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71

Because they are very lightweight and fragile. Therefore, thrust and propulsion mass must be distributed over span - a single, large propeller and motor would put too much force into the structure locally. Also, it would require a higher landing gear to give the larger propeller the required clearance. You are right about the higher losses. However, these ...

26

There is no airfoil with good lift in both flow directions, but one with some lift is conceivable. However, the lift-to-drag ratio will be nothing to write home about. One reasonable candidate would be created if we use the forward half of the venerable NACA 66(2)-415 and copy it again for the last half. Like that: As you might recognize from the plot, ...

22

You are right, the trailing edge does not need to point down. Take symmetric airfoils - here the trailing edge runs parallel to the airfoil chord. Or take reflex airfoils (like the HQ 34 of the SB-13 tailless glider): Here the trailing edge is indeed pointing upwards, and still this aircraft flies. But to create lift with as little drag as possible, it ...

17

For the same reason why we do not have one single type of aircraft flying all commercial and military missions worldwide: flight has many variables, and there is no single optimum solution. First off, you mention transonic flight in your question, which usually involves sonic flow at over at least part of the airfoil, which significantly alters the desired ...

15

I cannot answer all your questions, but maybe point you to some facts to come closer to an answer. Most important is the thickness of your parallelepiped - more than a few percent will just increase drag, without benefiting performance. It should be as thin as structurally possible. There are lots of model airplanes using flat plates for lift. The most ...

14

The idea of a stagnation point is an idealization. This point is infinitesimally small, and air particles flowing along a streamline which leads into it will slow down on their way. The closer they come to the stagnation point, the slower they flow, and in the end they never arrive at the stagnation point. In reality, air molecules have a finite size, so ...

14

Conventional commercial designs try to maximize kg-kms per dollar fuel cost or kg payload per dollar investment. Or minimize operating & maintenance cost. All these goals need an economy of scale that forces few large complex turbines. On the other hand, most solar planes focus on raw, dollar-agnostic performance / endurance metrics like max height ...

14

In aircraft, size does matter. Smaller aircraft flying at the same speed, air temperature and altitude than a larger aircraft have a smaller Reynolds number which characterises the boundary layer flow. A smaller Reynolds number allows for more laminar flow but demands a less steep pressure rise in order to avoid early separation. So there is a different ...

13

Introduction to Transonic Aerodynamics Roelof Vos, Saeed Farokhi (more) Written to teach students the nature of transonic flow and its mathematical foundation, this book offers a much-needed introduction to transonic aerodynamics. The authors present a quantitative and qualitative assessment of subsonic, supersonic and transonic flow around ...

13

I've got an aged book on pre-design of aircraft, which states that the B727 wing thickness was 13% at root and 9% at tip, average thickness 11% chord. $M_{M0}$ = 0.9, first flight of prototype is noted as 1963. The 727 was a derivative of the 707: first flight 1957, average thickness 10% chord, $M_{M0}$ = 0.9. So the 707 wing was even thinner than B727, ...

12

It does rotate. In fact, in aerodynamics, this rotation force has a name: pitching moment. Pitching moment changes depending on the angle of attack. Airfoils can be designed with almost any pitching moment and still generate lift, from positive (trailing edge rotates upwards as you describe), to zero (no rotation) to negative (trailing edge rotates ...

12

Don't forget the electrical advantages of multiple motors. No need to conduct all the current to a single place with long lengths of heavy wire, no need to control a large current. Also, many small motors and propellers provide redundant depth/graceful degradation in case of a failure(s).

12

The flaps and ailerons are "reflexed" on this glider. They have been raised to a setting above the normal zero position, above the airfoil's normal chord line. A number of flapped gliders have this feature. Two main benefits are a reduction in pitching moment as the pressure distribution on the wing is moved forward, so less downforce work for the tail, ...

12

First propeller use: A highly cambered airfoil would cause high pitching moments and twist the propeller blade. Of course you can pre-twist the blade so it will assume the correct shape in the desired operating point, but a propeller needs to work over a wide range of operating points, from take-off roll to high speed flight at altitude. In off-design points ...

11

The airfoil profile you've shown is called a supercritical airfoil. Typical airfoil sections are curved on the top and the bottom. The airflow over the top of the airfoil is accelerated i.e. the airspeed over the top of the airfoil is more compared to the free stream velocity. The associated reduced pressure helping to create lift. Source: www.symscape.com ...

