.

.

All things hang gliding. This is the main forum. New users, introduce yourself.

Moderators: sg, mods

User avatar
By designbydave
#61522
I was strapping the camera on my glider yesterday when Rob suggested it might be interesting to watch the Tufts during a stall.

[youtube]
[/youtube]

unfortunately the low resolution youtube video makes it hard to see the tufts so here are some screen shots.

The first one is of normal flight, note the direction of the Tuft:

Image


In the next 2 images the wing is stalled:

Image

Image

When the wing stalls, the flow along the surface of the wing become detached and the direction of airflow will reverse towards the trailing edge of the wing. This is what the Tufts are indicating.

Here are a couple of CFD images I generated last year for my senior project. The first one is of an airfoil at 12 degrees angle of attack, which in this case is not yet stalled. (Dark blue represents slow moving air, while the red is fast, unites are in m/s.) The blue "tail" coming off the end of this airflow is the separated flow I was referring to earlier.

Image


Now take a look at a CFD image of a stalled airfoil:

Image

I can't remember the angle of attack on this one, but I'm pretty sure it was 20 degrees (I don't think we could get solutions for anything over 20 degrees.) You can see here that the majority of flow has detached.

EDIT: attached higher resolution video. Works best in VLC
Attachments
(19.48 MiB) Downloaded 34 times
Last edited by designbydave on Mon Apr 07, 2008 12:42 pm, edited 1 time in total.
User avatar
By HangDiver
#61530
Dave,

Those CFC images are very impressive.... Are these wind tunnel simulations - curious how you generated these? They're really informative...

Rich
User avatar
By PilotGuy
#61533
As a fellow aerospace engineer, the still shots of the tufts and the CFD solutions get my knickers all a twitter. :mrgreen: Good work, man!

The ground doesn't look very far away in the stall images, by the way.
User avatar
By designbydave
#61538
PilotGuy wrote:The ground doesn't look very far away in the stall images, by the way.
Thats the mountain, I was flying away from it and in smooth air. There in more clearance than it looks like.
HangDiver wrote:Those CFC images are very impressive.... Are these wind tunnel simulations - curious how you generated these? They're really informative...
The CFD (Computational Fluid Dynamics) images were generated using the 2D structured version of Fluent. I have some other images I will see if I can dig out.
User avatar
By Lobido
#61546
PilotGuy wrote: the still shots of the tufts and the CFD solutions get my knickers all a twitter.
A disturbing thought, indeed, Jeff. :roll:
User avatar
By AP
#61588
Those images are tops. My knickers are also twisting.

I am really surprised how good the attachment is at 12 degrees. I suppose it is a efficient airfoil. Dave, have you done similar analysis on HG airfoils? We did a lot of 'tuft' analyisis on the Airborne C4 and to look at the footage was a real eye opener. Even at low AoA many tufts rear of the high point showed some separation. I was often left with the impression that nearly all AoA our HG airfoils are quite inefficient.

Or do you think this immediate boundary layer will always show a degree of separation? Or do you think our HG airfoils are a poor shape, being limited by the shape of the LE, flat undersurface etc?
User avatar
By knumbknuts
#61592
Lobido wrote:
PilotGuy wrote: the still shots of the tufts and the CFD solutions get my knickers all a twitter.
A disturbing thought, indeed, Jeff. :roll:
Isn't this about the time that Radwhacker says: "Did someone say 'tufts'?" and then link to gay pr0n?

Not that I am asking for that! :run:
User avatar
By designbydave
#61595
AP wrote:I am really surprised how good the attachment is at 12 degrees. I suppose it is a efficient airfoil.
Thats actually the airfoil used on the B-24, its from a family of airfoils developed in the 30s by a guy named Davis. The CFD solver assumes a smooth surface, where real aircraft have panel lines, rivets, or stitching. Thats one explanation for the amount of attachment.

AP wrote:Dave, have you done similar analysis on HG airfoils?
no, but I still have access to the software (:wink:) so I would be interested in testing hang glider airfoils. Another issue is, I have no clue what kinds of airfoils are using. Are the proprietary, or developed specifically for hang gliders? Or are they just sort of improvised to work with the types of wings we fly (65%, 85% double surface etc...)
AP wrote:I was often left with the impression that nearly all AoA our HG airfoils are quite inefficient.
Could be. Another issue with hang gliders is that the shape of the airfoil is a function of the load on the wing, which is a function of airspeed. Stuff the bar and the wing changes shape...
AP wrote:Or do you think this immediate boundary layer will always show a degree of separation?
Well the Tuft should lie at least partially within the boundary layer on the wing and my video clearly shows the direction of flow...
AP wrote:Or do you think our HG airfoils are a poor shape, being limited by the shape of the LE, flat undersurface etc?
yeah flat under surface, which actually becomes concave at higher speeds...
User avatar
By PilotGuy
#61628
I was thinking of doing some CFD stuff, also with pretty colors, but I ran into the same issue of not knowing what airfoil they use. I suppose maybe I could just call and ask, but then I wouldn't have such a good excuse to avoid doing the work.
User avatar
By Lobido
#61630
PilotGuy wrote:the still shots of the tufts and the CFD solutions get my knickers all a twitter.
AP wrote: My knickers are also twisting.
I suggest a duel, at dawn. Choose your knickers. gentlemen.
User avatar
By Gen.Tech.
#61658
Similar vid. with sailplane.


<object width="425" height="355"><param name="movie" value="
name="wmode" value="transparent"></param><embed src="
" type="application/x-shockwave-flash" wmode="transparent" width="425" height="355"></embed></object>[youtube][/youtube]
User avatar
By Gen.Tech.
#61660
Seem to have trouble adding youtube video to post. :roll:
User avatar
By knumbknuts
#61665
html is not enabled.

just paste the link in and highlight it and press the youtube button up there.
User avatar
By rallyant
#61669
designbydave wrote: no, but I still have access to the software (:wink:) so I would be interested in testing hang glider airfoils.
nice!
can the program you have test multiple airfoils at the same time like some programs can? ie washout.

I often wander if speed gliding was a big enough sport if they would make a hang glider with convex on the underside also.
User avatar
By designbydave
#61677
Gen.Tech. wrote:Similar vid. with sailplane.
ah, very cool

makes me want to go buy a spool of red yarn :thumbsup:
User avatar
By AP
#61724
DbyD
I have no clue what kinds of airfoils are using. Are the proprietary, or developed specifically for hang gliders? Or are they just sort of improvised to work with the types of wings we fly (65%, 85% double surface etc...)
The latter is my experience. I was quietly shocked in the begining to see the 'R&D' :wink: :shock: used by the HG manufacturers, at least one of them in any case. Development would usually involve getting the battens, placing each one in turn against the knee and simply bending away. Then go flight test the 'new' profile and if it worked, good, if it was an improvement even better. Certainly not rocket science. This method however has narrow tollerances for a given sail. Changing the camber, shifting the high points etc eventually misfits the sail which is fixed in chord etc.
I think Aeros were ahead of the game since they had acquired a wind tunnel (albeit a small one) off Aeroflot. For those first few years Oleg, their chief comp pilot, was winning everything. Don't know whether that was because of his superior flying ability though.
User avatar
By rallyant
#61924
I wondered if that was the case, while some of these euro companies have been computer designing there wings airfoils for years.

Brilliant study material! Searching for some extra[…]

This was a flight that ticked so many boxes for my[…]

Hang Gliding Lessons

Anyone interested in learning to hang glide, by o[…]

Dune heaven!

YEA, Johnny! . . . . 8) . . .