Gurney Flaps / BA's book cover

[obrut]

If you look closely on the front cover of Bruce A's book nearly all of the cars in the shop have a small extension to the trailing edge of the wing/tail. The extensions look like a 1 or 2" strip of perspex held on by little rivetted plates.

I assume they are some kind of Gurney flap arrangement? Anyone got any better idea as to what they are and if they help?

Chhers - Ryan

[TimT]

Yes thats what they are (gurney lips)..... has some effect of breaking turbulence and smoothing airflow after the wing or something.........

[jluetjen]

Obrut - You asked a very simple question -- the answer of which has most likely justified a number of PhD's!

I could just say that it tends to make the wing work better (increase downforce) under certain circumstances, but that wouldn't do the subject justice. For a sampling you might want to check out some of the articles on this site. They even have a paper dedicated to Gurney Flaps.

[avi8torny]

RE: the above post....the gurney flap does not increase lift...it is not an airfoil.


Gurney flaps have been used for years on aircraft and are used to spoil or reduce the amount of lift...in this case, downforce the wing produces at higher speeds. In other words, too much of a good thing (lift or downforce) can be undesirable. So the flap is installed in order "fix" this problem. It does not usually detract from the performance of the airfoil at lower speeds.

[911nut]

From the book "Competition Car Downforce" by McBeath: "The Gurney flap is a small, right angled strip bolted to the trailing edge of the rearmost element of a rear wing. It is a means of adding downforce with suprisingly little drag penalty, in spite of it's apparant crudeness.

"Recent research has now shown that the Gurney forms a pair of counter-rotating vortices behind itself, which has the effect of adding a verticle component to the velocity at the trailing edge. This deflects the flow upwards and increases the downforce. The effect is the same as adding more camber to a wing. One of the main advantages of the Gurney is that it is easily fitted and removed, or replaced with one of a different size and it is thus an important part of the fine tuning kit that racers take to the track.

" Small Gurney flaps give rise to increases in downforce for a small extra drag component, whilst larger Gurneys add smaller extra amounts of downforce, but bigger chuncks of drag. There are sensible limits to work to here and Gurneys are usually less than 5 % of chord in height."

[avi8torny]

There is a difference between a Gurney Flap and a deflector...I believe the one's on the cover of the BA book are deflectors which increase the downforce.

excerpt from yarchive.net
Newsgroups: sci.aeronautics.airliners
Date: 17 Dec 96 03:09:15
From: David Lednicer
Subject: Re: gurney flaps

nicole wrote:

> hi, i'm trying to find more about gurney flaps and don't know where to
> get started - they don't seem to be in any of my aero textbooks. anyone
> have any suggestions? could you please email me with them?
> thanks, nicole

Bob Liebeck of Douglas is credited with first bringing them to
the attention of the aerodynamics community, but they are named after Dan
Gurney, the famous race car driver. Several years ago, sitting in the
bar avoiding attending paper sessions at an AIAA meeting, I asked Bob for
the full story. He told me that Dan Gurney suggested to him the idea,
based upon the old "spoilers" added to the aft end of race cars in the
late 1960s. These "spoilers" were simply sheet metal tabs, added to the
end of the rear deck, projecting upwards. Gurney reasoned that if, they
worked there, why wouldn't they work on the trailing edges of wings?
They tried the idea and it worked! For a flap that was 1-2% of airfoil
chord, projecting perpendicular to the airfoil trailing edge, they got a
useful increase in lift (downforce for a car), without much drag
increase. Bob first published the concept in a 1976 AIAA paper (76-406).
This was later reprinted in the September 1978 AIAA Journal of Aircraft.

When the flaps first showed up at the Indy 500 on Gurney's cars,
competitors asked him what they were for, so Gurney had an explanation
ready. "When we push the car out, with our hands on the trailing edge of
the aft wing, we tend to get cut up, so we added this tab to blunt the
trailing edge." Competitors thought that this was a good idea, but they
put their tabs on projecting downwards, rather than upwards, as needed to
improve downforce! It was several years before they caught on.

Actually, designers have used strips on the trailing edge of
control surfaces before, which look much like Gurney flaps. However,
they were intended to reduce control surface oscillation caused by
wandering separation patterns.

-------------------------------------------------------------------
David Lednicer | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc. | email: dave@amiwest.com
2133 152nd Ave NE | tel: (206) 643-9090
Redmond, WA 98052 USA | fax: (206) 746-1299

[Tinton]