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Old 12-07-2019, 01:06 AM   #21 (permalink)
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Would this be suitable for Etsy?

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Old 12-07-2019, 03:09 PM   #22 (permalink)
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deformation

Quote:
Originally Posted by Hersbird View Post
Would the deforming be necessarily bad? If there was a high pressure area and it could push in vs a low pressure area that it could expand into. I guess the problem being the rigid parts that have to stay in one place.

How much does material make in the surface skin drag at say 70 mph? Would it be worth using a smooth fabric like Sunbrella vs rough duck canvas?

Actually Sunbrella is also woven, maybe PVC vinyl?
If you'd nailed a really clean shape,and then the airflow itself altered that form,you'd have a can of worms,as the deforming force would be varying as the square of the velocity and shape,all over the place,depending on elasticity.The stiffer the fabric the better.
It's why fabric-covered aircraft are shrunk-tensioned and doped to 'fix' the shape.You don't want the center of pressure moving around on you.
Race car driver,Bernd Rosemeyer was killed when under-spec,weak sheetmetal deformed at high speed,producing uncontrollable,destabilizing aerodynamic forces. Early V-2 development rockets crashed for the same reason.Wernher von Braun had to risk his life,close to the crashing in order to figure out what was happening.
Skin friction is only a minor portion of air drag.You could probably use carpet back there if you wanted.That entire area is already within a turbulent boundary layer,with the free,inviscid flow a couple inches above it,flowing laminar over all,as if it were on Teflon'd glass.Aeroelasticity is not your friend.
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Old 12-08-2019, 11:21 PM   #23 (permalink)
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Quote:
Originally Posted by freebeard View Post
This one looks taut:

Where this one looks much looser.

Is that a real difference or a trick of the light?

The tailgate bulkhead looks to be an opportunity to tension it. Fold into place and then pull back a fraction of an inch on the trailing edge.

There is a trailing step edge of 2-3" at the cab. It's reaching for the shape it wants to be. Is there room in the folding mechanism for curved front edges? Do you have a picture of the bare frame erected?
You are correct, Freebeard, in one of the pictures the cover is fairly taut, while in the other picture the cover is much looser. The material that I used for the cover was some sort of heavy nylon canvas which I bought from a local fabric shop. Unfortunately, this material is very sensitive to the temperature and it expands when it is cold and shrinks when it is warm or the sun is on it, which is not optimal for this application. My next version of this cover will have a much more robust material (convertible canvas) that should not have this problem.

I'm pretty sure I could curve the front edge - below is a picture of the bare frame.

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Old 12-08-2019, 11:24 PM   #24 (permalink)
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Quote:
Originally Posted by Vman455 View Post
This is freaking cool!

What might be going on is the air under the cap is stagnant, and at ambient pressure or close to it, whereas the air moving over the outside of the cap, even though it's regaining pressure as it moves back, is at lower pressure. High pressure inside/low pressure outside => inflation. That's not necessarily a bad thing; this article-interview with a GM aerodynamic engineer quotes her as saying,



I think you'll need sensitive measurement to see any difference between this and a hard cover, but if you can manage that the results should be interesting as to which is better. I'm not sure which I'd put my money on.
I was thinking the same thing, VMAN. It appears that the air outside the cap is a lower pressure than the air inside the cap causing the shape change. From my tuft testing, it appears that this is a good thing... I think.
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Old 12-08-2019, 11:31 PM   #25 (permalink)
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Quote:
Originally Posted by justinooo9 View Post
Id spend a decent amount of money on that if you ever thought about selling a kit someone could assemble (weld/bolt together at home).....

Seriously I would just a thought.
Thanks, justinooo9! The only problem with trying to sell something like this is that all pickup trucks have different bed lengths and cab heights, which affects the length and position of all the levers and pivot points of the frame. So each type of truck would have to have its own aerocap frame and cover design.
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Old 12-08-2019, 11:34 PM   #26 (permalink)
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Quote:
Originally Posted by freebeard View Post
If you were to sell a kit, the way to promote it would be to post a how-to on Instructables, and offer the kit if they don't want to fabricobble their own.

https://www.instructables.com/workshop/cars/projects/
Man, seeing these on more trucks would be awesome. Just not sure how to teach everyone to figure out the geometry of the frame....
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Old 12-08-2019, 11:39 PM   #27 (permalink)
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Quote:
Originally Posted by botsapper View Post
Intuitively figured that the downward airflow will lift the fabric, that's why you might need sleeved seams or hoops on your fabric top and they wrap around transverse bars. In my old concept I left the aero-profile side/nerf bars rigid and you could attach cross bars and the fabric top could be zippered into place.
There are tension fabric structures are easy to install and quite hardy.

Great shape and keeping close interest on your testing period...
Thanks botsapper - that's a good idea. I was also thinking that I am going to build more tension into my next version of the cap. By using a different cover material and beefing up the frame a little bit, I should be able to create quite a bit of tension which will resist the deformation. It will be interesting to see what happens.... I am kind of learning as I go.
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Old 12-08-2019, 11:54 PM   #28 (permalink)
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Quote:
Originally Posted by aerohead View Post
If you'd nailed a really clean shape,and then the airflow itself altered that form,you'd have a can of worms,as the deforming force would be varying as the square of the velocity and shape,all over the place,depending on elasticity.The stiffer the fabric the better.
It's why fabric-covered aircraft are shrunk-tensioned and doped to 'fix' the shape.You don't want the center of pressure moving around on you.
Race car driver,Bernd Rosemeyer was killed when under-spec,weak sheetmetal deformed at high speed,producing uncontrollable,destabilizing aerodynamic forces. Early V-2 development rockets crashed for the same reason.Wernher von Braun had to risk his life,close to the crashing in order to figure out what was happening.
Skin friction is only a minor portion of air drag.You could probably use carpet back there if you wanted.That entire area is already within a turbulent boundary layer,with the free,inviscid flow a couple inches above it,flowing laminar over all,as if it were on Teflon'd glass.Aeroelasticity is not your friend.
Thank you for your input on this, Areohead, I really appreciate it! I wasn't sure if the deformation was a good or bad thing, but a post like this helps me know what direction I should work towards for my next version.

The other day I was on the highway and I was driving next to a Porsche 911 convertible with its top up. I was looking closely at his convertible top at speed and I noticed that it did not deform at all. No wrinkles, no shaking, no bulging... it acted as rigid structure. Although I don't have the resources that an major auto manufacturer does, I think I should be able to create a aerocap that performs similar to that convertible top. I've learned a lot from this first version of my cap and I hope to use all these lessons to create a better version.
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Old 12-09-2019, 10:34 AM   #29 (permalink)
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It was a given with American convertibles 1965-1975 that highway runs were made with rear windows opened part way.

The expert in restoring Lincoln 1961-1967 four door convertible top mechanisms is an excellent subject in a series of videos and text (John Cashman, IIRC). Fascinating structure (back from when Americans were worthy of quality).

What’s the square footage of material that Lincoln roof?

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Old 12-09-2019, 01:10 PM   #30 (permalink)
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Quote:
Man, seeing these on more trucks would be awesome. Just not sure how to teach everyone to figure out the geometry of the frame....
That's the spirit. You could patent it and monetize it and sell a few, but show how and then offer the hard bits and you'll likely do as well if not better. The people who want one but can't afford a commercial product wouldn't be buying one anyway.

The geometry of the frame is the part I am curious about. How did you determine the dimensions you used? Feedback is available here.

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