Hot answers tagged

37

The engines may accelerate slightly differently, resulting in an asymmetric thrust. Spooling up slowly at first (normally to about 60% N1) prevents this. After that, you can accelerate up to full TO/GA (or press the button) without any significant asymmetry.


27

A pure engine-side viewpoint: Engines don't really like their power level being changed quickly. Doing so, one increases the thermal and mechanical stress over the engine parts, as well as the probability of engine failure, unstable operation or flame-out. One doesn't want some of these things happening during take-off. Aircraft engines are pretty much ...


26

Your question already contains the answer. As you say kinetic energy is proportional to velocity squared, so it is easier to accelerate air from 0 to 100 m/s than from 100 to 200 m/s. The same is true for the air flowing through the propeller disk. Even if we replace the propeller by a black box, or better a black disk, which simply adds a bit of pressure ...


16

High-powered single prop planes may react violently if the throttle is opened abruptly. Torque reaction and P-factor should be smoothly dealt with...


9

High performance sailboats can sail faster than wind speed in certain conditions. Therefore, your question is not as absurd as it might seem at first glance. However, the technique that enables sailboats to do so is not available to airplanes, at least not in flight. Sailboats use the speed difference between water and wind. Their keel keeps them from being ...


9

Indeed, exhaust gases can be expanded beyond Mach 1, and doing so will result in higher thrust from higher propulsion efficiency. The problem is that pressurised gas is expanded by constricting the cross-section area when lower than M1, and expanding it when higher than M1. So in order to achieve full expansion to ambient pressure $p_0$ at supersonic exhaust ...


8

Yes, thrust force vector must have a force equal to (but opposite) the combined gravitational force and aerodynamic drag force (from velocity) to climb at constant speed. In horizontal flight, thrust equals drag to fly at constant speed. But some of the drag comes from the wing creating lift. Since the larger wing moves "a lot of air a little", ...


7

Short version of (by now) two other answers: a sailing vessel needs a sail in one medium and a keel in another: a foil in the water, conventionally; ropes dragging on the ground; electromagnets in the storm wall of Jupiter's Great Red Spot, in a science fiction story; reaction wheels in inertia, in a solar sail. Slightly longer: it needs two things (airfoil,...


6

Choked nozzle Whether flying supersonic or subsonic, a jet engine's intake, compressor, and diffuser, all slow down the air to slow subsonic velocity so combustion can take place. When the temperature rises sharply, part of this energy is converted to velocity. Because of the high combustion temperature, the speed of sound is much higher, so the mass flow on ...


5

Without knowing more about what you are designing it is impossible to come up with a specific number. But there are a number of factors that you should take into account: the lower the thrust to weight ratio, the longer it will take to accelerate, so the longer the required runway will be. During the take-off roll the lift can usually be neglected, but the ...


5

On smaller planes, the way it was explained to me was a mix of letting the engine come up to speed more easily as well as smoothly applying the left turn tendency from the inertial changes from the prop spin. This left turn tendency is less of an issue on a dual prop plane with counter rotating props, but it's still a lot harsher on the engine and isn't ...


4

It should come as no surprise that flying an airplane is not like driving a car. When driving a car, you can come to a complete stop and nothing exciting will happen. You cannot do this in an airplane; coming to a complete stop is either the beginning or the end of a great deal of excitement. Flying near the ground in any circumstance is a hazard. As you say,...


4

The QRH (Quick Reference Handbook) contains a few tables for flight with unreliable airspeed. The one for holding comes closest to the situation you asked about: (Boeing 747-400 QRH 20.2 - Performance Inflight - CF6 Engines) As you can see, holding at 10000 ft altitude requires between 59.7% and 77.4% N1 depending on weight. You asked about 4 km, which is ...


4

For a glider to move forward using vertical sails, you would have to fix it to the ground somehow, as if it was on tracks, to provide the lateral resistance that allows the reaction force between lateral air movement and the vertical sail to occur. Something like a kite surfer. Replace the kite with a regular glider, with a tether line running from the ...


4

Okay guys, so I found some data from an old service publication from Lockheed. It says that the new C-130J propulsion system under standard conditions and flying 80 kts is rated "4,637 SHP, but produces 10,200 lbs". So using the equation for thrust available for a propeller aircraft I get: $T_a = \frac{\eta \cdot P_s }{V}$ with: $T_a = $Thrust ...


