slow flight, induced drag question

Jeanie

Jeanie

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Jeanie
OK. Here goes- Hope I can explain the question sufficiently.

Induced drag occurs at the highest percentage when the AoA is high as in slow flight. Has to do w/ the drag vector increasing off of the lift vector (perpendicular to the average relative wind) to match w/ the effective lift vector - see Aerodynamics for Naval Aviators page 67 -

So here's the question. As we put the plane into slow flight the underside of the wings and the fuselage - all that surface area is presented to the relative wind -- like a plank in the wind. So would you say that part of the configuration creates form drag (parasitic) also? Or are you going slow enough that it basicly doesn't count?

It would be easier to ask this if I could use my hands :rolleyes2:
 
Jeanie: Not an aeronautical engineer, but I think you question is true, that the parasitic drag increases also.

Stacey
 
You're in slow flight -- which reduces parasitic drag.

Induced drag increases due to high lift condition -- but at high cost (there ain't no free lunch)
 
Jeanie said:
OK. Here goes- Hope I can explain the question sufficiently.

Induced drag occurs at the highest percentage when the AoA is high as in slow flight. Has to do w/ the drag vector increasing off of the lift vector (perpendicular to the average relative wind) to match w/ the effective lift vector - see Aerodynamics for Naval Aviators page 67 -

So here's the question. As we put the plane into slow flight the underside of the wings and the fuselage - all that surface area is presented to the relative wind -- like a plank in the wind. So would you say that part of the configuration creates form drag (parasitic) also? Or are you going slow enough that it basicly doesn't count?

It would be easier to ask this if I could use my hands :rolleyes2:
Click to expand...

Why are you trying to explain this? Are you studying for the CFI?
 
I think one of the big "disconnects" is the very names - to me induced drag should be induced by something that's changing - like AIRSPEED. But NOOOOO, it's Parasitic drag that goes up with airspeed.

Someone had to choose a frame of reference, I know, but I still find the terms counterintuitive.
 
Thanks, ya'll. Thanks, Ron for the reference. And yes, Ben, I'm studying for the CFI. So my interest is in how I explain how/why it's like that.....
 
TMetzinger said:
I think one of the big "disconnects" is the very names - to me induced drag should be induced by something that's changing - like AIRSPEED. But NOOOOO, it's Parasitic drag that goes up with airspeed.

Someone had to choose a frame of reference, I know, but I still find the terms counterintuitive.
Click to expand...

Drag that is induced by lift... Just replace airspeed with lift and you will have it.
 
Jeanie said:
Thanks, ya'll. Thanks, Ron for the reference. And yes, Ben, I'm studying for the CFI. So my interest is in how I explain how/why it's like that.....
Click to expand...

Excellent. From a professor's perspective, I have a suggestion: try being able to explain difficult concepts such as this one from three angles. For example, you have the technical explanation down. Now, see if you can sketch it out frame-by-frame (like a cartoon) to show how it happens. Then try getting the student to explain it by asking him leading questions.

Doing this will help you, too!
 
Jeanie said:
OK. Here goes- Hope I can explain the question sufficiently.

Induced drag occurs at the highest percentage when the AoA is high as in slow flight. Has to do w/ the drag vector increasing off of the lift vector (perpendicular to the average relative wind) to match w/ the effective lift vector - see Aerodynamics for Naval Aviators page 67 -

So here's the question. As we put the plane into slow flight the underside of the wings and the fuselage - all that surface area is presented to the relative wind -- like a plank in the wind. So would you say that part of the configuration creates form drag (parasitic) also? Or are you going slow enough that it basicly doesn't count?

It would be easier to ask this if I could use my hands :rolleyes2:
Click to expand...
Jeanie-

you are correct in that the fuselage is presenting more area to the relative wind. While this should present greater drag, it is also at a much lower airspeed than cruise flight, so the increase in drag from the greater surface area is more than offset by the slower speed. Remember that drag is proportional to the square of the speed.
 
Cap'n Jack said:
Jeanie-

you are correct in that the fuselage is presenting more area to the relative wind. While this should present greater drag, it is also at a much lower airspeed than cruise flight, so the increase in drag from the greater surface area is more than offset by the slower speed. Remember that drag is proportional to the square of the speed.
Click to expand...
At slower speeds - I think as speed increases parasitic drag increases at an even greater rate - I recall the cube of the airspeed being mentioned in one text for high (>.6M) speeds.
 
Thanks Cap'n Jack - I figured it had to be something like that! Yea.... I love it when it makes sense....
 
Cap'n Jack said:
Jeanie-

you are correct in that the fuselage is presenting more area to the relative wind. While this should present greater drag, it is also at a much lower airspeed than cruise flight, so the increase in drag from the greater surface area is more than offset by the slower speed. Remember that drag is proportional to the square of the speed.
Click to expand...

And WRT the increase in projected area due to the larger angle between the fuselage and the relative wind at slow airspeeds, the speed in question is effectively very small relative to the total fuselage surface as it would be the total area times the cosine of that angle. In addition only when the lower surface was completely flat with sharp corners (often the case with metal airplanes) the actual drag would be far less than the equivalent flat plate of the same area. All of this conspires to make the effect that high AoA has on parasite drag pretty small.
 
TMetzinger said:
At slower speeds - I think as speed increases parasitic drag increases at an even greater rate - I recall the cube of the airspeed being mentioned in one text for high (>.6M) speeds.
Click to expand...
I don't think you'd see a cube effect unless you were getting into compressibility effects (transonic airflow). I suspect that what you're (mis)remembering is that the power required to overcome parasite drag increases as the cube of the IAS but that's because the drag increases as the square and power= speed * force (drag).
 
I'm glad that I don't have to completely understand aeronautical engineering to fly a plane... It's been a long long time since I took trig and calculus -

Guess the bottom line is: you play you pay - lift vs. drag; lift vs. fuel purchases...
 
Jeanie said:
Guess the bottom line is: you play you pay - lift vs. drag; lift vs. fuel purchases...
Click to expand...

Exactly.

Riding bicycles can really bring the whole equation into perspective.

When you ride slowly, most of your energy is overcoming rolling resistance.

As you increase speed, rolling resistance remains relatively constant, but form drag increases -- a lot.

Therefore it takes nearly twice as much energy to go 25 MPH than 20 MPH.

Airplanes add the cost of lift, whereas lift is negligible (and in fact not wanted in cycling).

Neat little app here:
 
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That app is pretty cool - I forwarded it to my ex husband who is the service manager for Richardson Bike Mart in Richardson (N. Dallas) - He's a racer type cyclist.

Nice drag analogy too...
 
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