11

There are no two (aerodynamic) forces. Force is not caused by the up- or downwash itself, but by the change from up- to downwash. The air is only accelerated downward and this change in it's momentum causes an upward force on the wing everywhere, there is no aerodynamic down force. Due to the pressure distribution however the centre of lift is about quarter-...

11

Your intuitive feeling that the fuselage could be used to create additional lift is correct. Indeed, some aircraft are specifically shaped such that fuselage could provide substantial lift (and that's excluding any 'flying wing' design where fuselage is completely blended with the wing). However, these are generally supersonic fighter aircraft. At ...

10

There are indeed several approximations, depending on the shape of the wing. Generally, the lift curve slope is $2\pi$ only for a flat plate in inviscid 2D flow (with Kutta condition fulfilled). With thicker airfoils, the lift curve slope in 2D increases slightly. It also increases with Mach number proportional to the Prandtl-Glauert factor $\frac{1}{\sqrt{1-... 10 Just three data points: The tail surfaces of the Pilatus PC-12 still use the venerable NACA 0012, even though a better alternative (from the Wortmann FX 71 L series) was proposed. It did not help that the Wortmann airfoil is used on many small airplanes, has more lift and less drag and an abundance of data exists on it: The (mainly British) engineers at ... 10 Every good design varies at least the thickness: While the root needs to be thicker to enclose an efficient spar, the tip can be optimised for aerodynamics, which usually results in a root thickness of 16% to 21% and a tip thickness between 9% and 13%. Look at Dave Lednicer's incomplete guide to airfoil usage for an exhaustive collection of aircraft and ... 10 By using a non-zero angle of attack. When the trailing edge is pointed downwards, and assuming the airstream leaves the trailing edge smoothly, the exiting airstream is deflected downwards. This causes lift via conservation of momentum. Increasing the angle of attack will increase your lift until such time as the airstream over the trailing edge becomes ... 10 A symmetric airfoil will generate no lift at no angle of attack, and negative lift at a negative angle of attack. However, cambered airfoils are curved such that they will generate lift at small negative angles of attack. 10 Obviously, drag should be smallest for symmetrical airfoils at zero angle of attack. However, most airfoils have camber, and then the lowest drag is at positive lift coefficients in case of positive camber. Where that point is exactly depends on many parameters; in case of laminar airfoils even local imperfections can have a noticeable effect. Generally, ... 10 You can't just cherry-pick aerodynamics and exclude everything else when it comes to aircraft design. But let's entertain flight dynamics alone for this instance, and use Selig S1210 or S1223 as examples. Selig-S1210 (for Re 0.2e6, 0.5e6 and 1.0e6) characteristics: First, notice that the linear range of the airfoil extends from$C_l\$ 0.5 to 1.9, which ...

9

Yes, it does vary slightly due to viscous effects. In inviscid flow, the flow speed would not affect the lift coefficient - angle of attack relation. However, increasing the flow speed will result in a thinner boundary layer and a slightly different shape of the airfoil - boundary layer combination as "seen" by the outer flow. This influence is captured by ...

9

The linear theory makes certain assumptions in its formulation: The flow is irrotational and isentropic. The perturbations are small; i.e. the bodies are small at small angles of attack. However, hypersonic flow violates these assumptions: For a body in hypersonic flow (blunt body with detached bow shock), an entropy gradient is developed that varies ...

9

Boeing designs its own airfoils for its aricraft. According to this site, the following airfoils are used in 737: Boeing 737-100: Wing root- BAC 449/450/451; Wing tip- BAC 442 Boeing 737-200: Wing root- BAC 449/450/451; Wing tip- BAC 442 Boeing 737-300: Wing root- BAC 449/450/451; Wing tip- BAC 442 mod Boeing 737-400: Wing root- BAC 449/450/451; Wing tip-...

9

All points you mention are correct (for the nitpickers: Point 5 is only true below transsonic speeds). That the pitching moment of the wing is drawn nose-up is probably because it is positive this way - nothing more. The sketch you posted in your question is rather poor, with the center of gravity located behind mid-chord when it should be closer to the ...

9

You can call the two opposite sides suction side (for face on the convex side of the camber curve) and pressure side (for face on the concave side of the camber curve). This has been used in a number of cases- for example in NASA Report CR-1767 Section Data for Thin, Highly Cambered Airfoils in Incompressible Flow: .. flow separation on the pressure side ...

8

When using an airfoil with a sharp leading edge in subsonic regime, you need to adapt the angle of attack so that the stagnation point occurs right on the sharp edge. At every flight condition there should be an AoA which achieves this. Higher AoA, and the stagnation point is too low. To low AoA, and the stagnation point is too high. However flight ...

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