# Tag Info

## Hot answers tagged theory

59

Anyone who has ever flown at Zar (EPZR) in Poland knows how much a runway slope can reduce the needed field length. For aircraft which need a 2 miles long level runway, a 1 mile long one with a 15% slope at its end will do just fine. Make the slope shallow at the bottom and increase it the further up the runway goes, just like the hillside runway at Zar. If ...

43

There is less gravitational force, but by how much? An insignificant amount. The gravitational force of attraction between two objects is given by, $\displaystyle F_{\mathrm g} = \frac{G m_{1} m_{2}}{R^2}$, where, $G$ is the graviational constant, $R$ is the distance between the object's centers, and $m_{1}$ and $m_{2}$ are the masses of the objects. ...

42

In subsonic flow the air can react to the approaching vehicle since the pressure field around it extends forward, too. This means that a suction area on one side of the fuselage or wing will already start to pull molecules in before they reach the vehicle itself which causes this suction. A round nose allows the flow to come from a range of angles without ...

40

Yes. The wing doesn't care what is causing air to flow past it. Headwind, propwash, 747 wake, gopher sneezes. If the airplane is restrained from forward motion, and the propwash over most of the wing is fast enough, let's say more than the airplane's stall speed, then the wing must generate enough lift to support the airplane, causing it to take off. It's ...

39

Aircraft design is full of compromises. In case of aircraft nose design, the main factor is to reduce drag. As you yourself noted, the main difference is in flow regime, i.e. subsonic and supersonic. There are different types of drag- skin (or friction) drag, form drag and wave drag (which becomes important only at supersonic speeds. For subsonic flow, the ...

38

Short answer: This design will probably work, but it will not be very efficient. It can be tweaked into flying, but when you start tweaking, you would continue such that the outcome would look differently. Now let's look at your questions one by one: How feasible is it to use a propellor larger than the wingspan? Is there any law of physics that ...

37

A glider and a paper airplane operate on the same principle: Exchanging whatever potential energy (altitude) they have for the kinetic energy (airspeed) required to keep air moving over the wings so they produce lift, giving a stable, controlled descent (glide). In both cases the airfoil will not stall unless it exceeds its critical angle of attack which is ...

36

Currently there is no rotary wing craft capable of supersonic flight. Combined with the forward motion through the air, the rotating blades attack the air on one side and retreat backwards on the other. As the aircraft moves faster this poses 2 problems: The retreating blade has a point of zero air speed, starting from the axis at hover and moving to the ...

30

Everyone collectively went "Oh god, not this one" because this same question has sparked some intense debates in the past. Aircraft rely on airflow over the airfoil (wings/tail etc) to produce lift - which is independent of the movement of the tires. This means that with enough air going over the wing, the aircraft will fly even if it isn't moving forward at ...

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

If such a pair of states does exist it ought to be straddling a Mach boundary (I think) e.g. A subsonic & B trans-sonic. Or A trans-sonic & B supersonic etc. Reasoning: To get from any point A to B in parameter space should usually be a state function i.e. path independent. i.e. You can give many tiny increments or one large increment. In our ...

20

Gravity itself @aeronalias is absolutely right. Given the gravitational acceleration of $g=9.81m/s^2$ on the ground, a perfect spherical earth of radius $R_E=6370km$ with homogenous (at least: radially symmetric) density, one can calculate the gravitational acceleration at an altitude of $h=12km$ by $$g(h)=g\cdot\frac{R_E^2}{(h+R_E)^2}= 9.773 \rm{m}/s^2$$ ...

20

The bypass air is accelerated by the fan at the front of the turbofan engine. This changes its velocity and therefore its momentum, which is the definition of a force (in this case: thrust): $$F = \frac{\text{d}}{\text{d}t} p = m \frac{\text{d}}{\text{d}t} v = m \cdot a$$ This thrust contributes to the total thrust of the engine. How much will depend on ...

19

I am a CFI who teaches at a large (+200 students) flight school in the United States. You might be surprised to hear this, but... We really don't worry about how a wing works that much. As far as I'm concerned, the technical explanation for how a wing works is a subject for the engineers who build and design such things. Private pilot applicants (at least ...

17

Clearly the plane must enter and exit through the belly of the larger plane Why so? If you drop this condition, things become much easier, because you could attach the smaller planes externally and avoid folding the wings. And this was indeed tried and even used operationally. The concept is generally termed as Airborne aircraft carrier. The first (?) and ...

