Rocket stability(center of mass)

AstronMike said:

*I'm* trying to follow all of this rather closely to assist myself in better *large rocket-powered glider* design!

The deal about the wing tip voritices is good info. If those are creating so much drag on a non-tipped wing, then wouldnt all that excess drag actually lead to increased chance of fluttering? Such 'drag production', since it would be occurring at the *furthest* portion of the wing, would have a larger moment arm effect and thereby help induce/worsen fluttering, wouldnt it?

Good time you guys are bringing this up! I have a strange looking cranked delta planform, with tip plates, that I intend to fly in a few days on a mid I motor. Wingtipwise, its very solid feeling, compared to the 'standard' deltas I have made over the years for HPR gliders. Its a larger version of what I'm posting below......

Will be following this discussion very closely although I may not have anything to add 'education-wise', only anecdotal in-field experience via large nonstandard gliders and thier behaviors.

Click to expand...

Induced drag is a funny thing, honestly. It actually reduces the total force on the wingtips, because it simultaneously causes drag and reduces lift towards the ends of the wing. I don't think it increases the chance of flutter because of this. Also, the induced drag, even though it is caused by the vortex at the tip of the wing, acts along a significant portion of the wing, although its strongest effect is at the tip.

This isn't a bad summary, and it shouldn't be too difficult to understand:
https://en.wikipedia.org/wiki/Lift-induced_drag

ClayD said:

You might as well just save your time, and say "I don't agree, shut up stupid"; leave it at that.

Click to expand...

Consider it "saided".

But to just leave it at that implies that this is just a disagreement over some "opinion". Things such as base drag also including can'ted fins (whoops, I mean canted fins). Or why the XB-70 had wingtips that folded down for supersonic flight. Those are examples of someone coming along mis-interpreting things, and when someone else points the way to the real answer, the originator calls them wrong, or claims they have ulterior motives for trying to correct the inaccuracies, or tries to play the "we disagree" card.

If someone says 2 +2 = 3, and others point out the answer is 4, then "we disagree" does not cut it. Neither does questioning the motives of anyone who says the answer is 4.

- George Gassaway

AstronMike said:

The deal about the wing tip voritices is good info. If those are creating so much drag on a non-tipped wing, then wouldnt all that excess drag actually lead to increased chance of fluttering? Such 'drag production', since it would be occurring at the *furthest* portion of the wing, would have a larger moment arm effect and thereby help induce/worsen fluttering, wouldnt it?

Click to expand...

The tip vortices occur when the wing is flying at an angle of attack, producing lift. At zero angle of attack, no vortices.

Since you mention flutter, and models, then for our models flutter is mainly an issue for BOOST, which is at very high speed compared to glide. In the most practical sense for delta winged models like yours, boost is effectively at a zero angle of attack. Possibly at very slight angles during boost, of plus or minus a very small angle of attack, as with fins during the boost phase of Alpha-type rockets.

I do not think that tip plates or anti-vortex devices at the wingtips would do anything good to prevent flutter on boost. They could actually promote more flutter on boost due to their mass. Especially if the plates are not symmetrical, such as on tip of the wingtip and not equal above and below.

To me, the most practical reason to add “tip plates” to a delta wing is to simply use them as rudder area for yaw stability. A great example of that is with the “SkyBooster” I made for the Orbital SkyDart Project. Or course one of the other reasons why I did that was because the 2X SkyDart on top prevented a large single rudder from being used on the SkyBooster (though it might also have had Yaw/Roll coupling problems with a large single rudder on top, so the low wing with twin rudders at the tips helped to cure that problem). The wing was strong enough, and boost velocity slow enough (two clustered G12’s), that flutter on boost was not a concern.

Another benefit of the tip rudders is that at a very high angle of attack during glide, the Delta wing could "blank out" a single large rudder in the middle. While the twin tip rudders do not get blanked out at such angles. And, done right, they usually look cool...

- George Gassaway

Hey MaxPayne

Were you able to pick up a copy of Handbook of Model Rocketry (I twisted the title in my last post)? Mine is the 7th edition.

Chapter 10 covers Model Rocket Aerodynamics. It describes base drag (and about other type of drag there is except queen) and how it can be reduced in language that even I can understand. But then I am not a rocket scientist nor do I play one on TV.

As a bonus it's hyperbole free.

Bones

georgegassaway said:

BTW - Powderburner's reference to a pig, perhaps he meant the one about not trying to teach a pig to sing?

Click to expand...

