Induced drag

bravoalphakilo

bravoalphakilo

Well-Known Member
Dear All,
This might be a very silly simple question. But i was asked the same in a interview so posting a few from that interview.

1. How can one profile(depict) a induced drag in a wing??ways to reduce induced drag?
2. Vortex generators - difference between the ones used in mil a/c's and civilian a/c's
3. Can a disturbance travel upstream in a supersonic flow??if yes why if no why?
4. flow generators in the engine(g/t engines)
5. shocks - different between subsonic and supersonic shock? - why are the shocks in a supersonic flow detached from the surface?
and many more...


Thanks in advance mates.
 
bravoalphakilo said:
How can one profile(depict) a induced drag in a wing??ways to reduce induced drag?
Click to expand...

You can tilt the lift vector slightly towards the rear of the aircraft. The horizontal component of this vector is the induced drag.

Vortex generators - difference between the ones used in mil a/c's and civilian a/c's
Click to expand...
The military ones are a thousand times more expensive?

Can a disturbance travel upstream in a supersonic flow?if yes why if no why?
Click to expand...
No. The disturbances flow at the speed of sound, so they can make no headway against flow that is traveling as fast or faster in the opposite direction.


flow generators in the engine(g/t engines)
Click to expand...
I don't understand the question.


shocks - different between subsonic and supersonic shock?
Click to expand...
There are many differences...are you talking about oblique vs normal? There are changes in pressure across the shock waves, density, temperature, etc.


- why are the shocks in a supersonic flow detached from the surface?
Click to expand...
If you're talking about the bow wave, it forms where the supersonic airflow transitions to subsonic. Since an area of high pressure forms in front of the leading edge of the airfoil, it has the effect of decelerating the air ahead of it, causing it to become subsonic. As the aircraft velocity increases, the bow waves moves closer and closer to the airfoil and would eventually become attached if the leading edge radius were zero, which is obviously impossible.
 
tgrayson said:
You can tilt the lift vector slightly towards the rear of the aircraft. The horizontal component of this vector is the induced drag.

The military ones are a thousand times more expensive?

No. The disturbances flow at the speed of sound, so they can make no headway against flow that is traveling as fast or faster in the opposite direction.


I don't understand the question.


There are many differences...are you talking about oblique vs normal? There are changes in pressure across the shock waves, density, temperature, etc.


If you're talking about the bow wave, it forms where the supersonic airflow transitions to subsonic. Since an area of high pressure forms in front of the leading edge of the airfoil, it has the effect of decelerating the air ahead of it, causing it to become subsonic. As the aircraft velocity increases, the bow waves moves closer and closer to the airfoil and would eventually become attached if the leading edge radius were zero, which is obviously impossible.
Click to expand...

Ahaa..cool!!!
Induced drag: same answer as I gave +1 for me :beer:
Vortex generators: I answered: civilian: winglets; Mil(fighters): some sort of projections on the wing surface. I didnt know the exact name so just answered this.
Disturbance travelling upstream: I was wrong. Didnt think logically. Simply speaking if an a/c is travelling at supersonic speed towards us, then we cannot hear it coming untill it crosses us.
Flow generators: I think what he meant could be the splitter plates at the intake nacelle of fighters seperating the actual intake and the fuselage(f-16, F/A-18 SH intakes)
Shocks: the basic normal vs oblique, he just wanted a diagrammatic explanation that's it. I answered normal and illustrated it then drew a oblique on a sharp supersonic aerofoil. so should be correct.
Shock detachment: I am confused as whether they detach from the surface or they get attached to it? I remember to have read in L.J.Clancy's aerodynamics that it gets detached due to some pressure difference or probably i read it wrongly. Some body clear me in this regard.
 
bravoalphakilo said:
Dear All,
This might be a very silly simple question. But i was asked the same in a interview so posting a few from that interview.

