Questions tagged [theory]
Questions about the theoretical aspects of aviation.
52
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1answer
143 views
Is it incorrect to say that “passenger jets like to cruise at higher altitudes because there's less drag”?
I hear it stated quite often that aircraft like to fly high because there's less drag at higher altitudes.
$$ \text{Range} = \frac{V}{g} \frac{1}{\text{SFC}} \frac{L}{D} \ln \left( \frac{W_\text{...
3
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2answers
151 views
Is the well known textbook-formula for net-thrust oversimplified or do I have a misinterpretation?
In order to understand how Jet-Engines works I came across the "famous" formula for net-thrust:
https://en.wikipedia.org/wiki/Turbojet#Net_thrust
$F_{thrust} = Q_{out} \cdot v_{out} - Q_{in}\...
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2answers
132 views
Why is the aerodynamic center for most airfoils located at the quarter chord location?
I've been reading several sources on the location of the aerodynamic center
https://web.mit.edu/16.unified/www/FALL/fluids/Lectures/f03.pdf
https://en.wikipedia.org/wiki/Aerodynamic_center
John ...
2
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3answers
204 views
Could gliders be a clean, practical alternative to other forms of transport?
Would it be possible to use pure gliders as an eco-friendly mode of transport?
From what I have read so far, gliders can travel at a speed of 100-170 km/h. That would not be able to compete with a jet ...
12
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5answers
5k views
Could propeller wash provide sufficient lift to take off - even in theory?
I know this seems a crazy thing to ask but it is not the same as lifting oneself by the bootstraps. If you try to lift yourself that way, there are two equal and opposite forces, yet there are ...
2
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2answers
191 views
Is the Batjet Design Physically Feasible?
I realize the plane is fictional. However, in the Batman Animated Series by Warner Bros, we see this Batjet.
So this got me thinking (and I'm not a physicist): is this air frame configuration even ...
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0answers
41 views
How to determine the number of copression stages given a compression ratio?
I have a personal project to design a gas turbine engine, the main problem that nags me is how do I determine the pressure rise in each stage of an axial compressor?
Say, I'm planning on a 38:1 engine ...
0
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1answer
87 views
Why does Vmca decrease with increasing altitude?
A question from a mock MEP class rating test:
Many thanks for your help.
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0answers
62 views
Is there any bibliography for triplane aircraft design?
I've been tasked with designing an RC aircraft that has the greatest lift force for a specified wingspan, total length and takeoff velocity, and thus have been studying the different wing ...
8
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3answers
3k views
How does the bypass air provide thrust?
To my knowledge, bypass air produces 80% of total thrust. But I don't understand to how it does that. By accelerating the air, by increasing the speed or increasing the pressure of the air? Is it ...
2
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1answer
296 views
Would adding a slat and a slotted flap increase the max CL of a multi element airfoil to 6.5?
Would adding a slat and a slotted flap increase the of a multi element foil to 6.5?
I understand, as a general rule, the max $C_L$ for a classic NACA airfoil is as follows:
Base: 1.5
Slat: + 0.7
...
0
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1answer
165 views
Pitching moment as a pure moment and its measurement with a homemade force balance
In mechanics, we learn that a force $F$ acting at a certain application point $P$ can exert a torque (also called moment of the force) about another arbitrary point $Q$ in the plane, i.e. $\tau= F \...
0
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1answer
83 views
Would turbine blade twist change if all conditions except the density of the fluid remained the same?
While designing a turbine, the blade twist is apparent from the velocity triangles, and this analysis only depends on the velocity.
However, would there be any difference for different media? When ...
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0answers
61 views
What is the relation between solidity and lift coefficients?
I've seen this graph in a couple of places. I'm not sure what it's supposed to mean, as the formulae I've seen in the papers cannot be found in any standard book or reference.
Please let me know if ...
0
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1answer
371 views
How to refute this geometrical method to find MH370? [closed]
A geometrical method to find MH370 has been published on the Net (see images below).
I am sure it is wrong (because if it were that easy, thousands of people would have had this idea back in 2014). ...
0
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2answers
523 views
What's a typical first lesson of glider flying like?
What is a typical first lesson for glider flying like? Does it usually include a pre-flight mini lecture on basic theories at all, or if not pre-flight, do instructors usually spend some time on the ...
