# Small wind tunnel for a model rocket

by leright
Tags: model, rocket, tunnel, wind
 P: 1,194 I need to test a model rocket which will have average airspeeds around 200 mph. I am hoping to construct a plexiglass box about 9 feet long, 3 feet wide and 3 feet tall. For these dimensions and airspeed requirements it looks like I'd need a 160000 cfm blower. Is this something I can feasibly obtain? This is supposed to demonstrate a model rocket roll control system, which will be my senior design project. Maybe I should look into a smaller rocket, eh? Sorry, I'm not an ME so this might be a dumb question.
 Sci Advisor P: 5,095 160,000 scfm is about 20 Lbm/sec. That's a lot of air. I would seriously look into scaling if at all possible. If you can get the scale of the rocket down and make the test section smaller, you can take advantage of the neck down to get your velocity up. Either that or find someone who already has a wind tunnel. Just to give you an idea of scale here, are you familiar with the blower units that janitors use to dry out carpets? Those put out, at best, about 1500 scfm. If you go to www.mcmaster.com and enter 3225k32 in the Find Products box, you'll see what I am talking about.
 Mentor P: 39,637 leright, this is mostly for vertical flight, right? I think I remember your thread on the control and feedback system. If the rocket is going to servo mostly vertically, can you just make the diameter of the tunnel only a few times the outer fin tip distances? Can you get it down to more like 1 foot in diameter? I don't know if that helps enough, though.
Mentor
P: 22,001

## Small wind tunnel for a model rocket

I recently did a study on an air handling unit in a skyscraper in Philly. The air handling unit was roughly the size of a small apartment (but taller) and had a capacity of 110,000 CFM and a 125 HP motor. The fan wheel itself was 6 feet in diameter.

Yah, I'd consider scaling it down too...
P: 1,194
 Quote by berkeman leright, this is mostly for vertical flight, right? I think I remember your thread on the control and feedback system. If the rocket is going to servo mostly vertically, can you just make the diameter of the tunnel only a few times the outer fin tip distances? Can you get it down to more like 1 foot in diameter? I don't know if that helps enough, though.
unfortunately, to house the electronics and servo motors/controllers the rocket 'tube' will need to be at least 6 inches in diameter I believe, and the rear fins plus tube diameter will be roughly 18 inches in diameter. I could get away with using a tube with a 20 inch diameter, I think. I could actually get away with using a 40,000 cfm blower if I were to do that. Ideally though, I would be able to get the wind speed to the maximum speed of the rocket, which is roughly 500 mph.
P: 1,194
 Quote by russ_watters I recently did a study on an air handling unit in a skyscraper in Philly. The air handling unit was roughly the size of a small apartment (but taller) and had a capacity of 110,000 CFM and a 125 HP motor. The fan wheel itself was 6 feet in diameter. Yah, I'd consider scaling it down too...
lol, thanks, I will consider that. sorry, I just have no intuition or feel for these types of things.

Maybe I will look into renting a university wind tunnel instead.
P: 1,194
 Quote by FredGarvin 160,000 scfm is about 20 Lbm/sec. That's a lot of air. I would seriously look into scaling if at all possible. If you can get the scale of the rocket down and make the test section smaller, you can take advantage of the neck down to get your velocity up. Either that or find someone who already has a wind tunnel. Just to give you an idea of scale here, are you familiar with the blower units that janitors use to dry out carpets? Those put out, at best, about 1500 scfm. If you go to www.mcmaster.com and enter 3225k32 in the Find Products box, you'll see what I am talking about.
ha, yeah, I'm going to look into using someone else's wind tunnel.

Problem is, I will need to empirically determine the airframe dynamics of the control system (relationship between roll speed and control fin angles) and this will require many different test runs. I assume renting a wind tunnel is not a cheap thing, even at a university.

