Flight profiles of the different airframes
These airframe characteristics can appear in many permutations and combinations.
Contents |
Wing position
High wing
This puts the CG under the lift, so the plane gains some pendulum stability. It also allows you to use struts to take some of the flight loads. The downside is that the weight, thrust and lift are all in slightly different places, so it doesn't respond symmetrically and flies best at the speed it's trimmed for. Full size planes with high wings have good visibility of the ground (useful for light aircraft) and lots of clearance for wing mounted engines (good for landing on rough ground or water)
Mid wing
With the wing in the middle the thrust, lift and mass can all act through the same point, so the plane will maintain the attitude it's left in. This is great for aerobatics but means the plane won't ever sort itself out if you make a mistake. For models, it's hard to make a removable mid wing, as you have to remove half of the fuselage at the same time, or make the wing in two seperate parts.
Low wing
Low wings put the weight above the lift, which is potentially unstable, so they usually have more dihedral than high wing planes (even for the same level of stability). It's easy to mount wide undercarriage on a low wing, and have a removable wing. Full size planes with low wings have good visibility upwards, which is good for warbirds.
Biplanes
Having two (or more) wings doubles the wing area without increasing the overall size, so it's good for generating lots of lift at low speeds. It's also possible to brace the wings together with struts and rigging, for a very strong, light structure. However all that bracing causes drag, so speed is limited. Also, two wings have four wing tips and wing tips are less efficient than the rest of the wing because air can flow around the ends, so a biplane is less efficient than the equivalent monoplane.
Wing shape
Wing Area
Aerodynamic forces are proportional to area, so a large will generate more lift and drag than a small wing.
Wing Loading
Wing loading is calculated by dividing the weight by wing area. A lightly loaded wing will need to generate less lift to fly at the same speed as a more heavily loaded wing and will thus stall at a lower speed. A more lightly loaded wing can generate more lift at any speed in prortion to the weight of the plane, so it can turn more tightly. Heavily loaded planes need to have more power and fly faster to compensate. Gusts affect a lightly loaded wing more than a heavily loaded wing.
| Wing loading | Will fly like this |
|---|---|
| <2 oz/sqr foot | indoors only |
| 4 oz/sqr foot | slow flier |
| 6 oz/sqr foot | park flier |
| 10 oz/sqr foot | fast aerobat |
| 20 oz/sqr foot | a missle.. |
Aspect ratio
This is the wing span divided by chord. The reason it's important is that high pressure air can 'escape' around the wing tip to the low pressure on top of the wing forming a Tip vortex, reducing lift. The longer the wing chord, the more lift is lost. Thus a long wing has a small tip and smaller losses. A wide, short wing can be built stronger than a long thin wing. The wing tip vortex delays the stall in the area it affects, so low aspect ratio wings have soft stalls. Very low aspect ratio wings never really stall - it keeps generating some lift but the drag keeps increasing as it slows down. Unless the plane has enough thrust to hover, it will reach a stage where full power isn't enough to overcome the drag, even though it's going very slowly (long after a normal wing would have stalled). The only way to recover is to put the nose down and gain speed.
Delta
A delta is a very low aspect ratio wing. It's almost stall proof, very easy to build but not very efficient. With enough power they are very easy to fly.
Dihedral
The height of the wing tips above the wing root has a large affect on Yaw and Roll stability. When a plane is rolled to the right, it will start to slip sideways (yaw) to the right and (if the plane has dihedral) the angle of attack of the right wing will increase. This will roll the plane level again. This stability is useful for trainers and relaxed flying but not for aerobatics (where, if you put a plane upside down, you want it to stay there). See also Dihedral
Swept wing
Wings are swept to reduce drag at supersonic speeds. And because it looks cool.
Wing Aerofoil
The aerofoil is the shape of the wing in cross sectional. It's the least obvious difference between planes but it makes a huge difference. A thick aerofoil has more drag than a thin one. A cambered aerofoil can generate more lift than a flat one, but will have more drag at high speed. An uncambered aerofoil is symetrical (the same either way up) and thus will fly inverted as well as it does the right way up.
Tail position
Conventional, tail last
Most planes have a horizontal tail at the back, read the alternative to see why.
Canard
Some planes put the horizontal stabilizer at the front. The canard develops an upward force to make the plane stable, which is more efficient than a rear tail that has to force the tail down. If the pilot applies too much 'up' elevator the canard will stall. This is usually arranged to occur before the main wing, and results in a plane that is resistant to stalling (once the canard is stalled the plane can't pitch up and stall) but also in poor elevator authority. Canards generally need the motor at the back for balance, which puts the prop very close to the ground when pitched up for landing and take off.
Tailless
Removing the tail reduces weight, drag and construction - resulting in cheaper, lighter, faster planes. The price is that you have to find some other way to generate stability, usually by sweeping the wings back and adding Reflex. This reduces the theoretical efficiency gains.
Flying wing
A flying wing is a tailless plane without a fuselage - the fuselage has little aerodynamic affect except drag but it's useful for holding the payload. If you can fit everything inside the wing you can eliminate the fuselage to reduce weight, drag and expense; if you have to make the wing thicker you loose the benefits.
Fuselage shape
A large fuselage can provide side area, which is useful for knife edge flight. The area needs to be balanced around the CG in the same way as wing and tail area - too much area in front of the CG will make the plane unstable. Too much area behind the CG will reduce control authority and efficiency. For knife edge flight, the area should be balanced top-to-bottom as well, with half above the CG (and thrust line) and half below.
