RE: Best way to avoid those ****** wing tip vortices ?
 

here is my first atempt, it has been anhilated due to inflight wing failure around 140 mph

it was strong, but not enough........

RE: Best way to avoid those ****** wing tip vortices ?
 

With the planform restriction, unless you can cheat and make the wing thinner, you're going to have outfly the other guy.
Adding -anything- will add drag.

RE: Best way to avoid those ****** wing tip vortices ?
 

For that kind of thing, I wouldn't bother with messing with winglets, for all the reasons mentioned above, and in those threads that were mentioned. But a lower-drag wingtip might help a little, or it might not.

However, I would see about smoothing out the engine/pipe installation, lots of drag there that can go away. And maybe a fillet along the wing/fuse joint to smooth that out as well.

RE: Best way to avoid those ****** wing tip vortices ?
 

Salmonbug, that cutoff wintip that you have already is probably a darn good option. I actually wouldn't mess with it. I would worry about all that profile drag that you have on that thing.

RE: Best way to avoid those ****** wing tip vortices ?
 

I agree, but if strykaas wants to, its all benefit for me

the only thing I did is to build it without the canopy

RE: Best way to avoid those ****** wing tip vortices ?
 

salmonbug & strykaas,

I'll assume you are both flying the same plane. Looking at the picture the salmonbug posted, I would say there are few things you could do to get some drag reduction. No wingtip treatment for induced drag reduction would be among them. You are optimizing your plane for high speed flight. At high speed, the CL will be small, and induced drag will be correspondingly small. There are several things you could do to reduce profile drag:

1) Put a nice fairing behind the engine, leaving a gap or channel for cooling flow.
2) Put nice rounded wing tips on the wings. Some people will tell you that a square tip, as is on the plane now, will reduce induced drag. I don't believe that, but I know that a rounded tip will reduce profile drag, which probably matters much more for your race.
3) Make some nice smooth fillets for the inside corners where the vertical stabilizers meet the fuselage. Some soft balsa stock could be glued there and carved/sanded to nice, smooth, rounded fillets.
4) Add some fillets where the wing joins the fuselage.

banktoturn

RE: Best way to avoid those ****** wing tip vortices ?
 

What Bank said, and...
Add lightness! Take out all the parts you don't need for THAT race!
The one's you must have, use the minimum amount of material.
The lighter plane has a bit less drag.
Move the c.g. back as far as you can stand..
That also cuts trim drag.
But if there's turning involved... you will have to outfly him.

RE: Best way to avoid those ****** wing tip vortices ?
 

Induced drag goes down as speed goes up, but it will still be the predominant form of drag on the aircraft at speeds below about 200 mph. (That REALLY depends on the value of C_Dp for your particular craft). In other words, reducing it will get you the greatest gains in drag reduction.

Lightening the aircraft's weight will effectively reduce the C_L needed to keep it aloft. Your trim will be different though and now you have trim drag to combat (off-design flight). The best option in this case might be to reduce the incidence of the main wing.... that may be going too far with modification to your aircraft though. You need to target a speed and lower the wing incidence to match it. I would think that the lower induced drag would really win out over any bad trim drag that you may encounter.

RE: Best way to avoid those ****** wing tip vortices ?
 

Ken,

I am pretty sure that this is incorrect. Why do you think that induced drag is the predominant drag component below 200 mph?

banktoturn

RE: Best way to avoid those ****** wing tip vortices ?
 

Induced drag is highest at the lowest speeds. Parasite drag goes up as speed goes up. When we are talking low-speed aerodynamics we are generally talking about speeds below about 30% of the speed of sound (about 200 mph, that's where i got that number). Induced drag is high in this aerodynamic regime.

The predominant form of drag really depends on how much of each form of subsonic drag that you have. Parasite drag will definitely become an increasing problem with increasing speed, but we need to consider "how much" of each form of drag that you have. If you have say, 70% induced drag at the speed you are flying at, it makes more sense to try to reduce that 70% rather than go after the 30% of parasite drag. See what i mean?

http://142.26.194.131/aerodynamics1/Drag/Page9.html

RE: Best way to avoid those ****** wing tip vortices ?
 