3
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1answer
159 views
Vortex Lattice Method with eliptical panels
I implemented vortex lattice method (or deeper here) into my aircraft simulation program. The problem is that sometimes trailing vortex filaments from Horse Shoe Vortexes of main wing hits panels of ...
2
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4answers
269 views
What is formula for induced drag in stalling regime?
I'm wondering what is the formula for induced drag in a stalled regime, i.e. in a regime where the $C_L$ (coef. of. lift) has started to decrease but is still nonzero.
I've a feeling that the ...
4
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2answers
286 views
If the the body and it's wing is mostly covered with solar panels and flies with electric motors, is it possible for that aircraft to fly?
First before I say anything else, this is indeed a stupid question.
It would be most likely heavy, bulky, and expensive.
But theoretically, is it possible to fly with only Solar energy?
What does it ...
10
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3answers
4k views
What is the physical meaning of circulation found in Kutta condition?
The Kutta–Joukowski theorem is applicable for 2D lift calculation as soon as the Kutta condition is verified. When this is the case, there is a circulation $\small \Gamma$ around the airfoil. My ...
4
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1answer
11k views
How are Cd and Cl calculated from Cp data?
Given pressure coefficient data for corresponding x/c (position along chord) values on a wing, I am looking to calculate coefficients of drag and lift. Here's what I have: b, MAC, S, sweep angles at ...
2
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1answer
267 views
How to demonstrate that an elliptical circulation distribution can be induced by an elliptical chord distribution?
I intend to demonstrate that a wing with an elliptical chord distribution, without torsion ($\alpha(z)=\alpha=\text{const}$) and constant section ($\beta(z)=\beta=\text{const}$, and $a_{2\text{D}}(z)=...
3
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1answer
1k views
Am I correctly calculating the density altitude?
I came across the following question while studying for the FAA PPL written test:
Determine the density altitude for these conditions:
Altimeter setting: 30.35
Runway temperature: +25°F
Airport ...
12
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6answers
7k views
Why does an aircraft pitch up when the speed increases?
Can anyone please explain why aircraft pitches up when the speed increased? (please consider an aircraft, Wing AC, CG and Tail AC lies on a line.)
The ideal explanation I'm looking for should have ...
6
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2answers
425 views
Can a gyro-copter soar?
If one centers the gyro-copter in a thermal and then simulates a hover facing headwind; can the gyro-copter climb up like that ?
Is it technically possible or is it advisable....
2
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3answers
823 views
Is there any engine that doesn't use a propellant to produce thrust?
As far as I know, all engines used in aircraft produce thrust by means of some propellant: kerosene, hydrogen peroxide etc. Maybe I've missed something, but could you tell me, are there any engines ...
38
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7answers
22k views
Is gravity less on an airliner at cruise altitude?
Is the force of gravity less on an airliner at cruise speed and altitude? I'm not talking about a special reduced gravity flight with a parabolic flight path, just a typical long distance flight.
...
10
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5answers
3k views
Are aircraft capable of sustained inverted flight at constant altitude?
Inverted flight as a part of manouveres, dogfights, acrobatics is fairly common but my question is:
Are any aircraft capable of sustained inverted flight without losing altitude? e.g. Could you fly ...
12
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1answer
740 views
What are the relative braking contributions from wheel brakes, spoilers and thrust reversers?
What is the relative contribution to braking forces in a typical large commercial aircraft from the three different devices contributing to braking after touchdown:
Wheel brakes
Speed brakes / ...
7
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2answers
1k views
Would a 90 kg object jettisoned from an aircraft create a detectable movement?
I was reading the description of an old hijacking which has this snippet of interesting information:
After takeoff Cooper told Mucklow to join the rest of the crew in the
cockpit and remain there ...
46
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2answers
23k views
Why/when is the blunt nose better?
Most large aircraft and some fighters have blunt, rounded nose cone. OTOH many fighters (that don't use the nose for air intake) and notably the two supersonic airliners (Concorde, TU-144) have the ...
7
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1answer
678 views
Do frequency response / Bode plots for an aircraft or helicopter come from modeling or empirical testing?
I was intrigued while reading an article by this snippet:
Examination of the V-22’s frequency response plots (Bode plots) in
2000 showed that the aircraft failed to meet the basic stability
...
4
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1answer
621 views
Where does upward momentum come from in leading edge vortex?
I believe I am beginning to understand how a leading edge vortex creates lift. Faster air creates a lower pressure above the wing making lift.
