Why does dive CG test work? What does 'work' mean? :-) - RC Groups
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Apr 02, 2003, 12:05 PM

[RCSE] Why does dive CG test work? What does 'work' mean? :-)

Hi Richard,

Dive tests do work.

Now what does the word 'work' mean in the context of fly RC sailplanes?
It 'works' if you mean that you too can end up with a model that is always
flying with up elevator to hold the lead in the nose up, versus a wing that
is happily soaring around.

If you mean it tells you something about balance, then you are right, it
tells you all sorts of things about balance, none of them of any value.

Keep focused for a minute and stop thinking, it gets in the way.:-)

When you are thermaling a sailplane, when does it ever get over 15mph?

Extremely seldom is the correct answer.

If anything we go slower than 15mph, stall then recover.

If you want to get argumentative and say they go faster than that in a dive,
I have to ask, why are you worried about balance in a dive? Isn't the point
of trimming and balancing to optimize the sailplane to take advantage of the
smallest amounts of lift?

Now if you are of the mind that while you paid for the radio, you really
didn't intend to use it to control your models and would prefer that your
models were set up as free flights that you could occasionally direct back to
the field...then maybe the dive test would a worthwhile thing:-) (notice
the word chose was not 'good ' it was 'worthwhile')

So if want to get anything at all of value from a 'dive test' do what you do
in real RC soaring in thermals (per Frank Weston). Gently pull the nose to a
stall (just like you would in a thermal turn by accident), then let the nose
fall, watch and see what it does in the first 30.' Beyond that the model's
speed goes past the range we thermal at. And yes of course no one has eyes
good enough to see what is going on in that first 30'.

Besides there is more to the model that is affected at high dive speeds than
lead in the nose. A fixed stabilizer like a V tail or stab elevator adds
another component to what will happen in a dive test. If the stab is
anything other than zero with the wing, the added air will give more power to
those surfaces set deflection abilities.

Tail boom flex, linkage play, hinge play, all sorts of things get jiggy under
dive speeds and affect the outcome of the models attitude during the dive.

Full flying stabs, takes the decalage thing out of the equation but they
don't fix boom flex or linkage or that sort of thing.

A simpler real time, readable/valuable/useful feedback for full flying stab
models is the Gordy's Balancing System (its only mine cuz I wrote the
article, the common sense belongs to guys who actually know stuff about
flying RC sailplanes:-).

Set you model up at 40% of root chord, that's from the leading edge. Then go
to the field on relatively calm day and give it a toss. Trim your elevator
trim so that the plane is flying slightly mushy but very flat. It will
balloon on the toss so make sure to get it flying. Okay to do this part a
few times.

Now once trimmed for flat flight (flying with its fuselage parallel to the
ground the whole time), not angled downward flight, give it a toss, the ONLY
part of this test is the last few feet of the glide. When the plane gets to
the end of its glide path, the airspeed bleeds off, the elevator stabs lose
power and the plane will eventually slow to a stop ... in the air....and the
nose will drop like a rock....made of lead.

Start pulling lead and tossing till the model flies and flies and flies and
flies and flies and then the first thing that touches is the tow hook. Then
you are pretty close to 'balanced'. The nose doesn't need the elevator to
hold it up...the wing is flying happy.

Notice there is no chance of tail boom flex, etc, no high speed to affect
anything, just loooooong glides to chase.

Next step to near perfect 'balance' will follow this post after you answer
this question.
Yes this was all in RC Soaring Digest Magazine, the "Journal for RC Soaring
Okay, so we do Gordy's Balancing System, first tosses the model comes to a
stop and plants its nose straight into the ground with each toss until
finally it just glides and glides and glides and well you get the point,
until the tow hook touches first.

Now we fly your sailplane and it indicates the lightest lift and flies like a
dream and turns on the slowest tightest dime on approach after setting the
balance point this way. (okay to re read this last sentence before typing
your replies to the question:-)

Then you take it home to find out it now balances ... say a full 2" behind '
where the plan said'....would you put that lead back in?


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