Ken,

Yes, I see what you mean, but I don't agree. Defining 'low speed' to mean below a Mach number of .3 is only valid if you are discussing compressibility effects. When dealing with induced drag, it is a bit of an oversimplification to say that it is only important at low speed. A more accurate characterization is that induced drag is important (large) when CL is high. Low speed flight is one situation in which a wing typically operates at high CL, but the definition of 'low speed' in this context has nothing to do with Mach number or compressibility. Induced drag is probably not an issue for a typical powered model plane flying above, say, 80 mph. Obviously, this depends, as you say, on the magnitude of the other components of drag (a very clean sailplane may be different, for example). For the plane pictured in salmonbug's post, I would be pretty confident of that estimate. Certainly, as you approach 200 mph with a model plane, induced drag is way down on the priority list.

banktoturn

RE: Best way to avoid those ****** wing tip vortices ?
 

Well, in a way we are discussing compressibility effects. Saying that we are at a low mach number (below about .3) can be used to say that we are dealing with low speed (incompressible) flow... which was my point.

As far as the magnitude of C_L goes, think of it this way... say that we have a model designed for flight at 100 mph (flight C_L). Below this we can maintain altitude by increasing C_L, above this we can maintain altitude by decreasing C_L. So to say that induced drag goes down as speed goes up is a common way of looking at induced drag and speed's effect on it.

You can extend the argument to situations where there is a momentary high C_L (turns, etc...), but we are talking about racing (thinking flat-out speed). OTOH... that exemplifies the point.... if there will be a lot of turning (high C_L maneuvering), induced drag will be an even more important factor.... because we will be losing speed in those turns due to induced drag, not parasite drag.

RE: Best way to avoid those ****** wing tip vortices ?
 

Easy... make removeable wingtips of various shapes. (I played with this....)

mix and match wingtips.

Example: (Sig Kobra)

Standard design wingtip on one side (as shown in the plans for the kit), and the carved one shown early in this discussion. (In my test... the modified tip added 1/4 inch to the span. I didn't make a spacer plate for the other sde to compensate)

The model would drop the wing with the standard tip first every stall. I couldn't force it to drop the other wing first.

Level cruise speed flight required significant rudder toward the modified tip (indicating less drag on that side) I also had to have a bit of aileron trim to compensate for a roll toward the standard tip... (indicating increased lift from the modifed tip.) How much of the rudder was needed due to th aleron being down (adverse yaw) and how much was due to just the wingtip's effect on the tip vortice? Hard to say.

******

Most people are afraid to try asymetric setups... but after you've lost 1/3 of a wing in a mid-air flying RC Combat... If you don't panic you realize the things can be controlled when one wing is defferent from the other.

RE: Best way to avoid those ****** wing tip vortices ?
 

No, compressibility is in no way involved. You have not explained how you determined that induced drag predominates below 200 mph.

Induced drag would be more important if turns were involved, but still not the most important. Profile drag also increases in turns.

banktoturn

RE: Best way to avoid those ****** wing tip vortices ?
 

as I mentioned you have significant changes in airplane behaviours, but can you prove your total drag decreased?



by the way I also somethime play with wingtips

RE: Best way to avoid those ****** wing tip vortices ?
 

The only ways to prove drag change...

Wind tunnel.

Speed runs though a radar trap.

A wind tunnel.. you can mount the aircraft to the platform which links to the scale measuring how much the wind is pushing the model backward. then turn the wnd speed up to a specific value. Change tips and do the wind again. You will KNOW the airplane was at the exact same attitude and the only change was the wingtips. The scale would give a definitive number to the change. (Who's got the money for the wind tunnel time?) Yo will als have a scale that is hooked up to the model that measures the lift generated.

The high speed runs... you have a less positive measure because wind might change a bit, altering the measured speed through the trap. If the top speed change is small, its going to be hard to prove anything, as the record setting speed models tend to have a difference in thier speed each run with no change to the model.

Low speed runs can verify increased lift... because the model will stall at a lowerr speed. (but again... wind may make this hard to quantify)

*********

I can easilly do the empirical test with one wingtip on one side... and a different one on the other.... and I can prove there is a change. And from the way the model reacts to different wingtips I can estimate what those changes mean for real world model performance.

Thats WHY I used the carved tips on the glider I mentioned earlier... I wanted to be faster then the kit designer... it worked.

