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Gurney flap on ducktail?

Inspired by Jack Olsen's Poor Man's Aero thread, I got to wondering: would a Gurney flap do anything for a ducktail? Anyone tried it?
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Old 09-14-2011, 01:48 PM
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You may want to cross thread this to the racing forum, a lot of guys over there that have aero experience.
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Old 09-14-2011, 02:00 PM
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You could try your own testing by using ticklets ( not sure about spelling), but I just mean small pieces of string attached by tape from the windscreen all the way down to the tail.

Record the whole thing with a camcorder and compare the results. The behavious of the strings should give you an indication whether the airstream would allow for more downforce being created this way.

Good luck.
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Old 09-14-2011, 03:06 PM
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I always thought that a ducktail WAS a big Gurney flap...or rather, that a Gurney flap was a ducktail attached to the back of a wing :-)

I guess we first have to agree if a ducktail is or is not a wing...

I don't know anything about this but it would be interesting to find out.
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Old 09-14-2011, 03:13 PM
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Me too. A ducktail looks like a Gurney flap. And I've never seen anyone try this... possibly because the wing is such a superior solution that further fooling with the ducktail sounded like a waste of time.
Jesper: what would I look for?
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Old 09-14-2011, 04:50 PM
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Skip the wicker bills. Either extend the current ducktail profile out the back with lexan or else do a Mary Stuart collar. The best thing to do is add a wing on top of the ducktail on some nice uprights like a Cupcar.
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Old 09-14-2011, 05:11 PM
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Old 09-14-2011, 05:30 PM
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...or like Jacks car!!!



(This thread suddenly became like looking in the mirror at a picture of looking in the mirror...)
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Old 09-14-2011, 05:31 PM
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Quote:
Originally Posted by moneymanager View Post
I got to wondering: would a Gurney flap do anything for a ducktail? Anyone tried it?
Nothing at all. One can extend the tail or wrap it (ala Mary Stuart RSR) for more downforce, at the expense of drag.

Gurney flap/wickerbills were designed and intended to make airfoils more efficient for a given chord & length.
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Old 09-14-2011, 07:33 PM
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The ducktail is a spoiler. Its shape is not really very important to its function. I believe that the factory engineers wanted it to go higher up (or at least, there was some additional benefit to making it taller), but styling considerations vetoed that.

And if my memory is correct, it was eliminated altogether because it was considered (through some technical interpretation of the German law) to be a hazard to pedestrians.

I've considered adding another few inches above the ducktail when I run the wing -- it could just be a flat cross piece that blocks airflow. But I haven't tried it.
Old 09-14-2011, 10:56 PM
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My understanding of a Gurney flap is that it helps trim a wing, depending on the angle of attack. Since a ducktail is fixed, I would think a Gurney flap is pointless. Then again, I'm not an engineer. Anyone have Adrian Newey's number?
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Old 09-15-2011, 05:33 AM
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When I was club racing a '71T with stock 2.2, I swapped ducktail with a fiberglass whaletail. Better high speed handling and I believe less drag.
Old 09-15-2011, 07:00 AM
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Incidentally, when I looked up a Gurney Flap on Wikipedia, they have an example shown on a Porsche 962 rear wing. It would seem that the ducktail, similar to the pop-up wing on the Boxster, causes some drag which creates lower pressure under the car, sucking it to the pavement at the cost of a higher co-efficient of drag.