1. How can one profile(depict) a induced drag in a wing??ways to reduce induced drag?
2. Vortex generators - difference between the ones used in mil a/c's and civilian a/c's
3. Can a disturbance travel upstream in a supersonic flow??if yes why if no why?
4. flow generators in the engine(g/t engines)
5. shocks - different between subsonic and supersonic shock? - why are the shocks in a supersonic flow detached from the surface?
and many more...


Thanks in advance mates.
Click to expand...

Hey there Bravoalphakilo,

Tgrayson gave you some great answers there, but your questions are a little vague, so it's difficult to answer.

For question 1, I think the guy asking you may have been asking about the "drag profile" for induced drag, or essentially the relationship between induced drag and velocity. It isn't a linear relationship, but as velocity increases induced drag decreases. The curve "opens" upward, to help you picture it. The second part of the question has a really easy answer then: you can decrease induced drag by speeding up.

I think Tgray's answers to #2 and #3 pretty much say everything there is to say on the subject. Vortex generators change the airflow from laminar (smooth) to turbulent. Generally turbulent flows are bad from a drag perspective... but they also separate later than laminar flows. Since separation is also bad from a drag perspective, it is kind of a case of the lesser of two evils. But as far as I know, no difference between military and civilian versions.

I have never heard of a "flow generator" in a gas turbine engine, either.

Like Tgray, I have a terminology problem on your question about "subsonic" versus "supersonic" shock waves. In my understanding (which is not perfect-- it's been 12 years since I really studied this stuff) when shock waves form the flow is at least locally Mach 1 or above, even though the aircraft may be below Mach 1. A good example of this is the flow over a wing when the aircraft is in the transonic regime. The aircraft could be below Mach 1, say .9 Mach for example, but because the airflow speeds up as it travels around the wing it may hit Mach 1 (or greater) at some point along the wing. As the top surface of the wing begins to bend away from the direction of the flow a shock wave will form. The laymen's explanation that I have always heard for why this occurs is because the supersonic flow cannot follow the curve of the wing and begins to separate and expand. This is the only context that I can place for the term "sub-sonic" shock wave.

For your last question, Tgray talked about shock waves being attached or detached. Basically, shock waves are "attached" when the body has a pointed leading edge-- imagine a sharpened pencil moving through the air at high speed (above mach 1) for example. When the leading edge is blunt, like the nose of the space shuttle, the shock wave is "detached."

Detached shock waves are curved and depend on the shape of the body forming the wave. Still, for some small portion of the shock wave the angle between the wave and the direction of the flow is 90 degrees. At this point, the flow following the shock wave becomes subsonic.

Attached shock waves (aka oblique shock waves) form at an angle to the flow that is greater than 0 (parallel the flow direction) but less than 90. Across those types of shocks, the flow still slows down, but not to below Mach 1. The equations that describe the pressure, density, and temperature changes across the two different types of shock waves are a little different as well.

One last point on shock waves... Tgray already brought this up, but his explanation may have been a little technical. Imagine that pencil example from before. The tip is pointed, so of course an attached shock wave forms at high Mach. Now imagine that you zoom in on the point of the pencil. No matter how sharp the pencil is, if you zoom in far enough it will look blunt. So really the "attached" shock wave in this case, is not really attached. There is still that "normal" region, even with a very sharply pointed object. It is just very small. So when dealing with real objects, scientists ignore the very small region where the shock wave behaves "normal" in many calculations. That is what Tgray meant about the radius becoming zero. No objects are PERFECTLY pointed all the way down to the atomic scale, but it is a close enough approximation to treat them that way for many applications.
 
