Blowing the Wing - what is it?

[Cam]

Actually, this link has probably explained the principle, along with examples, better than I could have imagined.

In a conventional blown flap, a small amount of the compressed air produced by the jet engine is "bled" off at the compressor stage and piped to channels running along the rear of the wing. There, it is forced through slots in the wing flaps of the aircraft when the flaps reach certain angles. Injecting high energy air into the boundary layer produces an increase in the stalling angle of attack and maximum lift coefficient by delaying boundary layer separation from the airfoil. Boundary layer control by mass injecting (blowing) prevents boundary layer separation by supplying additional energy to the particles of fluid which are being retarded in the boundary layer. Therefore injecting a high velocity air mass into the air stream essentially tangent to the wall surface of the airfoil reverses the boundary layer friction deceleration thus the boundary layer separation is delayed.

interstingly:

In production aircraft, blown-flap systems were found to be a maintenance nightmare. They were continually breaking down due to clogging with dirt, and were generally unreliable. This made blown flaps practically useless as a landing aid on many aircraft. They were removed from later production runs of some aircraft.

http://en.wikipedia.org/wiki/Blown_flap

[Tommy Cookers]

Flap Blowing in a key feature of the C-17 (an active large military transport aircraft program)

FB is not certifiable for civil aircraft (eg because it fails when there's engine failure)
propellor aircraft have inherent blowing effects on the wing etc, but cannot rely on them for certification


so aircraft have slotted flaps ie using natural airflow (the same as the thing F1 followers seem to call a wing)

but they are allowed to retract the flaps, indeed it's compulsory !

[gato azul]

Actually, this link has probably explained the principle, along with examples, better than I could have imagined.

Good I'm glad you like it, but I fear, that this is exactly the opposite of what the teams try to do.
The Aviation concept of blown flaps/wings tries,in a nutshell, to create as much lift as possible at an forward velocity as low as possible. (STOL etc.)
The "beauty" of an aircraft wing in general is, that he can morph/change/adapt to the task at hand. Using slats, slots, leading and trailing edge flaps whatever, to try and create the optimum lift at the condition at hand.
They are using a change of "camber" (curvature of the wing profile) and change in effective lift surface for a given wing span (AR) to account for low speed conditions (take off & landing), then as the velocity/speed of the aircraft builds, they can reduce this parameters, to maintain the lift they need (which is governed by the weight/mass of the aircraft).

F1 tries to do the opposite, they have a "high lift (downforce) configuration by default, and try to make it shed, as much drag as possible at higher speeds, without being able to do what aircraft's do.
That would be the equivalent to design an airliner (Boeing, Airbus etc.) who can take off & land safely and then try to make it cruise at high speed with all flaps (leading and trailing edge) and spoilers in max. lift position - no aircraft designer in his right mind would try to do that (nor does he need to).

Both applications are on polar opposites, therefore looking at the solution common in aviation, may does not lead to the answers you are looking for.

Aircraft wings in high lift configuration (highly cambered wing) tend to stall due to a lack of forward speed, therefore they try to increase the flow velocity around/over the wing/flap etc.
F1 tries to "stall" (even if I think, that this term is misleading) their wing (trying to reduce the lift/downforce it produces and thereby the drag which comes with that lift) at increasing speed.
I don't think, they suffer from flow separation at high speeds,therefore they don't want to delay it, they try to "create" controlled flow separation, to kill off some lift/downforce at higher speeds and the lift induced drag which comes with it.

[bhall]

Frankly, do we have this --- all backwards?

"Conventional" wisdom holds that the slit on the flap of the MP4-25's rear wing vented air which caused the wing to "stall." Fine.


Here we're told that blowing the starter motor hole in the exact same fashion serves to "energize" flow over the steeply angled (relative to the rest of the diffuser) center-section of the diffuser, which then keeps that flow attached. Fine.

