Author Topic: Speed Demon Wind Tunnel Test  (Read 46149 times)

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Offline Rex Schimmer

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Re: Speed Demon Wind Tunnel Test
« Reply #15 on: February 12, 2016, 04:14:07 PM »
If we have a race course next year I am sure that we will find out if the new car has the same or better performance than the old. As we all know George knows what to do with the pedal on the right!

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Offline robfrey

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Re: Speed Demon Wind Tunnel Test
« Reply #16 on: February 12, 2016, 06:09:58 PM »

I didn't think there was a way to make a concave surface "clean" when its both preceded and followed by a larger convex surface.  The body shape is pretty, but it would make an unstable bullet.  Wouldn't a straight profile have less overall drag? Tried and true. 

It's funny that you mentioned "tried and true". This car shape has more runs over 400 than all others put together. I'm sure they call this "tried and true."
Also, can you please share the data showing that straight profiles have less drag?
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Offline stay`tee

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Re: Speed Demon Wind Tunnel Test
« Reply #17 on: February 12, 2016, 07:11:03 PM »
"Drag" is also your friend,  :wink:
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Offline WhizzbangK.C.

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Re: Speed Demon Wind Tunnel Test
« Reply #18 on: February 12, 2016, 08:23:15 PM »
The shape of this car looks like they may have considered "area rule" in the design.  :?
Ah, this is obviously some strange usage of the word 'safe' that I wasn't previously aware of.  Douglas Adams

Offline robfrey

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Re: Speed Demon Wind Tunnel Test
« Reply #19 on: February 12, 2016, 08:29:06 PM »

The shape of this car looks like they may have considered "area rule" in the design.  :?
How do you figure?
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Offline UBRACER

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Re: Speed Demon Wind Tunnel Test
« Reply #20 on: February 13, 2016, 09:34:22 AM »
Are any considerations given to the effect of exhaust affecting airflow around tail ?

Offline Seldom Seen Slim

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Re: Speed Demon Wind Tunnel Test
« Reply #21 on: February 13, 2016, 09:46:50 AM »
Ub, if you could sit here and look over "there" on the wall you'd see the photo that I got from Pork Pie a few years ago.  It shows the Speed Demon chugging along at flank speed -- the 450+ run, if I remember correctly - and you can see without a shred of doubt the exhaust plume.  The exhaust escaped near the centerline of the car and no doubt fiddled around with the aero of the tail.

And if it's plain to me, sitting here in a snowstorm, then I would guess that the Speed Demon team knows it was there on the old car and used that information -- maybe even more refined than my armchair observation) to decide how to build the new one -- which has exhaust outlets much farther outboard on either side of the top surface - and before the tail.  I've got photos but they're at work.  If any of youse guys have a current photo -- from the GNRS, for instance - please post 'em.  Thanks.
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Offline WhizzbangK.C.

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Re: Speed Demon Wind Tunnel Test
« Reply #22 on: February 13, 2016, 04:02:43 PM »

The shape of this car looks like they may have considered "area rule" in the design.  :?
How do you figure?

When I was working on the B1-B program, I got to know several of the engineers who had been involved in the original design, as well as the XB70 before it. I asked them how the shape of the aircraft came about because I was pretty curious about it, and they took the time to give me a short hand explanation of area rule. The characteristic "wasp waist" or "coke bottle" shape of a lot of aircraft designed around that time is the most visible clue that it was factored into the design, but even today it is used, it's just not as visible to the untrained eye.

I went looking for a better explanation than I can give, since I can't even claim a rudimentary grasp of the subject, and found this. It seems to outline it pretty well. Note that area rule is specifically effective assisting in the control of shock waves in the higher subsonic and trans-sonic regimes, starting somewhere around the 500 MPH range. (Familiar number, huh?)

https://en.wikipedia.org/wiki/Area_rule

The change in profile from the driver's compartment area, through the engine compartment, and to the drive wheels, looks like it may follow this rule to some extent, is all I was saying. The speeds they are looking for may well be in the range where it can be effective.

Really it's just a W.A.G. on my part, but it looks almost plausible.  :-D
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Offline robfrey

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Re: Speed Demon Wind Tunnel Test
« Reply #23 on: February 13, 2016, 04:14:09 PM »
The reason for area ruling is to take into consideration of the area of the wings so that total cross sectional area grows and shrinks uniformly and smoothly. This car has no wings. Therefore there's no reason to neck the body down behind drivers compartment. There are no wings to compensate for.
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Offline 7800ebs

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Re: Speed Demon Wind Tunnel Test
« Reply #24 on: February 13, 2016, 04:44:35 PM »

Offline wobblywalrus

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Re: Speed Demon Wind Tunnel Test
« Reply #25 on: February 14, 2016, 01:47:45 AM »
Are they designing the body to provide drag at the rear where it is far behind the center of gravity?  This might give it more resistance to going sideways under power.

Offline UBRACER

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Re: Speed Demon Wind Tunnel Test
« Reply #26 on: February 14, 2016, 08:58:24 AM »
Any thoughts on how the dorsal exhaust outlets will effect airflow over rear afterbody,stabilizer  ?   Pete

Offline WhizzbangK.C.

