32 replies to this topic

[biercemountain]

Does air density have the same kind of effect on downforce that it has on engine performance? I've long heard about how races held at high altitude venues sap cars of their horsepower but I've never heard anything about it affecting downforce.

Anyone care to illuminate my understanding of the subject?

[McGuire]

Yes, air density does indeed affect the car's aero. For example, if you watch the Champ Cars at Denver or Mexico City, where the air has less density due to high altitude (above 5000') they will be running max aero package to recover downforce lost to the elevation. Also, since the thinner air carries away less heat they will also run much larger brake and cooler ducts at these venues.

...And in qualifying for the Indy 500 where one mph is everything, the sharper engineers will keep very careful track of the weather as in the tiny range they are working in, air temperature, density and humidity will affect both drag and downforce and can even change the car's balance. Some guys like Dr. Who will watch the dew point.

[Monstrobolaxa]

downforce reduction if proporcional do air density reduction.

[phantom II]

Absolutely. Have you seen that little bent tube on top the hood of a F1 car.



That is a pitot tube that measures airspeed. It has a static vent somewhere on the car that remains at ambient pressure. The comparison between the two pressure sources indicates airspeed. On a standard day, 15c and 1310 milibars or 60f and 29.92 inches of mercury, the airspeed will be the same as the ground speed. In Mexico city or Johannesburg, The airspeed will be lower than the ground speed or the shaft speed or GPS speed. In order the produce the same amount of lift or downforce, you wing would have to go faster. The indicated airspeed would stay the same but the true airspeed would be the ground speed.On a cold day in Miami compared to a high humidity mid summers day, the wing word perform as it does in Denver on a cold day. Aircraft need much less runway on a cold day than a hot day. Much more runway in Denver than Miami. Old 747s could not take off in Miami during the day in summer, they had to take off at night. even things like a cloud going over the track will decrease lap times. Surface temperature will effect the performance of the wing. That is why you will see pitot tubes at 2 or 3 different heights on the car. You would compensate for diffuser lift differently than you would for wings. You cant induce lift at the diffuser so the center of lift changes even if a cloud goes over the track. You would give the wings more angle of attack to induce more lift.
A wind on the track will also effect wing performance and depending on wing direction, the aero package is optimized.
Aircraft have pitot tubes in the cleanest air possible. Here it is on the tail of this F4.



Even thought the airspeed indicator is reading 670 mph the true airspeed will be 1400 or 2.2 mach. depennding on temp and pressure at 50 000 feet. OK that's extreme but an airplane performs at its indicated airspeed. On a low density days the plane would use much more airspace to turn around and wing would stall at the same indicated airspeed.
A 747 indicating 380KTS at 35 000 ft may have a ground speed of 500 kts plus or minus the wind.

Originally posted by biercemountain
Does air density have the same kind of effect on downforce that it has on engine performance? I've long heard about how races held at high altitude venues sap cars of their horsepower but I've never heard anything about it affecting downforce.

Anyone care to illuminate my understanding of the subject?

[saudoso]

The basic equation that drives both drag and downforce/lift is (1/2)*p*s*v^2*Cl

p - Ist's not p in fact, it is a greek letter that I can't name in english, and stands for fluid density
s - surface area
v - fluid speed
Cl for lift, Cd for drag

So roughly downforce is directly connected to air density.

I kow it gets more complicated than that, but this is (or was 15 years ago) the first thing you learn about fluid dinamics.


Ricardo

[phantom II]

Greek letter phi ϱ

Originally posted by saudoso
The basic equation that drives both drag and downforce/lift is (1/2)*p*s*v^2*Cl

p - Ist's not p in fact, it is a greek letter that I can't name in english, and stands for fluid density

Ricardo

[RDV]

Er... greek letter rho...

And standard atmosphere is defined as the sea level standard values of

Sea level pressure = 101325 N/m2 or (1013 mb) think 1310 a misprint or crossover from saying thirteen-ten instead of ten-thirteen..
Sea level temperature = 288.15 K
Hydrostatic constant = 34.1631947 kelvin/km

The sea level density of 1.225 kg/m3 is derived from the fundamental quantities above.

Conversion tables on= Standard atmosphere

As for setup you actually change ride heights to compensate for different temp and baro between morning and afternoon sessions... re difuser and wing effects from temp, running a diffuser a couple of mm off the ground will make it run in air which is at near asphalt temp (I.E. on a sunny day ambient temp might be 25 degrees but asphalt can be at 55 or 60deg C)... front wings and rear diffusers are running at a higher temp and rear wings are running in cooler air... so, car aero balance change... trim to suit.