(via Scarbs)

So, we have two different applications of the (apparently) exact same concept performing functions that are diametrically opposed to one another. What's the deal?

Were we wrong about the McLaren? Was the wing actually "stalled" by default and required a burst of vented air energy to attach air flow so that the wing would produce downforce? That wouldn't surprise me give the alignment of the slit versus the alignment of the slot gap, plus the fact that the flap is damn near vertical.

I've had the idea for a while now that the Mercedes W03's front wing is like this, that it's "stalled" by default and requires vented air to then be able to make downforce.

Or do we just not know what the --- we're talking about either way?

(Don't ask me how I personally reconcile these discrepancies. I think I believe in Voodoo.)

[gato azul]

good & valid questions and observations Ben.
On source of possible confusion is the "liberal" use of terms, which may mean different things to different people.

I will not claim, that I have all the answers, because I have not, but to me it seems, that the "blown" diffuser is
directly comparable with the aviation concept of a "blown wing/flap".
It allows, to run a steeper camber gradient ( AoA would not be the best term in regards to the diffuser) for a given
(lower) forward velocity.
The extra injected air, accelerates the flow, and helps it "around the corner" so to speak, makes it stay attached to the surface, it tries to follow.

If McL or any other team with an F-Duct would do the same, that would mean, that they would carry (drag )
around a wing in stalled condition for most of the time, just to get it to work, at high speed (straights) when the
least thing they need is extra downforce. - does not sound too plausible to me.

Keep in mind that a jet powered aircraft has "pressure on tap" if you like, but that he F1 car need to build up the pressure
via speed (or need to route the exhaust to the appropriate condition, maybe the Octopus was an attempt to do that)

Blowing is a broad term, you can blow tangential, perpendicular or towards (against) the oncoming flow. (and probably in may other ways ) with probably fundamentally different results, as what it would mean to the flow around the wing.
I would speculate that in the F-duct and VD-Wing solution the direction of the blowing is not tangential to the flow.

[superdread]

You can attach or detach a flow by blowing it, it depends in what angle to the surface you blow. When you have a small angle (as in the aircraft thingy some posts back) you pull the main flow back onto the wing. If it is perpendicular to the surface the main flwo is pushed away from the surface.

I think when they blow with sufficient flow to not only change the direction but also the energy of the flow (to a significant extend) then they call it energizing the flow (that's why they use multi-element wings after all). Of course this only works when you blow with the flow (small angle to surface).

[bhall]

[...]

Keep in mind that a jet powered aircraft has "pressure on tap" if you like, but that he F1 car need to build up the pressure
via speed (or need to route the exhaust to the appropriate condition, maybe the Octopus was an attempt to do that)

Blowing is a broad term, you can blow tangential, perpendicular or towards (against) the oncoming flow. (and probably in may other ways ) with probably fundamentally different results, as what it would mean to the flow around the wing.
I would speculate that in the F-duct and VD-Wing solution the direction of the blowing is not tangential to the flow.

How about a plenum for pressure on tap?

And with regard to the MP4-25, the following is an extremely crude depiction of the wake from the rear wing in different states of being "blown." Don't forget that this was actively controlled by the driver.

If those are totally inaccurate, just let me know. Otherwise, the driver had the choice to have his car trailing either pattern. The AoA never changed; only its effect. It was either drag or drag+downforce (more drag). The question is which was "blown," and which was not?

EDIT:

...this only works when you blow with the flow...

I knew a girl like that. Great gal.

[gato azul]

making the plenum expandable,variable in volume, like a balloon could be even better (kidding a bit)
I think you would like/need to maintain a relationship of velocity of the blowing flow (jet) to the velocity of the air flowing
around the wing.

There where some good comments on the subject and some example calcs/cfd animation in this thread:
viewtopic.php?f=6&t=8429&start=15

the wing profile chosen (very low camber) is perhaps not the best, but it shows that you can "separate" flow by blowing.

