Tip Tanks - Aerodynamics

[JDE]

This is for all the Lear drivers out there, and anyone else who can help. I'm doing a project on winglets for my aerodynamics class and I was wondering how tip tanks on the LJ 23/24/25/35 effect the aerodynamics. Do they act similar to winglets? I always assumed that they would create a lot more drag, but since Learjet hasn't changed the design they must help a little bit. Also, does anyone know why they can't/don't transfer fuel from the tip tanks to the fuselage tank during flight? Thanks!

 

[rausda27]

I gave a brief on winglets during my CFI training and I remember reading that the wing tanks act like end plates that are installed on some aircraft to reduce wingtip vortices. If because of space limitiations on the aircraft external tanks are required, it is thought that the wing tips are the best choice becasue although they increase parasitic drag, they may have some effect on reducing induced drag. So if you must have external tanks then, instead of a belly mount, the designers placed them on the wingtip in hope that they may reduce some induced drag.

 

[Ophir]

An interesting thing I just learned after a friend installed tip tanks on his C206H. The increase in wing span changes the amount of lift the wing produces and therefore they are designed to run the main tains dry before you feed the fuel from the reserves into the mains. The extra weight on the outboard part of the wing compensates for the extra lift gained by increasing the wing span. I guess they are not designed to be run dry but having full fuel in the mains becuase it puts too much stress/lift on the outboard part of the wing.

Are all tip tanks similar in this design/performance characteristic? I can't see the bulbous tanks that the Lear has as being able to produce lift.

 

[Eagle]

[ QUOTE ]
...how tip tanks on the L J 23/24/25/35 effect the aerodynamics. ....Also, does anyone know why they can't/don't transfer fuel from the tip tanks to the fuselage tank during flight? Thanks!

[/ QUOTE ]

In the L J all fuel that makes it to the engines comes from the Wing tanks.

Five places for fuel on the LJ.

Tip tanks (2)
Main (Wings) (2)
Fuselage (1)

normal (in the 35) is Take off from mains dump tips into the mains (uses a jet pump) then transfer to the mains from the Fuse depending on burn alt etc.

To the best of my knowledge the tip tanks add no value to the performance in lift or anything. they do not work like winglets.

another fun L J thing.

If you take off at Max Gross weight (about 18,000 pounds) you can't land again until you burn off 4000 pounds of gas, on a short trip, say from New York to Toronto, you won't burn 4k # in gas. ... and I won't tell you how I know that

 

[Ophir]

So what did you tell your pax while you were figuring out how to burn all that fuel?

“Ladies and Gentlemen, we going to need to take a low altitude deviation today due to complications with Air Traffic Control. On your right you will see Newfoundland . . . .”

 

[Cheechako]

[ QUOTE ]
a friend installed tip tanks on his C206H.

[/ QUOTE ]

Tip tanks on a 206??? That must look interesting! Do you have a picture or a link to one? I'd really like to see that. How much fuel does it add? I've got a couple hundred hours in the 206/207 and I seem to remember the amount of fuel being able to outlast my bladder, why would you want more??

 

[IrishSheepdog]

Something I noticed on the 25-series Lear is that the tip tanks on the ends induced a bit of dutch roll on final approach. Other things I'm sure helped, but with a pretty straight wing it should be hard to get dutch roll like that.

 

[ananoman]

When tip tanks are put on the Cessna aircraft, they just look like wing extensions. They are not like the ones on a Lear. If you were not told they were there you probably would not notice them.

Another advantage of tip tanks is that they reduce the wing bending moment. If you have a heavy fuselage supported by the wings, there is alot of stress at the wing root. Hanging engines on the wings or carrying fuel in the wings spreads out the load and reduces the stress imposed on the wing attach points. This is one of the reasons large aircraft have a zero fuel weight. This is the max weight the aircraft can weigh without fuel, anything over that weight has to be fuel.

 

[Ophir]

Yeah, I will get some photos and send them to you. Last time I saw it they were putting the tanks on. The integrate right into the wing and it would easy to miss them if you didn't know what to look for. I will interested to see how they come out.

The plane may be here in SLC sometime, if you are interested, let me know.