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KDR question

  • Tanser88
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Tanser88 created the topic: KDR question

Hi, just wondering if anyone could help me out with the KDR i have for Aerodynamics.

Part 61 MOS 1.3.1 CADC 2.4.1(A) (Lift and Drag Formula)

The question is:
Explain the meaning of the term used in Lift/Drag Formula "CL and CD depend upon the shape and the and of attack of an aerofoil"

Tried looking this up in the Bob Tait Aerodynamics book and couldnt seem to find an answer.

any help would be greatly appreciated.

Cheers,

Tanser
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  • John.Heddles
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John.Heddles replied the topic: KDR question

The question/MOS item probably go a bit too far into the deep and meaningful side of engineering things for basic pilot training .... however, if that's what they ask, guess you have to live with it ..

Point to note is that the coefficients depend on the actual wing characteristics (aspect ratio, camber, twist, etc) and angle of attack (incidence). These can be related to the forces (lift and drag) if one looks at air density, airspeed, a reference area (to make the coefficient non-dimensional) and both Mach Number and Reynolds Number (the last two generally being overlooked at the elementary level as their effect is not significant - however, they are quite important for, say, relating wind tunnel work to the real world, high speed flight and flight at high altitudes).

You can look up heaps of graphs either at your (better) local library or on the net.

A fairly basic discussion is at
www.grc.nasa.gov/www/k-12/airplane/liftco.html .

Another link
books.google.com.au/books?id=DPZYUGNyubo...g%20sections&f=false
is for one of the standard aerodynamics engineering textbooks which all of we aero engineers have on our bookshelves .. suggest you have a look at Fig 2 (page 4). This gives you a series of lines plotted lift coefficient against angle of attack for varying values of aspect ratio .. as you can see, AR has a significant effect on the coefficient. The basic lines show the typical variation of coefficient with alpha which you would have seen in numerous references. Don't sweat the rest of the link .. it is intended for the engineering folks rather than pilots at the start of their training work. On the other hand, if you can bear working your way through it over a cup or three of coffee, there is plenty of useful stuff there for newchums to think about.

Another link
www.calpoly.edu/~rcumming/Airfoils_Wings.pdf
in the final few slides, shows the typical effect of camber and (trailing edge) flaps. The BLC chart is similar to the effect of leading edge devices. Again, worth a read through the link over a coffee if you have the interest.

Appropriate graphs abound for drag coefficient ....

Without knowing just what the examiner's expected answer is these days (my theory training days are now long ago but Bob might be able to add something for that) I suggest that one should keep it simple and just observe that the coefficients principally are affected by the wing shape characteristics and the wing angle of attack, along with airflow parameters ... or something similar ..

Engineering specialist in aircraft performance and weight control.
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Tanser88 replied the topic: KDR question

Thanks mate, but yeah I'm after a simple explanation of what the question is asking. I think your answer is a bit more of an engineers perspective. Big respect to aero engineers by the way.
I've already passed the exam and all CPL exams. It's just a KDR report that my instructor has asked me to do. It's beneficial to me as well. But I also want to know for myself.

Cheers
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John.Heddles replied the topic: KDR question

That was the problem as I saw it .. at the most basic level, the question is its own answer. The next step up is my final comment in the previous post. Beyond that, you are into engineering territory.

I'll be interested to see if you can come up with something simpler than my "the coefficients principally are affected by the wing shape characteristics (camber, twist, etc) and the wing angle of attack, along with airflow parameters". That would be my answer with my pilot's hat on ... PPL through ATPL, pick whichever.

I presume Gavin is still in the seat but I have no idea just what sort of words he might be looking for .. I guess Bob/Richard will be along to offer some comment in due course.

Engineering specialist in aircraft performance and weight control.
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Tanser88 replied the topic: KDR question

John,

My apologies, my mate gave me an explanation that I think is close your explanation.

He explained it like this:

Shape: High Aspect Ratio (low CD and high CL, subsonic flight), Low Aspect Ratio (high CD and low CL, supersonic flight), Tapered (stability during turns), Delta (supersonic flight), Swept Back wings (large airliners, stability at slow flight)

Angle of Attack: High Aspect Ration wings require a larger AOA to achieve lift, flaps and leading edge devices used during taking off of large airliners (have a steep climb angle) allow for slow flight too at low AOA, therefore the CD will be low during slow flight and the best CL will also be at slow flight.
Low Aspect Ratio, also require a large AOA, but also require a greater airspeed, therefore the CD is high at even low speeds, (e.g.: Concorde) have to be at unusually high angle of attacks during takeoff and landing (military fighter jets) faster flight will be at a lower CD and still produce a greater CL

Low Aspect Ratio wings will include ur delta wing design, required greater AOA to achieve the same L/D ratio as a high aspect ratio wing.

High Aspect ratio wings are your swept back wings, e.g. commercial airliners, able to produce lift at slower speeds (e.g. gliders)


Would you agree with this explanation and/or that it's close to yours?

