Cd deletion
 

I think we'ce all seen the aero civic but I think the aircraft the AR-5 and it's follow on the AR-6 racer may have taken things a little further... and all starting about 16 years ago!

Now sadly I can't post a link officially until I've posted 5 posts here.. this is my fifth, so I hope the moderators will tolerate a little bending of the rules

the URL is

dubdubdub dot AR-5 dot com

Just to summarise why I've posted this.

This aircraft achieves 207mph on just 65hp!... by my calculations that's 32mpg @ 207mph (assuming 20% thermodynamic efficiency)... hence the title of this post. More importantly he seems to have gone further than just making it slippery, he did some very careful design around joins between sections etc.

It made me wonder if the same technique could be used on belly pans (keep the area of the gap beneath the car the same at every point along the length, so raise the pan a little at the axles.. if there's room of course).

Derek

Some very interesting reading there Derek. Thank you.

What a cool plane in more respects than one!!! Not only is it beautiful aesthetically, but it weighs less than 500lbs and has a almost a 600 mile range on 12 gallons of fuel. I WANT ONE!!!

4gph @ 175mph (air speed) = 43.75mpg
3gph @ 165mph (air speed) = 55mpg

And he calls this "thirsty" !!

He says the plane has a Cd of 0.016 and a frontal area of 0.88 ft^2, giving a CdA of only 0.01408!!! Imagine what the numbers are without the wings and aileron winglets!

He attributes most of his low drag design to meticulous reduction in interference drag. He worked very hard to make sure the drag of each component of the exterior did not effect other parts.

There is some very interesting reading there for sure.

Very light with hardly a flat slab to it (less than a square foot !) and excellent workmanship in building and finishing it.

Now compare that to your car.
Lots of flat slabs - often of the see-thru kind ;)
Rubbers sticking out into the breeze to hold those windows.
Panel and door gaps.
Windshield wipers that for the most part aren't aerodynamically covered (despite that it has been known for years that they also cause additional injury to pedestrians when exposed).
Sharkfins that are mounted as standard even if none of the equipment that requires them is installed in the car.
Roofrails standard with free option to delete them - rather than the other way around.
Dirty underside, though all manufacturers know it'd improve the mileage to clean up that mess.

The list is nearly endless.
They all like to keep something behind the cupboard to make future improvements, rather than apply known mods NOW and research others for future use.

Maybe governments should force the simple measures that are known to improve mileage upon the manufacturers as must-do measures, rather than do-as-you-please.

1. Note the engine cooling exit vent on the AR-5. At the very least, this is a good example of how engine air could/should be vented from a car engine bay back into the slipstream and past the belly pan.

2. Remarkable as it is, the AR-6 is still probably not as efficient as the Strojnik Laminar Magic.

3. Google for "Davis DA-11" and note that even without much aerodynamic refinement, Davis' little ~165 lb. plane got over 100 mpg cruising at ~130 mph on a 20 hp lawn mower engine. What would the DA-11 do with aero cleanup of landing gear, wing roots and tips, etc.?

I have a vhs tape set on this airplane. Very useful for construction techniques and aerodynamics. He shows how they repaired the airplane after a crash.

The big problem is that the work is done in composite and is not very healthy to work with and work proceeds very slowly while the epoxy is curing.

I certainly don't think the government should be the nanny behind society.

The 0.88 is equivalent flat plate area, which has a Cd of 1.0 or 1.1 depending on who you ask. So the CdA is 0.88 or 0.968. The convention with aircraft is to compute the Cd based on planform area, so 0.88/.016 = 55 sq ft of planform area (silhouette viewed from the top).

motorcycle
 

Kenny Lyon's Cd 0.11, Becker-Lyon motorcycle streamliner does 207-mph with 150 Bhp,in ground-effect with tires on the ground,so 207mph with 65 Bhp sounds very reasonable for an aircraft.
The mpg would vary as a function of load and BSFC map of course.
I don't know about the Cd figure.Abbott and Von Doenhoff ( sp?) warn about using any aeronautical values outside of 'flight conditions.'
My opinion is that Cd 0.016 should be 0.16.The fuselage cannot be lower than Cd 0.04 in free flow and that would put it at Cd 0.08 in ground proximity.Adding 'skinny' tires would push it to Cd 0.12,same as 1953 MG EX 181,or GM Sunraycer ( 1987 ).
If it's a 'laminar' aircraft you can increase the Cd significantly,as it is not possible to have laminar flow above critical Reynolds number in ground-effect.( 20-mph up ).