bravoalphakilo said:
Ahaa..cool!!!
Induced drag: same answer as I gave +1 for me :beer:
Vortex generators: I answered: civilian: winglets; Mil(fighters): some sort of projections on the wing surface. I didnt know the exact name so just answered this.
Disturbance travelling upstream: I was wrong. Didnt think logically. Simply speaking if an a/c is travelling at supersonic speed towards us, then we cannot hear it coming untill it crosses us.
Flow generators: I think what he meant could be the splitter plates at the intake nacelle of fighters seperating the actual intake and the fuselage(f-16, F/A-18 SH intakes)
Shocks: the basic normal vs oblique, he just wanted a diagrammatic explanation that's it. I answered normal and illustrated it then drew a oblique on a sharp supersonic aerofoil. so should be correct.
Shock detachment: I am confused as whether they detach from the surface or they get attached to it? I remember to have read in L.J.Clancy's aerodynamics that it gets detached due to some pressure difference or probably i read it wrongly. Some body clear me in this regard.
Click to expand...

Actually winglets are not vortex generators. Google "vortex generators" and go to "images" and you will see some examples of aircraft with protrusions that look like little fins that stick up from the wings. They are the same on both civilian and military aircraft. Actually, I can't think of any fighters off the top of my head that have any... but my KC-135 (a variant of a Boeing 707) has a bunch on the horizontal stab.

The "splitter plates" on the fighters that you mentioned are usually called "boundary layer separators." Their function is to ensure that the air entering the engine on fighters is out of the boundary layer from the skin of the airplane. BTW, the boundary layer is the region of the airflow where viscous effects are significant. Or to put it less technically, the air right next to the skin of the airplane slows down due to friction. This friction region grows bigger and bigger as the flow moves down the aircraft. Having those boundary layer separators near the engine intake helps to prevent most of that air from going into the engine.

I think the rest of it is already addressed.
 
Thankyou fish314 and thankyou tgray. In certain question even I was confused as to what the interviewer exactly wanted. There are few more questions in aerodynamics and in gasturbines, i am trying to remember them and post them here. so as and when i remember something i will put it across, probably it might save someone's day somewhere.

one question on G/T
1. How are the H.P compressor & turbine blades cooled?
a. bleed from L.P.
b. ram/by-pass
c. not cooled at all

2. The dove tail assembly to fix a blade is used
a. simplicity in assembling the blades
b. ease of (line)maintenance
c. avoid FOD
 
tgrayson said:
According to my jet engines book, the turbine blades are cooled by compressor bleed air. I didn't see any information that indicated that the high pressure compressor blades are cooled at all.
Click to expand...

I know that at least some turbine sections work that way for sure. (I don't know about ALL of them, though)

As for compressor sections, I don't think they are cooled by anything. They get hot, but probably no where near as hot as the air coming out of the combustion chamber towards the turbine section.
 
Sidious said:
Im interested as well
Click to expand...

Dear fellows,
Its was a interview for the post of research fellow for an under development fighter jet. thats all i am allowed to say, i am awaiting the result. hehehe
:D

Fish314: I have had this doubt from the day I learned about g/t engines. The turbines are located behind the combustion chamber, which is the max. temp region in the whole engine, so won't there be an effect on the blades? there is a nozzle which cools down the exhaust gas before it reaches the turbine, but still, imagine the temp at which the combustion takes places and how much can that gas be cooled before it reaches the turbine which would be just a few inches/couple of feet away, which means there has to be some way or other to cool the blades atleast the H.P ones..also i have learned during my g/t classes that blades are cooled using the inline lubrication or is it that they are used for cooling the shafts and hubs?? correct me. It would be of great help if some g/t mechanics can help me out.

Today's question:

Magnetic analysis is carried out on:
1. steel/ferrous
2. aluminium
3. titanium
4. carbon-fibre
 
I would never do this if I wanted the job, but it would be tempting to ask "what would you say is the practical value of knowing this information besides trying to stump people during an interview".
 
:yup:

From my point of view, I have finished the interview so I can't do anything about it. But for others they might know that even such questions do exists and can be asked in case any of us are going to be in a similar situation. Rest, it will surely be a brush up of the topics being discussed. As tgray brought a answer for the jet engines, by saying my engines book says "bla bla bla"..at times makes you get back to your books, and rest I believe you understand.
 
