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Author Topic: Frontal Area / Cd numbers  (Read 11719 times)

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Online Seldom Seen Slim

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Re: Frontal Area / Cd numbers
« Reply #285 on: June 16, 2019, 08:52:30 AM »
Quick turnaround:  I sure wish I could find pictures of the Burkland's turntable thingie for speeding the turnaround.  It was a slightly raised platform that Tom would coast the car onto after the down run.  The car stopped on the top - a turntable!

While the crew attacked the car the table rotated 180 degrees, some people walking backwards while changing plugs and filling tanks, some walking frontwards (on the other side of the car) - and stopped with the car pointing at the start line.  Finish the servicing, roll the car back to the surface, and viola, all ready.

It was quite the symphony in motion.  Thanks for bringing back the memory. :cheers: :cheers:
Jon E. Wennerberg
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Offline Stainless1

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Re: Frontal Area / Cd numbers
« Reply #286 on: June 16, 2019, 09:27:47 AM »
I'm with PJ, not only will that upset the apple cart... I don't think you will be able to build anything strong enough to move that much aero load.  Move the smallest amount of body that will get the chutes out.  Something that mounts on a parallelogram disrupting the minimum amount of air.
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Offline manta22

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Re: Frontal Area / Cd numbers
« Reply #287 on: June 16, 2019, 11:04:13 AM »
I think Pete has a good point. You might want to consider air brakes like the ones used on the F-86 Saber Jet. They extended out laterally on both sides so there was no pitching or yawing moment.

Regards, Neil  Tucson, AZ
Regards, Neil  Tucson, AZ

Offline Sumner

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Re: Frontal Area / Cd numbers
« Reply #288 on: June 16, 2019, 11:04:31 AM »
..... As speed increases the wing will lower to zero angle of attack producing zero drag and downforce. . Terry.

As speed is increased you still need downforce in some form, either weight or aero.  If it is aero you will need to adjust the angle of attack or it could be way too much at the same wing angle. 

The 'aero wall'.  I like to think that as the car has to overcome more and more aero drag to go faster it is like taking the car to a cement wall and putting the front bumper on it and burning the tires down.  The air is that wall and since it takes eight times more HP to run twice as fast you have to get that HP to the ground.  You need traction, one way or another, to get that done at top speed when you are against that 'aero wall' or the car is just going to spin the wheels/tire (if you have them) and go no faster.  The problem just isn't accelerating at slower speeds but also at terminal speed.

In our case with Hooley's car the weight added (over 2000 lbs) wasn't there so much to prevent spinning the tires under acceleration but to keep from spinning the tires at maximum speed.  The weight hurts when it comes to acceleration and I think that Rob is on the right track with the aero downforce but that can also be very tricky.  We added a wing when that was finally allowed but decided against the first big one as we had no idea how much downforce it might create.  So started with a small one that was pretty neutral and had plans to work up.  A wind tunnel and someone who really knows what they are doing would be a much better alternative but also more expensive,

Sumner
« Last Edit: June 18, 2019, 06:26:35 PM by Sumner »

Offline RaceEngineer

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Re: Frontal Area / Cd numbers
« Reply #289 on: June 16, 2019, 01:31:48 PM »
Simspeed,

What is the distance from the aerodynamic center of the new rear spoiler/wing (will be close to the geometric center) to the rear wheels and the front wheels?  This will determine the effect of the torsional moment (pitching torque) that will effect the load shift and possibly lift the front wheels off the ground.   The aero loads on a high angle of attack wing are very high at the targeted speed.   The  Lateral air brake flaps as Neil suggests would be preferred and can be made strong enough to handle the load (it's done on airplanes primarily made of aluminum).
IMHO it would be safer after throttling down the engine/motors to deploy a relatively small chute first.  Then flaps and second chute.  Then carbon-carbon disc brakes.
 
As was pointed out, Traction is most needed at max velocity in order to continue accelerating the vehicle.  Trading low drag for higher down force as speed and thrust increase is the technically preferred to adding weight but it is tricky and aero must be well sorted.