4

You should avoid dated units, such as the gram-force. Now, the area under your graph is in $MLT^{-1}$ units, force x time. These correspond to the variation of momentum $∆MV$ imparted to the air by the prop. Perhaps you could use that area as a reference, when testing other props...


3

I always thought of energy analysis as kind of cheating ;) I know that power input needs to balance kinetic energy increase, but it works like a black box and does not give me true understanding why it is the case. Inspired by Peter's answer I will try to analyze the situation by modelling the propeller as a disk. But not a black disk, but a moving piston ...


3

The general answer to your (general) question is: all airplanes can do that. With every air plane you can accelerate to maximum speed pull the stick until you reach 90°. The problem is that not all planes will be able to recover safely from that maneuver. Most planes will not maintain enough speed to continue the flight in a controlled manor. That's why the ...


3

Something has to provide the energy that is needed to accelerate the air. In the simplest combustion engines this is the combustion heat. Air expands when heated and when the route out the back is the easiest to take, it will flow that way. More complicated engines use this heat energy to raise the pressure of the air inside the nozzle so there is excess ...


3

As pointed out by @xxavier, the conversion between the indicated kilograms and the desired newtons of force is 9.80; more exactly, the local acceleration of gravity, depending on your location and planet. When used as designed, an object is hung on the hook below the scale. The force of gravity pulls on the object and stretches the spring, moving the ...


3

A stall is a stall. When the angle of attack exceeds critical, turbulent drag rises sharply and lift no longer rises with it. It doesn't necessarily imply that you fall; a kite (like a common child's toy) normally flies in a stalled condition. A stall can't happen without atmosphere, because the term implies that aerodynamic lift and drag are changing and ...


3

A "stall" may be terminology going back to the very origins of flight, when early aviators did not fully understand why there was a massive increase in drag (and accompanying loss of airspeed) when AoA exceeded a certain limit. The AoA definition lives on today, but it is interesting to consider airspeed loss due to insufficient thrust or excessive ...


3

In this case it's simply called: fall. In aviation, stall is associated with lift-driven machines, and it seems from your question that you are not talking about this type of machinery. You can find the definition of stall in this pdf document: Glossary of Terms from Flight Research: Problems Encountered and What They Should Teach Us, by Milton Thompson with ...


3

In rocket terms, maximum exhaust velocity is mainly a function of the fuel/oxidizer combination and chamber pressure, and produces a figure of merit called "specific impulse." This is directly proportional to exhaust velocity. Note that nozzle input pressure (chamber pressure, in a rocket) is a major contributor here. A Merlin engine (used in the ...


2

Sail-powered vehicles cannot be faster than the wind they travel in (along the wind direction) and need another medium (ground or water) that offers resistance. An airplane, to be in flight, needs to move forward w.r.t. the air sorrounding it and cannot be in contact with another medium. The two are mutually exclusive. A similar question got a similar answer....


2

It does not work for a couple of reasons. friction on the surfaces creates extra drag a diffuser will lower the pressure, and you're not adding energy to the stream. As a consequence the air going through the diffuser is not capable of overcoming the backpressure exerted by the sorrounding air, leading to a lot of turbulence and no thrust. So you're left ...


2

For the steady-state, uniform condition-- i.e. no changes in wind speed either over time or over space-- we can consider a glider flying within a moving airmass viewed from a reference frame fixed to the ground, as exactly equivalent to a glider flying within a stationary airmass (relative to the ground) viewed from a moving reference frame. There's no way ...


2

An rough estimation follows: in s/l flight, lift = weight and thrust = drag. If we assume a mass of 350000 kg and an L/D of 15, the thrust will be (350000 · 9,8)/15 = 228700 newton...


2

Why not make 94.5% read as 100%? 94.5% tells you the margin you have. It will be closer to 100% in high-thrust versions – or even higher, i.e. the limit is pushed as the thrust ratings increase. The PW1100G you're asking about, its thrust varies from 110 to 150 kN based on the installed version. For example, the A321's 150 kN CFM56-5B3 exceeds 100% near sea ...


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