You guys are too smart for me. I feel like the big fat guy from the trailer park sitting in on a nuclear physics international committee meeting. :beer:

Pretty cool stuff though, too bad my training didn't include anything related to supersonic flight other then it exists.
 
Re: 2. Vortex generators - difference between the ones used in mil a/c's and civilian a/c's

what came to my mind is maybe the interviewer was looking for the difference in the application on high speed and low speed aircraft.

in both cases, the vortices they generate help maintain an attached airflow to a lift producing surface. in small aircraft, this is to lower stall speed and mitigate its effects when the wing starts stalling.

in high speed aircraft, the same is true, but it is to raise the speed of a mach stall and mitigate the loss of lift when the airfoil goes critical. as the aircraft passes critical mach, there is a resultant loss of lift on the airfoils (wings AND tail feathers). the vortices help keep the airflow attached. they are often located forward of a control surface that are used at high altitude, as deflection of that surface will lower the aircraft's speed at which the local airflow speed goes supersonic.

so you may see them forward of ailerons on jet aircraft that have no roll spoilers, but not on aircraft whose ailerons lock out at high speed - unless of course there were other high speed issues with the wing that necessitated them. they are also likely to be found by the rudder and under the horizontal stabilizer as these control surfaces can wash out at high speed due to the separation from the shock wave.

i bet this is what the recruiter was looking for. in both cases, though, they are to reduce and mitigate the effects of stalls/airflow separation.
 
stratoduck said:
Re: 2. Vortex generators - difference between the ones used in mil a/c's and civilian a/c's

what came to my mind is maybe the interviewer was looking for the difference in the application on high speed and low speed aircraft.

in both cases, the vortices they generate help maintain an attached airflow to a lift producing surface. in small aircraft, this is to lower stall speed and mitigate its effects when the wing starts stalling.

in high speed aircraft, the same is true, but it is to raise the speed of a mach stall and mitigate the loss of lift when the airfoil goes critical. as the aircraft passes critical mach, there is a resultant loss of lift on the airfoils (wings AND tail feathers). the vortices help keep the airflow attached. they are often located forward of a control surface that are used at high altitude, as deflection of that surface will lower the aircraft's speed at which the local airflow speed goes supersonic.

so you may see them forward of ailerons on jet aircraft that have no roll spoilers, but not on aircraft whose ailerons lock out at high speed - unless of course there were other high speed issues with the wing that necessitated them. they are also likely to be found by the rudder and under the horizontal stabilizer as these control surfaces can wash out at high speed due to the separation from the shock wave.

i bet this is what the recruiter was looking for. in both cases, though, they are to reduce and mitigate the effects of stalls/airflow separation.
Click to expand...

:dunno: May be you are right. well, now I remember there was a "buy one get one" offer on this vortex question, it was, explain the difference between levcons and ailerons + elevators + slats and advantages of levcons for vortex generation and high speed wing profile.
As far as the questions go, they were quite confusing as I have mentioned earlier itself. I have tried my level best. Anyways, few more I remembered,

Explain mach-cone, mach-angle.
Characteristics of delta wing, what are canards.
these were quite straight forward questions, but in relation with mach, there was another question, critical mach number, though I answered the definition part of it, they asked to plot it on a airfoil which I couldnt do to their satisfaction.
next was, explain compressor stall, measures to prevent it.
micro-pulse jets, anybody heard of it??
 
bravoalphakilo said:
Dear fellows,
Its was a interview for the post of research fellow for an under development fighter jet. thats all i am allowed to say, i am awaiting the result. hehehe
:D

Fish314: I have had this doubt from the day I learned about g/t engines. The turbines are located behind the combustion chamber, which is the max. temp region in the whole engine, so won't there be an effect on the blades? there is a nozzle which cools down the exhaust gas before it reaches the turbine, but still, imagine the temp at which the combustion takes places and how much can that gas be cooled before it reaches the turbine which would be just a few inches/couple of feet away, which means there has to be some way or other to cool the blades atleast the H.P ones..also i have learned during my g/t classes that blades are cooled using the inline lubrication or is it that they are used for cooling the shafts and hubs?? correct me. It would be of great help if some g/t mechanics can help me out.