The turntable is mandatory if your going for a FIM record (turn around within an hour).
Feed back is always welcome!!!
Thanks

Offline Bratfink

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Re: Frontal Area / Cd numbers
« Reply #290 on: June 17, 2019, 02:01:51 PM »
Sumner makes some good points.

The angle of attack of the plane here is going to create some pitch issues that will make handling difficult. Not to mention the required strength of the system will make it both heavy and expensive. Even if it doesn't cause the vehicle to flip, the change in balance will be big enough that it'll unnerve you driving it. Half the battle of going fast is giving the driver confidence in the machine.

Aero done right is expensive, but you'd be surprised at how many people don't do Aero to save money then blow 3 times that amount on trying to get more power out of the engine. The problem in our field is available correct facilities. Layne and Tom did a great job trying to help teams with the understanding and providing a local facility. But it's tough business model. In the end the facility the teams really needed was much bigger and more expensive than the market is willing to bear.

Think that Gene Haas spent 80 Million on his facility for Nascar/Indy etc. And that facility is about 250mph too slow for what guys on this forum are doing! And those teams are pouring every dime they have into engineering other aspects of the car that are throwing up problems that no-one ever expected. It's truly a monumental task. So big a problem that it's no-one wonder teams spend more on driveline, at least there they can see the end goal.

Anyway different conversation for a different time and place (beer at my camper during speed week).

Keep up the good work.

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #291 on: June 18, 2019, 07:58:56 PM »
Hey Peter... your post went missing after the forum update but I did not look at the Target 550 air brake system.  Pretty slick I'd say.  Great use of space isn't it.  Thanks... Terry
« Last Edit: June 18, 2019, 08:00:45 PM by Simspeed »

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #292 on: June 18, 2019, 08:11:50 PM »
Sumner makes some good points.

The angle of attack of the plane here is going to create some pitch issues that will make handling difficult. Not to mention the required strength of the system will make it both heavy and expensive. Even if it doesn't cause the vehicle to flip, the change in balance will be big enough that it'll unnerve you driving it. Half the battle of going fast is giving the driver confidence in the machine.

Aero done right is expensive, but you'd be surprised at how many people don't do Aero to save money then blow 3 times that amount on trying to get more power out of the engine. The problem in our field is available correct facilities. Layne and Tom did a great job trying to help teams with the understanding and providing a local facility. But it's tough business model. In the end the facility the teams really needed was much bigger and more expensive than the market is willing to bear.

Think that Gene Haas spent 80 Million on his facility for Nascar/Indy etc. And that facility is about 250mph too slow for what guys on this forum are doing! And those teams are pouring every dime they have into engineering other aspects of the car that are throwing up problems that no-one ever expected. It's truly a monumental task. So big a problem that it's no-one wonder teams spend more on driveline, at least there they can see the end goal.

Anyway different conversation for a different time and place (beer at my camper during speed week).

Keep up the good work.

Hey James...thanks for the invite I hope to be there.  Your points about money invested in support infrastructure of other motorsports versus LSR is spot on.  There's just no where near the commercial appeal of LSR as there is almost any other form of auto racing, and I don't think there ever will be.  It goes back to a previous post I made on telling the racer's story in such a way that drives public interests and backing of corporate sponsors who want to sell products and services to them. Until we develop a scenario that draws everyone to the table of common interests LSR will advance only incrementally for decades to come in my opinion.  I'd sure like to see a breakthrough of some type that peaks the public consciousnesses.  Thanks... Terry.

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #293 on: June 18, 2019, 08:57:45 PM »
Considering everyone's thoughtful comments on the impracticality of placing an air brake wing behind the rear wheels I've shown here what mid mount active canard wings might look like just ahead of the quad drive wheels of the current design.  In this scenario the elliptical wings would tilt down as shown at the start of a run to add downforce/aero weight to increase traction as needed to build speed.  The wings would pivot back to reduce drag and downforce at higher speeds but could be electronically controlled to maintain just enough positive angle needed to pressure load wheel weight for optimum traction through control programming. As many have pointed out LSR cars hit a wall where aero drag can become greater than available traction and power limiting acceleration.  It occurs to me that through programmable active aero, in conjunction with traction control, acceleration can be increased for a given combination up to a certain point that should in theory allow the car to obtain greater speeds than one without such a system.  Your thoughts... Thanks.

