Wing and diffuser hook-up
Fellas;
There was another thread were Rennie and Stan Clayton were visiting about rear wing vertical and longitudinal position as to efficiently hook-up with the diffuser. Is this a function of the laminar flow of air over the top of the diffuser and how in acts on the rear wing as it is 'kicked up'?
I was under the impression the bottom element is for the mounting and the upper twin planes do the work. Wassup?
VR
Iverson
The lower rear wing has a big influence on the airflow through the diffuser but also creates downforce on its own. Getting the position just right is tricky, and, to reiterate what I said in another thread, you need a very accurate model of the entire car to simulate the flow in CFD. Absent that, you need a rolling road wind tunnel (expensive) or need to experiment on track.
For any car other than a late model VD or Radon Rn.10, I'd have to esimate where our lower rear wing should mount with respect to our diffuser. It will probably be fairly close, but the position is unlikely to be right for any other diffuser (especially since our diffuser is a double deck design with an internal airfoil and also has an extended center section).
The upper rear wing happens to be the least efficient downforce generating device on the entire car. You want to generate as much downforce as possible with the front wing and the undertray and as little as possible with the upper rear wing since that produces the lowest total drag.
Given that, we tested a single element upper wing design during the last couple of races in the pro series and at the Runoffs last year. It worked very well at faster tracks, and we'll be introducing an updated design next week at VIR (if the production wings are finished in time). I'll post photos as soon as I can, but it's a proprietary airfoil section with a very specific shape and change in angle of attack across the span to compensate for the flow field off the roll hoop. Although it's really optimized for the Rn.10 chassis, it should work well with any FC car.
And, to answer your other question, the airflow has separated by the time it reaches the rear of the car, so there is no laminar flow there.
Nathan
Nathan,
Can you post some pics of your diffuser / lower element set up so the rest of us understand what you are talking abut.
It is obvious that you know a whole lot more about this stuff than the average engineer.
Steve
Sorry, I'd posted them in the FC section a few months ago but should have posted them again. Here is a photo of one of the first production sets, the current ones are a bit cleaner.
Last edited by nulrich; 01.06.15 at 4:16 PM.
Rick,
There is a limited, but reasonable amount of predictive work one can do to estimate positioning of the rear wing assembly in relation to the diffuser exit. It's similar in concept to predicting optimum ride height based on the angle of your diffuser ramp - made more complicated by the fact that diffusers are rarely simple ramps, and airfoils these days tend to be proprietary. Granted, there are a number of assumptions one must make, most of which generically apply to FC's as a breed. The gross interaction effects are well known, however, and the placement in this region is nothing like the criticality of establishing good positioning between a mainplane and flap arrangement, for instance. Even done empirically, optimizing the position of an existing wing assembly can be done in a day or two of DOE testing on track assuming you have reliable data acquisition.
You are correct that the lower element of a typical FC rear wing assembly is not responsible for much downforce in itself - they are, by and large, low drag airfoils that are set at very low AoA. Worse, they are usually very poorly located relative to the upper wing pairing which is doing most of the heavy lifting, so to speak. This, in turn, affects the interaction between the diffuser and lower element, because the lower element is not being "energized" the way it could be, etc. One issue begets another, and it's hard to know where to begin, especially amidst the confusion of having different designers employing different approaches to the same problem. The placement of the rear wing, particularly in relationship to the diffuser is less about the detailed flow conditions, and more about gross field interactions that simply need to be in the right vicinity to work in harmony, rather than against each other.
The single element rear wing assemblies that seem to be all the rage lately with recent emphasis on high speed tracks like Road America, well... those are another story altogether! We prefer not to go down this design path for a number of reasons, though that is not to say that the inherent issues cannot be (or have not already been) overcome by others... just that we don't see a way through that particular tangle of difficulty at the moment.
With regard to laminar flow - forget about it. For all intents and purposes, it doesn't exist for a race car operating in a racing environment.
