17 replies to this topic

[Mr_Scooby]

From a discusion at a Danish phorum, which shape: Teardrop or spear are the better in aerodynamic perspektives ?

And why ? PLZ

Kindly regards

Mr Scooby

[Aubwi]

If you mean a teardrop or spear with equal volume, then I would say the spear because it would have much smaller frontal area. Unless the teardrop is so elongated it's actually shaped sort of like a spear. Basically, it all depends on the exact dimensions of the object. Saying teardrop vs spear isn't really precise enough.

[VAR1016]

For "spear" should we be reading "sphere"?

A sphere has a CD, so I read, of 0.15.

PdeRL

[Mr_Scooby]

Originally posted by VAR1016
For "spear" should we be reading "sphere"?

A sphere has a CD, so I read, of 0.15.

PdeRL

Maybe English my bad - but I mean spear as a javelin.

And off cause we are discusing CD/ CV or the most commen for streamlined.

But then again, why does a subsonic jetliner have a teardrop nose and a supersonic jetfighter have a spear shaped nose ?

Compare this with that a modern ship has a bulk-nose (teardrop) at the bows.

Is it posible to discuss steamline without to mention at what velocity it is aviable ?

[VAR1016]

Originally posted by Mr_Scooby


Maybe English my bad - but I mean spear as a javelin.

And off cause we are discusing CD/ CV or the most commen for streamlined.

But then again, why does a subsonic jetliner have a teardrop nose and a supersonic jetfighter have a spear shaped nose ?

Compare this with that a modern ship has a bulk-nose (teardrop) at the bows.

Is it posible to discuss steamline without to mention at what velocity it is aviable ?

No, not at all - I presume that you mean what is, in effect, a reversed teardrop?

I have always understood the teardrop to be very efficient although the CD of a sphere has always surprised me, which is why I quoted it.

PdeRL

[12.9:1]

Assuming Mr_Scooby means 'spear'
Then the advantage would depend on which speed range you intend to operate.
at the extreme low and the extreme high end, each will find a home

[Mr_Scooby]

Originally posted by 12.9:1
Assuming Mr_Scooby means 'spear'
Then the advantage would depend on which speed range you intend to operate.
at the extreme low and the extreme high end, each will find a home

And why ?

[12.9:1]

Low end; minimum surface area and laminar flow

High end; shock-wave management, volume distribution

[kusal]

Please some more details 12.9:1.
Could you develop a bit what you are just saying ???

Thanx

[kilcoo316]

In a purely subsonic regime, it is desirable to keep pressure gradients as small as possible, and try to keep the flow laminar for as long as possible. A man called Goldstein investigated using propulsion techniques and boundary layer ingestion to ensure laminar flow over bodys way back around the 50s (I think, but probably wrong). The teardrop shape is best in such situations.

However, in transonic and supersonic regimes, it is desirable to accelerate the flow as little as possible and to reduce the strength of shock-waves as much as possible. Here, the sharp point (Leading Edge) of the spear is prevalent. The flow is accelerated less, as it is displaced less from its normal path (quite simplistic view but easy to picture).

This is the reason why fighters and civil aircraft have vastly different aerofoil sections, one is designed for supersonic flight, the other transonic (with minimum drag as a result of the shock wave)

[joriswouters]

I was told it was the teardrop that was the most aerodynamic form in the world. I don't the coëfficient by heart but it's really low.

[Yelnats]

There's not enough information to answer this question. Are we reffering to objects of equal volume or frontal areas? If it's equal volumes then obviously the javilen wins hands down. If it's equal frontal areas the the teardrop would win simply because it lacks the drag caused by the much greater surface area of the javelin although in this case the comparison would be meaningless due to the huge differences in volumes.

Spear> <<<<<<<<<<<<<<<<< Teardrop> o of equal frontal areas.

[Jonas]

Originally posted by VAR1016
A sphere has a CD, so I read, of 0.15

You can't say that a sphere has a general value of CD. This value is dependent on Reynold's number, that is Rho*V*l/My
Rho = density of the surrounding (for instance air)
V = characteristic velocity
l = characteristic length (say for instance the diameter of the sphere)
My = dynamic viscosity of the surrounding

For a smooth sphere with a diameter of 56 mm in an airstream with a velocity of about 15 m/s and other conditions assumed to be "normal" (-> Re = 42.6*10^3) a CD of 0.6 is achieved.
Now a teardrop formed object with a frontal area diameter of the same as above, 56 mm, and a length of 150 mm gives a value on CD of 0.1.

This shows that the teardrop form is far better streamlined than the sphere!

[Greg Locock]

Ah, but if you decide to slow things right down the sphere will have a better Cd than the teardrop. And if you go very fast indeed they will have the same (to within experimental error) Cd.

If you think about it even in nature the teardrop is rarely (ever?) used as an aerodynamic form, despite all the lies you were told at school. Sure, fishes are a somewhat teardropped shape, but very few fast swimmers have the blunt nose and forward maximum chord of a classic teardrop. Hey, that's funny, I was just thinking they are probably flattened side to side to improve the efficiency of the tail flapping, in which case why aren't dolphins flattened top to bottom?

[Mr_Scooby]

Aint there something we are forgetting about the spear/ javelin, that what is gained at the nose can be lost way down to the tail as the surface staying in contact (surface friction) with the air will be a lot bigger as in comperation with the teardrop ?

[john_smith]

one thing that has always made me wonder, and this may be an amature question...

but do rain really fall in the shape of a teardrop while in the air?

[12.9:1]

Good Questioning !

No, they have the classic shape just as they fall from the faucet, once up to speed you see a blob with a flattend forward end.

[Chevy II Nova]

I like your name 12.9:1. You running racing fuel with that CR?