JMillar
Well-Known Member
I know everything below has been presented here before, but I don't think it's ever been put together like this - if it has I guess I just wasted my time. I wanted to crystallize in my own mind some of the aspects to picking a design, so I typed up a bit of notes, I'd like to hear corrections/additions/comments, or "oh, that's already over here" :
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Aircraft design is necessarily an exercise in compromise – no aircraft can be all things to all people. The job of the designer, in addition to the engineering involved, is to weigh and balance the various goals in such a way that the craft best suits its intended purpose. Here I try to give a brief summary of some of these factors:
Range – is a function of the fuel onboard, and the efficiency with which it is consumed. To make an long-range aircraft, it needs to
a) carry a lot of fuel – with the size and mass that involves
and / or
b) Travel a long distance per unit fuel – IE efficient engine(s), and low drag. Since drag is proportionate to speed, an efficient plane is usually a slow one.
One issue with regards to range is, how long do you want to sit in the plane at a stretch? Endurance is a logical thing to compromise on, to an extent, because few people want to sit still (with no bathroom!) for more than a couple of hours. Another note, when you talk about efficiency, you have to account for useful load, too – think of fuel per person-mile, for instance, instead of straight mpg.
Cruise Speed – Means low drag, and powerful engine(s), as well as an airframe that can take the stresses. High speed rules out rotorcraft [not that I was really covering them, I don't know enough].
Useful load – in terms of weight and/or passengers carried. The more passengers, the bigger it has to be (obviously!). More weight means more power (for the same performance) and more wing area to keep the same characteristics
Rate of climb – varies proportionately to power and inversely with weight. Affects what fields you'll be able to use, and (combined with glide ratio) your “safety margin” for flying low.
Stall Speed – determined by the aerodynamic configuration (airfoil, wing area, etc) and weight. Sometimes limited by regulations (required to be low). Again, affects the fields you can use.
Rough- and short-field capability – Do you want to be able to land at out-of the-way rough airstrips? If so, you want good prop clearance, a high wing, and sturdy undercarriage. Also, you want plenty power-to-weight and a low stall speed to take off in a short distance.
Cockpit visibility – different cockpit designs greatly affect how well you can see out. Wing placement also has a big influence on the view.
Cost – As it gets more capable, the cost goes up!
Aerobatic capability – The more agility you want, the more touchy it will be to fly. The strength required means weight, eating into performance and useful load.
Gear configuration – [Can't comment here yet.]
Easy to fly – An easy-to-fly plane should be stable (not need constant adjustments), Have a slow stall speed, handle well without being twitchy, etc.
Aesthetic appeal – largely subjective, although many people think along vaguely similar lines
Construction material – The designer's choice, but certain materials are better suited to better tasks – Composites are sleek, good for fast aircraft. Fabric over tube or wood is well suited to slow-speed aircraft due to light weight and the low drag being less important. Metal is somewhere in the middle – a little higher drag than the composites due to rivet heads etc, but sleeker than a fabric-covered design.
Gross weight – The lower the gross is, the easier it is to make it perform well. This requires efficient design and use of high strength-to-weight materials to cut gross weight without reducing the useful load
Layout – Conventional / canard / tandem, monoplane / biplane / more, pusher vs tractor, style of tail, etc. Can't comment much yet.
[it's written in OOo and the formatting didn't come out so smooth when I copy/pasted]
------------------------------------------------------------
Aircraft design is necessarily an exercise in compromise – no aircraft can be all things to all people. The job of the designer, in addition to the engineering involved, is to weigh and balance the various goals in such a way that the craft best suits its intended purpose. Here I try to give a brief summary of some of these factors:
Range – is a function of the fuel onboard, and the efficiency with which it is consumed. To make an long-range aircraft, it needs to
a) carry a lot of fuel – with the size and mass that involves
and / or
b) Travel a long distance per unit fuel – IE efficient engine(s), and low drag. Since drag is proportionate to speed, an efficient plane is usually a slow one.
One issue with regards to range is, how long do you want to sit in the plane at a stretch? Endurance is a logical thing to compromise on, to an extent, because few people want to sit still (with no bathroom!) for more than a couple of hours. Another note, when you talk about efficiency, you have to account for useful load, too – think of fuel per person-mile, for instance, instead of straight mpg.
Cruise Speed – Means low drag, and powerful engine(s), as well as an airframe that can take the stresses. High speed rules out rotorcraft [not that I was really covering them, I don't know enough].
Useful load – in terms of weight and/or passengers carried. The more passengers, the bigger it has to be (obviously!). More weight means more power (for the same performance) and more wing area to keep the same characteristics
Rate of climb – varies proportionately to power and inversely with weight. Affects what fields you'll be able to use, and (combined with glide ratio) your “safety margin” for flying low.
Stall Speed – determined by the aerodynamic configuration (airfoil, wing area, etc) and weight. Sometimes limited by regulations (required to be low). Again, affects the fields you can use.
Rough- and short-field capability – Do you want to be able to land at out-of the-way rough airstrips? If so, you want good prop clearance, a high wing, and sturdy undercarriage. Also, you want plenty power-to-weight and a low stall speed to take off in a short distance.
Cockpit visibility – different cockpit designs greatly affect how well you can see out. Wing placement also has a big influence on the view.
Cost – As it gets more capable, the cost goes up!
Aerobatic capability – The more agility you want, the more touchy it will be to fly. The strength required means weight, eating into performance and useful load.
Gear configuration – [Can't comment here yet.]
Easy to fly – An easy-to-fly plane should be stable (not need constant adjustments), Have a slow stall speed, handle well without being twitchy, etc.
Aesthetic appeal – largely subjective, although many people think along vaguely similar lines
Construction material – The designer's choice, but certain materials are better suited to better tasks – Composites are sleek, good for fast aircraft. Fabric over tube or wood is well suited to slow-speed aircraft due to light weight and the low drag being less important. Metal is somewhere in the middle – a little higher drag than the composites due to rivet heads etc, but sleeker than a fabric-covered design.
Gross weight – The lower the gross is, the easier it is to make it perform well. This requires efficient design and use of high strength-to-weight materials to cut gross weight without reducing the useful load
Layout – Conventional / canard / tandem, monoplane / biplane / more, pusher vs tractor, style of tail, etc. Can't comment much yet.
[it's written in OOo and the formatting didn't come out so smooth when I copy/pasted]