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Re: Speed Demon Wind Tunnel Test
« Reply #27 on: February 14, 2016, 12:41:35 PM »
The reason for area ruling is to take into consideration of the area of the wings so that total cross sectional area grows and shrinks uniformly and smoothly. This car has no wings. Therefore there's no reason to neck the body down behind drivers compartment. There are no wings to compensate for.

As I understood it, the purpose is to minimize total cross sectional area change, and to control where on the vehicle this change occurs in order to limit the effects of the shock waves, or possibly take some sort of advantage of them.

On an airplane the wings are sort of necessary, and obviously must be accounted for in area rule design. I don't see how it would necessarily follow that just because a vehicle doesn't have wings the basic tenets wouldn't still apply.

Like I said, I'm no how no way an expert, and it was just a W.A.G. on my part.  8-)
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Offline hotrod

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Re: Speed Demon Wind Tunnel Test
« Reply #28 on: February 14, 2016, 01:41:05 PM »
As mentioned above the area rule came about when they were trying to understand why there were huge changes in drag as an aircraft entered the transonic speed range (ie some local air flow might be very close to going super sonic even though the overall aircraft was still below supersonic speeds).

This was finally validated in the design of the F-102A aircraft, the original design F-102 could not achieve supersonic speeds but after shifting to an area ruled design they reduced drag by approximately 25% at transonic speeds and the new A version easily went supersonic.

Again this really only applies at near supersonic speeds (about 0.7 mach or about 500 mph) when certain areas on the body might force the airstream to accelerate to high subsonic speeds and incompressable flow has trouble getting out of the way of the car.

One other consideration, is that boundary layer thickness builds as the air moves down the body, creating an effectively larger and larger car body as the air must move around not only the car body but also this thickend boundary layer. One way to re-attach flow is to put a bulging section in the body to create a favorable pressure gradient (ie increasing pressure as you move down the body) and this can allow the detached flow to re-attach to the surface.

Without knowing the specific design considerations in that body shape it is just speculation but that rear bulge would also help move the center of pressure to the rear from where it would be on a uniform body. Like the skirt on a pellet gun pellet (or a badminton shuttle cock) moves the center of pressure well behind the nose to make it more stable.


http://history.nasa.gov/SP-4219/Chapter5.html

Offline robfrey

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Speed Demon Wind Tunnel Test
« Reply #29 on: February 14, 2016, 04:36:27 PM »
The reason for area ruling is to take into consideration of the area of the wings so that total cross sectional area grows and shrinks uniformly and smoothly. This car has no wings. Therefore there's no reason to neck the body down behind drivers compartment. There are no wings to compensate for.

As I understood it, the purpose is to minimize total cross sectional area change, and to control where on the vehicle this change occurs in order to limit the effects of the shock waves, or possibly take some sort of advantage of them.

On an airplane the wings are sort of necessary, and obviously must be accounted for in area rule design. I don't see how it would necessarily follow that just because a vehicle doesn't have wings the basic tenets wouldn't still apply.

Like I said, I'm no how no way an expert, and it was just a W.A.G. on my part.  8-)

Hi Wizzbang,
First off, if I sounded short with you on any of my post, I apologize. Somethings my fingers don't put down in writing in the same tone as my brain.
As I understand it, when area ruling a body whether a plane or car, you want the growth and shrinkage of the total cross sectional area to be smooth with no lumps. High spot of the cross sectional area including appendages should be somewhere near the center or slightly aft. I only say slightly aft because you must include jet or rocket plume in your modeling. There should not be two or more high spots as in coke bottle design. I'm pretty sure this will increase drag.
Imagine if you have a airplane and you slice it up lengthwise into 6" segments. You next measure the cross section of each of those segments and record them from front to back. Then you layout these numbers on a graph. You will quickly see if you have a problem. From here you can adjust the model if possible to improve the graph or just plug the "smoothness" into the equation that is needed to calculate "wave drag".
Now, to make matters more complicated the above mentioned method can only be used to calculate wave drag at Mach 1.0 Coming up with graph to calculate wave drag (the majority of your drag in a supersonic car) let's say Mach 1.5, these cross sectional area need to be calculated in swept back cones from the centerline of the body. The higher number you want to calculate for, the sharper the cones will need to be. Eric (Blue) knows or knows where to find the the angle of the cones for a given Mach number.

Eric has been a good and patient teacher to me and Eric if I have said anything that is wrong or inaccurate, please correct me.
Eric is a good teacher because he does not let me take his word for anything. He makes me read the text from where he has learned it.

What I'm also trying to say in this long winded text is that if the designers of the Speed Demon were trying area rule this shape, they messed up. I'm pretty sure they were not.

Off the subject slightly, there are a few high end Ultimate (supersonic) car designers have a theory that it is totally impractical to build an Area Ruled LSR car. I was lucky enough to be able to help Eric prove that theory incorrect. This car is area ruled to Mach 1.5.
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