Me and my big mouth..
I have been trying to ge a friend into turbine aircrafts, and he said he liked the Hotspot,
and would have bought one if they came with winglets.
Thinking that he would never get serious, I say "If you buy that plane I will make winglets for you".
That was half a year ago, but last week he ordered a Hotspot, and has of course not forgotten my words.

There are some small drawings in the Andy Lennon book, but if anyone could provide me with some more
information, I would be grateful.

Lars

I have a couple technical papers about winglet design for sailplanes - but I haven't had a chance to read them yet. I would attach them but haven't found out how to do so.

All 3 reports are about 500 Kb comobined, let me know if you want me to e-mail em.

Other than looks there's no possible valid reason to put winglets on such a model. In fact it could be argued very favourably that they will HURT the performance at certain points.

Winglets are used on commercial airliners for one very good reason. They promote more wing efficiency and less fuel consumption AT ONE SPEED. Go back and read the capitals again.....

There was a very good and full article in Model Aviaton lots of years ago that summarized the NASA report. The basics were that the winglets are small twisted wings that are turned up at a sharp angle. The winglets are also severley washed out or twisted such that the vortices are minimized at their tips. The amount of twist and the angles they are set at overall are directly tied to one cruise angle of attack for that wing. On a model that will presumably be used for the usual jet style aerobatics they make no sense whatsoever. Also the size of the winglets affects the vertical stabilizer size as the winglets can be shown to have a slight destabilizing effect depending on the wing's sweepback.

Even sailplanes have largely tossed the idea of winglets because they realized that overall efficiency over a wide speed RANGE is more important that a higher efficiency at a small range of speeds. The currently popular crescent or Schumann type tips provide good flow over that wide speed range.

So if he just wants some little shark fins for looks then just stick shark fins on the wingtips. Carve 'em out of balsa and profile the base to fit, glue with epoxy and use a bit more epoxy resin, glass cloth and microballoons to reinforce and fair the finlet into the tip. Shoot on some primer and paint and you're good to go. Just don't make them too large.

If it'll get you out of trouble with your buddy when you suggest that you shouldn't bother with them print this out and show him....

Just to reinforce what Bruce has said, I built a series of F1A gliders that featured Whitcomb winglets. Nicknamed by the locals as 747-400's because of their size (they were high aspect ratio in the late 1970's, standard now)

You could see these winglets working from the flightpath, a very flat fugoid - from the top (slowest - no stall) the model nosed slowly over, accelerating, then the winglets would cut in and the model would climb, again very slowly, with the nose coming up as the speed reduced. A full cycle would take perhaps 3 or 4 seconds, and would have the model moving through an altitude change of perhap 12 inches. The speed differential was not noticeable but has been deduced as the explanation of the flight path.

How did we know that they worked? By knocking them off one side to see what happened to the glide. In every case the model turned torward the side with no winglet. And I mean turned - the standard circle for these models was about 50m diameter, without one winglet, they were turning in about 20m.

Did they make any difference to the performance of the models? In my head - a big yes. My heart does still have reservations.

Of course, the hotspot is on of the delta-wing turbine sport planes, so winglets would just add to the directional stability. If you are going just for looks, the tech details just don't matter that much. The winglet should be about as tall as the wing chord at the tip and I prefer to use the shape of the vertical fin as a guide for the tip shape.

The fairing from wing to tip should be smooth and rounded in the inside corner as weell as the outside. This will help avoid tip stalls as drag from turbulence being generated at the root of the winglet.

Oops, didn't realize the Hotspot was a delta. So then they aren't technically winglets but tip fins.

Went and found a pic of the model.....



GOOD GRIEF! Why would you want to add winglets to a model that looks like this.

If you DO add winglets then I think it's safe to say you should reduce the verticals by about the same amount that you add to the tip just so the spiral stability stays the same..

I really think he's nutz to want tip fins on a model of this appearance. But to each their own.

Bruce; I totally agree with you. Of course I will try to talk him out of it,
and if it dont work, I will go for detachable shark fins.

Thanks guys!

Lars

Winglets are actually washed IN, not out, the twist is to keep the local angle of attack constant along the span of the winglet, the local flow is most angled at the tip of the wing, and the angle gets smaller and smaller as you go up. Therefore the winglet washes in to try to keep the AOA constant.... with that said, winglets may screw up the aerodynamics of this plane, deltas often need that vortex for high aoa lift, and lateral stability.

