aircraft construction and modification suggestions 1

[Higgler]

I'd like to learn about aircraft construction and modification and would appreciate a reference book suggestion. Modern military aircraft primarily.

I am proposing cutting long thin slots (from 10"x0.01" up to 200"x0.1")in the top of aircraft for adding sensors and need to learn the basic construction techniques. I know, sounds ridiculous with the 200" length.


Probably the best option would be to speak with an aircraft designer or someone who has experience modifying aircraft. Maybe I could pick your brain in a few emails. Knowing me, it'd be about 20 emails over the next 45 days, lots of what if's and how about this's.

Thanks,
kch

 

[rb1957]

effectively you are introducing a 200" long crack into the fuselage ... ok, the stress concentration is slightly less.

I'm assuming the plane is pressurised.

I would strongly suggest that you reinforce the slot with a doubler, presumably external; possibly a triper around the ends to help with the hoop load there.

What do you plan to do with the fuselage frames ? ... 200" long, you're probably cuting through between 8 to 12. The slot will deflect quite significantly (getting wider in the middle), is this an issue ?

You'll need to provide longerons to react the skin shears, to carry them around the slot.

How much OEM data will you have ?

What's the certification level of the plane ? FAR23 ?

Can you get an experimental permit for the modified plane ?

 

[Higgler]

Thanks rb1957 (if that's your birth year, I too am a model 1957)

Yes, aircraft is pressurized.'

Can the frames in a modern aircraft, or part of the frame be changed from metal to non-metal (can't be carbon fiber either, too conducting). If that is too ridiculous to ask a military customer, then I won't dare show any cuts through a frame. Maybe that long continuous 200" cut length is for new aircraft design only and not a retrofit. I really want to propose realistic options.

I'd like to make a sketch in Powerpoint showing a few slots and ask for help to annotate it showing added hardware to maintain structural integrity in the added slot areas? I'd then draw it up in Solidworks for use in a proposal. Anyone care to assist with that?

thanks,

kch

 

[rb1957]

how much space do you need inside the slot ? ... some sort of sensor you mentioned. the point is that it would be WAY preferible to provide a load path as close as possible to the slot; I'm envisioning some sort of channel (Top-hat section) with flanges attaching to the skin on either side of the slot and your sensor being placed inside the channel. This could also work as pressure containment if you don't want to cover the slot with glass (saves that nasty hissing sound !). If you can work with a channel, then you'll only be modifiying the fuselage frames (cutting off the skin flange) rather than cutting them completely ... WAY better.

If you need a non-conducting region around the slot, cut a wider slot in the fuselage and add a non-conducting doubler (glass fiber?). Same with the frames.

consider if you REALLY need a 200" uninterrupted slot ... if you could work several shorter slots (allowing some frame flanges to span the slot) would be WAY better (than one long slot).

 

[KENAT]

Higgler,

If you just want to get a general idea of structural layout general interest aircraft magazines & books often have beautiful drawings of the general structures of aircraft etc. I'm not sure how accurate all of these are but may give some idea. This is the best I found with a quick google search

http://adg.stanford.edu/aa241/structures/structuraldesign.html

As rb1957 mentions if you need to be cutting through more than the skin there may be considerable other structure (or even other systems) in the way.

Can you add the sensors in an 'external' fairing.

I believe this is how the sensor housings on aircraft like JSTARS

http://www.fas.org/irp/program/collect/jstars.htm

, ASTOR

http://www.airforce-technology.com/projects/astor/

, RC135

http://www.globalsecurity.org/intell/systems/rivet_joint.htm

etc are done.

There are obviously aerodynamic concerns but especially if modifying existing airframes this may be a better idea.

(Note my aerospace experience was in defence/aircraft weapons systems so take my comments with a pinch of salt!)

KENAT, probably the least qualified checker you'll ever meet...

 

[Higgler]

Let's start with a small slot, between the frames.

I don't need any area inside the slot, cut the slot, then add FR4/G10 board inside of the aircraft to close out the slot. The circuit board can also protrude into the slot if that helps. This circuit board can be very thick, 1/4, 1/2, 3/4 inches, and if the slot is 20" long the circuit board could be 22"x6"x0.75" in size if needed.

I wouldn't want a U channel under the slot.

On top of the aircraft, ?would you need to add another circuit board and connect the inner and outer circuit boards with mounting hardware, or is just one thick inner circuit board ok? You'd use non conductive gap filler to cover the slot.

kch

 

[rb1957]

was that 20" a typo ? or can you work with shorter slots.

i don't think you can count on a circuit board being a structural member (closing the slot).

i really liked KENAT's idea of an external pod ... that would be much easier to integrate with the fuselage.

i take it that your sensor viewing thru the slot is something like a phased array ... not much space needed for a sensor head, PCBs for wiring ... sounds easy to integrate for a systems perspective ...

 

[Higgler]

rb1957, as large a slot as possible, I picked 20" as the ballpark spacing between the braces. We've used G10/FR4 circuit boards as structural members before for outdoor structures.

What type of non conducting material do you suggest to close out the slot? I'll check it for electrical parameters.

Kenat, thanks that first link in Stanford shows a nice structural layout that makes me thing slots around the circumference may be a bad idea too.

Regarding the pods, that would be a good option for present day usage, but this is a bit pie in the sky future developments.

Thanks for the inputs,

kch

 

[KENAT]

I believe some of the stealth aircraft may use 'slot' antenna etc. This may be an area worth investigating of how it's done.

KENAT, probably the least qualified checker you'll ever meet...