George, my “pig” reference was to a saying that goes something like: You shouldn’t try to catch a greased pig, because you will only get dirty and the pig would enjoy the massage too much. (Hmmm. Oddly appropriate, if you substitute ‘troll’ for…&#8230 I do love the Enigo quote, though; that is one of my all-time favorite shows

jsdemar, I’m glad you enjoyed my little dab of “calm down” sarcasm, my dry humor seems to have passed cleanly over everyone else

We have been attempting to explain fairly advanced aerodynamic concepts to someone who defers to the wisdom of Napoleon Dynamite, is convinced he has seen a drop-down wingtip on an SR, and seems willing to fight to the death to back up his “handle” (or at least attempt to kill us with spelling and grammar errors). I yield to his incredible miracles of logic, and the careful use of reversible terminology. You just can’t win a battle of wits with an unarmed man. (I hope someone keeps a close watch on this guy when he’s on the range.) Before I get called a “dummyhead” or something equally disturbing, I have to end my participation on this thread so I don’t get all upset and lose sleep.

MaxPayne (and AstronMike), I apologize for whatever part of the derailment for which I am responsible. If we did not answer your questions here maybe we should start over on a clean sheet of paper. One of the mods needs to hit the ‘flush’ handle on this entire swirling thread.

cjl said:

To be fair to Clay, the sumo example was indeed base drag. It did involve flow separation at the base of the rocket, which both stabilized and added a tremendous amount of drag to the rocket. It also added some form drag due to the air hitting the front of the plates, but without running some CFD, I couldn't say whether the increased base drag or form drag is more significant.

Click to expand...

I have run such simulation before and shape drag (oncoming air against a ring plate) is at least an order of magnitude larger than the additional base drag from increasing the base diameter.

cjl said:

Also, I'm not sure that I would describe base drag as having anything to do with cavitation - cavitation involves a phase change of the fluid flowing around the object, and that doesn't occur in rockets.

Click to expand...

Cavitation can be a turbulent drop in pressure in a fluid without a phase change. But, you're right, it's most often used as a term in turbomachinary. I thought I'd throw it out there as a red herring for somebody, not you.

MaxPayne said:

You guys confused the hell out of me.

I second that cobra1336.

Click to expand...

Perhaps you'd like to give it another try? Please post a new thread about what parts have not been clarified or have you confused, and hopefully everyone will stay on-topic and not get out of hand.

- George Gassaway

powderburner said:

George, my “pig” reference was to a saying that goes something like: You shouldn’t try to catch a greased pig, because you will only get dirty and the pig would enjoy the massage too much. (Hmmm. Oddly appropriate, if you substitute ‘troll’ for……)

Click to expand...

Ah, do recall that one now that you mention it. The teaching a pig to sing one is the one I have remembered best. But indeed the greased pig one may be more appropriate here.

- George Gassaway

jsdemar said:

I have run such simulation before and shape drag (oncoming air against a ring plate) is at least an order of magnitude larger than the additional base drag from increasing the base diameter.

Click to expand...

Fair enough. That definitely sounds plausible to me.

Bone Daddy said:

Hey MaxPayne

Were you able to pick up a copy of Handbook of Model Rocketry (I twisted the title in my last post)? Mine is the 7th edition.

Chapter 10 covers Model Rocket Aerodynamics. It describes base drag (and about other type of drag there is except queen) and how it can be reduced in language that even I can understand. But then I am not a rocket scientist nor do I play one on TV.

As a bonus it's hyperbole free.

Bones

Click to expand...

I havn't got a copy yet, but it will surely be my next purchase. BTW, does anyone have an ebook? I could post a thread requesting the same, it it is allowed.

MaxPayne said:

I havn't got a copy yet, but it will surely be my next purchase. BTW, does anyone have an ebook? I could post a thread requesting the same, it it is allowed.

Click to expand...

I've never seen a PDF eBook version of the Handbook of Model Rocketry. There's a Kindle edition on Amazon for $13.35.

Here's the old Centuri TIR-30 paper on Model Rocket Stability. It's pretty much the same as the chapter in Stine's book.
https://www.spacemodeling.org/JimZ/manuals/tir-30.pdf

Here's Barrowman's original R&D report on calculating CP:
https://www.apogeerockets.com/education/downloads/Barrowman_report.pdf

And here's a newer paper that talks about all the forces on a rocket in flight: https://cambridgerocket.sourceforge.net/AerodynamicCoefficients.pdf