# Questions tagged [propulsion]

Questions about the various methods used to propel aircraft.

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### Why does adding more blades to a propeller/fan not show a decrease in efficiency?

According to this answer increasing blade solidity increases the blade area by either enlarging chord or increasing blade count both of which reduce efficiency via reducing the lift coefficient and ...
256 views

### What is a spool in any jet engine, what is its funtion?

I am a student and I'm trying to determine how a spool works in an engine. What effect does it have depending on the number of spools on the performance of the engine? I tried to find it on Google but ...
1k views

### Is it possible for a ramjet to start from 0 velocity?

I was wondering if it is possible for a ramjet to start from 0 velocity. I know its working principle and all, but is there any missile or aircraft that completely uses ramjet, with no rotating part ...
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282 views

### What are the advantages and disadvantages of fluidic thrust vectoring on aircraft?

The Rockwell XFV-12 was a research project for combining supersonic speed with Vertical Take-Off and Landing, From wikipedia: The XFV-12 used a thrust augmented wing concept... in which exhaust would ...
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### Could small ramjets on the tip of propellers allow for supersonic spin of turbine blades?

Well, of course, if it was practical, someone would've already done that, but I'm still curious. As you can imagine, I'm an illiterate on the matter of aerodynamics, and even though supersonic ...
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### Thrust varaition of a Turbofan engine with altitude and mach number [duplicate]

my classmates and I are trying to find a mathematical model for a low bypass turbofan engine, we ended up finding a model given by Snorri on "General Aviation Aircraft Design", but the ...
86 views

### How to determine the length of the CD nozzle?

I am designing a convergent nozzle as well as a CD(Converging-Diverging) nozzle. How do I find out the length of those nozzles? If the throat of the CD nozzle has 0 length then what should be the the ...
755 views

### What is a ram recovery point?

As aircraft speed increases, thrust tends to decrease somewhat; as the aircraft speed reaches a certain point, ram recovery compensates for the losses caused by the increases in speed. The inlet must ...
2k views

### Why does a rocket engine increase power with speed if the burn rate is constant?

A rocket produces constant thrust with speed, and that means increasing power with speed. Where does this increase in power come from if the fuel burn rate is constant? How can we explain this in the ...
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2k views

### What makes an engine suitable for supersonic flight?

I would like to know for example if someone can take the "core" of an CFM56 used on A320, redesign it by adding a supersonic intake, a convergent-divergent nozzle, and reduce its bypass ...
56 views

### Which would be the best placement of an engine for a supersonic buisness jet that has an intake of 2-D ramp? [duplicate]

I would also like to know if possible which would be the advantage/disadvantage of placing the engine under or over the wing for example.
66 views

### Is a ducted fan considered a jet or propeller for the performance parameters?

Right now I am working on a UAV design which will use ducted fan. But the performance calculations are a bit troublesome. For some calculations jet and propeller results are almost same but for the ...
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122 views

### Do aircraft using the push pull configuration always use the same engine?

I am currently doing research for a “white paper design” aircraft and I have always been interested in push pull aircraft as they seem to be rare and I wanted to see some challenges in making a very ...
68 views

### How are Fan pressure of and Static pressure in the turbofan engine related?

If the fan air is ducted to the rear, the total fan pressure must be higher than the static gas pressure in the primary engine's exhaust, or air will not flow. By the same token, the static fan ...
78 views

### Performance metrics of a propulsion system - optimising a propulsive fan system [closed]

I'm designing an electric fan propulsion system for a hybrid-electric business jet. The hybrid business jet uses a series hybrid configuration and distributed electric propulsion. The electric fan ...
141 views

### Relation between weight of the aircraft and mach number [closed]

Is there any formula that relates weight of the aircraft with mach number?
1 vote
89 views

### Is there a limit to a subsonic ramjet's pressure ratio?

Subsonic ramjets have high specific fuel consumption. This indicates to me they must have a low pressure ratio. Is there a limit to how high pressure ratio one can achieve with subsonic air? Why can't ...
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### Can thrust be produced only using the forward speed of a nozzle?

Why can't a nozzle that is already in relative motion to the air produce thrust by speeding up the air that passes through inside it? So let's pretend you're in a glider with two converging nozzles (...
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240 views

### What is the reason for different number of front nozzles on commercial aircraft? [duplicate]

As we observe in commercial aircraft, that there are four or two front end nozzles for gas emission in the air. What is the reason for this variable of 4 or 2 nozzles? Why is it not a constant?
639 views

### Why is thrust said to be constant over speed for a jet engine?

I'm just interested in some basic facts regarding flight. On reading the rather "low level" book "Understanding Flight, David F. Anderson, Scott Eberhardt), I recently came up to the ...
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75 views

### Is hybrid propulsion widely used by UAVs?

Is it common to have hybrid propulsion in UAVs used for terrain mapping with long endurance, short range and altitude? Is the technology and research mature enough to make hybrid propulsion? ...
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1 vote
248 views

### How can jet engines be more efficient than propellers? [duplicate]

If jet engines are smaller in diameter than propellers, is it fair to say that propellers accelerate a lot of air a little while a jet engine accelerates a little air a lot? Basically, a jet engine ...
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1 vote
63 views

### What are some first-order drag approximations for airframe/propwash interactions?

What are some first-order drag approximations for airframe/propwash interactions, both for dynamic flight and for static thrust, in a traditional tractor configuration?
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1 vote
150 views

### How to avoid shock (particularly Oblique shock) inside Cd nozzle?

Using the Area ratio (A/A*) and for the desired exit Mach number I have designed a cd nozzle. The Inlet total pressure (Po) and the total temperature (To) is calculated using Isentropic relations for ...
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### Does jet engine exhaust velocity relative to itself remain constant despite speed changes?

