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Old Oct 30, 2013, 03:33 AM
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Sydney Australia
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Drop Tanks

There seems to be a useful accumulation of aeronautical knowledge hereabouts and so i thought that i would ask a question that is not strictly speaking about models:

You see, I want to put a pair of drop tanks on an aircraft ( a Long EZ). The issue that I have, is that I understand that putting them well away from the wing minimises aerodynamic interference with the wing, but this also means that the mounting system will also see a lot of side loads in handling as well as from gusts etc. This means that wear and tear might become an issue. So, i would like to mount them as close as practicable to the bottom of the wing, but not so close as to become draggy. So, does anybody have any idea on how close such things might be mounted to the underside of a wing before aerodynamic penalties start to be incurred?

Thanks in advance

Carolyn.
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Old Oct 30, 2013, 01:08 PM
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Palmdale, CA
Joined Oct 2000
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Toy airplane builders and designers can NOT give you information about full-scale design and safety!
Talk to Rutan.
If you're doing a model, then just about anything will work.. as long as it's strong enough to hold the tank on.
But dropping anything from a model.... not a good idea.
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Old Oct 30, 2013, 01:24 PM
Grad student in aeronautics
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I would actually just suggest to do what others have done here. I can almost guarantee their objectives are the same as yours:
"mount them as close as practicable to the bottom of the wing, but not so close as to become draggy"
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Old Oct 30, 2013, 01:52 PM
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Quote:
Originally Posted by Carolynne View Post
but this also means that the mounting system will also see a lot of side loads in handling as well as from gusts etc. This means that wear and tear might become an issue.
The wear and tear isn't just on the mounting system. The side loads will impose bending loads on the wing structure... probably not a big deal. The fore-and-aft loads (due to hard braking for example), however, will impose torsion on the wing structure. Unless you know the wing structure can handle these loads, the wing could be compromised. That could make for a particularly bad day.
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Old Oct 30, 2013, 02:18 PM
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Sparky said it best. What we do with our models has no bearing on the issues encountered on a full size aircraft.

Questions that come to mind are:
  • the fin area effects based on the location and what it will do to the aircraft stability.
  • How to safely install mounts in the wings to hold the tanks that will not create stress riser points that could lead to skin and sub structure failure.
  • CG considerations of the tanks and gas.

There's likely other issues as well. To me the big one will be in adding a significant stress riser on the wing's structure. There's no doubt that some sort of additions to the basic wing would be needed to spread this out so it's fully integrated into the wing. And since I seem to recall that these wings are foam and composite construction I'm guessing that the proper integration of such "hard points" would have started with spreader stringers in the foam before the first layer of cloth was brushed on.
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Old Oct 30, 2013, 06:22 PM
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The airplane in the picture apparently is a full sized one that has been fitted with baggage pods (http://en.wikipedia.org/wiki/File:Ru...g-wily.arp.jpg). Assuming that the conversion was successful I'd try to contact the owner for some insight. I know that the fuel tank selector has already been the cause of at least one accident too, and I strongly doubt that you could legally fit actual drop tanks, though supplemental ground removable tanks might be allowed. I also think each tank would need to be fitted with some form of fuel pumping arrangement, since they wouldn't quite work for a gravity fed arrangement, which adds more complexities to the system. It might be simpler to find a way to fit conformal fuel tanks above the wing central section, after all wider body EZ'es seem to fare well.
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Old Oct 31, 2013, 04:46 AM
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Hi guys and thank you for all of your points. Let me address a few of them now;

Structurally, pods such those seen above have been in use for awhile now in the full size canard community and don't fall off in regular usage. They are generally mounted with 6 hard points let into the structure. 2 on the upper leading edge and 4 disposed fore and aft of the spar and either side of the mounting pylon. All of this is tied together with a long fibreglass strap which wraps from the leading edge to aft of the pylon. Where the pods are used for fuel, liquid is pumped electrically, up the pylon into the respective main tanks in each wing root when required. Most pods are used for baggage or just to look nifty.

As far as the comment about "toy airplane" enthusiasts is concerned, I beg to differ. There are on this forum a number of individuals who clearly have a great deal of both practical and theoretical experience in aerodynamics. This is clearly shown in their clear, concise, well reasoned and referenced comments. It is this expertise that i am hoping to access. Of course there are many other comments which may be less reliable. Please let me say also, that I am only looking for comment here. I fully accept that the course of action that I ultimately take, and the engineering that I apply, May or May not bear any relationship to the comments given, but will definitely be my own decision in its entirety. ( how's that for a disclaimer!)