Cheers,
Rennie
Hi Nathan,
Can you share any info regarding the effects of the "Gurneys" on the trailing edge of your wing end plates? I realize that this is a common practice, but I have not seen any published test data regarding the effects of such devices.
Thanks,
Rick
Nathan isn't overstating the issue one bit. In many cases, if you are off by even 1/4", the gain can be zero.Originally Posted by nulrich
Hi Rick:
Not our endplates, we simply modified the mounting of the current common aluminum endplates.
We do have a new endplate design, but it hasn't quite made it into production yet (busy with tooling for the new chassis). We should introduce it at Road Atlanta in about four weeks, though.
Rennie, not sure if single element wings are "all the rage" since I've only seen ours, but apparently others are trying them? You are absolutely correct that they are difficult to get right. I think we ran about thirty cases in CFD before we were satisified (20M+ meshes, since you have to do the entire car for a rear wing, with a lot of computational cost). I have heard of racers just trying the main element from a two element wing, and we know that won't work!
Since the F2000 series runs at mainly aero tracks (six of the seven venues) we have focused on aero efficiency as our primary design goal. Our parts will work anywhere and make as much downforce as anything we've seen (but with lower drag), but they are especially effective at aero tracks. We've toyed with doing a really high downforce kit for Mid Ohio, and have some designs we've run in CFD, but I'm not sure if we'll put them into production this year. The Rn.10 already makes much more downforce than any existing car, so it would mainly be for Van Diemen retrofits.
Nathan
This not a technical answer,but..
...be careful of the Law of Unintended Consequences.Hook the wing to the defuser and when one gets hit they both get wiped out. Ok,back to the techie stuff.....
Last edited by D.T. Benner; 04.03.10 at 9:31 PM.
I believe they are talking about a "hook-up" of the flow from the diffuser and wing rather than a mechanical attachment. An aero-handshake, if you will.
Ken
This is it! Nicely said, Bud.
I think this started when "they" moved away from composite endplates.... For the aluminum one to work - you have to bend it someplace to give it some rigidity. The composite ones would shake and eventually start failing....Plus they were $$$ over $$ for aluminum
2006
2007
Dr. Nulrich;
Niki Coello has a circa RF-01 as well as a Polecat. This combination has given him a few National Championships, so this is probably where I should start.
Rennie;
Are your rear wings movable on both planes?
RP, I was doing fine until you mentioned this. Cripes sakes, I should'a stayed with FFs.
I've only seen photos of Niki's car, so I don't have complete data. However, based on what I know from track, CFD and wind tunnel testing of the parts he's been running, he won despite his aero configuration, not because of it.
I do have some (anecdotal) radar data from the 2009 Runoffs which would indicate Niki was giving up a fair bit of top speed despite enjoying a slight power advantage with his Pinto engine. Those of you who were there might be able to confirm if that was the case.
I understand his car preparation and setup is first rate, and he's clearly a very talented driver. As we all know, you can have the fastest car in the paddock, but that doesn't mean you'll win!
Nathan
I saw several different types of single plane wings at Road America last year.
I have yet to see anyone give a concrete number for what kind of improvement there is to be had. 20 lbs more downforce? 50 lbs less drag at the same downforce levels? Is there really enough to be had to let someone challenge Niki?
From our wind tunnel testing on a '02 Van Diemen, our complete package (front wing assembly, diffuser, rear lower wing and crush structure/mount, two element upper rear wing) showed a gain of roughly 12% (depending on how you measure it) over the next best configuration, which was the stock Van Diemen wings.
Aerodynamicists, especially those with an aeronautical background, tend to talk about L/D (lift over drag) ratios as a figure of merit. That is reasonable for aircraft, where your lift (in steady state) is defined by the weight of the aircraft and you only care about improving drag.
Formula cars are different. There is no defined lift (downforce) requirement, so improving -L/D ratios doesn't necessarily decrease lap time. Since there is a substantial amount of drag, from the tires, cockpit, driver's helmet, etc, that can't be eliminated, you can always increase -L/D by simply adding more and more downforce. And go slower.