Same as for engines, roughly each 3 degC worth 1%.

So dont get excited about solo winter test times being faster than the last race there.... very cool air and high density helps... you look very good, come the day mid summer you are just as far off the pace as last year!

Re clouds ... quite common to see engineers standing at pit entrance looking at the sky and waiting for that lone cloud scudding along... ready to launch car at appropriate moment during quals, always worth that little bit.

[Bill Sherwood]

Originally posted by phantom II
A 747 indicating 380KTS at 35 000 ft may have a ground speed of 500 kts plus or minus the wind. [/B]

FWIW at 35,000' our typical indicated airspeed would be more like 290kts - 380 kts at 35,000 is faster than Vmo.
290kts indicated works out to about 500kts true airspeed. Head or tail winds affect that to give us our groundspeed. I've seen as low as 325kts GS going into Narita in a big jetstream, then a couple of days later 698kts.

[desmo]

Um... this raises a question I've briefly considered but never asked about: I know the aero CoP's fore aft location can be important, how about the aero CoP's height? All else equal does it matter within reason?

[biercemountain]

Thanks for the response everbody. What a brain trust this forum is.

[phantom II]

Yep, typo. All my posts have typos. Only find them after they have been posted.

Originally posted by RDV
Er... greek letter rho...

And standard atmosphere is defined as the sea level standard values of

Sea level pressure = 101325 N/m2 or (1013 mb) think 1310 a misprint or crossover from saying thirteen-ten instead of ten-thirteen..

[phantom II]

A few more typos in that post. Can you spot them?

Originally posted by Bill Sherwood


FWIW at 35,000' our typical indicated airspeed would be more like 290kts - 380 kts at 35,000 is faster than Vmo.
290kts indicated works out to about 500kts true airspeed. Head or tail winds affect that to give us our groundspeed. I've seen as low as 325kts GS going into Narita in a big jetstream, then a couple of days later 698kts.

[saudoso]

rdv, phantomII, thanks!

There is something else to consider, not regarding aero: At the same pressure, an engine will give more horsepower since it needs less energy to compress the mixture. That's also realted to the late night takeoffs...

Ricardo

[phantom II]

The height has nothing to do with lift or CoL. except for ground effect considerations.
Out of the 3 aero devices that create lift, front wing rear wing and diffuser, the diffusrs does not have a moment that effects traction as it creates a vertical force at the CG . The front and rear wing have a moment that augments lift caused by drag. The diffuser's CoP is very close to the CG of the car on one plane. The front wing is placed in a box as is the rear wing which is determined by the FIA. The front wing has a moment not only with the downforce CoP on the individual wing and the distance from the CG and front wheels on the x axis but also a moment on the Y axis below the CoG. The moment of the rear wing caused by drag adds additional load to the rear tires. Aircraft use center of thrust and center of drag couples for stability. Race cars use it for additional downforce.
You would assume that the front wing is always mounted as low as it can be and the rear, as high as it can be, but on low downforce tracks only changing the heights of the wings reduce the effects of drag.

Originally posted by desmo
Um... this raises a question I've briefly considered but never asked about: I know the aero CoP's fore aft location can be important, how about the aero CoP's height? All else equal does it matter within reason?

[Bill Sherwood]

Originally posted by phantom II
A few more typos in that post. Can you spot them?

Nope, there are none.

[phantom II]

Well for a start, sentence structure, grammar and spelling is awful as usual. On a standard day, 15c and ...1013 milibars or 60f and 29.92 inches of mercury at sea level ....., Even thought the airspeed indicator is reading 670 mph the true airspeed will be 1400 or 2.2 mach. depending (sp.) on temp and pressure at 50 000 feet. OK that's extreme but an airplane performs at its indicated airspeed. On a low density days the plane would use much more airspace to turn around . The wing (New sentence) would stall at the same indicated airspeed.
A 747 indicating 300KTS ( what I meant to write)at 35 000 ft may have a ground speed of 500 kts plus or minus the wind.

Originally posted by Bill Sherwood


Nope, there are none.

[phantom II]

Yet another one. ...., Even though....

