Servers by jDrones

L/d test flight

(James Miller) #21

That’s not a short story.
If you calculate the proper dimensions of vortex generators for a wing of 22 cm chord at 14 m/s with the corresponding reynolds number, you find that most of the historical tests were performed wit generators that were far too large.
I found a product that bills itself as a turbulator that actually was a vortex generator tape in self-adhesive form and was pretty close to the proper size and spacing- 0.6 mm high instead of the 1mm that would have been ideal, so I used that for the generators. We all know the nasty tip stalls that some otherwise great aircraft have, and I was trying to find out if that problem could be reduced with devices to delay the stall at the tip.
In short, it worked partially on my much-abused but still true X-8, with the placement in the most effective position of 27% chord aft and a length of around 25 cm, with no detectable reduction of the L/D. The tip stall was reduced to the point where my fairly heavy X-8 would stall at around 7 M/sec. and a pitch angle held to around 5 degrees, with only a slight rolling tendency. I balanced the roll by varying the length of the generator strips until I got it as close to a straight-ahead stall as possible, but it never got to the point where you could say the problem was cured- only to the point where an alert pilot could dump the stick for less than a second to break the stall, still in stabilized mode, and it would fly right out of it—far better than the original condition. Here’s the link to the product.

Was this product truly making vortexes that would draw the free-stream air flow to the surface and keep it attached? Can’t really say, but it definitely worked to retard the stall.

(Jace McCown) #22

Thanks for the brief!

(Jace McCown) #23

So we did the glide L/D tests and it seemed to work okay. We didn’t ever pin down a number but got enough data to show that flying at x speed increases our L/D significanty over flying at x+ speed. It would be really hard to pin a number down to even a tenth because of the variance even in a single descent. A well tuned aircraft might do a better job at holding speed and maintainign a constant descent, but we saw quite a bit of variability. We also flew in some failry stiff winds, which didn’t help in acheiving prescise results, but didn’t really hurt the gross understanding we were tryign to achieve.

(James Miller) #24

The real world sure makes it tough to nail down a firm L/D number, I know. Here’s a well thought out tool I used to get a starting point for my estimates. My numbers matched this fairly well.
I was not particularly concerned about absolute L/D, but stall. Also, I looked at my notes, and the pitch angle was 15 degrees, not 5 degrees.