 

[KENAT]

More background on why simplistically cutting slots in a modern aircrafts skin can be problematic.

http://en.wikipedia.org/wiki/Monocoque

KENAT, probably the least qualified checker you'll ever meet...

 

[Higgler]

Kenat,
your link says
"that is then covered with a non-load-bearing skin". Wouldn't that make it easy to cut a slot, then add some plastic to cover the hole, especially if it's non load bearing?
I realize there are pressure differentials, but maybe I can take a full section out between ribs and replace it with a circuit card. Most skins are ? 0.030" and if I used a 0.060" or 0.125" FR4 to cover the slot should be a stronger structure than the original skin???

In a previous email I realized that going to a large open area or a large add-on results in temperature change material matching that's a real problem.


kch

 

[KENAT]

Higgler, I think you misunderstood,

Monocoque (French for "single" (mono) and "shell" (coque)) is a construction technique that supports structural load using an object's external skin. This stands in contrast with using an internal framework (or truss) that is then covered with a non-load-bearing skin.

Read down to the section talking about aircraft.

Someone with more experience feel free to correct me but simplistically most modern aircraft are Monocoque so the skin is load bearing. This is the point I was trying to make.

Also take a longer look at the stanford link. It gives some numbers for 'typical' skin thickness.

KENAT, probably the least qualified checker you'll ever meet...

 

[Higgler]

Thanks for the pointer, poor reading by me.
Seems the skin thickness is 0.016" to 0.060".

Maybe that says that the ribs play less of a role than in older aircraft.

Interesting Stanford analysis curve about failure predictions and that computers haven't given significant improvement compared to the old fashion methods in the pre computer days. Makes me repeat that an experienced/old engineer is worth his(or her) weight in computers.

kch

 

[sjbjcl]

AIrcraft skins are consided structural. The rule of thumb we use is to ensure that the area of material that is removed from the aircraft skin is replaced with doubler plate of the that has enough material and fasteners to support the load capacity lost by the hole. If you cut a 20" slot you will need at least 20" of doubler material of the same type and thinkness as the skin as well as a sufficient number of fasterners either side of the hole to support the loads.

 

[KENAT]

How would you propose attatching your GRP doubler/panel.

sjbjcl mentions fasteners but you may have to consider adhesives and any implications from this.

Also how does cutting slots into composite compare to slots in aluminium (aluminum)? Is aluminum easier to re-enforce?

KENAT, probably the least qualified checker you'll ever meet...

 

[Higgler]

sjbjcl,
I assume a "doubler plate" means one sheet external, one sheet internal, total thickness of the two sheets equals the thickness of the material removed. Is there any method to avoid the external doubler? Can a small section of skin be removed (I realize skins may not be small sections but are large sections) and replaced that keeps it flushmount, small section being maybe 20"x10" or whatever the internal support structure spacing is? I'd like to propose a design that doesn't change the external aerodynamic shape of the aircraft.


kch

 

[40818]

Am i the only one here slightly worried that (with no offence) somebody is creating a proposal that entails the task of making up to a 200" slice into the skin..with no structural input probably barring this forum????

Higgler, firstly i will give some answers/advice:

In a typical semi-monocoque aircraft, as the skin thicknesses are generally similar as are frame pitches, then the skin takes abour 80% of pressure membrane tension, and the remainder is in the frame.

As for using a circuit board as a load bearing member to span the cut-out is a path that you should cease. You would never find anyone willing to sign that piece of engineering off.

What aircraft platform is your proposal for?? My thinking is that you are thinking about a sensor type system that can be rolled out across differing aircraft types?

You would be better served looking at putting your mod on the outside of the aircraft, especially if it doesnt protrude too much.

 

[KENAT]

A doubler plate is usually a single plate on one side of the original panel/plate.

The trouble with putting it internally in your instance is the amount of modification it may require to other structural components etc.

I'd seek confirmation from a more experienced aircraft structures guy but a thin doubler will probably be in the boundary layer anyway so have minimal aerodynamic input. There are peope on here who could give more information.

KENAT, probably the least qualified checker you'll ever meet...

 

[thruthefence]

Harbor Freight has a big sale this weekend-
Saws-all's are on sale!!!

 

[KENAT]

Higgler,

I'm inclined to agree to thruthefence & 40818 sentiments.

I think you need to talk to a qualified party in detail, I think you may need more than just 20 emails or. You'll probably need to explain what you're trying to do in some detail and work with them to make some baseline assumptions etc.

That said I love these kind of pie in the sky exercises and probably wont be able to resist responding to posts here, even though my qualifications to do so are marginal at best.

Going back to your OP.

You say Modern Military Aircraft, what do you mean by this? Do you mean fast jets (fighters, attack aircraft) or more sedate performers (Heavy bombers) or some of the converted civilian types in military service (most refuelling aircraft, many maritime patrol & recon platforms) or do you mean something else (UAV, space platforms)?

Also how modern? Currently in service (F15, 16, 18, Tornado, Harrier) or entering service/upcoming (Typhoon, F22, JSF etc).

Both these factors will make a difference in the types of structure and the materials etc used.

You talk about modifying aircraft. Do you mean modifying stock aircraft that are already built and in service? Or do you mean modifying aircraft designs so they are built with your sensor system?

If you mean the former I really strongly feel an external fairing is probably a better way to go.

If the latter then doing it internally may be more possible but still probably expensive & time consuming.

You are talking about major structural modifications, imagine if you were proposing to put these sensors with their slots in the pillars or other structural member of a bridge or something. Still sound like a simple/good idea?


KENAT, probably the least qualified checker you'll ever meet...