aircraft construction and modification suggestions
aircraft construction and modification suggestions
(OP)
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
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





RE: aircraft construction and modification suggestions
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 ?
RE: aircraft construction and modification suggestions
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
RE: aircraft construction and modification suggestions
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).
RE: aircraft construction and modification suggestions
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 ht
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
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...
RE: aircraft construction and modification suggestions
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
RE: aircraft construction and modification suggestions
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 ...
RE: aircraft construction and modification suggestions
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
RE: aircraft construction and modification suggestions
KENAT, probably the least qualified checker you'll ever meet...
RE: aircraft construction and modification suggestions
http://en.wikipedia.org/wiki/Monocoque
KENAT, probably the least qualified checker you'll ever meet...
RE: aircraft construction and modification suggestions
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
RE: aircraft construction and modification suggestions
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...
RE: aircraft construction and modification suggestions
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
RE: aircraft construction and modification suggestions
RE: aircraft construction and modification suggestions
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...
RE: aircraft construction and modification suggestions
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
RE: aircraft construction and modification suggestions
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.
RE: aircraft construction and modification suggestions
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...
RE: aircraft construction and modification suggestions
Saws-all's are on sale!!!
RE: aircraft construction and modification suggestions
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...
RE: aircraft construction and modification suggestions
40818, I think for now the 200" slot has to be incorporated in a new aircraft design, so we'll stick with a 20" slot for now.
I would certainly think there'd be a method to make a 20" thin slot and inside the aircraft add a 24"x4"x2" thick piece of ?aluminum, steel or titanium with a similar thin slot in the center 20" section, then cover that slot with thick (1") solid non metallic material to create essentially 10x the strength of the original 0.060" thick aluminum skin.
External fairings would gernally have to be very tall for sufficient results, I can't propose them as a general solution.
I could add a part that's say one inch thick externally for most of the aircraft, the part would be constructed of metal and plastic that would be say 24" square and curved to fit the contour of the aircraft. To mount it on the aircraft and make it work, I'd need to cut out a 20" square hole between the fuselage frames and mount the 24" square rigid part to cover the hole, mount it to the frames if that helps. Does that description sound more feasible? The 24" square part would be very rigid.
I'm on travel today, back Friday.
I have a sawsall already, they're a bit rough don't you think?
Thanks all,
kch
RE: aircraft construction and modification suggestions
get professional help (though not of the psychoiatric kind !) ... we started with a 200" slot, went to a 20" one (one only, or repeated), started with longitudinal, mentioned circumferential; and now we've added a 20" by 20" hole. it's fine that this is early days in your project, and pretty much we're blue-skying ideas, but (pretty much as you anticipated in your OP) we're all speculating about applications and attendant limitations.
Basically, almost anything you want can be accomplished one way or another. I would suggest that a 2" thick piece of Al probably isn't going to find it's way onto a hi-performance plane (abit too argicultural !)
good luck with your project !
RE: aircraft construction and modification suggestions
htt
http://www.eurofighter.com/et_cu.asp
http://
http:/
http://www.publishingsolutionswww.com/air_cut.htm
Hopefully gives you some idea that you aren't just looking at skin. There are structures and even systems directly under the skin. If you make major changes to a stucture you've got to look all sorts of things not just the quasi static loads you seem to be considering. What about the effect on fatigue/vibration etc.
Also how are you going to get the access to modify the exisiting airframe, sometimes easier said than done.
KENAT, probably the least qualified checker you'll ever meet...
RE: aircraft construction and modification suggestions
thanks for the cutaways, scary looking collection of metal.
It seems some aircraft have small spaces on top.
Maybe towed sensors would be easier.
kch
RE: aircraft construction and modification suggestions
Plus they're mostly artists impressions, may not be totally accurate.
I've seen a Eurofighter on the assembly line and several other types in maintenance, there is not a lot of spare room.
Towed sensor may have potential, I assume you mean like a towed decoy. Depending how expensive your device is, people may not be so keen on dangling it out the back. I think towed decoys can probably be jettisoned if need be, is your sensor a bit expensive to be jettisoned?
