Parasite Glider Question

[Mike]

Looking at Parasite gliders where the glider "hangs on" to the side of the airframe such as the Orbital Transport. This obviously creates a high drag on one side, does the drag need to be balanced out on the other side?

If not, can this assymetrical (!) drag cause a model to go unstable?

 

[hokkyokusei]

One solution is to put a parasitic glider on the other side too, like teh Estes ARV Condor.

 

[shreadvector]

All good questions.

Yes, it causes drag on one side.

Yes, it *could* make the rocket unstable during boost, but it can also make it arc over and crash.

I have a model called the Shadowcat with parasite Glider. The regular Shadowcat rocket uses a 12 inch BT-50 body tube. For the special version with the large parastie glider, I use an 18 inch length. Since the Shadowcat uses a birch wood nose cone which weighs more than plastic or balsa, the longer tube helps move the weight of the nose cone farther forward. The glider is nested far back on the body with the wings resting jut above the fins and the fuselage with horizontal and vertical stabilizers ("tail") hanging far behind. The C.G. is forward of the wing and the boost is pretty damned straight.

I had experimented in the early design with the 12 inch long body tube and it resulted in a C.G. at about the mid point of the wing. This resulted in some great boosts and some arcing crashes. Longer tube solved that.

Never mount any wings in front of the C.G.

Photo of the glider is on the second sheet of the attached product info - it's the one on the left.

Originally posted by Mike
Looking at Parasite gliders where the glider "hangs on" to the side of the airframe such as the Orbital Transport. This obviously creates a high drag on one side, does the drag need to be balanced out on the other side?

If not, can this assymetrical (!) drag cause a model to go unstable?

 

[Mike]

Thanks for the responses guys.

So will increased stability mean it can handle greater differences in drag between the two sides?

 

[shreadvector]

Stability is a 3-Dimensional thing. It should be stable enough and move fast enough to fly upward and not arc over. If you look really closely, a glider like my parasite design will fly a tiny bit like a Space Shuttle off the pad: it will keep pointing straight up but will travel a tiny bit sideways as it climbs. Since it's travelling pretty fast, you will not notice the sideways movement and it never looks like it's arcing over. The lift from the glider wing and the force created by the angled horizontal stabilizer will cancel the moment created by the offset drag of the glider. You can draw a force diagram and imagine how a small force far away from the C.G. can create havok (like small fins near a nose cone that are NOT glued on straight).

Originally posted by Mike
Thanks for the responses guys.

So will increased stability mean it can handle greater differences in drag between the two sides?

 

[JRThro]

Originally posted by shreadvector
You can draw a force diagram and imagine how a small force far away from the C.G. can create havok (like small fins near a nose cone that are NOT glued on straight).

Wouldn't that be more like "a small force far in front of the CG can create havoc"?

A small force far behind the CG would be self-correcting, as happens with weathercocking.

 

[flying_silverad]

Do you have the "Handbook of Model Rocketry" by Stine? If you do, go to chapter 13. There is some info there that might help as well.

 

[JRThro]

Silver,

Was that meant for me or Mike?

 

[flying_silverad]

Originally posted by JRThro
Silver,

Was that meant for me or Mike?

For Mike

 

[shreadvector]

"yes" and "no".

If the small force is behind and is always 'pushing sideways' it will make the rocket arc over/loop/crash.

With fins on the rear (aft of the C.G.) if they are glued on straight, they stop generating a force when the rocket is pointed straight into the wind. They do generate a force when at an angle of attack to the wind. That is what corrects the flight path of a fast moving rocket after it leaves the rod and also what makes a slow moving rocket weathercock.

The faster you move, the less effect the sideways blowing wind will have. Calm weather is still the best for any rocket.

Originally posted by JRThro
Wouldn't that be more like "a small force far in front of the CG can create havoc"?

A small force far behind the CG would be self-correcting, as happens with weathercocking.