Ty

Ty, I guess it's all in your perspective. The article I have showed them strongly washed IN at the root near the wing tip and twisting strongly towards the tip of the winglet as wash OUT. The angles would be set as you say by the local airflow so perhaps it all depends on which aircraft the winglets are designed for. They ARE operating in a largely 3D region of air after all

The concept was to maintain the angle of attack of the tip of the wing around the corner and onto the winglet base. The idea being that the strong lift would match the pressures of the tip of the wing with the base of the winglet. The winglet was then strongly washed OUT so the lift it produces tapers off to close to zero at the upper tip to reduce the strength of the tip vortex. The actual angles used may vary from wing style to style based on the spanwise airflow and the size and angles of the votex flow at the tip. The key element according to that article was that the Cl goes from matching the wingtip at the root of the winglet to close to 0 or even a trifle negative at the tip.

This matching of the lift of the fixed angle winglet root areas to the wing tips is whay makes them largely a single angle of attack cure for drag. I called them a single speed solution before and that wasn't right. It's more about the angle of attack and thus the lift coefficient.

But none of this changes the fact that on a Hotspot I think they would look hideous. The twin canted fins already imparts a nice techy and serious look. Tip finlets or winglets in this case would be like having dandelions in a tulip garden. Just way too many vertical bits...

Bruce, the article I used for my experiments with Whitcombe tiplets gave the following -

Tiplet root - -11*
Tiplet tip - -4*

The most successful set that I made were jigged on the wingtip at -7* > 0*. I tried the reduced toe-out on the basis that the out-flow angle of the tip vortex would be smaller and tighter than with a fullsized a/c (scaled naturally).

What I think most people who try these things lose sight of is that they are NOT just "glued on" at 90* to span line (right angles to the wing).

My earlier F1A;s tried this and it did not work a dang. So I tried setting the wing design to give sufficient lateral wing area with a 12* dihedral. Now why 12*? Because the Whitcombe tips are set at 78* to the spanline, plus 12* = 90*, ie vertical to the horizontal plane. Why do this? So that I didn't have to factor the tiplet area into the total wing area.... "Part of the wing? Nah!!! They are forward fins..."

But one thing that I did prove was that the combination of rake (sweep), camber(from spanline looking from the front), and angle of attack (toe-out) was critical to them working.

BTW the wing airfoil used was ThomannF4.

Probligo, a wonderful real world example.

I'm surprised that you used toe OUT (as in wash out with a strong twist to wash IN at the tips of the winglets) since the article I have showed they should be lifting quite strongly inwards in order to generate the pressure needed on the outer surfaces to force the air from under the wing to stay there. And your -7* to 0* seems to be at complete odds to the original article.

Or maybe I'm just remembering the theory wong after all these years. It was some 10 years ago that I last read the darn thing.

I think I'll just sit back and watch where all this goes from here on in....

In one of my wind tunnel experiments in college I built three winglets, one with no twist, one with washout, and one with washin. The winglets were mounted to the end of the low aspect ratio test wing such that they could be adjusted at varrious angles. The tests showed that the winglet with washout was less efficient then the straight and washed in winglets... and there was very little difference between no twist and the one with 5 degrees of washin. The anlgle that the winglet is mounted to the wing does make a huge difference on efficiency. As the angle of attack increases the trailing edge of the winglet must be moved inward to optimise performance. The goal of the experiment was to determine if in flight adjustable winglets warrented the complexity... for some missions where good short field and long range are both needed, such as the C-17, I think such a system would be appropriate. For airliners they just care about one angle of attack, cruise. It's interesting why airliners use winglets... it has nothing to do with efficiency... they just need short wingspans so they can fit more airplanes into the gates. That infromation came from a boeing wind tunnel guy that later taught my aerodynamics classes... he designed the highly swept wingtip extensions that look like winglets but are layed down flat... they are actually more effiecient then winglets, but they increase the wingspan so very few boeing aircraft use them.

Ty

Bruce,

Not by any means am I trying to say that you are wrong - hell man I am just an ol' accountant.

The way that I got my setup to make sense was to picture the airflow as it came out from under the wingtip, and how the winglet was placed inside of that local flow. [sm=idea.gif]

The towout makes sense then.

Remember too that the Whitcombe is positioned from the wing TE, and has a fairly severe rake - the TE is 38* from vertical if I recollect and you have to figure that into the mix as well.

I can tell you that it makes a very good mentate for those long evenings around this time of year when there is nothing to watch on tv...

Here is how it looks with "Winglets" Have flown it some time now.
But not without the winglets. It flyes extremely well, and looks great in the air.
Don`t think this has a great effect on the model, other than the looks.
The only thing missing now is to find å nice coloursheme.
Thanks for the help guys.
LN-JET can`t get me to take them off now.