 Sci Advisor P: 5,095 You never know! Play the "poor college student" routine to the hilt. You may be surprised. When I did my senior project, there were a couple of companies that donated time and a few items to our cause. After hours time on a small tunnel doesn't seem like it would be too much of a donation.
 P: 175 Well just a couple of ideas that might help a) get a real wind tunnel, b) if not a, gravity might be your friend -- terminal velocity might be around your 200mph velocity regime, so if you can find somewhere to drop the streamlined thing off a cliff while it's running a data collection / controls actuation test that you can gather data from, that'll get you empirical data that's at least in the realm of being accurate for your problem. Intact recovery shouldn't be that infeasible depending on your test site and setup. c) flight test the thing under its own power if that's easy; if you're collecting data and have appropriate exercises programmed in to the actuators, I'd think you'd be able to gather some useful data before the velocity decreased too much. Start out with neutral controls and get increasingly adventurous as altitude increases then let it parachute (or whatever your design does) home after the high altitude / high velocity period is over. d) If you tethered it to a rotating arm or cable it seems like it shouldn't be too hard to use a conventional motor to spin it up to reasonably high airspeeds safely. Rather than using a giant propeller with high speed blades to push air at high speed down a wind tunnel, it simplifies things a bit to just use the airspeed at the tip of a rotating arm itself if you can stand the centripetal accelleration and the radius of body turn in the airstream is within the realm of servo control that you wanted to test anyway. Clearly unrealistic for 500mph, but clearly do-able for 100mph (45m/s), and somwhere between the two is your limit. e) you can use a nozzle (just like .... a rocket, hey!) to convert high pressure/temperature air to low pressure high velocity air... So it could conceivably be within the realm of engineering feasibility to generate low pressure 200mph to 500mph air on the output of a static combustion chamber with a nozzle, but, of course, you'd need to have some reasonable balance of temperature vs. speed vs. pressure (which would need to remain above atmospheric, of course). Certainly nothing to attempt for a huge diameter / length of test chamber, but possibly something that would be very do-able for a very restricted size. f) Would it be that bad to just use computer models to estimate the forces involved, then to use some kind of self-calibrating servo algorithm using feedback sensors etc. to implement something like a PID loop or whatever? How far to turn the fin? Well start turning it a little, if that's not enough, turn it some more... etc.
 P: 175 g) drop it from a small weather balloon and use a terminal velocity fall for some calibration tests. That's something you could repeat several times a day if you had a safe test area with little cross-wind and launched/recovered the balloon and test payload once every hour or so and limited the altitude to some modest few thousand foot level.
 P: 69 The Russian's used large compressed-air banks to test supersonic flows. Could this concept be of use at lower velocities?
 P: 8 Fred Garvin; Can I get that SCFM to LBs equation from you? I've been looking for that for sometime.
P: 175
 Quote by momentum_waves The Russian's used large compressed-air banks to test supersonic flows. Could this concept be of use at lower velocities?
Actually, that's a great idea for certain flow regimes!

I suppose I didn't put it out as an idea since they'd
been tossing around 100,000.00 CFM numbers a few
posts back talking about the "pie in the sky" dimensions
for an "I wish I had" wind tunnel. You'd run out of
compressed air pretty fast at 100,000.00 CFM, but then
that kind of flow is impractical for any simply built
wind tunnel, you'd need a full size jet engine or something
to get those numbers.

But if the tunnel test could be scaled down, certainly,
one could get a nice clean cool stream of several
hundred MPH air out of a few SCUBA tanks exhausting
2200 to 3300 PSI air through some nozzles into a
chamber a very small number of cubic feet (or less)
of volume.

You'd have no real control or stability of flow velocity
in that the tanks pressures would drain pretty quickly,
but for testing automatic servo mechanisms, I'd guess
you could learn a lot in only 5 seconds of test time.

And if you staged 4-5 tanks and had a few people open
the valves in a staggered arrangement e.g. start to
open the next tank valve when the first tank is
down to half-pressure, you could prolong the test time
and get somewhat more stable air flow rates since as
one tank emptied and lowered pressure you'd be adding
the boost of another.

And, actually, having quickly varying air-flow rates would
be perfectly realistic of a rocket with high accelleration
anyway since it'd be going from something like 0 to
200MPH in just a few seconds, so constant air-flow rate
would NEVER occur in a real world environment for his
setup anyway.

You can get SCUBA tanks of compressed air up to
120 CU FT capacity (in STP CF equivalents) with a peak
pressure in the 3200PSI range, so figure at least 60CF
will be available at a "pretty high" pressure and it'd
diminish in pressure for a single HP120 tank from there
as it got more empty.

You can get much bigger industrial gas cylinders that
have several hundred CF capacity at ~ 2000PSI max.
pressure, and it's not uncommon to put a few of those
in parallel for higher capacity.

So it's an option if you can figure out a way to use
pretty cold fast air in the range of 50CF to 500CF total
ambient air pressure CF capacity delivered at 2000-500
PSI pressure decreasing fairly rapidly in time.

You might need to use DIN valves (for SCUBA tanks)
or some other kind of valve that lets the flow rate you
need escape; the normal tanks have relatively narrow
orifices so that a large fraction of the tank can't escape
in less than several seconds.

Of course you'd also have do be sure to do this kind of thing
SAFELY so that you don't have any hazards from freezing
air, accidental air pressure surges causing ruptures of any
restricted flow areas (make sure the exit flow tubes
can NOT get blocked so that no high pressure can
develop), and that normal high pressure tank/gas
handling precautions are observed.
P: 5,095
 Quote by cliff Fred Garvin; Can I get that SCFM to LBs equation from you? I've been looking for that for sometime.
It is simply $$\dot{m}=\rho*Q$$ where

$$\dot{m}$$ = mass flow rate
$$\rho$$ = density at reference condition
$$Q$$ = volumetric flow rate at reference condition