We have a pretty good idea, from track testing and lap simulation, how much downforce is optimum at a given efficiency for each type of track. So the goal becomes to improve efficiency around a specific downforce value.
Put another way, if you put a set of FA wings on a FC car, you will undoubtedly increase downforce substantially, and even improve the -L/D ratio of the entire car, but your lap time will be slower because the additional drag will cost you more than the additional downforce gains you.
In real numbers, a stock Van Diemen with a standard Van Diemen wing package makes about 177 lbs of downforce at 90 mph in the wind tunnel, and has a drag of 111 lbs at a medium downforce setting. The same Van Diemen with a full Radon aero package makes 203 lbs of downforce with a 109 lbs of drag (when we roughly match drag). Or 177 lbs of downforce and 106 lbs of drag (if we match downforce).
It's always harder to reduce drag than increase downforce on these cars, which becomes obvious when you think about it. That's why our designs don't look anything like a Formula 1 car (that has 800 hp). That seemingly small drag reduction of 5 lbs at 90 mph, by the way, becomes 13 lbs and almost 5 hp at 140 mph.
And that's without our single element wing, which provides even more of a drag reduction for equivalent downforce.
I can't answer that question, it's not really subject to aerodynamic analysisIs there really enough to be had to let someone challenge Niki?
I will say that given the improvements we've managed on a Van Diemen chassis just by changing wings and diffuser, you can imagine how much better the Rn.10 will perform, which was designed from scratch to improve aerodynamic performance. I don't think Niki would have a chance at Road America against a decent driver in an Rn.10, but my opinion doesn't count, no matter how well informed. What matters is how fast it is on track, and we'll just have to wait and see.
Nathan
Rick - yes, the assembly can be moved fore / aft as well as vertically up / down. Our original design work was done against a Pennon diffuser, and that's how our current track testing is being done as well - I think this is the same configuration as your car, yes?
So... L/D is not a figure of merit for formula cars because they do not have steady-state lift requirements... but your explicitly stated goal here is to improve L/D around a specific lift requirement? I'm unclear on the distinction you are trying to draw here.
I take it from this and your other comment earlier in the thread about having seen only your own single element rear wing assembly, that you don't make it out to the track often? As Wren stated, there were several single-element rear wing assemblies at the Runoffs which were (continue to be??) the subject of a great deal of discussion. Granted that the Runoffs are not the Pro F2000 series, so it might be outside of your area of concentration. Each example that I saw at the Runoffs made explicit and unavoidable tradeoffs to make them work (or not work, as the case may be); it will be fascinating to see where it all unfolds in terms of development, and if anybody truly cracks the code on them.
Regarding Niki, the goalposts have moved a bit with regard to what you will be competing against, equipment-wise:
http://www.rfrcars.com/thecars/f2000/
http://www.rfrcars.com/gallery/
Cheers,
Rennie
Read it again. For a given efficiency there is an optimum downforce value for each track (not for all FC cars everywhere). If you make more downforce without increasing drag, then lap time will clearly decrease. If you make more downforce and increase drag by adding a bigger wing or more elements, the L/D of the car will improve. Depending on the actual values and the track, you may go faster or slower.
I spent 27 days at the track in 2009, and will probably spend more than 40 this year. I don't know if that qualifies as "often" or not!I take it from this and your other comment earlier in the thread about having seen only your own single element rear wing assembly, that you don't make it out to the track often?