Originally posted by phantom II
Well for a start, sentence structure, grammar and spelling is awful as usual. On a standard day, 15c and ...1013 milibars or 60f and 29.92 inches of mercury at sea level

....., Even thought.....

the airspeed indicator is reading 670 mph the true airspeed will be 1400 or 2.2 mach. depending (sp.) on temp and pressure at 50 000 feet. OK that's extreme but an airplane performs at its indicated airspeed. On a low density days the plane would use much more airspace to turn around . The wing (New sentence) would stall at the same indicated airspeed.
A 747 indicating 300KTS ( what I meant to write)at 35 000 ft may have a ground speed of 500 kts plus or minus the wind.

[phantom II]

...and another thing. Max afterburners at sea level and at 50 000ft will give you Mach.2 and Mach 2.2 respectively with the same indicated airspeed..

Originally posted by phantom II
Well for a start, sentence structure, grammar and spelling is awful as usual. On a standard day, 15c and ...1013 milibars or 60f and 29.92 inches of mercury at sea level ....., Even thought the airspeed indicator is reading 670 mph the true airspeed will be 1400 or 2.2 mach. depending (sp.) on temp and pressure at 50 000 feet. OK that's extreme but an airplane performs at its indicated airspeed. On a low density days the plane would use much more airspace to turn around . The wing (New sentence) would stall at the same indicated airspeed.
A 747 indicating 300KTS ( what I meant to write)at 35 000 ft may have a ground speed of 500 kts plus or minus the wind.

[saudoso]

My dear Phantom II,

This is not about you, it's about discussing things. People are expected to say silly things, do stupid ones. My last post in this thread is something I think I'll regret soon, but anyway, I'm not to state that 'phi' for 'rho' is a typo. It's plain, human, mistake.

We have gone deep into some polictical argues in the past but I have to say that I allways read what you post. You have some experience that I can't even dream of. I really like it.

I like your thoughts, you just can't get that personal about being corrected, that's thread jacking at it's worst.

Hope you get me right.

Ricardo

ps. My writing really su**s and I'm aware of that.

[phantom II]

I don't understand what you are saying but I admire your courage to participate on a forum that is not of your tongue. I think the mistake is all mine. RDV corrected me in his usual gentlemanly manor. I don't do diplomacy like he does.
I thought that I was being helpful.

Originally posted by saudoso
My dear Phantom II,

This is not about you, it's about discussing things. People are expected to say silly things, do stupid ones. My last post in this thread is something I think I'll regret soon, but anyway, I'm not to state that 'phi' for 'rho' is a typo. It's plain, human, mistake.

We have gone deep into some polictical argues in the past but I have to say that I allways read what you post. You have some experience that I can't even dream of. I really like it.

I like your thoughts, you just can't get that personal about being corrected, that's thread jacking at it's worst.

Hope you get me right.

Ricardo

ps. My writing really su**s and I'm aware of that.

[saudoso]

At least one of us is completely nuts. I really can't tell if that's me, you, or the both of us.

Nice image, anyway.

Ricardo

EDIT: You where, indeed, being helpfull - IMO.

[phantom II]

Oh it's me alright. My own 30 year old son says that I should drive a little bit more carefully than I do.

Originally posted by saudoso
At least one of us is completely nuts. I really can't tell if that's me, you, or the both of us.

Nice image, anyway.

Ricardo

EDIT: You where, indeed, being helpfull - IMO.

[saudoso]

Don't be so sure, I do have some loose nuts & bolts doing some kind of weird noise inside my head!

Ricardo

[Bill Sherwood]

Originally posted by phantom II
Well for a start, sentence structure, grammar and spelling is awful as usual. On a standard day, 15c and ...1013 milibars or 60f and 29.92 inches of mercury at sea level ....., Even thought the airspeed indicator is reading 670 mph the true airspeed will be 1400 or 2.2 mach. depending (sp.) on temp and pressure at 50 000 feet. OK that's extreme but an airplane performs at its indicated airspeed. On a low density days the plane would use much more airspace to turn around . The wing (New sentence) would stall at the same indicated airspeed.
A 747 indicating 300KTS ( what I meant to write)at 35 000 ft may have a ground speed of 500 kts plus or minus the wind.

Dude, I don't know WTF you're on about, but a typo is a spelling mistake. I have made none in this thread.
And all that other stuff you carry on about in the above quote is nothing that I wrote.

Want to try again?

[phantom II]

I'm talking about the 'errors' in my post/s.