"If it's a 'laminar' aircraft you can increase the Cd significantly,as it is not possible to have laminar flow above critical Reynolds number in ground-effect.( 20-mph up )."

Do you have a reference, or a rationale for that? Thanks.

Bob,

I was thinking of you when I went to the AR-5 website and was reading about just how *smooth* the aircraft was shaped.

Flashed back to our earlier discussion of how laminar flow can be easily *tripped* into turbulence by any roughness in the surface finish.

Jim.

I don't know if he said this on the web site but he mentions that waves are worse than surface fineness. I remember a part of the tape where he was measuring and plotting sagitta along a line on the airplane. He used 3 points about an inch apart. He then graphed it and compared it to other commercial aircraft.

laminar
 

Bob,just now catching your post.Am away from my references here at the copy center so relying on memory ( a scary proposition!),here's what I recall.
----------------------------------------------------------------------------
A laminar structure is designed to operate under 'flight conditions' which for aircraft is at altitude,in free-flow environment,with turbulent-free air.
A laminar structure is only 'laminar' up to the first point of minimum pressure ( center of lift ) after which the flow is necessarily turbulent due to the un-favorable pressure gradients present.
Also,in aircraft applications,drag is customarily governed by skin friction and embody zero flow separation,whereas in automotive applications,skin friction plays an insignificant role,with flow separation responsible for the largest portion of drag.
----------------------------------------------------------------------------
For road vehicles above 20-mph,and for the surface roughness conditions obtainable in automotive finishes ( or even polished surfaces ) the given surface grain size versus vehicle length/velocity relationship mandates that the Reynolds number go 'critical' and boundary layer transition to ' turbulent' with a ground/air velocity of 20-mph.Which is all good for road vehicles,as the turbulent boundary layer allows the separation point to be moved rearward,allowing pressure regain and reduction of pressure drag,the largest component of profile drag.
All air in ground proximity is considered turbulent and cannot support the environment necessary for laminar boundary layer,unless the velocity is held below 20-mph.( i.e. college teams pulling off 7,000 mpg as they creep along a closed-course ).
From memory,I will credit references I cited in my stickys: Fluid Mechanics by Dauherty and Franzini,Boundary Layer Theory by Schlichting,Aerodynamic Drag by S. Hoerner, Theoretical Aerodynamics ( can't remember author ),I think that Abbott and Von Doenhoff get into it also in their Theory of Wing Sections.
In my Fluids text there were some sample problems for road vehicles.Critical Rn occurred around 500,000 which was triggered at very low ground velocity.
That's all I can do off the top of my head.
At Battle Mountain I grilled the Cal Poly team( I think it was).They were using CFD for engineering their bike and believed that Cd 0.11 was probably the 'range' for bikes running,like Varna Diablo III.The day I was there,Sam went 81.6-MPH in it.
Matt Llewelyn's (sp?) Sylph was designed around something like a Cd 0.0066 laminar wing section and also scored Cd 0.11,in ground proximity in the Guggenheim tunnel at Cal Tech.
It appears that,on the ground, the 'laminar' forms cannot exploit the efficiencies they enjoy in free flight.

Regarding surface texture, about 20 years ago Dan Somers at NASA Langley did wind tunnel tests of factory painted, optically smooth and shiny, and sanded (in direction of flow) wing panels.

Guess what?

Draggiest was the shiny surface, next most draggy was factory painted surface, and least draggy was sanded-in-direction-of-flow surface. This is also essentially the reason 3M skins were used on America's Cup sailboats, to good effect if aligned with flow or no more than ~15 degrees off. Has about the same texture as a phonograph record.

For our purposed, probably best to forego attempts at laminar flow, but try for turbulent but attached flow, which is much less draggy than turbulent and detached flow.

GM (Aerovironment) claims that they were able to achieve laminar flow with their Sunraycer:

"In the GM wind tunnel, we were able to put a stethoscope against the body and hear whether the flow was laminar or turbulent. If it was laminar, the sound was smooth, a whooshing; turbulence burbled. We were able to confirm that the flow over the front third of the car was laminar everywhere except for two thin wedges just above the side edges starting back about the middle of the front wheels. It was not clear why the flow was not laminar there, but as it represented less than one percent of the drag of the vehicle, we were not overly concerned."