Also think again about a conformal external fairing or even a pod.
Think about when manufacturers want to increase fuel on F16, F15, Typhoon. They don't rework the internal structure they add on a conformal tank. Minimal structural change at the expense of modifice aerodynamics.
KENAT, probably the least qualified checker you'll ever meet...
RE: aircraft construction and modification suggestions
the sensor isn't too expensive in high volume. Less than a typical decoy.
Haven't heard of a conformal tank, makes sense to avoid a large wing pod structure.
What would the typical dimensions range. i.e. 4" thick in center, 36"x48" width and length and triangular shaped?.
Is the tank always on the bottom?
kch
RE: aircraft construction and modification suggestions
It's the 'odd' shape both sides where wing joins fuselage.
htt
It's on the side of the 'intake' under the wing called a FAST pack. If I recal correctly they are permanent on F15E and may have been made structural members but on the F15C they were conformal tanks.
http://www.fas.org/man/dod-101/sys/ac/f-15.htm
Only a wind tunnel model but hopefully pretty obvious where the tanks are.
h
In addition many older aircraft developed bulged bellys or large spines in later versions to increase fuselage volumes. I don't know if most of these were incorporated in the primary structure though.
KENAT, probably the least qualified checker you'll ever meet...
RE: aircraft construction and modification suggestions
I'll probably use a small slot in the skins for one set and for the long slot I'll have to think about it more.
Thanks others for the assistance and the warnings.
kch
RE: aircraft construction and modification suggestions
That said, I just want to amplify some of the warnings one more time and think it would be worth getting someone who really knows what they are doing to take a look before you present to a customer.
Ken
KENAT, probably the least qualified checker you'll ever meet...
RE: aircraft construction and modification suggestions
I copied your reference on external pods from the other thread too, in case I need to use one of those companies for sensor placement.
I realize aircraft are touchy area's and finding experts is key. As an Engineer, I've always thought by the end of any job, "why didn't we just talk to him/her in the first place!".
kch
RE: aircraft construction and modification suggestions
When I interviewed there they'd just done extensive modification to a quasi military 737 to add a big radome dome on the rear end. Ended up looking a bit like the Nimrod AWAC tail.
I'm sure there are other places out there but they're the first to spring to my mind.
KENAT, probably the least qualified checker you'll ever meet...
RE: aircraft construction and modification suggestions
You are playing with fire, attempting make long slits in metallic fuselage skin(s).
Suggest You consider the following...
NOTES:
Assume the sensors are NOT Ribbon-like materials [thin] that could just be bonded to the exterior.
Assume that stealth [low observables] technology is NOT an issue.
Assume that modification-stealth is NOT a factor [IE: OK if obvious to a casual observer].
Brain-storm A. Fasten sensor strips to the crown-skin exterior in airflow direction for minimum drag. "So-what" if they look like scabbed-on strips... as-long-as performance does not suffer significantly.
Brain-storm B. Overlay [sealant-bond] a low-stiffness [composite, rubber, plastic, etc] sensor-skin on the structural skin. This material can contain the sensor-strips.
Brain-storm C. Trim-out the crown-skin, width A/R, and insert a structural composite section with the skin, stiffeners, frame-segments and sensor-strips embedded.
Brain-storm D. Build sensors into external antenna blades distributed all over the acft [fuselage crown, upper wing-skins, H-stab upper skins and/or tips, etc... or into one-big-spiney-back-like antenna, such as on the Australian B737 AWACS.
Regards, Wil Taylor
RE: aircraft construction and modification suggestions
http://www.fas.org/man/dod-101/sys/ac/e-737.htm
http:/
htt
htt
And a story of how not to modify an aircraft which includes bit radomes.
http://www.spyflight.co.uk/Nim%20aew.htm
KENAT, probably the least qualified checker you'll ever meet...
RE: aircraft construction and modification suggestions
htt
This has a little on conformal antenna
http://www
http://www.edocorp.com/AntennaMilAirborne.htm
KENAT, probably the least qualified checker you'll ever meet...