Quote:

>I flew my Arreaux yesterday on an E16-7. Great boost, no ejection. I

>held my breath and watched it fall horizontally from an estimated 850

>feet (probably less because of weather conditions, etc). It landed

>horizontally on the sand and loosened two fins. I've repaired it, but

>I'm still wondering why it didn't come in nose down? I guess the only

>way it would have stayed horizontal is if it was moving forward at a

>relatively high speed (IOW it was remaining stable as usual, but just

>sideways). But it didn't drift very far to be moving very fast, and

>why didn't it arch over at apogee? Can wind cause it?? TIA

This is exactly what the Bumbling Brothers' (my brother Bob's and my) NARAM R&D

report was about.

Our goal was to make this a deliberate recovery mode.

The secret is in the angle of attack.

I'll assume you know about Center of Pressure (CP) and Center of Gravity (CG).

CP is the "average" point that air pushes on the model. CG is the balance

point of the model, and the natural pivot point of the model in free flight.

If the CP of the model is behind the CG, the air pushes on the model (on

average) behind the pivot point. So if the model points away from straight

forward, the air pushing on the model tends to push the rear of the model back

behind the CG. The model is stable, and all is happy.

You can find the CG of a model by balancing it on one finger. The CP is

trickier. The ancient method was the cardboard cutout method. Essentially you

cut out a profile of the model from cardboard and balance the cardboard cutout

on a pencil. Technicaly the balance point is the "center of lateral area

(CLA)." But this method turns out to be accurate only for a model travelling

directly sideways--a 90-degree angle of attack. Forunately, a model that tests

stable this way, with the CLA ahead of the CG, is stable--the method is very

conservative.

Then came the great and powerful Jim Barrowman, who showed us the way to

clculate the CP for models that were already moving the way they were

pointing--at low angles of attack. and lo, Barrowman showed that models that

looked unstable with the cardboard cutout method wer indeed stable. And fins

became smaller, and models flew straight, and it was good.

But some models that pass the Barrowman test--the CG is in front if the

Barrowman CP (BCP), fail the the cardboard cutout test--the CG is behind the

CLA. This can be bad if you launch from a short rod in high winds. An

otherwise stable model goes unstable, and all hell breaks loose.

But our R&D hypothesis is that this condition, a CG ahead of the Barrowman CP,

but behind the Center of Lateral Area, leads to a miracle. A model that is

stable is stable going forward as long as the angle of attack is low, but one

that can experience a high angle of attack if it is pointing straight up at

apogee. For then it experiences a high angle of attack for a moment, and loses

its stability.

When the air hits the model sideways, the model wants to point so that its CG

is ahead of the CP. But now the CP is the Center of lateral area, which is

ahead of the CG. The model wants to point backwards. But if the model goes

straight backwards, the angle of attack goes low again, and now the CP is ahead

of the CG. Argh! the poor model wants to crash, but backwards or forwards?!?!

The poor confused thing compromises somewhere in the middle, typicaly in a

something of a backwards glide.

And so your Arreaux fell sideways, just as a swing-stability tested model

sometimes kind of just wanders in direction as you swing it around your head,

kind of wanting to go backwards, but not having the heart to, but will go

foreward with great certainty if you start it right.

The trick is to make this happen in flight deliberately. Bob and I use a

little hole punched at the front of the body, so the ejection gasses throw the

model into a tumble.

This got us 4th place in a single flight in D R/G at NARAM and 1st place in

R&D.

Peter Alway

Saturn Press

PO Box 3709

Ann Arbor, MI 48106-3709

http://members.aol.com/satrnpress/saturn.htm

Free scale data at:

http://personal.physics.lsa.umich.edu/alway/space_rocket.htm