# Questions tagged [aerodynamics]

Aerodynamics is the study of how air moves and interacts with solid objects. It is an essential part of aircraft design.

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### What's the relationship between mach number and drag force on an airplane?

From what i've read drag coefficient at mach 1 is several times (up to 10x) the drag coefficient below drag divergence mach number. It drops as the mach number increases. It's easy to find plots ...
27k views

### Does lift equal weight in a climb?

This subject keeps coming up in the discussions and questions such as this one, which asks if lift equals weight in level flight. Good answers there, pointing out that upwards force has many sources. ...
119 views

### Is a stream of air / jet exhaust low pressure?

Inspired by this answer, is a moving stream of air (Jet exhaust, rocket plume, etc) low pressure? I’ve always thought that a higher pressure area (say the combustion chamber of a rocket engine) will ...
520 views

### What is the axis of rotation in a high alpha roll?

When doing an aileron roll at a high alpha (say 25 degs), which axis will the aircraft rotate about? Is it the incoming air-stream vector or is it more the body X axis? And does the pitch inertial ...
100 views

### How do I choose Cl during cruise flight with a given MTOM and variable S?

Currently I'm challenging myself to build an airplane from scratch. I have a background in aeronautical engineering but I have no experience in actually designing an aircraft from the ground up. (FYI: ...
79 views

### Why doesn't the Coanda effect also pull air from inside the jet / streamline?

As explained nicely in this answer, the Coanda effect will make a streamline of air draw from the surrounding air outside of the streamline. This picture shows it nicely : So my question is what ...
3k views

### Why does drag coefficient decrease with Mach number in the supersonic regime?

I'm trying to understand why the drag coefficient decreases in the supersonic regime with Mach number. While it is easy to show this using supersonic, potential flow theory, I'm looking for a more ...
417 views

### Is an airfoil's critical angle of attack measured using "geometric angle of attack" or "effective angle of attack?"

One of the first things we learn about stalls is that they occur when a wing exceeds a certain critical angle of attack, which depends on the specific design of the wing. But if a given wing has a ...
1 vote
33 views

### Does air create a high pressure zone once the low pressure zone is filled in? [migrated]

So say you have a sphere of low pressure air (let's say a 0.5 atm sphere in a 1 atm environment). Air would rush to fill in the low pressure, therefore it would accelerate to the center of it. Wouldn'...
308 views

### Pitch of a glider with altitude

I'm designing a high-altitude glider. Just to clarify a few points before my question: The glider will weigh close to 200 grams. It will be using the A18 airfoil. The glider is dropped from 80,000 ...
892 views

### What causes the "dead band" phenomenon during takeoff on the Boeing 737?

During takeoff, the Boeing 737 encounters a distinctive "dead band" phenomenon after the initial pitch reaches approximately 10 degrees. Essentially, a slight increase in back pressure is ...
230 views

### For a fixed density altitude, how does climb performance depend on pressure altitude?

Below is the climb performance from the 1978 Cessna 152 POH, which I've plotted as climb rate versus density altitude. It shows that higher pressure altitude gives decreased performance even when the ...
124 views

### Conundrum about designing a glider relating to coefficient of lift

I'm designing a high-altitude glider. Just to clarify a few points before my question: The glider will weigh close to 200 grams. It will be using the A18 airfoil. The glider is dropped from 80,000 ...
131 views

### Which is more efficient, thickening an airfoil or adding a bottom-surface curve?

So if you were to take a standard airfoil and increase the thickness, you’d increase the lift (Bernoulli’s principle), but it would also increase the drag. Now imagine you have the same airfoil, but ...
5k views

### How I determine numerically the roll rate of an aircraft?

I'm a Spanish student of aerospace engineering and I'm doing my final bachelor work that consists in designing an unlimited aerobatic plane, and I wonder if it is possible to calculate an ...
57 views

### How I determine numerically the pitch rate of an aircraft?

I have done aerodynamic analysis on a UAV. Is it possible to calculate pitch rate and yaw rate of my UAV depending on design parameters? (I have aerodynamic analysis of the wing, CL, CD, MAC, W, etc.)
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### VTOL Propeller Sizing and Executing a transition from VTOL to Forward Flight mission profile

Hi I am an engineering student, currently in the process of designing a compound UAV drone capable of both vertical and forward flight. I've made significant progress, having completed initial gross ...
58 views

### What happens to a shockwave when it goes through different temp/density air?

Specifically I am asking what happens to a shock as it travels through different density air. This answer's comments has some good information about it. As said in the linked answer, a shock will bend ...
120 views

### How to maximize L/D ratio by design in a high altitude glider? [closed]

EDIT: Some initial confusion with TAS and IAS confused me, I now understand this better, thanks for everyone who helped. I'm designing a high-altitude glider, and I'm wondering how the angle of attack ...
213 views

### Why can you only hear an aircraft after the shockwave passes?

Why do you only hear engines and other noise from a jet going supersonic after the plane has passed? A lot of resources use this picture to illustrate : This is cool and all, but say you changed the ...
1 vote
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### Intersecting Boundary Layers in XFLR5

I am analyzing an airfoil called RevE-HC. While doing a type-1 analysis, I encountered that the BL is intersecting towards the trailing edge. The panels are refined and the geometry is also smooth. ...
655 views

### Why does turbulence over a wing make drag?

Drag is the result of air ‘pushing’ the opposite direction of flight, if you want to think of it that way. The skin friction drag is made from the air trying to pull the wing opposite of the flight ...
189 views

### In a plane with airbrakes, such as a glider, how do slips and airbrakes interact?

While adding a slip to enhance my descent rate, I got to thinking that the airbrakes might be losing effectiveness as the slip twists them out of the relative wind. This thought arose because I did ...
1 vote
232 views

### How are shockwaves able to refract?

How are shockwaves able to refract? As said in this, When two shock waves collide, they interact with each other and produce complex patterns of compression, rarefaction, and reflection. The ...
50k views

### How do wings generate lift?

Just the basic question that every aviation enthusiast must be curious about: exactly how does a wing generate lift?
58 views

### Flow separation in inviscid flow

I watched this video about CFD ( ) where flow was assumed to be incompressible and inviscid, but how can vorticies occur behind the ball at the start of the video, shouldn'...
3k views

### How does the A350 variable camber system decrease drag?

Just curious how the A350 reduces drag/fuel burn by extending the flaps ever so slightly during cruise. Everything I've read about extending flaps says that extending them pushes the centre of ...
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### What should be decalage angle for high lift to drag ratio biplane glider?

