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Author Topic: Speed Demon Wind Tunnel Test  (Read 12726 times)
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« Reply #30 on: February 14, 2016, 04:38:25 PM »

Thanks to 7800ebs for posting video! It's a good one.
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« Reply #31 on: February 14, 2016, 06:19:08 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.  cool

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.


No offense taken, or implied.  grin I'm just enjoying engaging in a conversation on a subject that interests me, but is way over my head in actuality.  cool

I can see that the Speed Demon design isn't optimized for lowest possible drag, and that makes sense, since it has to be a compromise that takes into consideration a whole raft of conflicting issues. What I thought I saw a possibility of was that they were using the basic principles to induce and manipulate shock waves to their benefit at the speeds they are targeting. It looks to me like the air will accelerate in the area behind the cockpit, and then encounter the flared wheel area, possibly forming a shock wave at the rear of the vehicle and inducing stability enhancing drag behind the CG at target speed without having to push a larger tail fin through the air while getting up to that speed, thus helping with acceleration.

Like I said, just blowing smoke and making Wild A** Guesses.  cheers

Eric's design looks awesome. Someone needs to really hit the Lotto and finance that.  cheers
« Last Edit: February 14, 2016, 06:22:02 PM by WhizzbangK.C. » Logged

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« Reply #32 on: February 17, 2016, 08:48:47 AM »

Hotrod, great info thanks for posting!
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« Reply #33 on: February 17, 2016, 02:59:48 PM »

Speaking of the SD wind tunnel test, Layne Christensen of the Darko Tunnel called yesterday and we're going to put the latest video of the new #715 car in the tunnel -- just last week or so.  I'll get it as soon as I can.  I hear it's over on Mike Cook's FB page, so you might have already seen it.  I'll put it here ASAP.
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« Reply #34 on: February 17, 2016, 03:16:42 PM »

http://hotrodenginetech.com/smokin-the-speed-demon/  cheers

Mike
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« Reply #35 on: February 18, 2016, 05:42:21 AM »

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.  cool

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.

Rob, everything you have said is correct.  Please post the M1.5 volume distribution graphic, plot, and USAFA shock distribution.  The surface porosity vs. Mach data may be requested by other teams, we will publish if and when a credible attempt on the ALSR is mounted.

So far there isn't one.
« Last Edit: February 20, 2016, 02:58:09 AM by Blue » Logged

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« Reply #36 on: February 20, 2016, 03:02:04 AM »

"Drag" is also your friend,  wink

never.
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« Reply #37 on: February 20, 2016, 03:56:57 AM »

"Drag" is also your friend,  wink

never.

"never", in relation to  huh huh
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« Reply #38 on: February 20, 2016, 02:08:25 PM »

OK, I'll modify that.  Drag is our friend when we want to slow down, not when we want to go fast.
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« Reply #39 on: February 20, 2016, 02:32:10 PM »

Without drag there will never be a CP.
  Sid
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« Reply #40 on: February 20, 2016, 03:47:51 PM »

with stablity, we go faster,,
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« Reply #41 on: February 23, 2016, 04:06:15 PM »

Yes and no.
All cars will have a certain amount of drag. The key is to have drag increase on the rear as well as air deflection as soon as yaw starts.
A small parachute hanging from the back of the car might keep it straight but that would be a terrible way to achieve what what we are looking for.
Btw, the aero drag that give you stability also gives you instability if it is on the wrong end of the car.
We could build a car that is stable that could run in a vacuum (assuming we had onboard oxygen for engine).  Space craft do it all the time.
But I regress. The reason for the vertical fins that we use is that as  soon as any yaw is encountered, it starts to redirect air like a wing in an angle of attack. The displacement of that air is what tries to straighten up the car keeping it stable. We never want to increase drag at the rear of the car, we want to manipulate the air to make the car stable.
I can't think of any instances where drag is good.
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« Reply #42 on: August 19, 2018, 10:40:38 PM »

Without drag there will never be a CP.
  Sid
There is a fundamental difference between forces and moments.  Cp (as is used in LSR) is a function of moment in yaw, drag is a separate force.
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« Reply #43 on: August 19, 2018, 11:12:24 PM »

Like many on this forum, I'm always willing to learn. Can you explain that?
  Sid.
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« Reply #44 on: August 20, 2018, 08:32:26 PM »

Mr. Blue,

In my little handbook of airfoil sections, Cp is the reference for Center of Pressure for the airfoil.

Did I miss something in the language of transformation for aero structures and references?

Thanx in advance,
HB2 smiley
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