Krokodil

Mark, as I wrote in my previous post, although not completely, the Xfoil data shows the base Cd (Cd0) and not the complete Cd for the wing. The complete Cd is a combination of the base Cd (skin friction and from) and induced drag. Induced drag is combination of Cl, wing plane shape (e.g., square or elipse), and aspect ratio (Length/chord). This results in a final Cd estimated by the following equation:

Cd = Cd0 + Cl^2 / ( pi * Ar * e)

Cd0 = base Cd (skin friction and form)
Cl = lift coefficient
Pi = well, Pi
Ar = aspect ratio (length / chord for a square wing)
e = efficiency factor (1.0 for ellipse, 0.7 for square)

This is just one more reason why we cannot conclude anything with certainty from the GF data presented. The Ar of the wings tested in your charts is likley not the same as the Cup, and studying the equation above shows that the a longer wing (higher Ar) has less induced drag, and therefor less total drag, for the same profile than a wing with a smaller Ar. Add in the size of the endpates, if any, and you have very different conditions. Again, I am not saying your data is incorrect, I just think it may not be as clean as you conclude. It also yields that the base Cd0 shown in the charts get dwarfed by the induced drag - making the Cd much less speed sensitive than the xfoil graphs would indicate.

Here is a link to an simple discussion: http://wright.nasa.gov/airplane/drageq.html

Here are the Cl, Cd0, and calculated Cd and L/D numbers for an E423 with a chord of 299mm and a length of 1700 mm length at -1 to 12 degree AoA:

AoA -1 0 1 2 3 4 5 6 7 8 9 10 11 12
Cl 0.9908 1.0957 1.2000 1.3021 1.4018 1.4998 1.5923 1.6811 1.7659 1.8456 1.9141 1.9754 2.0178 2.0389
Cd0 0.0093 0.0094 0.0095 0.0099 0.0101 0.0105 0.0102 0.0109 0.0117 0.0127 0.0142 0.0161 0.0193 0.0242
Cd 0.0881 0.1057 0.1251 0.1459 0.1678 0.1910 0.2136 0.2377 0.2619 0.2860 0.3082 0.3293 0.3460 0.3578
L/D 12.7 11.7 10.8 10.1 9.4 8.9 8.4 8.0 7.6 7.3 7.0 6.8 6.6 6.4



Cheers,

mark kibort

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Well that makes much more sense. you can see the reason for my confustion . ive never seen a simulator output that shows Cd0 and calls it Cd usually it's induced drag, or drag due to lift) its a little misleading
So, its only the base Cd. Gotcha!
either way, the point really is, where the lift max is.... we know past that, the drag continues to increase. maybe not as fast as the Cd0 might, but still, based on the other wings, it increases at a pretty high rate.
the take aways here, are that if we get beyond 10degrees AoA with the high lift air foils. (e423, or even Crawford), the drag consequences are pretty high. the only thing as a big variable, is the AoA, based on roof line deflection of the air flow. (angle change). Even if we split the difference of roof line angle and horizontal, still we get a 7-8 degree value and if the wing is set at anything beyond 3, that's 10degrees. It seems with all the high lift wings, anything more than that will just give drag and reduce lift slightly.

I do think the GF tests, on 3 very different wings, come to the same conclusions where changes in size wont effect the outcome. on low AoA, the GF absolutely produces more drag, and there is no logical reason to thin that any configuration or shape will change that. The interesting point, is where the cross over points are. it looks like unless you are beyond max Cl, there is no reason to use a GF and by using one at low AoAs, the drag consequences are quite high.... I think many would underestimate the effects of the drag for using one. in mike's case, I would think that at 0 AoA, which is near 0 because with the Ferrari, the air flow to the wing is nearly horizontal, the drag consequences are even higher than we approximated.
(0 angle of incidence of a clean wing vs 5 degrees angle of incidence with a GF) (Crawford or cup car wing)

do you have the calculations for up to 16 AoA? that's were i was and it looks like from the chart you posted earlier that the lift was going down, but drag skyrocketing.. since that was Cd0, it looks like the Cd total was actually not that bad. i want to use the same caclulations to see the difference. I think if lift is not any higher, and i don't expect it to be, then i have not much to lose going down 5 degrees to a net AoA of near 11, since i suspect the lift coefficient will be near the same.