But I am hoping to get another view of this in terms of ...
11
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1answer
1k views
What is the meaning of “Docile pitch behavior” in the context of Pugachev's Cobra?
In another answer on SE Aviation it is mentioned that the ability to perform Pugachev's Cobra in a non-thrust-vectored aircraft needs "docile pitch behavior of the airframe up to approx. 110° angle ...
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6answers
3k views
What is the optimal bank angle to accomplish a given turn in a glide?
For an aircraft in a glide, what—if any—is the optimal bank angle to make a given turn while maximizing glide distance and maintaining published best glide speed?
In other words, at a constant ...
6
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2answers
405 views
Would it be feasible for a typical passenger jet to carry along its own Oxidizer?
We know that many Space Shuttles / Rockets etc. carry their own oxidizer with them. This is partly because at the heights they operate the atmosphere is too rarefied to have enough oxygen to sustain ...
11
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6answers
26k views
Can planes take off from a treadmill?
Note that this is more of a Physics question than an Aviation one.
This topic has been extensively covered on the Physics Stack Exchange site, and the answers there are recommended reading for ...
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11answers
18k views
Why can gliders fly for so long?
When you throw a paper airplane, it flies for a couple of seconds before losing speed and stalling. With a glider, it has a very long wingspan, but you take off with another plane or winch tow pulling ...
22
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4answers
3k views
Would an aircraft with contra-rotating propellers longer than the plane's wingspan be able to fly?
I enjoy taking the time to try and visualize how fictional aircraft might actually work in reality. Unfortunately there are some that seem highly impractical from an engineering student's point of ...
26
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8answers
4k views
Is there an aerofoil that gives reasonably good lift for both air flow directions: forward and backward?
Could an aerofoil be designed to give reasonably good lift for both flow directions?
I was motivated by this related question on Aviation SE: Can a plane fly backward if its propeller is in reverse?
...
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3answers
4k views
How long would a cruising aircraft take to crash if everyone spontaneously vanished?
I recently saw a movie in which what appeared to be a Lockheed C-130 Hercules was suddenly without occupants. They did not move or exit the craft, they simply vanished. I assume that the craft ...
18
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2answers
2k views
When does the use of afterburners save fuel?
An afterburner uses a lot more fuel for a little more thrust (rule of thumb: 50% more thrust for 5 times the fuel flow with full dry power). Is a flight maneuver possible where using the afterburner ...
7
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1answer
1k views
Are Aircraft yokes self centering in non-auto pilot mode?
In a typical car steering wheel if you release it while the car is in motion, it will return to its center, neutral position.
What's the corresponding situation in an airplane? Is a yoke self ...
18
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7answers
5k views
How to build a 2 mile long runway on a 1 mile square island?
On Puzzling.SE I found this question:
You have a 1 mile long x 1 mile wide private island that you wish to
turn into a resort. A plane requires a 2-miles long runway to take
off. What do you do?...
30
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4answers
6k views
Principle of aerodynamic lift: are misconceptions also taught in flight schools?
Describing lift as the result of "equal transit time" on both sides of an airfoil is a fallacious theory widely found in technical books and articles for general public (see below for details about ...
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1answer
529 views
Is there an equation which allow us to perfectly match a helix propeller and an engine?
Is there an equation which allow us to perfectly match a helix propeller and an engine? I'm looking for an equation that allow me to correctly dimension an helix propeller for my engine. I mean that ...
6
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2answers
4k views
Is the maximum lift-drag ratio found at minimum drag?
Figure 10-5 of the FAA's Pilot's Handbook of Aeronautical Knowledge shows:
I didn't think $L/D_{MAX}$ coincided with $D_{MIN}$. Is this Figure accurate?
10
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1answer
509 views
Why are the power and drag curves called polars?
See Wikipedia:Drag polar and Wikipedia:Polar curve (aviation) for example.
These curves are not on a polar coordinate system. Why are they called polars?
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2answers
9k views
What is the definition of “best L/D”?
What does "best L/D" mean? Is it meaningless without context? Do you need to know "best for what?"
Or, is it synonymous with max L/D?
13
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1answer
855 views
How can I get the velocity-power curve for a particular aircraft?
The answer for How does wind affect the airspeed that I should fly for maximum range in an airplane? refers to a velocity/power-required curve.
As far as I can tell, this curve can't be deduced from ...
