# Tag Info

## Hot answers tagged theory

58

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 ...

58

Let me first give some background information on the art of inflating an evacuation slide. There are two ways to fill them: Expand gas from a high-pressure bottle: This will cool the gas as it expands, so ice may form at the aspirator outlets where sucked-in air and the expanding gas mix, blocking the flow of outside air into the slide. Also, storing ...

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. ...

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 ...

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 ...

36

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 ...

33

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 ...

31

The $\text{CO}_{2}$ is used as a diluent. The pressure vessels (for inflating the slides) usually have nitrous oxide (or similar) compounds, that undergo exothermal thermal decomposition during inflation. For example, $\text{NO}$ reaches temperatures of $\sim 650\,^{\circ}\text{C}$ during thermal decomposition. As $\text{CO}_{2}$ absorbs heat during its ...

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 ...

28

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 ...

25

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, ...

21

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 ...

21

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 $$g(h)=g\cdot\frac{R_E^2}{(h+R_E)^2}= 9.773 \rm{m}/s^2$$ ...

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 ...

15

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 ...

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 ...

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

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

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 ...

12

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 ...

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 ...

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

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 (...

9

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)...

9

The Republic XF-84H "Thunderscreech" was a USAF jet fighter modified with a turboshaft engine and a propeller designed to operate at supersonic blade speeds. It first flew in 1955, and the result was a noise that was literally deafening. During ground engine runs, "the prototypes could reportedly be heard 25 miles (40 km) away." Whereas there may be some ...

9

If the autopilot was on, it's a question of how much fuel is on board as related in another answer (unless high ground was in the way). If the autopilot is off, even if in perfect trim, the aeroplane will usually gradually drop a wing (this process will start within 15-30 seconds) and develop an ever steepening spiral dive and crash at high speed into the ...

9

In order to keep an aircraft flying, you need to keep adding energy. In an aircraft with an engine, the energy is primarily in the form of fuel. In a glider, the additional energy comes from the lifting action of rising air. Glider pilots will find areas of rising air (they have sensitive instruments to assist with this), and circle in that area to increase ...

9

The glider and a paper aircraft are designed and operated in different ways which affects their stalling characteristics. Both aircraft trade one form of energy (potential energy or altitude) in order to gain another (Kinetic energy or forward speed). In general, the gliders are in (a controlled) descent unless they encounter thermals or rising air. This ...

9

"Docile" means no big pitching moments over the angle of attack range. Since the control power with fully stalled control surfaces is rather small, the airframe must not create strong pitching moments, but keep a small pitch-down moment so the airplane returns eventually to normal angles of attack which allow regular flight. The maneuver is dynamic, so the ...

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