J richard

...

mark kibort

Could you find a wing that was more unlike anything that any of would care about?

even still... GF produces more drag at moderate wing AoAs, and no one uses GFs of 1.44%.... i think the smallest GFs ive seen on wings have been 1/4" , but usually 1/2". (3-5% of cord)

this graph you posted is for a multi element wing, used on some of the very old indy cars.

8
Effect of adding a 1.7% chord–long Gurney flap on the lift and drag coefficient of a
rectangular wing (AR = 8, NLF 0414 airfoil). AR, aspect ratio. (Data from Myose et al. 1996.)
levels of gains in the lift. Carrannanto et al. (1994) followed with numerical analyses of
Gurney flaps to validate these results, to calculate streamline shapes near the trailing
edge, and to compare them with the water tunnel flow visualizations of Neuhart &
Pendergraft (1988). Numerous other studies followed, such as Papadakis et al. (1997),
revisiting the effect of a tab both at the trailing edge and inside the gap between the
airfoil’s two elements. Three-dimensional effects, demonstrating the effectiveness of
these tabs for practical airplane wings, were reported by Myose et al. (1996). Typical
lift and drag data on the effect of such a short flap are shown in Figure 8. In this
case, Myose et al. (1996) used a rectangular wing with AR=8, a NLF0414 airfoil,
and a 1.7% chord high Gurney flap. In general, the lift increases with the addition
of the flap, as well as the drag, throughout the whole range of angles of attack. The
case of drag reduction, as reported by Liebeck (1978), is present only with very thick
airfoils and not present with modern low drag airfoil shapes (as in Figure 8). Also,
an increase in flap size (sometimes up to 5%) will show lift increase and occasional

improvements in L/D.The applicability of the device to race cars’ front wings was revisited by Zerihan
& Zhang (2001), who investigated the effect of ground proximity, and Jeffrey et al.
(2001), who extended this work to two-element airfoils. Because of the simplicity

J richard

If you define any problem so there is only one answer you pretty much know what you're going to get...

mark kibort

actually, you have a point, but it wasn't made with that wing, as kooky as it is for our discussion.
still at moderate lift levels, the gurney flap has more drag at the same lift levels so, you might want to try again.

mark kibort

http://www.grc.nasa.gov/WWW/k-12/airplane/foil3.html

Krockdil, using this simulator, it seems that the closest I could create to the e423, the lift maxs out at about 2.0 and the L/D is about 4.0. however, after that the L/D actually goes up and gets better as it starts to stall. I was under the impression that the lift would continue to go up as the graphs we have show.

TrackDays247.com

See my quasi-gurney on wing in my avatar

mark kibort

side pic??

chartersb

This weekend we tested 3 different cars with a total of 12 different configurations, including wickers at speeds of 100kph, 140kph, 180kph and 220kph for each configuration in the ACE wind tunnel at the University of Ontario Institute of Technology. All of the aero forces were measured in pounds, in real time, at the tire contact patches, and in three axis (vertical, fore-aft, left-right). The % change in chassis pitch and roll was tracked as well.

After spending most of the day in the UOIT wind tunnel, here’s what we concluded. First, anything affects everything else. Second, getting Front down force is a lot more difficult than adding Rear down force. Specifically we found that by increasing the AOA of a GT3 Cup wing by a few positions, the Rear down force was increased, Drag was increased and Front down force was slightly reduced. No surprises there. But by running the wing at its original, more neutral AOA and simply adding a wicker, we had the same measured down force with far less of an increase in drag. We were running an extended front splitter and as the Rear down force was increased, we saw a reduction in the Front down force. We changed the chassis angle of attack (adding rake) and brought the Front and Rear down force values back into a better balance with a negligible increase in drag. We found similar results using a variety of wicker heights in a second car and a third car with a fixed wing We also tested the cooling efficiency of a car with a reduced front air intake. The air intake had been dramatically reduced to increase measured front down force. The car was “driven” in the tunnel at 6000 rpm loaded for over 5 minutes at a wind speed of 160kph to see if it would overheat or not. It didn’t.

