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Author Topic: Winglets  (Read 566 times)
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bbdave
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« on: April 27, 2017, 04:23:16 PM »

I have built the wing of my 2m RES glider the kit has winglets but I'm unsure of the effectiveness of them on a model I know this may be a can of worms but what are the opinions on winglets?

Dave
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Konrad
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« Reply #1 on: April 27, 2017, 04:45:37 PM »

Winglets work but only in narrow coefficients of lift. In the rest of the flight envelope they are a drag.
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bbdave
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« Reply #2 on: April 28, 2017, 01:16:08 AM »

Thank you I did wander as the big competition gliders don't have them so I guessed maybe they might not be worth going for.

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« Reply #3 on: April 28, 2017, 01:43:47 AM »

Winglets definitely can have a positive effect on a span limited class like the 2 metre F3-RES. They can reduce induced drag and act like a slightly bigger model. That said design of them is very involved and its easy to add more drag than you save.

As Konrad said they tend to be efficient at only a narrow speed range but this may not be a problem in F3-RES where the launch speed, thermal speed, and penetration speed are all fairly close. The winglets you see in F3-RES are often there to provide some yaw-roll response but in my opinion this is less efficient than having sufficient dihedral in the first place.

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USch
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« Reply #4 on: April 28, 2017, 08:58:28 AM »

For me that is the right question in the right moment.

I am about building 2 new E-36, electric FF with 36" span limit. These models fly at 15-18m/sec in climb and 4-5m/sec in glide. So winglets may not be the best solution if the winglets work best only at one specific air speed. But I have in mind some of the CLG from Olbill which also do away with dihedral, actually the tip plates substitute the dihedral. Indeed I was thinking of doing one wing with "normal" dihedral, about 10-12% of span and the second one with tip plates and just about half of the dihedral of the first.

Question is how to mount these tip plates,  I thought of building in about 1-1,5° of toe out (bigger span at the trailing edge). What does the scientific community think of such an idea in respect of lower dihedral and the way to mount it  Huh

Urs
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Konrad
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« Reply #5 on: April 28, 2017, 11:39:27 AM »

Winglets, I assume we are talking about those things out on the wingtips that look like the wing is giving the world the finger. Not the wing tips that are some derivative of a Hoerner wing tip like those found on the Cessna 182 but rather the wing tips that look like Rutan VariEze wing tips.
https://en.wikipedia.org/wiki/Rutan_Long-EZ
https://en.wikipedia.org/wiki/Wingtip_device

As much as I like the theoretical math, some times it is best in the real world to follow the money. I’d like to put forward a short history of the winglets.
 
In the 60’s the gliders guys in limited span classes wanted to gain more efficiency from the lift to drag ratios. And started to move away from simple wing tips that only addressed the tip vortex destroying lift at the wing tips. They were seeking a tip that could harness the tip vortex to generate thrust. In the 70’s some of these guys wanted know if the added cost of the winglets actually were worth the lightening of 40 thousand pounds to their wallet and asked Boeing to run some wind tunnel test. The data and test results on the winglets was inconclusive and that the winglets in all likelihood weren’t worth the effort.

But the big boys in the glider comps that kept winning with their german gliders, more often than not, were sporting these winglets. So the winglet stayed alive even finding their way onto business jets. In the 80’s Bombardier  had a very successful jet in the Challenger and RJ series. In the 90’s Boeing again comes into the picture wanting to enter the business jet market. The Boeing marketing guys said that the 737BJ was at a marketing disadvantage without those thingies on the wings. Engineering was told to find a justification for those winglets.

Through the magic of Boeing's marketing management edicts, it was found that the winglets do help at the high CL portion of the flight envelope. The benefit of this is that the aircraft can now climb a bit faster putting distance between the noisy engines and the airport microphones. Saving the owners a lot of money from noise fines imposed by Cat III noise regulations.

Southwest Airline found that the higher climb rate saved fuel costs with their short hop structure as it got the aircraft to altitude faster where the jet engine is much more efficient on fuel burn. So the winglets were worth the airframe drag for the cruise on short hops, as the engine were able to use less fuel at altitude for any given Southwest Airlines trip.

You might have noticed that on the 747-400 there are small winglets but these have shown to be all but worthless and were removed on the 747-8. Even in the days of the 747-400 these winglets were NOT on the MEL (Minimum Equipment List).

So at high Cl numbers the winglet is a benefit but more as a result of financial concerns than aerodynamic. So for our toys the answer is the same Boeing gave us in the early 70’s.
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Olbill
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« Reply #6 on: April 28, 2017, 12:56:08 PM »

. But I have in mind some of the CLG from Olbill which also do away with dihedral, actually the tip plates substitute the dihedral. Indeed I was thinking of doing one wing with "normal" dihedral,

Urs

My gliders with winglets have dihedral. My rubber powered LPP and F1M models don't have dihedral.
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USch
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« Reply #7 on: April 28, 2017, 01:04:57 PM »

My gliders with winglets have dihedral. My rubber powered LPP and F1M models don't have dihedral.

Do your gliders have less dihedral than a tipplateless glider? And do they have some inclination lenghtwise ?

Urs
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Konrad
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« Reply #8 on: April 28, 2017, 01:54:39 PM »

As there is spanwise flow, inclination (wider at the rear) is a low drag placement. Most wings loose dihedral effectiveness if the plates extent downwards. Conversely most wing show increases dihedral effect with plates that extend above the wing. Now these are gross generalization.
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Olbill
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« Reply #9 on: April 28, 2017, 03:05:46 PM »

My gliders with winglets have dihedral. My rubber powered LPP and F1M models don't have dihedral.

Do your gliders have less dihedral than a tipplateless glider? And do they have some inclination lenghtwise ?

Urs

The gliders have normal dihedral. I'm not sure what you mean by inclination lengthwise. My tip plates are vertical but I usually have camber on the inner surface and some toe-in front to rear. I don't know if any of this is aerodynamically correct but both gliders have national records. It could just be luck.

Incidentally I just completed a reconfiguration of BigPoker to make it more compatible with the 44' ceiling at Rantoul. Weight was reduced from 11 grams to 9.4.
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USch
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« Reply #10 on: April 28, 2017, 05:53:17 PM »

I'm not sure what you mean by inclination lengthwise. My tip plates are vertical but I usually have camber on the inner surface and some toe-in front to rear. I don't know if any of this is aerodynamically correct but both gliders have national records. It could just be luck.

Thank you Bill. With inclination lenghtwise I meant what you call toe-in/out, but was not sure if comprehensive.
Certainly your set up with camber on the inside and toe-in is against instinct, but your results prove that it works  Grin

Urs
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