Bart Hibbs, Lecture 2-2: Sunraycer Aerodynamic Development, p 28, in GM Sunraycer Case History, 1990.

PS: Speed was 40 mph.

I would agree that Georgi's bikes are anything but laminar flow bodies. Yet they are quite practical vehicles and can be ridden in 20+mph gusty conditions. I have two hanging on my wall (Varna 2, the grandfather of the diablo3 and a Mephisto clone (unfinished)) and have personally witnessed the Varna2 being ridden on a closed course in dust devils at 50+ mph for an endurance race.

Of course Georgi doesn't use any engineering or computers, just what looks good and experience. Georgi has told me that as long as the surface is not wavy then the finish doesn't make that much difference at the speeds that human powered vehicles run. I believe him. He goes out at night with a flashlight to refine the shape of his plugs using tangential lighting looking for waves.

Back to the subject.

Aerohead writes "All air in ground proximity is considered turbulent and cannot support the environment necessary for laminar boundary layer,unless the velocity is held below 20-mph."
I have always been skeptical of claims of laminar flow on vehicles, and this is another common consideration, but it seems that when the wind is legal for a speed record, the air might be still enough for some laminar flow.

Anybody trying for it will want to look at longboards for sanding - flexible, two-handed sanding blocks approaching a meter long.

Hi Otto,

Did the article mention anything about what grit sand paper was used?

Thanks, Jim.

Hi Bob,

Do have any recommendations for this type of sanding board? Maybe some pictures of one's you may have?

Thanks, Jim.

We used sanding belts and slid a section of formica or masonite a bit longer than half the length of the belt. Making in effect a bow. Sanding is done fore and aft using the open side. It works good with two people. Start with 36 grit and work toward finer. Belts can be gotten from Mcmaster for a good price.

Here is a hand written page describing Georgiev's sanding and some construction techniques: I used the last one described.
http://www.veloliner.com/sorcerer/sandinginfo.jpg

I usually use a thin wooden board, ripped to the thickness suitable for the contours I'm working on. Spray glue is pretty good for holding on sandpaper until it is time to change it. End handles can be screwed or glued on.
If you are really into making smooth shapes, have a look at foamed glass blocks. They wear into any contour as you work, cut like 40 grit, and finish like 100 grit. They are often called Fartstone, being blown with hydrogen sulphide.
I'm also very fond of a 1" hand-held belt sander with all the junk removed so that I can use the unsupported side of the belt.

laminar
 

Patrick,did Bart say that the boundary layer was laminar,or the flow outside the boundary layer?

laminar
 

I think that the general flow outside the boundary layer ( the 'ideal' or 'inviscid' flow ) is laminar,as with any structure free of 'stall' or 'separation.'

320-grit sandpaper
 

Abbott and Von Doenhoff report of an investigation conducted by NACA in which aircraft surfaces,sanded in the direction of airflow with # 320 sandpaper exhibited no more drag than highly polished surfaces,and that this became their 'standard' for the high speed tunnel work.

new thread with supporting data
 

I've compiled some data which concerns 'laminar' aircraft technology as it might pertain to EcoModding and will post as a separate thread 'Laminar' Aircraft and EcoModding.
I thought the topic deserved its own thread and will probably pop up from time to time.
Hope it's okay.

He didn't say, but since they were listening with a stethoscope pressed against the body, my guess would be that it was the boundary layer.

AR-5 videos
 

Bumping this old thread as i have been watching these excelent videos full of design and construction details... heres the link https://youtu.be/U2PX1p7nbw8

Mike uses sandwich construction with foam and fiberglass/epoxy to build this slippery one seat plane, which would transfer to aero caps or covers nicely.

Look up The Arnold Company on you tube. Many excellent vids like "moldless low drag wheelpants"

That's quite the hot-wire foam cutter. Here's mine:

http://ecomodder.com/forum/member-fr...e6263-tool.jpg

And here's the Lionel transformer my folks used to cut foam around the time Burt Rutan was starting out:

http://ecomodder.com/forum/member-fr...ransformer.jpg

I'm only 42 minutes in, now to go back and finish.

edit: The wheel pants thing is great. There are interesting design decisions from offsetting the strut to turning the split laterally.

The whole process in miniature.