As for the illustration of the McL, it's a bit simplified, but we are coming closer to where we want/need to go I think.
It would be nice/interesting to see some "wake" patterns (wake shape&height) from races in the rain, maybe Spa will
show us something.

Mind about the velocity (kinetic energy of the air molecules) and the direction in which they travel when the are "leaving" the underside (low pressure side) of the flap.
And how these parameters velocity & direction would be affected by the different system (F-Duct / VD etc.).



you would need to mirror this graphic along the horizontal axis, to make it race car relevant.
mind about the function of ɛ (down/up wash angle)

[bhall]

making the plenum expandable,variable in volume, like a balloon could be even better (kidding a bit)
I think you would like/need to maintain a relationship of velocity of the blowing flow (jet) to the velocity of the air flowing
around the wing.

[...]

How about a jet that's fed from a plenum that's "regulated" by a diffuser that's "controlled" by more or less the same flow that's around the wing?

[hardingfv32]

How does the green diffuser above 'regulate'?

Brian

[superdread]

How does the green diffuser above 'regulate'?

Brian

Well, it would lower the pressure in the plenum.

I still do not buy the theory that Lotus can not come up with something better than a simple cavity. There are fluid switches that switch at a precise pressure much more abrupt. They could also be tuned better.

Also here is not the place for that discussion, but it came up, so what...

[hardingfv32]

I can understand that the diffuser performance is correlated/proportional to the exterior flow, but how does the relate to the prevailing theory that the duct at the diffuser needs to become restricted for the system to function? What attributes does the diffuser add to the control of the system's flows?

Brian

[olefud]

I think the answers are not really knowable, but it’s fun to speculate. I assume that the team(s) with the better answers is piling on the misinformation. And, unlike aircraft, the “wing” performance is not speed related. What’s needed is low downforce/drag on the straight and the opposite through turns with the latter often at high speed.

It would seem that what is required is a bistable airflow switch that can attach or detach the airflow to or away from airfoil. So maybe we should be looking to pneumatic logic devices that can produce a 0 or 1 state with a small input air flow, i.e. blowing. Pneumatic computers may be the place to look for answers.

The diffuser hot blowing is probable a different animal in that appreciably heated gas has characteristics differing from ambient gas and forms more adherent and controllable boundary layers that in turn confine and allow the cooler gases to stay attached through rather abrupt direction changes.

At least that’s what I see.

[superdread]

I think the answers are not really knowable, but it’s fun to speculate. I assume that the team(s) with the better answers is piling on the misinformation. And, unlike aircraft, the “wing” performance is not speed related. What’s needed is low downforce/drag on the straight and the opposite through turns with the latter often at high speed.

It would seem that what is required is a bistable airflow switch that can attach or detach the airflow to or away from airfoil. So maybe we should be looking to pneumatic logic devices that can produce a 0 or 1 state with a small input air flow, i.e. blowing. Pneumatic computers may be the place to look for answers.

The diffuser hot blowing is probable a different animal in that appreciably heated gas has characteristics differing from ambient gas and forms more adherent and controllable boundary layers that in turn confine and allow the cooler gases to stay attached through rather abrupt direction changes.

At least that’s what I see.

That's not strictly true. The cars operate in the area of 80 to 300 kph, with the fastest turn exits somewhere on the good side of 200 kph. So they need downforce at rather high speeds (10 kph more carried through the corner are more worth than 10 kph more top speed). They would like to have switching aerodynamic characteristics (and did with F-Ducts), and operating the rear wings at stalling AoA is too risky (i.e. no precise switching speed).

Pressure switches are a great idea and have been used (experimental passive front wing stalling of Mercedes last season). I also see no reason for Lotus to not use such a pressure switch in their currently developed passive rear system.

PS Aren't pneumatic (same as hydraulic) logic devices all fun and games with no real implementations?
(At least none you could call a computer.)

[rjsa]

The original McLaren F-Duct was a pneumatic logic device, wasn't it?