I didn't go to the Runoffs, so I didn't see any other examples. Justin Pritchard ran our aero package, and put on our prototype single element wing (machined from aluminum) the morning of the race. The test session Gibby was going to use to tune aero balance was rained out, so he ended up with quite a lot of aero push. Not sure if he could have beaten Niki with better balance, but I understand he was hitting the rev limiter at the end of the straight.As Wren stated, there were several single-element rear wing assemblies at the Runoffs which were (continue to be??) the subject of a great deal of discussion. Granted that the Runoffs are not the Pro F2000 series, so it might be outside of your area of concentration. Each example that I saw at the Runoffs made explicit and unavoidable tradeoffs to make them work (or not work, as the case may be); it will be fascinating to see where it all unfolds in terms of development, and if anybody truly cracks the code on them.
From track and wind tunnel testing we know our single element wing works. I can't comment about any others, I haven't seen them.
Yeah, those concept artist's drawings were circulated last November prior to PRI. I understand the actual car looks quite a bit different (which is good, since the design shown in those drawings doesn't meet FC rulesRegarding Niki, the goalposts have moved a bit with regard to what you will be competing against, equipment-wise:
http://www.rfrcars.com/thecars/f2000/
http://www.rfrcars.com/gallery/
Regards,
Nathan
Life is good. PM sent this AM.
Iverson
Nathan,
Thanks, I get that, but I was actually addressing your assertion that L/D is not a figure of merit for Formula Cars. The question of larger wings / more drag is a non-sequitur, because it changes the frame of reference with respect to L/D. Even with aircraft, using a larger wing or more flap angle can result in an "improved" L/D, while still resulting in a slower aircraft. Obviously, this is counter productive, but it's also beside the point.
In order to properly evaluate any L/D figure (be it aircraft or car), one must constrain the discussion by fixing certain variables - provide a frame of reference. For aircraft, it's typically constrained by lift and speed. If we take you at your word, i.e., "So the goal becomes to improve efficiency around a specific downforce value" - then you are speaking strictly about increasing L/D of the car around a specific lift requirement, presumably at a certain speed. This is a drag reduction effort - same as the aircraft example you cite.
My point here is that L/D is a dimensionless number that doesn't have much relevance outside of a point of reference, regardless of whether the topic of discussion is aircraft or cars.
Rick,
Drop me an email, haven't received your PM as of yet! rennie@dauntlessracing.com
Cheers,
Rennie
Rennie;
Sorry about the delay, but was on holiday in Destin for three weeks. You are correct, Sir, an RF94/95 with a Polecat.
How are the track trials going? Any relevation you might want to share?
VR
Iverson
L/D for aircraft
L/D max is an essential for pilots in understanding their aircraft. It is the speed which provides max endurance. It does not provide max range. It is useful if I need to loiter, but irrelevant if I need to cover some territory. If I add flaps, I have changed the airfoil. Flaps typically increase lift and drag dramatically. Nathan is accurate in his assessment of why many aircraft concepts do not directly translate to race cars. It took me quite a while to break away from some of the typical aviation guidance as not pertaining to race cars. As airspeed increases, aircaft in straight and level flight reduces induced drag and increases parasite drag by changing the angle of attack. We don't have the option of changing the angle of attack while the car is moving.
Larry Oliver
Larry Oliver
We do - it's just not recommended
I think this video is proof that race cars, and aircraft, do at times have alot in common...
Enjoy
http://www.youtube.com/watch?v=ptmg3Q0sqWI
Dumb question, but here goes:
The two wings on a 747 aircraft generate about one million pounds of lift?
It just doesn't seem possible.
HooooooooooLyyyyyyyyyyyy She-ott!
Man, that vid is unreal. It looks like the Cloverfield monster just picked that hing up and chucked it!
"Whooooooooooooa" - K. Reeves
B-747
Yep! One common measurement used in aircraft is wing loading, or weight of the aircraft divided by the number of square feet of wing. Aircraft like the 747 have HUGE flap sections that increase wing area tremendously, and are used to get the plane off the ground.
Larry Oliver
Larry Oliver
If each wing is 40 feet long and 10 feet wide, that's 400 square feet times two wings.
Dividing one million by 800, I come up with more than 1,000 pounds of lift/downforce per square foot of wing.
