Fuel Efficiency - High rake v Low rake

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bonjon1979
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Fuel Efficiency - High rake v Low rake

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Having seen reports in AMUS regarding mercedes Fuel efficiency being far superior to that of the other cars, I was wondering if this might not just be due to the engine. Could the low rake of the car in comparison to some of the high rake concepts provide greater efficiency? I only suggest it because logically speaking, if the car is angled up more would you not expect more drag then a car that has a flatter aspect? Be good to hear what anyone with greater technical knowledge than me thinks about this!

cramr
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Re: Fuel Efficiency - High rake v Low rake

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Increasing the rake generally creates more front downforce but also more drag so could be a bit related. But you should be able to generate more downforce for frontal ground effect, therefore more efficiently and maybe you can also reduce drag from other places that creates downforce not as efficient.
Not a straight an easy answer

Jolle
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Re: Fuel Efficiency - High rake v Low rake

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No, rake by itself got nothing to do with it. Downforce efficiency does.
There is another thing at play here. A small part of the Mercedes fuel economy comes that their GP’s take shorter then other teams (because of the fuel flow limit). But the main reason is that they are able to have a bigger recovery of energy then the other PU’s.

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gcdugas
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Re: Fuel Efficiency - High rake v Low rake

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Downforce generated by the floor (diffuser OR rake attitude) is "ground effects" and has a MUCH greater efficiency that wing generated downforce. Therefore the drag penalty is far less per unit of downforce gained. So high rake is more fuel efficient because there is marginally less drag. Plus floor generated downforce is less affected by dirty air.
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Moose
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Re: Fuel Efficiency - High rake v Low rake

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gcdugas wrote:
Wed Mar 14, 2018 4:22 pm
Downforce generated by the floor (diffuser OR rake attitude) is "ground effects" and has a MUCH greater efficiency that wing generated downforce. Therefore the drag penalty is far less per unit of downforce gained. So high rake is more fuel efficient because there is marginally less drag. Plus floor generated downforce is less affected by dirty air.
This assumes that high-rake cars generate more downforce with the floor. Mercedes' assertion is that they have found a better way to utilise the floor with low rake, not that they're sacrificing floor-generated downforce for some other benefit.

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gcdugas
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Re: Fuel Efficiency - High rake v Low rake

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Moose wrote:
Wed Mar 14, 2018 4:25 pm

This assumes that high-rake cars generate more downforce with the floor.....
Correct. And it is obvious that high rake cars do generate downforce via the floor but too much rake invites the inflow of air from the sides and nullifies the effect in the same manner that a skirt seal lapse upset the ground-effect cars of bygone days.
Innovation over refinement is the prefered path to performance. -- Get rid of the dopey regs in F1

Moose
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Re: Fuel Efficiency - High rake v Low rake

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gcdugas wrote:
Wed Mar 14, 2018 5:06 pm
Moose wrote:
Wed Mar 14, 2018 4:25 pm

This assumes that high-rake cars generate more downforce with the floor.....
Correct. And it is obvious that high rake cars do generate downforce via the floor but too much rake invites the inflow of air from the sides and nullifies the effect in the same manner that a skirt seal lapse upset the ground-effect cars of bygone days.
It's obvious that all cars generate downforce from their floor. Exactly how they're doing it is not known very well by anyone without a CFD model of the exact car.

What's *not* obvious, is that a high rake car will automatically generate more downforce with its floor than a low rake one. To be able to determine that, we'd need to see those CFD models.

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gcdugas
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Re: Fuel Efficiency - High rake v Low rake

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Moose wrote:
Wed Mar 14, 2018 5:08 pm

What's *not* obvious, is that a high rake car will automatically generate more downforce with its floor than a low rake one. To be able to determine that, we'd need to see those CFD models.
Do you need to see a CFD model to determine a car with a wing has more downforce than a car without a wing? Some things *are* obvious. The main thing is to avoid "porpoising" (longitudinal tilting) when the throttle is applied exiting corners and to still have a pliant suspension.
Innovation over refinement is the prefered path to performance. -- Get rid of the dopey regs in F1

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jjn9128
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Re: Fuel Efficiency - High rake v Low rake

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Yeeeeeeah I'm not convinced by the argument that a higher rake automatically equals more underbody downforce. It moves different parts of the car closer or further from the ground - i.e. front wing and front bib will be closer, but rear diffuser and rear wing will be further so downforce balance can become forwards biased. To maintain downforce at the diffuser requires a load of effort in the bargeboard area to induce vorticity to 'seal' the edges of the floor - prevent flow ingress raising pressure under the car - to that end induced drag around the middle of the car will be higher.

If you run identical front and rear wings with a high rake concept drag would be higher, just because the blockage of the car increases (marginally - we're talking a couple of points of drag), you also get a little benefit from the AOA of the rear wing increasing. Dropping the front wing increases the venturi effect so you can run a lower incidence for the same downforce, you may also get a thicker wake with a lower incidence, so again you have to work the bargeboards to prevent it impacting the sidepods and floor, though there may be a benefit over the suspension to the tops of the sidepods. You also change how much of the front wheel is shadowed by the endplate so again you have to work the air harder to reduce the front tyre drag by the same amount.

