26 replies to this topic

[Ali_G]

Take a look at ths drawing I made.



Why is it that the teardrop shaped red coloured section would be more aerodynamically efficient than if the whole shape with the extra bit at the front would be included.

I just don't get it. Wouldn't the more pointy front lessen the drag??

Niall

[Manson]

I think it has something to do with the penetration myth but someone whith more education than myself will have to expand.

[Bluehair]

The most efficient shape will change as speeds increase. NASA's "supercritical" wing looks more like the pointed teardrop, than the blunt teardrop. Sort of a compromise between your two shapes. As speeds go up, I think the needle nose begins to work better, but I'm talking about supersonic speeds here. Strangely enough, the ever slowly moving canoe has that sharp front end on it, possibly because the fluid dynamics are totally different in water than air.

[Ali_G]

To be honest I cannot understand why a pointy frontend wouldn't suit all conditions.

Niall

[Paolo]

Very interesting question, Niall; I had to think about this one...
The drag of a subsonic body is due to two factors : friction drag and pressure drag. Friction drag grows with exposed area ; your pointed shape has more exposed (or "wetted") area than a teardrop.
Pressure drag depends on the relative pressures on the front and rear part of the body.
For "streamlined" that the body may be , there will always be a part in the rear of it in wich you end up with flow separation, and low pressure. More or less, the bigger the area of separated flow, the bigger the drag (keep in mind, anyway, that it does not just depend from the area but also from how much the pressure distribution is altered by the separation; but it's pointless to go that deep now)
Has the teardrop an earlier separation point then your pointed shape ?
No. The separation point almost only depends on the pressure gradient at the back, wich depends from the shape of the rear part (provided you don't have separation at the front, wich happens only when the flow meets quite a big obstacle ; on a teardrop, it does not happen).
It's even possible that the pointed shape encounters a slightly earlier separation, since the flow is more "tired" because of the greater wetted area in front.

This, in case the flow comes at angle zero.

The pointed shape you propose would for sure be less efficient as soon as you have any angle of yaw : the sharp edge would cause flow separation, and pressure drag would increase too.

[Ali_G]

So, basically its becasue with the pointed one the air flow goes over more of the wing and hence this causes drag ?

Niall

[condor]

Aerofoil shapes (such as your red teardrop are called) are designed for particular conditions e.g aeroplane wing to maximise lift and reduce drag. Thick aerofoil section where wing attached to plane mainframe and thin aerofoil section at wing tips. so upside down aerofoil sections on a racing car will maximise downforce and at the same time reduce drag - however your red teardrop design isn't efficient for either purpose - there would have to be more camber. But either way - still far better than your black outline.

[Ali_G]

No, that is just a drag reduced cross section of a suspension member.

Just to get the point accross.

Niall

[Paolo]

Niall :

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Basically yes.

[Ali_G]

Paolo: That seemed pritty easy to you.

have a look at my other thread abot Ground Effect Wings below.

Trust me, its a tad harder.

Niall

[condor]

Sorry - I'm new here - so hadn't realised this was just a question which you already knew the answer to. My mistake.

[Ali_G]

Originally posted by condor
Sorry - I'm new here - so hadn't realised this was just a question which you already knew the answer to. My mistake.

jees what did I say. Sorry

Niall

[condor]

Not a problem - I should have spent more time reading the posts rather than being impulsive and responding to one straight away.

[condor]

I haven't quite understood this - Do you know the answers and this is just a test - or do you really want to know the answers?

[Ali_G]

No, I just wanted to figure out why one shape is more aerodynamic than another.

Niall

[Dan_G]

Paolo,

I understand the idea of the increased surface area means increased frictional drag.

Im a little shakey on the idea of the flow seperation. Are you saying that the greater the distance that the air flow over one side is seperated from the airflow over the other side, the greater the drag? If this is the case, is it due to a difference between air pressure between the two airflows? Does the seperation inherently cause differences in airpressure between the airflows, or is it possible to ensure equal pressure on either side of the shape, thereby reducing or eliminating that aspect of the drag?

