Boundary Layer Airflow

There is some stuff on turbulators in the old MIT journals. Some of these are available from NARTS, others are out of print.

Sounds like a good R&D project. You'll need to track down a good wind tunnel.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

For more information I suggest you find SoarTech #8 which covers airfoils for models, mostly RC stuff. It's the definitive reference on low Reynolds number stuff. Also check out Michael Selig's web site at

Investigating turbulators is one of the things mentioned at A database of Mike's airfoils is at You might also wanter around Mark Drela's work at and an archive of his airfoil data at Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

Except for gliders that should have turbulators on airfoil apexes, generally the smoother the better.

A smooth airframe rocket can require a 2 second longer delay than a "naked" rocket.

Jerry

I worked on the Cal Poly subsonic wind tunnel (while taking econ as a major). I have developed a lot of "rules of thumb", many of which I have already posted to rmr over the years.

Including another today to this thread.

Jerry

Making the airflow turbulent has a good effect on base drag, but a bad effect in friction drag. In a blunt body like a golf ball, there's a lot of base drag. In fact, it exceeds friction drag. The tradeoff there is good. In a fine body like a rocket, the tradeoff is bad.

In 1993, I took measurements from repeated flights of two Estes Big Berthas. One was painted and trimmed. The other was left rough. My measured flight summary Cd's were:

Big Bertha Painted: 0.48 Big Bertha Unpainted: 0.67

Precautions: I hired undergraduates at CMU to take the field data under my supervision. In this regard, the experiment was blind. More obfuscation resulted from the fact that, on a C6, the unpainted rocket flew higher because of the lighter weight. (On more powerful motors, the altitude advantage reversed.) Finally, this was a secondary result of a larger experiment with another purpose, so no one was pushing for these results. Those doing the measurement were too far away to know what I was putting on the pad.

Disclaimers: A better design would have involved multiple physical models. Both numbers are really accurate to only one decimal point.

I recommend Estes technical report, TR-11 _Aerodynamic Drag of Model Rockets_ for a more complete discussion of this subject.

Regards,

-Larry Curcio

Sad thing is, that while I was in the service, I had a beauty just down the hall....

Could hit just over mach 1.2 with a 12" cross section.

20/20 vision and rose colored glasses

Oh well.

Anybody got a good scan of the report Larry references? I checked JIMZ and he does not have it.

This article isn't bad:

Rich Kroboth Webmaster Metropolitan Rocketry Association (METRA) Northern New Jersey Prefect #94

At one time, these were on the Estes Educator web site as PDF files. maybe not. I see many other ones, but not this one:

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

Very little. Please write a good R&D report.

If your rocket is sort of like a golf ball, it may well benefit from inducing the boundary layer to transition to turbulent. However, if your rocket is like more typical rockets, it will benefit from a smooth will finished surface.

Alan

"THREE" "TWO" "ONE" "FORE" (couldn't resist it...!!!)

I would think the only way to get meaningful data is a FIXED way to measure WITHOUT requiring a rocket motor.

I would think the simply variations from motor to motor would dilute the data ???

Chris Taylor

Some years back I saw an article about researchers who were experimenting with covering airfoil surfaces with zillions of tiny holes and sucking the turbulent boundary layer air down through these holes to reduce drag.

Always wondered if you could use the partial vacuum at a rocket's base to do the same thing, IF you had a way to drill (laser?) tens(hundreds?) of thousands of tiny holes in the BT.

I ain't doing it with a Dremel! :-0

+McG+

It should be possible to laminarize the boundary layer by perforating the body and nose cone and dumping the flow through a vent to the low pressure in the base area of the rocket. You will increase overall drag if you bring too much flow on board, though. Any step, such as at the shoulder of the nose cone/body tube, or surface roughness (like a bug splatter) will trip the boundary layer to turbulent. It is impossible to relaminarize the boundary layer downstream of one of these without a heroic effort that isn't worth the trouble. Once it goes turbulent you are effectively done.

Brad Hitch

Brad,

Looks like you came up with some good info.

I have investigated using "Zig-Zag strips" and Riblets (no longer available) for my Bowling Ball rocket.

I have read some reports for "Bluff bodies" where they trip the boundary layer early which causes some turbulence and reduces base drag.

Any ideas???

Thanks, Rich Kroboth Webmaster Metropolitan Rocketry Association (METRA) Northern New Jersey Prefect #94

It depends a lot on the Reynolds number--a rocket, for most of its flight, is flying at relatively high Reynolds numbers (especially the body, which is much longer than the fins). At Reynolds numbers over

40,000 or so (pretty easy for a rocket, except at the end of the coast phase), you probably don't need a turbulator, because (unless everything is just absolutley spiffy-perfect) you're going to have turbulent flow anyway.

Scott Orr

There's a whole chapter in Martin Simons' Model Aircraft Aerodynamics (which is about small planes and gliders, not rockets, but the same principles apply).

Scott Orr

True. In fact, forget Rn. The gap between the nose cone and body tube will trip the boundary layer. Roughness thereafter will have an unambiguously deleterious effect.

Hmmm, sounds like something BMS could do...

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!