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #294 on: June 18, 2019, 09:03:01 PM »
more...

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #295 on: June 18, 2019, 09:03:34 PM »
more

Offline RaceEngineer

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Re: Frontal Area / Cd numbers
« Reply #296 on: June 29, 2019, 11:24:53 PM »
Terry,

I like the wing w/ dihedral, combines 3 things ---the roll stability of the fin,  the downforce of an inverted wing and drag brake when rotated and it's in the right place.  Detail is not real clear but the bottom surface of the horizontal wing should be convex (wing shaped) and the top should be flat and the near vertical "wing" should be convex on the interior surface and flat on the outer surface (left wing rolls body right and right wing rolls body left. Lift increases the more the body rolls (self righting or one wing counter acts the other hopefully to keep it shiny side up.  Downside-- if one wing fails the other will cause the vehicle to roll.  How will these act in a strong side wind?  The  near verticals should probably have a short chord length to minimize there area to cross wind?
Hydraulic control for the air brake function?
What is  the current frontal area? (with the correct engine diameter?
Fewer comments, I guess folk are busy getting ready for SW!!  I've been busy.. 

 

Offline Peter Jack

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Re: Frontal Area / Cd numbers
« Reply #297 on: June 30, 2019, 12:07:47 AM »
How will the wing affect your bailout procedure? In an emergency I'm sure there would be no hesitation to step onto the wing but how will you do during tech?

Keep going, I'm enjoying the evolution.

Pete

Offline RaceEngineer

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Re: Frontal Area / Cd numbers
« Reply #298 on: June 30, 2019, 06:12:17 AM »
Pete,

Valid question.  The final position of the wind (within the wheelbase) needs to be adjusted based on detailed aero work.  When I said "it is in the right place I meant generally (ie: behind the rear wheels won't work).  Final position may likely be more forward to balance downforce on front vs. rear wheels (for stability).  Which is the linear ratio of position. The weight of the driver is a low load relative to the aero loads so the wing could be built robust enough to have a "step plate" area.  Thanks for highlighting this detail.  Lots of design considerations to balance. 

Don

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #299 on: July 01, 2019, 07:48:35 AM »
Terry,

I like the wing w/ dihedral, combines 3 things ---the roll stability of the fin,  the downforce of an inverted wing and drag brake when rotated and it's in the right place.  Detail is not real clear but the bottom surface of the horizontal wing should be convex (wing shaped) and the top should be flat and the near vertical "wing" should be convex on the interior surface and flat on the outer surface (left wing rolls body right and right wing rolls body left. Lift increases the more the body rolls (self righting or one wing counter acts the other hopefully to keep it shiny side up.  Downside-- if one wing fails the other will cause the vehicle to roll.  How will these act in a strong side wind?  The  near verticals should probably have a short chord length to minimize there area to cross wind?
Hydraulic control for the air brake function?
What is  the current frontal area? (with the correct engine diameter?
Fewer comments, I guess folk are busy getting ready for SW!!  I've been busy..

Hi Don...I appreciate your comments.  I used a simple ellipse shape for the wing/fin cross section.  I used the NASA FoilSim app to calculate downforce for the various parameters and angle of attack the wing would go through from a max of -14.5? forward pitch producing roughly 2500 lbs of downforce at 250 mph back to 0? angle producing zero lift and 19 lbs of drag at the same speed.  250 mph is the max speed setting for the app.  An ellipse shape offered the best performance characteristics for this configuration based on the app's various settings.

At zero angle of attack the fins should produce neutral left/right loading acting like normal stabilizer fins.  At negative pitch angles the fins would produce offsetting outward loading to hopefully help stabilize directional control.  No doubt if one fin fails at speed the car will roll.  Fin construction and support structure will need to be well engineered with safety factors.  I think this design will respond well to side winds given that the body itself acts as a wind block for the low wing placement.  Yes, I think electro-hydraulics is likely the best control system for this design.  Total frontal area of V.4.8 with wings is 6.388 sq.ft.  Thanks... Terry