The Motorsports Research facility at UOIT will be available in 2015 at competitive rates. Contact me (brion@bbxracing.com) or Ernie (ernie@mantissport.ca) if you are interested.

KaiB

WOW! Thanks for posting this!

winders

So maybe these teams aren't as stupid for using Gurney flaps as Kibort implies......

KaiB

Scott, you obviously don't understand the math...

mark kibort

ignorance can be taught, stupidity is forever.

Look, there is NO graph plotted so far that shows that the wing with a GF makes less drag than without for a given downforce. as interesting as these tests conducted are, I don't see the tests testing equal downforce and the resultant drag values for a flat wing.... we have already narrowed down the condition that even at near 0 angle of incidence, that that can be near 10 degrees for the cup car wing, which is maxed out Cl. for the Ferrari, the difference is 10 degrees AoA... see the picture on the thread??? (911 vs ferrarri 458?) HUGE difference.

at any other position for equal lift (with vs without GF) the drag is at worse, equal, UNLESS the wing has maxed out without the GF. it is total possible that even with this test, where I think the "more neutral" wing setting still could have been 3-4 degrees angle of incidence, even more if they were changing rake at the same time.

The graphs are already shown. the interesting part here is that now its on a car, where we can determine what the real AoA is.




KaiB, its not math, its something that even you might understand... PICTURES! (or RTF Thread)

mark kibort

Can you clarify the test sequence. you ran the rear wing at its 'normal" setting. what was that (angle of incidence)?. then you moved it and tested several more positions. (what positions were those and how far apart angle degrees were they?)
Then, when you moved the wing to "its more neutral " position. what was that? true 0 degrees angle of incidence (or 0 AoA relative to the body line)?
or was it a few degrees inclined....... my wing almost looks flat now, but its at 4 degrees. the 928 has much less of a roof line deflection (16 degress) vs a 911 which is more like 20)
Anyway, you put it to this more neutral position and added a wicker and the downforce was near equal to , I don't know , 1 or 2 of the more inclined positions, with less drag. that makes perfect sense and is consistant with the charts for the GF drag vs lift, if the angled wing was anything over max lift, which its very likely it was.

as suspected, all the changes will effect everything else. a lot of those you expected. did you do any hood venting to gain some front downforce? did you play with the dive planes to get a drag vs downforce idea?

what was the front end downforce vs rear end downforce when you left the Wind Tunnel and at what speed..... can you give us the drag too. (any random config would be great! (just want to see if the calculated overall drag of a race car vs downforce is close to what you saw. 120mph or near 180 or 220KpH if you can tell us. It also sounds like the nose of the car was blocked off from air flow and instead of that air going in to the radiator,and then under the car, with it blocked off it would roll over the car increasing flow over the car, creating more downforce. hoodvents could do that same thing If you designed the opening correctly..

I know you might not have had the time, but with no wings or splitter, it might have been cool to get that data.

Thanks for the info... great stuff.

This next part is for WINDERS and KiaB....... if the angle of the wing was set to 7 degrees and then 12 degrees for example....... 7 degrees plus roof line deflection would be near 17 to 25 degrees (we don't know) if so, as we all know now, the GF allows for much greater angle of attack with in line, proportional L/D ratios.

im guessing that in the tests, the downforce of his near "neutral " wing, was set at near 3-4 degrees. the moved "positions" were probably 5 degrees apart. the effective AoA could have been 15, 20 and 25 degree tests.
in such tests, because the wing is far over max Cl, of course using a GF at 4 degrees might give much higher lift, and a lot less drag. WHY, because this is why you use a GF... it extends the use of the wing AoA effective range.

Even if the wing was at 0 which can equate to 10 to 15 degrees AoA, the cup car wing (e423) maxes out at 10-12 degrees. if that's the roof line deflection for flow, its simple to see that a GF on a 0 angle wing (incidence) would be a more efficient set up.

Now, look at the Ferarri 458 air flow..... its almost parallel. my challenge to the Ferarri team still stands.... if 0, then angling up 5- 7 degrees would have no downside with out a GF, and much less drag.