Today's question:

Magnetic analysis is carried out on:
1. steel/ferrous
2. aluminium
3. titanium
4. carbon-fibre
Click to expand...

Hey the BAK. Sorry I haven't gotten back to this in several days. The way that turbine blades are cooled that I am familiar with is that some bleed air is taken off the compressor section and fed through the center of the turbin blades themselves. This air comes out of tiny pin-holes in the turbine blades, to create a very thin zone of air around the turbine blade that has a lower temperature than the surrounding air.


As for your question of the day, I would have to say "1. Steel/ferrous" as my guess, because I think there are only about 3 metals out there that can be magnetized: Iron, cobalt, and I forget the third.... Nickel, according to wikipedia (and they say some rare earth metals can also be magnetized). Therefore, I would choose #1 since steel is an alloy of iron, and "ferrous" means "composed of iron."

bravoalphakilo said:
:dunno: May be you are right. well, now I remember there was a "buy one get one" offer on this vortex question, it was, explain the difference between levcons and ailerons + elevators + slats and advantages of levcons for vortex generation and high speed wing profile.
As far as the questions go, they were quite confusing as I have mentioned earlier itself. I have tried my level best. Anyways, few more I remembered,

Explain mach-cone, mach-angle.
Characteristics of delta wing, what are canards.
these were quite straight forward questions, but in relation with mach, there was another question, critical mach number, though I answered the definition part of it, they asked to plot it on a airfoil which I couldnt do to their satisfaction.
next was, explain compressor stall, measures to prevent it.
micro-pulse jets, anybody heard of it??
Click to expand...

Afraid I can't be of any help on any of these, except that possibly regarding "micro-pulse jet." That may be another term for a Pulse-detonation engine. Essentially what that refers to is an engine where the fuel is "detonated" instead of "combusted." A detonation is an extremely fast combustion that results in a much greater release of energy than simple combustion. There are some ideas for engines out there where the fuel is released in a series of very short bursts, or pulses, and then detonated. I am hardly an expert, but here is a Wikipedia article to start you off in that direction. According to that article, there has been at least one aircraft that has successfully demonstrated the Pulse detonation technology for a few seconds, with a pulse rate of 80 Hz (80 explosions per second). The goal is somewhere up in the thousands of pulses/second. That's really all I know about it. Hope that helps.
 
Oh, apparently there is also such a thing as a pulse jet (which doesn't use detonation, but rather deflagration... just a fancy word for normal combustion). Here's the wiki. This other wikipedia article about "valve-less" pulse jets especially, mentions that some have been used for missiles, model aircraft, and go-karts, so maybe that's where the "micro" comes in.
 
Fish314: thankyou mate. Actually the question micro-pulse jet came up because, those fellas heard me saying " I had done an working valveless pulse-jet engine during my second year at the undergrad" :D I am quite familiar with these air breathing engines, but micro-pulse jet, hmmm what I thought would be similar to micro-ram jets used as thrust augumenters or probably the combustion chamber is designed based on ramjet principle for efficient combustion or something on those lines. :dunno:

Magnetic analysis was a straight question and I gave the same answer, so should be correct ;) Actually after the interview I thought I screwed up the opportunity, because I answered the shock part of the question wrongly and also the supersonic disturbance question too. After about 50 mins of grilling, the final question from them was, How good are you in mathematics mate?? can we talk something about it. I was like waat??? :eek: they took about 15mins for the guy before me and 25 mins for the one before him. hehehe probably I was their target for the day. From the morning they had interviewed about 12 fellows and i was the 12th man and the only man who got so much of grilling :D
 
You must log in or register to reply here.
Back
Top