Help! Aerodynamics

[pilatus028]

Why does a higher aspect ratio generate less drag? (Don't understand, jepp book doesn't go into detail)

Why does VA increase with weight? (I know an answer, but I don't think it's the best technical answer)

Why does speed not effect the load factor in a turn? (Question might be phrased wrong)


Thnks in advance,
Im stumped.

Clem

[E_Dawg]

1) Smaller wingtip vorticies (small chord at the tips) and less induced drag due to less upwash / downwash along the span of the wing

2) More weight = higher AOA for a given airspeed. Va is a speed you fly to achieve a specific angle of attack, so when you add weight you have to add speed to stay at the right aoa

3) Load factor = Total wing lift / weight.
When you slow down in a turn, the wings still need to create the same amount of lift as before, therefore the load factor stays the same (but turn rate and AOA will increase).

Hope this helps

[Wm226]

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Why does a higher aspect ratio generate less drag? (Don't understand, jepp book doesn't go into detail)

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The aspect ratio is basically the comparison between the length and width of the wing. It can be summarized in a mathematical formula as provided below:

length of the wing / width of the wing = aspect ratio

Basically, the goal of an aircraft engineer is to make the length of the wing larger in proportion compared to the width of the wing... this would provide an ideal "Aspect Ratio" which would therefore have the tendency to increase lift and generate less drag.

If you had an aircraft with a low aspect ratio (say the length of the wing was closer in proportion to the width of the wing), then the aspect ratio would be lower... which means increased drag (not ideal).

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Why does speed not effect the load factor in a turn? (Question might be phrased wrong)

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Just like what SkyGuyEd said, Load Factor = Total Wing Lift / Weight

Your velocity is not a factor in this equation.

When you make a turn, however, your stalling speed is increased because your load factor is also increased due to the angle of your aircraft's bank.

[flyguy]

The only one I understand well enough to be able to explain is #2. Remember VA is the speed at which you can make full and abrupt control movements without causing structural damage to the airframe. Below VA you will stall before damage occurs, which is a good thing because you can recover from a stall easier than you can repair a broken wing in mid-air. When you increase weight, you make it easier for the airplane to stall at a higher airspeed, so increased weight means you can go faster and still stall before damage occurs if you make full and abrupt control movements.

We aren't doing your homework for you are we?

[jetman]

1]Think of an infinitely long wing ,no wing tips no wing tip vortices [less drag]

2]What is easier to accelerate a lighter or a heavier weight?
VA is a function of stall speed[1.9 times stall @ gross W.]
stall speed increases as weight increases and so does VA

3] Speed in a turn does not affect loading,angle of bank does

[ananoman]

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Why does a higher aspect ratio generate less drag? (Don't understand, jepp book doesn't go into detail)

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The wing tip vortice is caused by the higher pressure air below the wing spilling over the wing tip to the lower pressure region above the wing. This destroys the pressure differential (lift) at the wing tip.

To see why a high aspect ratio has less drag, we will compare 2 wings of the same total wing area, but of different aspect ratios. Our low aspect ratio wing has a short span and a wide chord. It has a relatively large area at the wing tip that is affected by the wingtip vortices. Since the wing tip vortices destroy the lift created at the wing tip, the wing must be flown at a higher angle of attack to create enough lift. This creates more induced drag.

The high aspect ratio wing has less area concentrated at the wing tip since it is relatively long and narrow. The wingtip vortices affect a smaller area and less lift is lost. This allows the wing to fly at a lower angle of attack compared to the low aspect ratio wing. Since the wing needs less angle of attack to create the same amount of lift, it has less induced drag.

We know that at low speeds induced drag predominates, and as speed increases induced drag is reduced. At higher speeds parasite drag greatly increases. This is why you usually only see high aspect ratio wings on gliders or other low speed aircraft. High aspect ratio wings can also have a weight penalty. It is hard to make a long, thin wing light and strong.

Winglets can be used to increase the efficiency of lower aspect ratio wings, by lessening the effect of lift destroying wing tip vortices. They allow the use of a lower aspect ratio wing for better high speed performance (shorter span = less frontal area = less parasite drag) while helping to reduce induced drag.Tip tanks also have much the same effect.

[ananoman]

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VA is the speed at which you can make full and abrupt control movements without causing structural damage to the airframe.

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Be careful here. This is the definition that is used in most text books, but it is still possible to damage an aircraft below Va. Va is based on positive limit load factor. You can still exceed the negative G limit or damage the vertical stabilizer below Va. Unless you are a real nut, this should not be an issue, but it is worth knowing.

[flyguy]

Well, for purposes of explaining why VA goes up with weight, I like my answer better. Thank's for the tidbit though.

[CAVOK]

"Why does speed not effect the load factor in a turn?"

***Draw a picture of this...it helps.

If it helps to think of it from a geometric/physics point of view (yeah right)....just remember that as you "tilt" the lift vector in a turn, the cosine of the angle you are banking determines the load factor.

Ex: 60 degree bank = cos60 = .5
So the load factor doubles; whether at 70kts in a C152, or at Mach 1 in an F16, you are gonna feel 2 'g's at a 60 degree bank (and holding altitude of course).

I think I just confused myself....if this explanation doesn't help you, forget everything I just said.

[ready2fly]

Yeah, that cleared it up professer... thanks.

[E_Dawg]

We're just pilots here