In the end I don't think the attitude of the car makes a massive difference to either total drag or total downforce, so long as the concept for the car is followed through from front to back. Mercedes run a massive wheelbase to both get the front tyres away from the LE of the floor, but also to increase the planform area of the floor. They get a benefit from their bargeboards and LE VGs which maintains downforce along the floor, the diffuser is also closer to the ground so their peak suction at the diffuser throat will be higher. But maybe they don't have the same positive front axle as the Red Bull (from aero so in medium speed corners) because the front wing is comparatively high up in the air.
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Moose
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Re: Fuel Efficiency - High rake v Low rake

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gcdugas wrote:
Wed Mar 14, 2018 5:30 pm
Moose wrote:
Wed Mar 14, 2018 5:08 pm

What's *not* obvious, is that a high rake car will automatically generate more downforce with its floor than a low rake one. To be able to determine that, we'd need to see those CFD models.
Do you need to see a CFD model to determine a car with a wing has more downforce than a car without a wing? Some things *are* obvious. The main thing is to avoid "porpoising" (longitudinal tilting) when the throttle is applied exiting corners and to still have a pliant suspension.
You really do actually. The 2016 F1 cars generate *far* more downforce with far less wing than the 1995 ones.

More importantly though - the 2017-18 cars are *far* more complex than the 1995 ones. You can't just look at them and go "oh look, that's what the aero is doing*". Trying to assert that a high rake car "obviously" generates far more downforce than a low rake one is crazy when the championship winning car from last year was a low rake one, and the only car able to go flat through turn 9 in testing was the low rake one.

* Unless you're Adrian Newey, apparently

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gcdugas
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Re: Fuel Efficiency - High rake v Low rake

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Actually the effectiveness of high rake cars generating downforce via the floor is a function of the added width. With a narrow car the amount of leakage from the sides left only a narrow area of low pressure in the center. With the wider cars, assuming the leakage from the sides is constant, the area of low pressure gets wider and therefore more effective. And that is why you started to see it in 2017 and not earlier. Think about it.
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Just_a_fan
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Re: Fuel Efficiency - High rake v Low rake

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gcdugas wrote:
Wed Mar 14, 2018 5:30 pm
Moose wrote:
Wed Mar 14, 2018 5:08 pm

What's *not* obvious, is that a high rake car will automatically generate more downforce with its floor than a low rake one. To be able to determine that, we'd need to see those CFD models.
Do you need to see a CFD model to determine a car with a wing has more downforce than a car without a wing? Some things *are* obvious. The main thing is to avoid "porpoising" (longitudinal tilting) when the throttle is applied exiting corners and to still have a pliant suspension.
High rake only works if you can control the flow along the edge of the floor. Excessive inflow will kill any benefit very quickly. Good edge control is usually by way of vortices and these do add drag. More rake requires more vorticity. More rake therefore adds more drag. It's likely less draggy than a barn door rear wing, true, but it's not drag free.
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Just_a_fan
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Re: Fuel Efficiency - High rake v Low rake

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gcdugas wrote:
Wed Mar 14, 2018 11:27 pm
Actually the effectiveness of high rake cars generating downforce via the floor is a function of the added width. With a narrow car the amount of leakage from the sides left only a narrow area of low pressure in the center. With the wider cars, assuming the leakage from the sides is constant, the area of low pressure gets wider and therefore more effective. And that is why you started to see it in 2017 and not earlier. Think about it.
Assuming the leakage from the sides is constant with width changes might be one assumption too far.
Turbo says "Dumpster sounds so much more classy. It's the diamond of the cesspools." oh, and "The Dutch fans are drunk. Maybe"

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gcdugas
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Re: Fuel Efficiency - High rake v Low rake

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An aerodynamicist doesn't have omniscience. Neither does all the CFD in the world. He has to look at things and already have a general set of knowledge that steers him in where to look for gains. Reducing front wheel wake, barge boards, intakes etc. and so on he goes down the car. It is this general set of knowledge that my assumption is based upon. The floor width to wheel tract is mostly constant with the wider cars so the floor edge seal is affected more or less the same by the front wheel wake. Assumptions are unavoidable. The good ones just have better assumptions as where to look for gains.
Innovation over refinement is the prefered path to performance. -- Get rid of the dopey regs in F1

Moose
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Re: Fuel Efficiency - High rake v Low rake

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Just_a_fan wrote:
Thu Mar 15, 2018 1:16 am
High rake only works if you can control the flow along the edge of the floor. Excessive inflow will kill any benefit very quickly. Good edge control is usually by way of vortices and these do add drag. More rake requires more vorticity. More rake therefore adds more drag. It's likely less draggy than a barn door rear wing, true, but it's not drag free.
That's not really true - we've saw RedBull running the high rake concept for years (many before the 2017 regulations) at this point. It's just one of several ways you can go on producing downforce, and not necessarily the best (we don't know that without seeing accurate modelling of all cars).