By way of explanation, I am hoping to execute a configuration similar to the photo. The centroid of volume of the pod will be on the longitudinal cg of the aircraft and because I am thinking of using a laminar flow body, the distribution of side area of the whole aircraft will change only marginally. The big issue though is how long the pylon can be to optimise strength vs drag. Your comments remain welcome!

Conformal tanks as far as I am aware have not been tried, though LE extension types have been done by no less than dick rutan and others. I don't really want to go that way as the loading calculations on a great long proboscis hanging off the leading edge is more than my maths can cope with!

Best regards

Carolyn
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Old Oct 31, 2013, 09:36 AM
I bail out, anywhere, anytime
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The Netherlands, OV, Almelo
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Why not turn around the question?

I am familiar with jet fighters and know the structural configuration of pylons and details of internals. If you want to design a wing pylon there have to be accomodated (some?) equipment inside. That's why you first have to decide how you want to operate these fuel tanks hanged on these pylons, and control the content say fuel. Also you have to decide if you really want to drop (empty) tanks.
When the dimensions are pretty good determined to make an useable pylon and accomodate all equipment the distance to the wing probably also will be enough to guarantee a low drag.
Enlarge the minimal needed vertical distance as needed for internal equipment will enlarge torque forces on the D tube (if there is) of the wing structure so there will be less reason to do that, especially when you want to drop the empty tanks.
Dimensions of the tanks can be the next step in this case, accepted torque forces on the wing and totall weigth of the plane.
Enlarge the minimal needed vertical dimension of this (practical) pylon also can have a negative effect on the method you want to transport the fuel

Not dropping? Less equipment in the pylon and in that case probably important to determine the vertical dimension. In that case the dimensions of the tank and wing will be facts to count with and have you to design the dimensions of the tank in the first place to calculate and measure deformation of the wing under load before making the tanks.

In all cases, how to control the content of fuel!!


Taurus Flyer
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Old Oct 31, 2013, 01:11 PM
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If this is full-scale installation, ask Rutan!
The stuff a toy airplane can get away with can be fatal in full-scale!
Is this full-scale?
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Old Oct 31, 2013, 02:03 PM
I bail out, anywhere, anytime
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Quote:
Originally Posted by Sparky Paul View Post
If this is full-scale installation, ask Rutan!
The stuff a toy airplane can get away with can be fatal in full-scale!
Is this full-scale?
Full scale.
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Old Oct 31, 2013, 06:56 PM
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I've got 3,000 hours flying with "drop" tanks (F-18). In all that time I've never once dropped a tank. They probably should be called "keep" tanks.
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Old Oct 31, 2013, 07:05 PM
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Drop tanks have aided an entire industry!!!!!
http://www.flickr.com/photos/abi-nick-tour/161754560/
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Old Oct 31, 2013, 07:51 PM
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Quote:
Originally Posted by ShoeDLG View Post
I've got 3,000 hours flying with "drop" tanks (F-18). In all that time I've never once dropped a tank. They probably should be called "keep" tanks.
Come on, 'fess up. In all that time you've never looked over and wondered "Hmmmmm.... I wonder..... "

To bad there isn't an "evilgrin" or devil smilie....
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Old Oct 31, 2013, 08:07 PM
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Oh, I've dropped plenty of other stuff...
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Old Oct 31, 2013, 08:28 PM
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I recall watching a firepower demonstration at Camp A.P. Hill when my father was the C.O.... some of the demo included a flight of F-86s from Langlely AFB, headed by Col Robert L. Scott of "God is My Copilot" fame..
Demoing a rocket pod ground attack, Col. Scott came swooping down, and.... dropped the whole pod!!! instead of firing the rockets in the pod!
Blamed it on a switch problem in the cockpit.
We went out to where the pod had impacted and the ground was littered with broken rockets!
I could see F-84Fs practising bombing on the range at A.P. Hill from my home room at Bowling Green High School. Watching a "Loft bomb" attack with a simulated nuclear shape (about 15 feet of blue painted concrete filled bomb) going up upon release, with the plane Immelmanning out to scoot away beneath it was awesome... knowing that in real life, there was no way to stop that bomb from wiping out the countryside...
Another time, my dad got a complaint from a farmer whose backyard had several practice bombs land in it... The pilot had set his computer for the loft bomb situation, he thought.. but it was tossing the bombs along his flight path just after he began his pullup.
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