Cheers
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John.Heddles replied the topic: KDR question

Considering the original question .. “Explain the meaning of the term used in Lift/Drag Formula "CL and CD depend upon the shape and the and of attack of an aerofoil” one has to observe that it is a bit average so far as comprehension is concerned. I presume the requirement is to consider the effect of shape and angle of attack on CL and CD ?

Some comments on your mate’s thoughts …

Shape:
High Aspect Ratio (low CD and high CL, subsonic flight), Low Aspect Ratio (high CD and low CL, supersonic flight)

I think he needs to provide a bit more meat to clarify what AR is doing to CL and CD and describe why there is a relevance to subsonic/supersonic flight .. rather than just using throwaways. Either one should answer the question as the examiner wants it answered (if one has that knowledge), or briefly to expose oneself to the minimum, or in plenty of detail if one ventures into detail .. the old "fly the check captain on the check" trick.

Probably better to say that a higher AR is good for lower speed flight providing the design envelope doesn't require a higher speed cruise at lower levels. Certainly the induced drag is lower (although section drag can be a tad higher and parasite drag will be higher) and lift is good.

Structural considerations fit well with low speed and lower gust environments. As the speed increases gusts become more of a concern and the structural problems will predicate against very high AR wings. Hence one sees sailplanes with very high AR operating in the low speed regime but higher subsonic and supersonic aircraft curtailing AR somewhat.

To be fair, for commercial aircraft, airport infrastructure limitations dictate that wing spans be kept a little in check – that is an external dictate imposed on the designer.

For light aircraft, the design standards restriction on stall speed is a significant driver in wing design.

Stores considerations (where do I put the undercarriage if wing mounted ? fuel tanks ?) can have a significant effect on AR (ie wing thickness) decisions.

Other considerations come into play as speed increases - eg, aeroelasticity/flutter related, bending, gust response, thickness/chord, mach cone angles and these will drive lower AR values especially for supersonic design.

Tapered (stability during turns)

Perhaps you can indicate just what your mate means by turning stability ? and how taper might influence such a consideration ?

Generally, taper design considerations are driven by things such as keeping the lift distribution not too far off elliptical, stall characteristics and so on. On the other side of the table, the designer needs to keep wing thickness in mind for things such as undercarriage and fuel tanks and the effect of taper on wing structure weight.

Delta (supersonic flight)

I think your mate needs to put some effort into explanation. The delta has some utility for high speed flight .. but he needs to offer some thoughts on what it might do for CL and CD. One needs to keep in mind that delta planforms generate lift somewhat differently to how the straight wing works so that tends to confuse the issue a little.

Swept Back wings (large airliners, stability at slow flight)


Is your mate suggesting that slow speed stability is better/worse with a swept wing ? Suggest he needs to describe some influence on CL and CD.

Was your mate going to mention things such as twist, LED/TED, camber, leading and trailing edge shapes … ? The check captain (examiner) is going to be wondering a little as to your mate's depth of knowledge ..

Angle of Attack: High Aspect Ratio wings require a larger AOA to achieve lift, flaps and leading edge devices used during taking off of large airliners (have a steep climb angle) allow for slow flight too at low AOA, therefore the CD will be low during slow flight and the best CL will also be at slow flight.
Low Aspect Ratio, also require a large AOA, but also require a greater airspeed, therefore the CD is high at even low speeds, (e.g.: Concorde) have to be at unusually high angle of attacks during takeoff and landing (military fighter jets) faster flight will be at a lower CD and still produce a greater CL
Low Aspect Ratio wings will include ur delta wing design, required greater AOA to achieve the same L/D ratio as a high aspect ratio wing.
High Aspect ratio wings are your swept back wings, e.g. commercial airliners, able to produce lift at slower speeds (e.g. gliders)

Maybe I’m just having a senior’s moment but this all sounds like so much gobbledegook .. perhaps your mate might like to rewrite it a bit and we can have another go at making something of it .. ?

Would you agree with this explanation and/or that it's close to yours?

Sorry, the answer has to be “no” on both counts ..

Engineering specialist in aircraft performance and weight control.
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Tanser88 replied the topic: KDR question

Hi john,

Before reading your latest reply, I had another read of what my mate wrote and I was having trouble of what he meant and I don't think he knows how to answer the question. I asked him to clarify and he had trouble.
After reading your latest reply I would have to say that I agree with you. To be honest I'm a bit confused about the question; as you stated before the answer is the question itself but what the check captain/ testing officer wants as an answer I do not know. I really don't know how to answer this question in a way that is not in the form of multiple pages. I want to be able to answer it in a way that's it's comprehensible, and relatively a short explanation. I've read up on what's in Bob Tait Aerodynamics and it talks about low AR having low CL and high CD, and High AR having High CL and Low CD. Similar to how my mate explained it. But I don't think I can make enough sense of what's in the book in regards to the question to be able to answer it.

In pretty lost at the moment.

I wonder if Bob or Richard can explain and answer the question of the KDR.
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