Planes with truncated tails

[skyking]

Originally Posted by drmiller100

airplanes operate at higher speeds then our cars - higher speeds imply different reynolds numbers which means they get to use different degrees of taper.

An example - airplane wings "stall" if you go too slow.

A stall can happen at any airspeed. It occurs when the wing reaches its critical angle of attack.
The taper in the example plane in the OP is very far from ideal and is an example of form following function. It will look like a mess in the wind tunnel.

[SvdM]

Makes one wonder why they didn't just truncate it with a hard edge for clean detachment, but I guess even this bad stub tail is better than no tail for drag?

[aerohead]

Originally Posted by drmiller100

We must be talking about different things, and I would appreciate your patience in helping me understand what you are saying.

Where does Reynolds Number fit in with what you are saying?

Are you implying that all objects with all reynolds numbers all stall at about 22 degrees normal to the surface?

Surely that is not what you are saying????

*The Reynolds number is related to a structures length and velocity.At a 'critical' velocity,the smoothest surface which can be manufactured will transition from a laminar boundary layer,to a turbulent boundary layer.
*A turbulent boundary layer cannot follow a slope which is steeper than 22-degrees without separation.This was determined on a 16.13% thickness fuselage.
*All structures which are moving at a velocity at,or above critical velocity are limited to the 22-degree slope angle in the aft-body.
*'Laminar' wings are 'laminar' only up to the position of minimum pressure and immediately transition to turbulent boundary layer.
*Wings are 2-dimensional flow devices and are different than the fuselage which is a 3-dimensional flow structure.
*All structures in 3-D flow will suffer flow separation if the aft-body convergence exceeds 22-degrees.
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*Automobiles,due to their size achieve a turbulent boundary layer at about 20 mph and from there on their drag coefficient is constant.
*Automobile drag is ruled by pressure drag,which is ruled by separated flow.
*To eliminate separation,the cars body is extended with curvelinear taper both in elevation and plan just as the 'Template' depicts.
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*Aircraft design virtually 'presumes' attached flow and skin friction and induced drag are the bogeymen of aircraft performance.
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I am saying not to go beyond 22-degrees on any car unless you can provide a 'blown' or 'suctioned' aft-body to stabilize the boundary layer.
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I'd say the same thing for a fuselage although their drag is of little consequence to the overall drag of a plane.
You can chop off the last 20% of a fuselage on an aircraft and it will never show on the fuel gauge.This is why the tails on the cargo planes matter very little to overall performance.

[aerohead]

Originally Posted by SvdM

Makes one wonder why they didn't just truncate it with a hard edge for clean detachment, but I guess even this bad stub tail is better than no tail for drag?

In World War-II,Heinkel cut almost 9.25 feet off the tail of their He-177 and replaced the boat tail section with a machine gun ball-turret.This only increased the drag coefficient of the fuselage from Cd 0.120,to 0.122.
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On Messerschmitt's Me-109 "G",the fuselage of the plane made up only 12% of the planes drag,whereas the wing contributed 81%.
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You could have put a small drag chute on the back of the plane and it would have hardly noticed.I think this is why cargo plane designers can play fast and loose with the fuselage.It's drag kinda gets lost in the signal-to-noise ratio of testing.

[drmiller100]

interesting. You gave me a great direction of research, and I'm digesting it all.

one of my fundamental mistakes was confusing turbulent flow and flow separation.

doesn't your template have greater then 22 degrees at the back?

headed for the big apple. won't check in for a few days.

thanks again!

[euromodder]

Originally Posted by aerohead

*All structures which are moving at a velocity at,or above critical velocity are limited to the 22-degree slope angle in the aft-body.

I've measured the Shuttle tail cone on pics - it's also around 22°.

You can chop off the last 20% of a fuselage on an aircraft and it will never show on the fuel gauge.This is why the tails on the cargo planes matter very little to overall performance.

That's when starting with a fully streamlined fuselage.

On the Arava and Noratlas, the clamshell doors are at or very near the full fuselage width - not nearly the last 20% of the streamline.
On the C-82 it was a bit better, and best on the C-119 .
The operational drawbacks were many, and serious.

Not surprisingly, this WW2 design hasn't stood the test of time and was replaced with the rear loading ramp - pioneered on the Budd-ugly Conestoga (RB-1 / C-93) - which has become the preferred lay-out.

[aerohead]

Originally Posted by drmiller100

interesting. You gave me a great direction of research, and I'm digesting it all.

one of my fundamental mistakes was confusing turbulent flow and flow separation.

doesn't your template have greater then 22 degrees at the back?

headed for the big apple. won't check in for a few days.

thanks again!

It took me a long time before I could sort out what people were writing about,and it was hard to grasp the concept of laminar flow over a turbulent boundary layer.And it was only after reading Schlicting's book on boundary layer theory that I got a handle on separation.
The tangent angle hits 22-degrees at around 70% of the template aft-body,then maintains 22-degrees out to where you'd have to cut the body off,respecting the 10-degree SAE Departure angle of the diffuser.
Enjoy N.Y.C.!

[aerohead]

Originally Posted by SvdM

Makes one wonder why they didn't just truncate it with a hard edge for clean detachment, but I guess even this bad stub tail is better than no tail for drag?

I've never seen a technical term for it so far,but it appears that a mediocre tail can performing a 'wake stuffing' function,creating a physical boundary for which the turbulence cannot enter,sort of reinforcing the flow from the inside out.
Carl Breer did this with the 1934-1/2 De Soto Airflow test mule on which a hypo-template boat tail helped the car achieve Cd 0.244.
Hucho has mentioned that by simply placing mannequins inside an open convertible,that lower drag can be measured in the wind tunnel.