Planes with truncated tails

[Cd]

As Aerohead pointed out in a previous post, some planes get by with rear airflow which is less than ideal.

He had mentioned the C-119 Flying Boxcar

Looking at the plane from the top view seems to show quite a bit of taper that would result in flow separation, yet the plan view seems to make up for this with all the right curves. I had wondered if the airflow was pulled into a smaller wake through a combination of the two angles.

I've always wondered how that this plane got by with such a truncated tail, and in many cases with the tail cone removed all together. ( For missions like cargo drops etc - see this video at 00:18 )

[skyking]

I'm sure that somewhere out there are some performance charts in operation manuals that tell you how well or badly it performed with the tail off.

[euromodder]

Drag increased significantly on both, and all similar designs, when the rear clamshell doors were removed.
This resulted in more power and fuel required, less range, more buffeting, lack of pressurisation in transit - i.e. lower transit height, cutting down range even further.
It was also extra work to remove or refit the doors, and that work needed to be done before the actual transport job - the clamshell doors couldn't be opened in flight.

The ultimate result was the disappearance of such designs in favor of the rear loading ramp design on all modern tactical and practical transports C-123, C-130, C141, C-5, C-17, C-27, C-235, C-295, A-400M and all their CCCP counterparts.

[aerohead]

Between Mair and Buchheim the limit for attached flow will lie around 22-23 degrees of tangent angle.Anything beyond that and we'll have separation unless we provide blowing or vacuum in critical areas.
These 'steep' curvatures may have been a compromise for the life-cycle-cost analysis of the particular project.
If you have Kate Winslet in the can,you might get some extra $millions re-releasing her films in 3-D.Or is that Double-D?
If you'
re hauling freight and 115/145 AVGAS is costing you 18-cents a gallon as when I used to pump it,you may not give a d--- about a little extra drag and a mediocre boat tail.I wouldn't try and read too much into 'why' certain things got done unless you have access to the designer.
And with respect to the Packet and Flying Boxcar,a number of parachutists were injured attempting to jump with the tail in place,so they just removed it and lived with the inefficiency.

[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.

[hawk2100n]

I can tell you that these designs of aircraft are far from ideal and that there is probably separated flow on them towards the point. You would have to look at the local Reynolds number and put them in a tunnel to see. I would guess that flow around them is far from perfect, especially with the state of aero research then.

You can tell a lot about an aircraft by looking at it closely. On some aircraft, you can see where the flow is expected to become turbulent by the rivets. Laminar flow has smooth rivets due to the thinness of the boundary layer but after the switch the structure will use stronger and cheaper raised rivets because they still reside within the boundary layer and have almost no effect on drag.

Generally lower speeds mean more attached flow at a receding surface. Also, you can think of detached flow at a rear window of a car almost exactly the same as a stall. In both cases the flow detaches from the surface and drag increases significantly. Except the car doesn't loose lift and fall out of the sky.

[Fat Charlie]

Originally Posted by Cd

As Aerohead pointed out in a previous post, some planes get by with rear airflow which is less than ideal

I've always wondered how that this plane got by with such a truncated tail, and in many cases with the tail cone removed all together. ( For missions like cargo drops etc - see this video at 00:18 )

It sure would be nice to have attached flow with a tail that short !

You've got to wrap your head around the way the military views the word "efficiency." Dropping a few paratroopers somewhere is a more efficient way of getting people there than having an armored division fight its way through. Air dropping a few pallets of ammo is more efficient than having an isolated unit run out. Most military equipment has so many oddball performance requirements in its design that it's amazing that it works at all in any normal way.

It's not the rear end of an airplane, anyway. It's a cargo loading door. Any thought given to aero was with an eye to not interfering with loading and unloading. The overall feeling here shouldn't be "that's not really good aero," it should be "Wow! it even flies!"

[botsapper]

Reminds me of my favorite 'aerodynamics lesson' movie of all time! (Not the 2004 remake, though still OK) Who's a Heinrich Dorfmann on Ecomodder.com?

http://www.youtube.com/watch?v=V0hIoLNecqI

Unfortunately, the movie's stunt pilots really suffered what everyone thought would happen.

http://www.youtube.com/watch?v=n82nN_lqn58

[aerohead]

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.

I want to tread lightly here,but according to boundary layer theory,once critical velocity is attained,and the boundary layer is turbulent,all structures will follow the 115-degree rule,regardless as to whether they're a plane,car,or submarine.
If air,or any 'fluid' is attacking a particular curve,of any given radius,if 'zero' is the forward stagnation point,the 'fluid' MUST separate at 115-degrees of rotation beyond the stagnation point.
Example: if an analog wristwatch is moving from right to left,and the air first strikes at the 9:00 position,the air will not be able to remain attached beyond 4-seconds after 12:00.By the way,the angle at the separation point is 22-degrees.

[drmiller100]

Originally Posted by aerohead

I want to tread lightly here,but according to boundary layer theory,once critical velocity is attained,and the boundary layer is turbulent,all structures will follow the 115-degree rule,regardless as to whether they're a plane,car,or submarine.
If air,or any 'fluid' is attacking a particular curve,of any given radius,if 'zero' is the forward stagnation point,the 'fluid' MUST separate at 115-degrees of rotation beyond the stagnation point.
Example: if an analog wristwatch is moving from right to left,and the air first strikes at the 9:00 position,the air will not be able to remain attached beyond 4-seconds after 12:00.By the way,the angle at the separation point is 22-degrees.

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????