Theory behind W13's 'blister' RW flap and other RW questions.

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Hutchie.91
Hutchie.91
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Theory behind W13's 'blister' RW flap and other RW questions.

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So one thing I've noticed about the W13 that I'm hoping someone more aero literate could explain, is why Mercedes have given the pressure side of the flap such a pronounced convex surface to the point it looks like a blister bubble that then tapers back down before the leading edge, unlike other flaps with a convex pressure surface (such as the RB6).

A photo for reference along with a crude MS Paint sketch of the assumed profile:

Image

Image

This photo from Australia shows the surface well (look at the curvature of the 'R', 'N' and 'A' on Petronas), and notice how this 'blister' tapers off along the Y-axis towards the DRS actuator mount and towards the end of its span where it meets the 'endplates'/DRS pivot pins.

Image


And finally, on a slightly different topic, why is it common to see the pressure side of the main plane of a rear wing profile be cambered so much that it's concave?

And what I mean by that, is that if the force vector imparted by the pressure differential is normal to the surface it acts on, surely on a concave pressure surface this would then create a horizontal component which would form either drag or thrust?

Is this horizontal component so small relative to the vertical component that it can be disregarded as negligible? Likewise im guessing the majority of the force normal to the DRS flap is horizontal and therefore mainly drag force, but that is offset by the main benefit of having the slot in the first place to keep flow attached on the suction side for longer while having such aggressive trailing edge camber and flap AoA which is a net increase in downforce at the cost of it being much less efficient downforce that brings a lot of drag with it?

I also know the caveat that the suction side contributes more to downforce by a ratio of like 3-5:1 relative to the pressure side and of course, solutions seen in F1 aren't necessarily the most efficient or even the best with regards to peak downforce/drag reduction, it's just the best they can do within the confines of the regulations they have to work with (as well as time, budget, man power, manufacturing facility considerations), and of course, talking about the RW in a vacuum isn't ideal considering it's working as a system along with the beamwing and diffuser and is affected upstream by things like engine cover/intake, sidepods, floor space infront of rear wheels etc.
Last edited by Hutchie.91 on Mon Sep 12, 2022 8:34 pm, edited 1 time in total.

Just_a_fan
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Re: Theory behind W13's 'blister' RW flap and other RW questions.

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The top surface is concave because it increases the overall camber and hence downforce.
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wesley123
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Re: Theory behind W13's 'blister' RW flap and other RW questions.

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If I recall correctly a thicker leading edge helps prevent flow separation, essentially allowing you to run higher angles of attack.
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Hutchie.91
Hutchie.91
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Re: Theory behind W13's 'blister' RW flap and other RW questions.

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wesley123 wrote:
Mon Sep 12, 2022 7:16 pm
If I recall correctly a thicker leading edge helps prevent flow separation, essentially allowing you to run higher angles of attack.
Yeah but from what I can tell, the blister sits behind the leading edge and tapers back down to it's 'normal' surface before the LE, so it isn't increasing it like you say, to reduce the sensitivity of leading edge AoA.

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Stu
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Re: Theory behind W13's 'blister' RW flap and other RW questions.

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It could be something that has an effect when the DRS is open (changing the effective AoA of the flap), it may reduce upwash in the ‘closed position’
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johnny comelately
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Re: Theory behind W13's 'blister' RW flap and other RW questions.

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From a distinctly non-aero bloke
1. Gurney flaps the lower wing
2. Reduces drag when closed as the speed increases by bouncing (told you I wasnt an aero bloke) the air over the rest of the wing (deliberate separation?)

Question: what is the total weight of the DRS system?

Hutchie.91
Hutchie.91
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Re: Theory behind W13's 'blister' RW flap and other RW questions.

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So while researching some LMP prototype aerofoils out of curiosity, I happened to find this page which seems to answer my question with regards to the W13 flap.

However, for the '22 or even '21 technical regs, there's no rule stating the gurney flap has to be perpendicular to an imaginary plane that connects the trailing edge to the top of the pressure surface like there is for LMP1 at that time. Only rule is that it can't be more than 20mm. And of course, at least in that Jeddah photo, Merc weren't always running a gurney flap. So the idea of increasing the angle of the gurney flap is out of the window.

The only other benefit they mention is more of a flow-conditioning measure as seen on the leading edge of whale's fins to increase it's efficiency through a fluid, however Merc's approach is having two large bulbous surfaces either side of the DRS actuator across the majority of the flap's span, as opposed to multiple smaller bumps as seen by the 2011 Oak Racing Pescarolo LMP1.

Funnily enough Peugeot had a similar idea which looked a bit more like Merc's flap, but again, without that specific gurney flap rule, there wouldn't be a need for it still...

Image

johnny comelately
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Re: Theory behind W13's 'blister' RW flap and other RW questions.

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Hutchie.91 wrote:
Mon Sep 12, 2022 11:55 pm
So while researching some LMP prototype aerofoils out of curiosity, I happened to find this page which seems to answer my question with regards to the W13 flap.

However, for the '22 or even '21 technical regs, there's no rule stating the gurney flap has to be perpendicular to an imaginary plane that connects the trailing edge to the top of the pressure surface like there is for LMP1 at that time. Only rule is that it can't be more than 20mm. And of course, at least in that Jeddah photo, Merc weren't always running a gurney flap. So the idea of increasing the angle of the gurney flap is out of the window.

The only other benefit they mention is more of a flow-conditioning measure as seen on the leading edge of whale's fins to increase it's efficiency through a fluid, however Merc's approach is having two large bulbous surfaces either side of the DRS actuator across the majority of the flap's span, as opposed to multiple smaller bumps as seen by the 2011 Oak Racing Pescarolo LMP1.

Funnily enough Peugeot had a similar idea which looked a bit more like Merc's flap, but again, without that specific gurney flap rule, there wouldn't be a need for it still...

http://www.mulsannescorner.com/Peugeot9 ... 11-SC2.JPG
Funnily enough the new Peugeot hasnt got one (rear wing) at all (9X8) :wink:

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Vanja #66
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Re: Theory behind W13's 'blister' RW flap and other RW questions.

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Hutchie.91 wrote:
Mon Sep 12, 2022 5:35 pm
So one thing I've noticed about the W13 that I'm hoping someone more aero literate could explain, is why Mercedes have given the pressure side of the flap such a pronounced convex surface to the point it looks like a blister bubble that then tapers back down before the leading edge, unlike other flaps with a convex pressure surface (such as the RB6).

A photo for reference along with a crude MS Paint sketch of the assumed profile:

https://www.racefans.net/wp-content/upl ... 6-26-5.jpg

https://i.imgur.com/hmnYjjR.jpg

This photo from Australia shows the surface well (look at the curvature of the 'R', 'N' and 'A' on Petronas), and notice how this 'blister' tapers off along the Y-axis towards the DRS actuator mount and towards the end of its span where it meets the 'endplates'/DRS pivot pins.
Nice catch! I've noticed how thick the flap is at that point, but I just assumed this was a thick leading edge.

First thing that comes to my mind, this shape reduces pressure on the top of the flap, so leads to less downforce. But, more importantly, leads to lot less drag on the wing as well. Unfortunately, this thickness also increases drag with DRS on, so it's a trade-off for sure.
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OO7
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Re: Theory behind W13's 'blister' RW flap and other RW questions.

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Yes, that bulge was reported at launch or just after, so it's been incorporated into other flap designs for the W13 as well.