Maybe this is a stupid question. I seem to have been told, the velocity of the exhaust leaving an airbreathing jet engine relative to the engine, is constant despite changes in airspeed. However, to ...
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1 vote
277 views

I am familiar with tip jet powered helicopter experiments. Is there any aircraft that use/used this mode and would it be useful to use a tip jet driven prop on aircraft instead of piston or turbine ...
379 views

### why some airplanes have their exhaust gases vectored above the wing?

I was looking at images of the SkyCourier shown below and i have noticed the asymmetrical exhaust pipes in both engines (one pointing up and one is straight), doing my research on the internet, i ...
335 views

### What is wrong with my understanding about total pressure for a rocket nozzle?

I’m using the ideal rocket equations and isentropic flow but cannot determine what is wrong in my understanding about total pressure. Can anyone explain where I am wrong using the following example? ...
211 views

### How to select and electric propulsion system for my experimental Ultralight?

I've been trying to design and build, my experimental Ultralight using electric propulsion system, My plane designed base on Slaler and Gensis ultralight , following is the primary calculation Empty ...
194 views

### Which is better: a rotor / prop followed by a stator, or contra rotating rotors / props?

It is all in the title. I'm interested in power absorbed/used for a given thrust, or thrust generated for a given power (same diameters of course). Just thrust and power. Ignore materials, electric ...
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### Is a chambered fan more efficient than a ducted fan?

A chambered fan is not exposed to the oncoming airflow in forward flight. I am working on a VTOL design that uses a chambered fan. There will be a wing attached to top of the thrust arms. Could it ...
122 views

### Why are some exhaust cone exposed?

Some engines have an exposed tail/exhaust cone at their exit, protruding aft the nozzle. how does that affect the overall thrust and pressure distribution along the walls of the cone? is it's role ...
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### Are subsonic nozzle fully expanded?

this is about converging subsonic nozzles. Are they fully expanded? meaning exit pressure=ambient pressure? What about diverging subsonic nozzle? Are they fully expanded? what would happen if the ...
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1 vote
235 views

### what is the physical explanation for additive drag?

from what i understand, additive drag is not really drag. it is a way to account for the variation of inlet condition due to conservation of mass. but i still don't get why it create drag(negative ...
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1 vote
269 views

### Why don't we see diverging nozzles or diffusers?

When it comes to fully subsonic propulsion, all outlets are converging nozzles (I did say fully subsonic so no converging diverging). From what I understand, they are flow regulating devices (changing ...
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### Are there modern airplanes with two propellers driven by one single engine?

Except 1903 Wright flyer, are there any other airplane using one single engine driving two coplanar propellers? Coplanar means push-pull and coaxial designs are not in the question's frame. Tilt ...
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### What are the relative efficiencies of the various stages of a counter rotating propeller?

After going through the CFD results of a drone project I am involved in, I noticed that the data shows significantly increased (+10%) thrust generation at the second stage of a counter-rotating, 2-...
1 vote
324 views

### Propeller effect on the L/D of an aircraft (Pusher Propeller configuration)

I recently performed an aerodynamic analysis of a fixed-wing UAV with a pusher propeller configuration. I found out that the L/D reduced by about 15% when compared to the body alone without propeller. ...
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### Why can’t electric motors be used in aviation [duplicate]

Why can’t electric motors be used in aviation? Is it a problem with batteries, torque, or what? From what I’m aware motors at high torque are expensive and need a massive footprint, than a high rpm ...
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827 views

### How fast do four Tire-Propellers have to rotate to lift 2000 lbs? [closed]

I just saw this come out, Goodyear's Tire-Propeller Concept – a combination tire that can also act as propeller (well, more like a ducted fan I guess). There's a neat video that goes with it, ...
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### Why Engine Exhaust Nozzle can not be totally buried in aircraft fuselage? [closed]

Why The CD nozzles of the fighter aircraft are always outside the fuselage, Why the whole engine is not buried in the fuselage?? If engine shroud also can be made variable area and varies with CD ...
223 views

### Can a turbofan engine have less noise so that it doesn't disturb our ears?

A few years back I have seen that Pratt and Whitney 's Geared Turbofan Engine(GTF) has reduced it's noise levels to about 50%. But recently I was having a discussion with a friend and he told me that ...
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### Hydromechanical/Electronic fuel control system [closed]

Can someone explain the picture below which represents hydromechanical/electronic fuel control? What's the function of the solenoid on the picture below? What are the differences between solely ...
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441 views

### Is the Meredith effect utilized in the cowling design of air-cooled engines?

The P-51 is famous for its advanced-for-its-time aerodynamics, the prime example of which is the radiator design under the belly which utilizes the Meredith effect to, in effect, turn it into a mini ...
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### What are the advantages of using a ramjet in an air-to-surface missile?

Which are the advantages of using a ramjet compared to a rocket engine in an air-to-surface missile? I suppose the cost rises a lot, so why do some countries use ramjets as a way of propelling ASM ...
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### How does the debris rejection system in the CFM LEAP engines work?

How does the debris rejection system in the CFM LEAP engines work? LEAP - The debris rejection system
1 vote
1k views

### How is a propeller diameter for given application determined?

let's say I need 1000 N of thrust for a given aircraft and flight velocity. I have available a fixed pitch propeller with a given shape, and thus given performance to advance ratio curves. Let's say ...
1k views

### Can motorjets be a viable means of propulsion?

A motorjet is a very early type of jet engine where intake air passes through a compressor driven by a separate piston engine, and is then mixed with fuel and burned to provide thrust. They were used ...
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483 views

### How does the load transfer from a prop to the airframe?

I was just watching this video... Restoration and startup of a Wright 1820 Cyclone ...and it got me thinking: a propeller shoves back air, and this creates a forward force on the prop. The prop ...
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