Posted: Fri Aug 15, 2008 10:21 pm Post subject: Structual Integrity of Helicopter Airframes - n00b question!
Firstly, apologies in advance for what might be the stupidest question ever asked on these forums
I'm pretty well educated in all areas of fixed wing aviation, but I admit my rotary experience is limited to 7 or 8 joy flights and zero theory knowledge, so go easy!
I am wondering what structual requirements govern the viability of a helicopters design and build. For example, from a laymans point of view, contraptions like the M.A.S.H. helicopters and even moreso some of those rather disturbing little homebuilt (ultralight) helicopters seem to have very little in the way of solid structure.
I understand that they are a little stronger than they look and by use of framework in the tailplane of the MASH style choppers adds strength, but I'm wondering how important a helicopters fuselage/tail boom/engine mount structure really is and what more there is to it (structually)?
From a laymans point of view, a helicopter like an Squirrel for instance seems to consist of nothing more than a fairly flimsy cabin, an appropriately braced and frame worked engine mount/bay area and a tailboom/rotor assembly that most likely would have some internal re-enforcing beneath the thin aluminium outer skin...
But am I completely underestimating the engeneering that has been put into this or are helicopters relatively easy going on airframe stress/fatiuge?
Oh and im asking this because I've come accross a rather amazing Series II Cobra mock up that was being displayed at a really little known, out of the way and unexpected war museum/skirmish field near my place.
I'm not realistically considering anything too dreamy dont worry, but I must say, my aircraft recognition (down to fine detail) is pretty respectable and the detail that has gone into this mock up has me in two minds as to wether this is indeed a mock up, or if it somehow made its way into private hands many years ago and is actually a static display restoration!? It did get me wondering however, what is stopping somone from taking a project like that one step further and fitting a Jetranger engine/flight control system to such a beast?
SO now you've seen the whole crazy picture, it might help in answering a little better
Thanks and apologies again for the leftfield question!
Joined: Feb 20, 2008 Posts: 1059 Location: New York
Posted: Fri Aug 15, 2008 11:51 pm Post subject: Structure
Well, I am not a helicopter engineer, but can share some thoughts.
A helicopter by it's nature needs to be as light as possible. but at the same time, must be capable of supporting the engine and related components, tail rotor and drive, and of course the occupants, and payload.
History has shown, that "tubular" (whether hollow tube or solid rod) construction with a sheet metal or composite covering works best.
The Mash or Bell 47 helicopter used the tubular framework to support the drive shaft, tail rotor, gearing and other components. The pilot compartment was also formed by tubing and fitted with a plexiglass "bowl".
Anyttime you use "solid" frames, you add weight which requires additional horsepower if the same load is to be maintained.
Even most homebuilts use a tubular frame of sorts
Since you stated that you have fixed wing knowledge, then you can see the similarity between fixed and helicopter. Fixed wing use tubular construction as well with coverings from the old time fabric to modern day composite. Little difference between constuction.
OH bye the way........It's never the question that is "stupid", only some of the answers. _________________ "A Copter Pilot's Life has it's... ups and downs"
Bell 47-206, Schweizer 300/500, Citation 525
Fixed wing aviation has a range of different in flight stresses on the airframe that cant apply to rotary obviously, its rather clear that bolting a afterburning turbofan to a cessna is going to end up in disaster (after everyone stops laughing of course hehehe) - but I'm just getting my head around the applied forces on the helicopters actual airframe in flight (not taking into account the obviouse need for precision and strength in rotor blades of course).
So just clarifying in relation to my second post/reason for question, can you see any reason why it wouldnt be possible (provided the airframe weight was within operational limits of engine power) that a well built fibreglass mock up helicopter airframe such as I've described could be retro-fitted with a JR engine/flight control system - essentially just providing a rather eye catching chassis? Or do you feel that the forces involved in helicopter flight would deem the idea totally unfeasable?
Once again, its a hypothetical - but its fun to think what they could have done with such a good looking display
Joined: Jul 10, 2008 Posts: 61 Location: Australia - North Queensland
Posted: Sat Aug 16, 2008 1:57 am Post subject:
I'm not an engineer either...but my theory is.....one needs to consider the flight 'forces' difference between helicopters and slab wings. In a helicopter the main forces,lift & thrust are focused on the mast....and antitourque on the T/R so I reckon as long as the transmission is well secured you're pretty safe and I gues the tail boom and attachment needs to be able to cope with some fair forces..mainly lateral.
Not a very technical summary but.....
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