RE: RE: Best way to avoid those ****** wing tip vortices ?
 

I agree for the wind tunnel, I mentioned that in my third post on this thread

it's impossible to quantify the effects of wingtips on total drag with our conventional RC equipement


for the radar speed mesurment I don't agree. not reliable enough. you should make your pass at exactly the same weight (fuel consumption), exactly same altitude and azimuth (paralax error) and exactly the same RPM to be able to prove something.
I forgot also to mention that the wind would also be the same

RE: Best way to avoid those ****** wing tip vortices ?
 

OK... check out this simple graph on the following web page:

http://142.26.194.131/aerodynamics1/Drag/Page9.html

Basically, it shows that induced drag dominates the low speed regime in aerodynamics. Obviously it really depends on the aircraft. Can i quantify it for the particular aircraft in question? No. I dont think that anyone could without the wind tunnel tests mentioned. It's a question of where that minimum lies for the particular craft in question.

Nonetheless, induced drag is an important drag force on model aircraft... simply due to the fact that we are operating at low mach number. I mention mach number because induced drag goes to zero as we approach the speed of sound. It is at it's highest at the lowest speeds that we would fly at.

Profile drag does not change much (if at all) in a turn because the frontal area of the aircraft does not change significantly... not nearly as much as the lift changes... which directly affects the induced drag.

I tried looking, but i could not find a good source that showed the types of drag present for the stuff we are interested in.

RE: Best way to avoid those ****** wing tip vortices ?
 

Nice site :D Lots of good reading there.

RE: Best way to avoid those ****** wing tip vortices ?
 

"Induced drag goes to zero as we approach the speed of sound"? I'd be a little careful with that suggestion.

RE: Best way to avoid those ****** wing tip vortices ?
 

altough the drag/eas curve is one of the most remarquable one in aerodynamic. I believe that in our application it's more intersting to base our speculation on the PN/Eas curve.
99 procent of us fly with internal combustion engines turning a conventional propeler after all......

here is a synthesis of the comparaison of the two curves I made 15 years ago. If you want more explanation and theory about some interesting points on the curve, feel free to ask.

compressibility drag has been omited in those curves

RE: Best way to avoid those ****** wing tip vortices ?
 

The way the original question was asked it appeared that a technical response was appropriate, but as it unfolded it seems that a race is the primary goal. Most of the respondents have good suggestions, however if the intentions are to win a race with similar aircraft, I suspect that victory will go, not to the one having some small technical advantage but to the one, who on that day is the best flier.

RE: RE: Best way to avoid those ****** wing tip vortices ?
 

I AM GOING TO WIN THEN !!!!! HANDS DOWN !!!

RE: RE: Best way to avoid those ****** wing tip vortices ?
 

No, to get back to the initial post :

- My understanding is that these tip vortices are somewhat linked to drag. As you all know, they are generated through pressure difference between upper and lower part of the wing. So the higher CL, the higher the pressure diff, the bigger the vortices... Induced drag, which increases as CL increases, corroborates this.

- So my question "how to reduce them ?" : indeed, I want to reduce drag (I have something to sort out with SalmonBug, a young inexperimented modeller :)). I don't know if it's actually impossible to reduce them (some of you stated this) and that it is only possible to move them away from the wing, but eh, that 's exactly the same : If we can limit their impact on wing airflow, useful wing area will get closer to its geometrical size, thus reducing induced drag, though virtual wing area increase.

Regarding SalmonBug's experience, this was a joke, he 's ultra-experimented, and without a little help from you fellows, I would say I've already lost the race... Let alone the fact he 's about to build his third Shrike, so he definitely knows this airframe quite a bit.

Too bad he's noticed this thread :( ! Interesting fight, though :D ...

RE: RE: Best way to avoid those ****** wing tip vortices ?
 

Strykaas,

Yes, there is a way to reduce the effect of these vortices on your plane. You are already doing it. It is to fly very fast, so that the wing is operating at a low CL. I assure you, although Ken Litko does not agree, any effort you expend to reduce induced drag would be better spent reducing profile drag. Fairing behind the engine, or better yet, cowling the entire engine, would be your biggest drag reduction, by far.

banktoturn