16

The primary difference between a glider and a paper airplane is that most folded paper airplanes have very bad aerodynamics. Most folded paper airplanes have short stubby wings. This in itself makes it very draggy. The ideal wing with absolute minimum drag has a wingspan of infinity. Obviously it's not possible to build that ideal wing (we don't have ...

16

What you ask is: If a model glider is thrown at a higher speed than its trim speed, why does it pitch up? Short answer: Because the rear horizontal surface produces less lift per area than the forward surface. When flying at a speed different from its trim speed, the combined center of lift of all surfaces is shifted such that it creates a pitching moment ...

15

Yes. Airplanes get their thrust by using the air. The wheels are not powered. The drag from the wheels will limit how fast the treadmill can go before the plane won't be able to take off anymore. It's simpler to understand if you pick a different frame of reference. Assume the treadmill is standing still but the air is moving around it in any direction ...

14

It is an historical name. The first polars were drawn by Otto Lilienthal in polar coordinates. Here (sorry, German link) we find an example:

13

There is a far more important question (and the real deciding factor) How much fuel is on board? If left unattended a plane will fly until it runs out of gas quite literally, the more gas in the tanks the longer you have. This has happened on occasion before. The Helios Airways Flight 522 is about as close as you can get to this hypothetical situation in ...

13

Yes they are! Aerobatic aircraft can have symmetrical wing to improve inverted performance. So with these aircraft there is no any problem at all. Other agile aircraft, gliders can fly inverted as well. Of course is far from optimal but possible. Don't forget that lift depends of angle of attack as well. So if you fly inverted angle of attack and drag will ...

12

In principle yes, but why would you do it? For vertical take-off this would be grossly inefficient. Lift is produced by deflecting air downwards. This becomes easier as more air is available for deflection since the amount of deflection needed for a given lift can be reduced. However, when all air movement is provided by the propeller, why reduce efficiency ...

11

Yes It's not just weight, it's also aerodynamics. First let's assume that the aircraft was trimmed slightly nose-heavy (no passengers), therefore it's trimmed slightly "tail down" anyway When the stair was first opened, it would have been held partially closed by the airflow under the fuselage - it's not heavy enough alone to extend fully against a 100 ...

11

Yes there are. Although they are not for commercial aviation. One example are solar powered engines: Source Wikipedia Alternatively, there are other means of storing energy - rubber bands, but this one is just for model aitplanes. Source: Pinterest But in either case, you need store energy somewhere, and best way is to use oil-based fuels.

10

I can only speak on behalf of the Australian Syllabus as put out by CASA (Civil Aviation Safety Authority - Australia), but we are expected to teach both Bernoulli's Theorem, and the Theorem of Air Deflection (I believe this is the second theory you are talking about in regards to Newton's Laws). BUT, we are also expected to teach these items as theories, ...

10

A seaplane would be a suitable solution. Then the only infrastructure required on the island is a jetty and fuel/servicing. This also enables a much larger aircraft and so easier cargo resupply and a longer range on the aircraft.

10

Well, for all L/D curves and D curves, the assumption is that the weight of the aircraft is constant and that there is no acceleration. Therefore the lift equals the weight (neglecting the small vertical component of thrust). So lift is a constant in these curves. The rest is simple mathematics; the maximum of $\frac{1}{f(x)}$ occurs at the minimum of \$f(x)...

10

Does this vortex draw in air from above, pulling it downward and as a result lifting the plane up? Yes, the bound vortex in combination with translational flow accelerates air downwards. Or is the vortex shed downward off the tail of the wing? How does this work? A vortex has no beginning and no end. When the wing begins to move, the vortex starts out ...

10

Most aircraft use a cambered airfoil. Such an airfoil only gives you a higher stall speed, otherwise it will just cope fine with prolonged inverted flight. The wing's twist will most likely increase the induced drag since the circulation distribution over span is designed for upright flight, and maybe the airfoil will operate outside of its laminar bucket (...

10

A bypass fan provides thrust in the same way a propeller provides thrust: by increasing the energy content of the gas mass passing through the disk. The added energy is most effectively converted into thrust by allowing it to expand until internal pressure is equal to ambient pressure, so all added energy is converted into kinetic energy. This expansion ...

Only top voted, non community-wiki answers of a minimum length are eligible