Assuming hypothetical biplane variant of a modern conventional glider, wingspan of 8m, 1m separation between the wings, no stagger, no struts, what should be the decalage between the wings assuming ...
1 vote
87 views

### Wouldn't the induced downwash "pull" the wing of a plane down?

I asked a similar question on this recently, (here), and the answer said this: Downwash is not a force. It is a small increment in the velocity vector. Downwash can change the local angle of attack --...
1 vote
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### Elevator / horizontal tail effectiveness at high speeds?

Take for example a jet fighter, when at low speeds vs high speeds, airspeed will differ a lot. As the turning moment is due to the lift force from the tail and lift increases quadratically with ...
3k views

### Why does this curved wind tunnel rotate?

Why does this curved wind tunnel rotate, why is it not just curved? A curved wind tunnel has a radial pressure gradient, but this pressure gradient doesn't exist in a real turn, so how do you get ...
63 views

### How is the pressure constant across the boundary layer but density isn't?

In $P=ρRT$, ρ is density, R is the gas constant, and T is the temperature in Kelvin. In the boundary layer, the temperature and density varies, so why doesn't the pressure? If you have a slip line in ...
399 views

### Is this method to calculate a biplane's center of gravity using 57% of the biplane's gap and 23% of each wing's MAC correct?

Once you have the MAC for both wings and the CG balance point as a percentage of the MAC. connect the two CG positions with a line. Measure up between the wing Gap distance and place your center of ...
157 views

### Does obstruction in one side of a Y/bifurcated inlet can cause asymmetric thrust?

In an airplane with Y/bifurcated inlet If for whatever reason one side of the inlet is obstructed would this cause to roll/yaw or the aircraft would stay stable. And in another setup, If instead ...
1 vote
59 views

### What way does induced downwash make a wing roll?

Say you have a wing, no sweep or anything to simplify things. One side of it, let's just say the left, magically doesn't have a tip vortex, therefore no induced downwash from the tip vortex. Would the ...
1 vote
58 views

### Why does induced downwash vary across a wing in a sideslip?

in this excellent answer, it states this : A well-known effect of wing sweep is the variation of induced downwash along the span from the trailing wake that produces an additional lift distribution ...
247 views

### What happens to tip vortices in a sideslip?

Say you're in a right sideslip. What happens to tip vortices? I'm fairly sure that it would just reduce the strength of the vortex (for the left wing), because there is a force trying to keep the high ...
1k views

### Is it recommended to forward slip an aircraft wth a v-tail?

Is it recommended to forward slip an aircraft wth a v-tail just above stall speed at a maximum angle of attack? A forward slip requires crossed controls, eg. Full right rudder and full left aileron ...
165 views

### Is the math behind my approach to design a propeller right?

I'm trying to design a fixed pitch propeller of diameter 1.8 m flying at 10000 ft going at forward velocity(85 m/s). What I've done is take a series of airfoils (from root to tip), found the ...
628 views

### Does climbing or descending affect the rate of turn?

In the past, I used to have this idea that if all other factors remain constant, then climbing increases the rate of turn while descending decreases it. This idea was most likely planted into my mind ...
71 views

### How do tip vortices make downwash? [duplicate]

As seen in this picture, the tip vortices are said to make downwash. Why is that? I’m also not sure why that’d tilt the lift vector backwards, but that might require a separate question. Link to the ...
1 vote
133 views

### What causes the vapor cone when an aircraft is traveling at M ≤ 1? [duplicate]

What causes the vapor cone of an aircraft going M ≤ 1? I know that the vapor is from the air expanding, but why is it in the cone shape? What shock system forms it into that shape?
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### Why is the wave drag of a straight wing less than that of a swept back wing?

According to NASA's claims, when the swept back wing is inside the shock cone and the shock is not attached to the leading edge of the wing(subsonic leading edge), the wave drag of the swept wing is ...
1 vote
103 views

### Why don't shockwaves expand rearwards when they expand out?

Why don't shockwaves expand rearward when they expand out? In this for some reason really big picture, you can see the shocks expanding out past the actual plane. Why don't they also expand rearward? ...
3k views

### Why are supercritical airfoils not used in GA aircraft?

Why are supercritical airfoils not used in GA aircraft? The 787 is assumed to use a supercritical airfoil, so why don't GA aircraft use them? My guess is that they're harder to manufacture, and a lot ...
199 views

### What are the effects of wing washout and anhedral on induced drag?

I am studying for my ATPL and was curious if and how induced drag varies based on wing anhedral and geometric washout. I have a basic understanding that spanwise flow affects induced drag since there'...
14k views

### Is it possible for an airliner to safely fly with doors open?

From another question asks about the possibility of dropping bombs from converted airliners. My question: is it possible to safely fly aircraft with a door open for the whole flight envelope? Would ...
1 vote
61 views

### How does a shear layer make vortices?

How does a shear layer make vortices? If you have 2 streams of air that are moving at different speeds or opposite directions right beside each other, how would that make a vortex? What causes the ...
1 vote