Also, if i understand correctly, this is the cross-section of a suspension member. If you increased the angle of attack of the member vs the oncoming airflow, then you would want a teardrop shape because it would be more efficient than the pointed shape. That is to say, the pointed shape MAY be more efficient and create less drag if the member is exactly parallel to the airflow, but as soon as the angle of attack changes up or down compared to the airflow, then the pointed shape immeadiately becomes less efficient than the teardrop shape. This is due to the air swirling and eddying immeadiately behind the leading edge of the member.

Does that sound right, or am I completely missing the mark here?

[condor]

Your red teardrop is symmetrical - just at a glance it is similar to the NACA0012 aerofoil section. Aerofoils sections are tested in two-dimensional wind tunnels. The NACA0012 is the section used to calibrate all two-dimensional wind tunnels. Therefore should be fairly easy to get results that are similar to your shape. - Sorry about the mix-up earlier.

[condor]

Well I might be a little impatient - but I think I deserve a response.
ALI_G :D:D:D:D:D:D

[Ray Bell]

Where can we get more on these NACA stats?

[condor]

That's a typical 'old-hand' poster response - I'm a newbie - find out for yourself. Use your SEARCH ENGINE - :D:D:D:D:D:D

[condor]

Ok - if I go and look for it- will i get into the paddock?

[condor]

No reply from the mods - so obviously not - well ALI_G do some work and get searching - cos I ai'nt getting a reward for it. These aren't homework questions you're posting here are they?

[Ali_G]

Condor: Thanx.

BTW: I am no aerodynamicist. I'm doing Economics in College. !!!

Niall

[Ray Bell]

Wonder what Mr A thinks?

...or knows?

[Wolf]

I must admit tooo that I never understood why it should be so. I applied a bit of deduction and the only answer I came up with is as follows. One is inclined to think that liquid will when moving (unconstrained) will adopt to the lowest-drag shape. But the fault I find in this reasoning is simple-gravity. If e.g. inverted-teardrop shape was most-efficient, the liquid could not adopt that chape, simply on account of its static unstability. It's only logical that raindrop falls heavy-end-down, not because of any aerodynamical laws, but because of statics (stability of that shape).

Perhaps we would be better off (sorry Paolo, but to eliminate chord length, you were reffering to, but I think the L/D factor would reduce drag, at least per lenght of the chord ;)), if we considered same length of the shape: red shape with the biggest thickness near front end, and second same thickness in the middle.

[Ray Bell]

Yes, raindrops falling in air adopt that shape... is that the premise?

So we get a less from learning how bearing balls are made?

[turin]

The main question is what you call "efficient".
If you just want to produce the less drag and les turbulence behind the element, you may want a pointed shape paralell to the flux lines. I'm not pretty sure, but Williams two years ago used to have the front suspension in that shape, so as McLaren in 1998. They looked very thin, but had the same transversal area as other's circular shaped suspensions, plus the extra benefit to put some sponsors on it! ( Goodyear, in Williams case). More common, and easy to understand, is the case of bridge piers, when you want that the water flux tends to be as smooth as possible, reducing the turbulence and eddying.
On the other hand, if you want more downforce and the less drag, you need the teardrop shape slightly not paralell to the flux lines. This produces that two adyacent particles, once they find the element, and let's say that one goes by the upper side and the other on the lower, they will travel at different velocity because they must get at the end of the element at the same time ( in order to meet the continuity law). Thus, the upper particle will travel slower because it has less distance to cover in the same time. Then, there will be less pressure on the lower side and resulting downforce. If you use a pointed shape to this model, you will have a lot of turbulence in the lower side due to the abrupt change in the flux line, and the particles won't meet at the end of the element, and the whole principle of function will be messed up.

I hope that someone understand this, because once I read it back I don't. I think faster that I type, and in other language.
Sorry

Pato