Gurney flap - Wikipedia, the free encyclopedia
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Old 09-15-2011, 07:15 AM
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Jack, have you tried the wing w/o the ducktail? Should be less drag.
Old 09-15-2011, 07:25 AM
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I always thought wicker bills added downforce but were rather inefficient, making a lot of drag- hence why the wicker bill height on the rear wing is what is used to trim out prototype speeds on the Mulsanne straight. I thought that it would be more efficient to get the same lift by using a more highly cambered wing, but after reading the Wiki article it sounds like if the wicker bill operates in the boundary layer then it will be more efficient than the more highly cambered wing. But this means you need to do Reynolds number calcs to determine the boundary layer thickness at the critical speeds, right? And it cannot be more efficient at all speeds therefore?
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Old 09-15-2011, 09:29 AM
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Argh. Why do we have two threads like this? I think my reply should really be in Jack's wing thread.
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Old 09-15-2011, 09:31 AM
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Max... two threads so as not to hijack Jack's. But I agree, now they'd be better together.
I think I'm going to put my aluminum angle, epoxy and fiberglass away. It seems pretty clear that supplementing the ducktail with a Gurney flap is a waste of time! Those of us who don't want or can't add proper wings will just have to live with that stomach-sinking rear end lightness.
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Old 09-15-2011, 10:49 AM
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Quote:
Originally Posted by moneymanager View Post
Me too. A ducktail looks like a Gurney flap. And I've never seen anyone try this... possibly because the wing is such a superior solution that further fooling with the ducktail sounded like a waste of time.
Jesper: what would I look for?
Sorry for the late reply. I see you have got execellent advice already.

My aerodynamics experience is not from racing, but from aviation. So I can not speak from a racing point of view. Just apply what I know from flying.

The gurney flap generally just helps to increase the differential in pressure that is created when the airfoil is moving through the air at a given angle of incidence. Generally referred to as lift, or downforce when it is used "upside down" in this application.

When relating this to the ducktail, we cannot really talk about a pressure differential, since the ducktail is part of the car and, as I see it, the entire car is in this case seen as the airfoil, hence the ducktail is effectively the gurney flap.

There is no real flow below the ducktail, hence we cannot create this pressure diff. When looking on the ducktail as an isolated unit.

When you put on a wing like what Jack Olsen has done. Then we can apply all of the techniques that a plies to a basic airfoil. Gurney flap etc.

Back to what I call ticklets:

What you would be looking for is basically to see if the strings are nice and steady in the flow if they are flapping around randomly, it indicates that the airflow has separated and the car is creating more drag than would be if the flow was nice and smooth around it. The thing is, if the flow is too smooth, as in fast flowing, it means that we have increased the dynamic pressure by increasing the airstream, this increases the pressure differential between the upper part vs the lower part of the car, creating lift. This is what we can try to overcome by using a wing to created downforce.

Back to the ticklets again
These small pieces of string will indicated if you have got enough energy in the airstram to effectively utilize a wing on the tail. If the airflow has separated too far ahead of the tail. The wing will not be really effective.
This can be mitigated by applying energy to the airflow. This can be done using vortex generators. It is basically just small pieces if sheet metal that is placed at an angle in respect to the airflow to constantly create small "local" accellerations to the airstream. It can however also be fancy sharkfinn type profiles.
This prevents the airflow from separating and enables you to reduce the overall drag on the car and maybe use it to create more downforce on a tail mounted wing.
This technique is used on everything from airliners to race cars. I think Mitsubishi has done on some of their EVOs.

I hope this makes sense, just a bit. It is a little hard for me to find the correct English terms.
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Old 09-16-2011, 05:13 PM
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There is still a pressure differential with a ducktail, otherwise there would be no point in having it. The ducktail sort of dams up the airflow and separates the flow so there are turbulent eddies underneath.
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Old 09-16-2011, 05:53 PM
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Quote:
Originally Posted by Flieger View Post
There is still a pressure differential with a ducktail, otherwise there would be no point in having it. The ducktail sort of dams up the airflow and separates the flow so there are turbulent eddies underneath.
I know, I am just trying to say that seen isolated the ducktail can not be seen as an airfoil that could benefit from mounting a flap . Its purpose is to reduce some of the lift that is created on the aft part of the car when traveling at speed. This is done by slowing/spoiling the airflow.
One could say that the duck tail serves to reduce the overall pressure differential on the car as a whole. Hence reducing lift on the aft part of the car.

When placing any body in a stream of air it will always create a press diff. by applying aerodynamic principles we can utilize this to our benefit.

The use of the strings(ticklets) would in fact illustrate the difference, if let's say you recorded some shots with and with a ducktail. My guess is Porsche has already done this.
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Old 09-16-2011, 06:59 PM
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