Originally posted by Bill Sherwood


Dude, I don't know WTF you're on about, but a typo is a spelling mistake. I have made none in this thread.
And all that other stuff you carry on about in the above quote is nothing that I wrote.

Want to try again?

[phantom II]

To increase braking efficiency on an airplane on the landing roll, a deflection of the elevator will load the main gear because the CG is fore of the gear. The friction on the tires keeps the nose wheel on the ground(no brake on the front wheel) If you released the brakes, the tail would hit the ground. I reworded the text below. You, Greg or RDV could do a better job of it.

Originally posted by phantom II
The height has nothing to do with lift or CoL. except for ground effect considerations.
Out of the 3 aero devices that create lift, front wing rear wing and diffuser, the diffusrs does not have a moment that effects traction as it creates a vertical force at the CG . The front and rear wing have a moment that augments lift caused by drag. The diffuser's CoP is very close to the CG of the car on one plane. The front wing is placed in a box as is the rear wing which is determined by the FIA. The front wing has a moment not only with the downforce CoP on the individual wing but also from the distance from the CG and front wheels. The moment of the rear wing caused by drag adds additional load to the rear tires. Aircraft use center of thrust and center of drag couples for stability. Race cars use it for additional downforce.
You would assume that the front wing is always mounted as low as it can be and the rear, as high as it can be, but on low downforce tracks only changing the heights of the wings and not the AoA will reduce the effects of drag.

quote:
--------------------------------------------------------------------------------
Originally posted by desmo
Um... this raises a question I've briefly considered but never asked about : I know the aero CoP's fore aft location can be important, how about the aero CoP's height? All else equal does it matter within reason?

[phantom II]

This pitot tube on the virtical fin is actually a pressure head for the 'feel' system of the stabilator (A stabilator is simply a combined horizontal stabilizer and an elevator). As you transition from low to high speed and pressure rise regions and high and low altitude, different stabiliator elevator deflections need additional augmentation and pilot inputs must remain effortless or have the correct 'feel.' This was all new stuff in the mid 50s. The two airspeed indicators had similar looking pitot tubes one is placed on the nose. See avatar. I just thought that I should mention this before certain members become picky ;) and you know who you are.

A F1 car doesn't have an airspeed indicator for the driver. This info is transmiited to the pit for more important people to deal with.

Originally posted by phantom II
Absolutely. Have you seen that little bent tube on top the hood of a F1 car.
Aircraft have pitot tubes in the cleanest air possible. Here it is on the tail of this F4.

[Fat Boy]

Originally posted by phantom II
I just thought that I should mention this before certain members become picky ;) and you know who you are.

Sorry, what was that? I'm still looking for the 'hood' on an F-1 car.

[phantom II]

Bonnet?

Originally posted by Fat Boy


Sorry, what was that? I'm still looking for the 'hood' on an F-1 car.

[wegmann]

Originally posted by desmo
Um... this raises a question I've briefly considered but never asked about: I know the aero CoP's fore aft location can be important, how about the aero CoP's height? All else equal does it matter within reason?

Absolutely. Both the CoP's fore-aft location and height, relative to the CG, create a torque around the y axis. Assuming they are both on the centerline of a symmetrical car, they essentially have the same overall effect of changing the front/rear aerodynamic balance. If you add something creating drag way up above the CG (perhaps a camera on top of the airbox), it's going to shift the balance to the rear.

[phantom II]

That's what I meant to say.

Originally posted by wegmann


Absolutely. Both the CoP's fore-aft location and height, relative to the CG, create a torque around the y axis. Assuming they are both on the centerline of a symmetrical car, they essentially have the same overall effect of changing the front/rear aerodynamic balance. If you add something creating drag way up above the CG (perhaps a camera on top of the airbox), it's going to shift the balance to the rear.

[turin]

I've never seen a pitot located there. I wonder if it goes along the lines of what phantom II mentioned before, to have cleaner air and to compare the differences with the other readings. Perhaps the other readings were messed up, altering all the calibration parameters (i.e., for wind tunnels?)

Here is the pic I'm referring to

[no-clue]

That pic is of a yaw pitot, used to determine the attitude of the car to the air in a turn. It can be used to determine if any crosswinds are present by referencing what the speed laser sensors are telling the engineers, by simply subtracting one from the other, the angle of the car to the local airflow can be determined. As the cars experience different aero behaviour between straight line and turning conditions, this also helps the engineers to determine if the crosswinds have any adverse effect on the car as it laps.