Aircraft Question - Induced Drag

[OrangeRedSooner]

Hi everyone. I am new to this board. For anyone who might have a question about my name, I am an OSU graduate who married into a HUGE OU family, and fell in love with going to OU games. I am married to StoupTroop, who many of you know, or should know with the amount of time he spends here...lol!

I know that many times he has posed questions here, and the wealth of knowledge obtained has been amazing. Therefore, I now place before you a question that I will need to answer before tomorrow night for a test that I will be taking. Bonus points are on the line!

Thrust in a helicopter is opposed by drag, specifically 3 types: Parasite, Profile and Induced. Both Parasite Drag and Profile Drag increase with increased airspeeds, but Induced Drag decreases with increased airspeed.

My question is "Why does Induced Drag decrease with increased airspeed?" I have searched, and found 3 possible reasons (because the helicopter moves into undisturbed air, or because of the rotor tip vortices, or because there is less angle of attack due to increased velocity).

Can anyone assist me in answering this question?

Thanks! :twinkies:

[sanantoniosooner]

Just ask your husband what a typical reply from me would be like.

That'll save me the trouble of typing it.

[slickdawg]

Well, initially, I would think that the fact that once the helocopter is moving, the airflow would become more laminar, resulting in a decrease in induced drag.


That said, I've asked two aeronautical engineers I know to answer this.

[GottaHavePride]

Hmmm. I'm assuming induced drag is drag on the rotors "induced" by the turbulence resulting from the previous rotor's passage? If that's the case, then at a greater airspeed there would be less time for turbulence to develop (like drafting in car racing) plus the speed (and angle-of-attack) would mean that rotors aren't following the path of the previous rotor as closely, so there would be less drag.

Of course, I'm just a musician who took a lot of math and physics for some idiotic reason. If I'm mis-interpreting what induced drag means, ignore everything I just said. ;)

[StoopTroup]

I was thinking it might have to do with the efficiency of the rotor due to blade twist being reduced as airspeed increased.

BTW...My Wife is a wonderful woman. :D

[proud gonzo]

http://www.commercialcloset.org/images/data/COMCLOSET_PICS/picture/7131.JPG

[GottaHavePride]

No, that's induced vomiting.

[StoopTroup]

http://www.commercialcloset.org/images/data/COMCLOSET_PICS/picture/7131.JPG
That's really twisted...lol

[afs]

increased thrust to go faster gives you a higher ratio over drag that is naturally occuring and not relative to speed.

[Ike]

as a guess...I have no idea what these terms really mean, so I'm taking a stab in the dark.

I am assuming that induced drag isn't really drag at all. That it's the component of the lift vector parallel to the aircrafts motion. Thus, for a helicopter, the induced drag is always negative, since to move forward, the the rotors have to tilt forward, and the lift vector along with them. Thus as airspeed increases, it is only natural that the induced drag goes "more negative"

The way I guess, its basically the same thing as thrust for a helicopter.

[StoopTroup]

Induced drag must be added to the parasitic drag to find the total drag. The minimum drag speed occurs at the speed where the induced drag is equal to the parasitic drag.

This we know....

But why does induced drag decrease as airspeed increases while parasitic increases as airspeed increases.

My answer is...

Induced drag is inversely proportional to the square of the airspeed whereas parasitic drag is proportional to the square of the airspeed.

[afs]

"Induced Drag is drag due to the production of Lift.
When a wing “flies” at the zero lift angle of attack there is no lift and therefore no induced drag. All the drag is parasite in that case. (Of course you can’t fly with no lift.)
As the angle of attack increases the wing produces a force, as shown to the right. The component parallel to the relative wind is the induced drag."

So as the helo flies faster the rotors tilt to an amost level position b/c changes in altitude aren't needed. as a wing approaches level flight - induced drag narrows down to zero.

and the helo starts to decend. pull on the stick, rotors tilt, helo climbs, induced drag increases and airspeed slows.

[StoopTroup]

So as the helo flies faster the rotors tilt to an amost level position b/c changes in altitude aren't needed. as a wing approaches level flight - induced drag narrows down to zero.
and the helo starts to decend. pull on the stick, rotors tilt, helo climbs, induced drag increases and airspeed slows.
Another good answer IMO.

[StoopTroup]

http://upload.wikimedia.org/wikipedia/en/1/12/Drag.jpg

[Ike]

in pictures:

airplane induced drag:
http://selair.selkirk.bc.ca/aerodynamics1/Drag/Graphics/Induced1.GIF

for a helicopter moving forward, the lift vector points up and forward instead of up and backward, thus making induced drag decrease as a function of airspeed.

[GottaHavePride]

I would definitely take afs's answer over mine.

[StoopTroup]

How about this....

At a hover, or at lower airspeeds, induced drag is highest. It decreases as airspeed increases and the helicopter moves into undisturbed air.

[49r]

Jeez.

Doesn't anybody want to know the best ways to update metadata tags on mp3 files any more?

:mad:

[Okla-homey]

All I remember from Aero for Dummies in flight school is this. Total drag increases as you fly faster. Therefore thrust must increase to overcome the effect of that drag. I am reasonably certain that will be of absolutely no use to you.;)

[KABOOKIE]

Very nice answer folks.

Induced drag is porportional to the AOA (angle of attack). As airspeed increases the AOA required to maintain a set amount of lift is reduced therfore, reducing the amount of induced drag.

[KABOOKIE]

Well, initially, I would think that the fact that once the helocopter is moving, the airflow would become more laminar, resulting in a decrease in induced drag.


That said, I've asked two aeronautical engineers I know to answer this.


Laminar flow has more to do with parasitic drag than induced. An aircraft wing can be at a relatively high AOA and still have laminar flow over the wing.

[StoopTroup]

Thanks for the help all.

[JohnnyMack]

Can you have sex on it?

[OrangeRedSooner]

Thanks to everyone (especially for the "special" DRAG photo!) for replying to my question. The correct answer was that Induced Drag is reduced when airspeed increases due to a reduced angle of attack.

I truly appreciate everyone's help!

Have a great 4 months until Football Season! ;)

[KABOOKIE]

Very nice answer folks.

Induced drag is porportional to the AOA (angle of attack). As airspeed increases the AOA required to maintain a set amount of lift is reduced therfore, reducing the amount of induced drag.



BAM!!!!

[crawfish]

I thought induced drag was your second reefer...

[OKC Sooner]

I thought induced drag was when you paid a guy to dress up in women's clothes.

[Flagstaffsooner]

Quit hangin' yo big ole funky butts out the window.;)

[LoyalFan]

Self-induced drag. http://www.gasolinealleyantiques.com/celebrity/images/Artwork/tv-geral.jpg