straight line
without stalling, no matter how slow you throw it. Remember to
always toss it at a point on the ground about 20 feet in front
of you.

7. Now you want to induce a turn. Add about 10 degrees of
horizontal stab
tilt to the right to induce a left turn. Add about a half gram
of clay to the left wing tip to get the turn started. Give it
another slow toss slightly down. If it glides into a left turn
that is pretty flat, you are very close to perfect. If it
turns too fast, remove to tip weight. If it won't break into
the turn, add a touch of left rudder.

8. Time to throw it. Throw it up at about a 60 degree angle
and tilted
slightly to the right. It should arch up, go briefly inverted
at the top and roll out in the opposite direction from which
you threw it. Give it a real firm throw.

If it kind of slid up and did not arch back, you have the CG
too far back. Add half a gram of clay to the nose and go back
to step 6.

If it definitely looped back on you, try again with a throw
that is a little more horizontal. If you just can't get much
height because it wants to loop back (usually into the
ground), the CG is too far forward. Remove a bit of nose
weight and go back to 6.

If it seemed to launch OK and pretty much stalled at the top
with a really slow roll out, add just a bit more weight to the
left wing tip and/or a touch more left rudder.

9. The glide after HL should be a big gentle circle to the
left. If you are
not getting a turn, but the launch looks good, give it a bit
more left rudder. That should help it into the turn. If the
model tends to glide too fast in the turn, add a bit more up
elevator. If it seems you just can't get everything working
quite right between the launch and the turn (e.g., glides
fine, but wants to loop on HL) add wash-in to the left wing
tip. That is, bend the trailing edge down on the outter 1/4
of the left wing. Now, as the speed builds, the lift of the
outter portion of the left wing wing increase more than the
rest of the wing and forces it to straighten out the glide a
bit, slowing it down. As the model slows, the wing tip weight
and rudder will tend to turn the model back into the turn. Now
you can back off a bit on the elevator. Use this
sparingly. You can over do it and cause the model to tip
stall. If the glider builds speed as it glides, with no real
recovery in a second or two, the CG is very likely too far
back. Add a half gram of clay to the nose and go back to 6.

That's about it. From this point, you will just have to try
it, varying each of the parameters to get a feel for what
works well. If you work this well, you should get very good
hand launches and transitions to glide.

-Kevin McKiou 1994, 1996 US Spacemodeling team member

visit the Vectoraero website (http://members.aol.com/kmckiou)
for RC Rocket Glider kits and free plans

visit the US Spacemodeling website
(http://www.spacemodeling.org) for info on competition model
rocketry.

Comment from Bob: I don't know if Kevin tried anything between
12 and 15, but I've been using 14 for many years. Why 14?
Well, it's actually 14.0362... Which is the arctan of
.25. It's simple to lay out. Build a right triangle that is 4"
on the base, and 1" high. That's a 14 degree angle. I use that
guide to build my dihedral joints. It works for Micro Maxx or
D sized models.

Here's a crude drawing. If the block is 1" high, then the
extension is 4". By making the base of 1/2" plywood you've
got a good sized backstop for a sanding block to sand the
dihedral bevel into the wing root.

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previous page: 8.2.14 The pod stuck on my boost/glider and the thing crashed. What is wrong?
  
page up: Model Rockets FAQ
  
next page: 8.2.15.1 How do I trim a Flexie?