A wing the size of a beer coaster should be sufficient to keep my car stuck to the ground.
I know that airplanes are faster than cars, but the airplane is off the ground at about 100 mph, right?
Peter,
The larger 747 variants have a maximum takeoff weight of 910k lbs - so yeah... they damned well better have in the neighborhood of 1 million lbs of lift!
Cheers,
Rennie
a Boeing 747-400 has 5825 sq ft of wing, max takeoff weight of up to 910,000 lbs.
From http://www.airliners.net/aircraft-da...ts.main?id=100
So, 1 million / 5825 = 171 lbs/sqft.
Gary
Gary Tholl
#24 BlurredVisionRacing
Peter;
747-100: 5,500 ft² (510.95 m²) = 181.8 lbs/ft²
747-400: 5,650 ft² (524.90 m²) = 177.0 lbs/ft²
Throw on a craphouse full of flaps, and these numbers take a plunge. Look at the lift formula.
Lift = (1/2)(coefficient of lift)(wing surface area)(air density)(velocity²)
Although there is but one lift formula to handle both symmentrical and asymmetrical wing designs, on any given day with these two designs of identical SA and tested in identical air densities (slugs/ft^3), lift really comes down to the CL (inherent wing design) and V² (THIS IS IT!!).
Think of it this way: a 1 degree angle of attack on a symmetrical RW will most certainly produce LESS lift than a 1 degree angle of attack on an asymmetrical wing of pronounced camber.
Iverson
OK, downforce seems to be figured out. I'm not even gonna ask why I need more wing surface than two LP record album covers, in order to hold my car to the ground (but not my Rolling Stones Sticky Fingers album cover with the Andy Warhol drawing of Mick Jagger's pants).
The point is....whenever there is an aero "breakthrough", I assume it has to do with drag.
Then consider the load on each fanjet blade at full takeoff power at max rate of climb....
“Racing makes heroin addiction look like a vague wish for something salty.” -Peter Egan
747 take-off speed?
I believe they take off at around a buck-fifty? That would provide more lift than 100, correct? Like double?
Novice aero guy here...but have several commercial pilot clients.
Lawrence Hayes
Hayes Cages, LLC
Sagle, ID.
Bernoulli said it best: P1V1=P2V2.
A molecule of air figuratively 'splits' at the leading edge of an airfoil, where one half travels over the top, and the other half travels under the bottom. But they must meet and join at the trailing edge.
To watch this process on an asymmetrical airfoil with a pronounced camber, the molecule on the top has a greater distance to travel, and to meet its counter-part at trailing edge, it has to travel faster. The faster it travels, the less pressure above the wing. Hence, lift.
Iverson
A 747 has 1/2 million horsepower (according to my 30s google search). That might explain why the higher angles of attack are acceptable.I forget the exact numbers right now, but for kicks I once ran my wing at it's max angle of attack for a test session at Roebling. My spotter said that I appeared out of the back section of track 2 seconds earlier, but crossed the finish line 4 seconds later.
Iverson,
Track trials are going very nicely, and giving us quite a bit of information. We are seeing a very consistent gain in top speed of 1.5-3 mph, despite the fact that most of our testing has been conservative to the point of carrying too much rear wing for perfect balance. We're at the point where we're ready to really start doing proper setup optimization to see what else we can squeeze out of the wings, and based on what we've seen to date, we're quite optimistic that more improvements are coming.
Now the fun truly begins - the first production run for the rear wing assemblies is finalized now, and the front wing prototype is being fabricated as I type...
Cheers,
Rennie
747 lift per sq inch
At 171 pounds of lift per square foot, a 747 has about 1.2 pounds per square inch of lift.
When you get down to PSI it is not that much.
Also, airplane wings as was stated earlier have all sorts of tricked up adjustable surfaces to generate high lifts with tons of added drag. But with all the thrust one of them has, they can get moving in the high drag configuration.
Then when speed picks up, change the wings to a less draggy configuration.
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