0
$\begingroup$

From experience everybody knows if you put your arm out of car window at high speed(150km/h), it's a lot easier to hold arm angled backward then hold arm straight out(prependicular to airlfow).So conclusion will be that drag force is reduced..

Can wing sweep in same manner reduce total drag force at speeds up to 250km/h?

enter image description here

Pulling angled "beam" with rope in water will be easier than if "beam" is prependicular to flow.Right beam has smaller lift force but also less total drag force.Is this correct? enter image description here

$\endgroup$
6
  • $\begingroup$ While the question is valid, the experiment you describe is flawed. You reduce the effective "wingspan" by the cosine of your arm angle, and thus the moment felt at the shoulder joint by the cosine squared. At 45° this cuts the moment in half even if the drag per unit of of wingspan is the same. $\endgroup$ – Sanchises Aug 30 '20 at 10:21
  • $\begingroup$ @Sanchises Shoulder will feel lower moment by 1,4times with angled arm at 45degrees,cosine 45=0.707 (1:0.707=1.4) if drag force remain the same.But I think drag force will also reduced. $\endgroup$ – member2017 Aug 30 '20 at 13:17
  • 1
    $\begingroup$ related: Is there any benefit of using a swept wings at low-subsonic speeds? $\endgroup$ – Manu H Aug 30 '20 at 13:46
  • $\begingroup$ @ManuH This topic was about swept wing in general,here hope for detail explanation about how sweep affect on drag in low-subsonic.. $\endgroup$ – member2017 Aug 30 '20 at 13:54
  • $\begingroup$ @member Yes, total drag will also be reduced (the frontal area is reduced because your arm doesn't "stick out" as far), hence the cosine squared so half the drag not 0.707. Also in your second image, is it on purpose that the wing span is smaller on the right-hand side? For a more fair comparison you should make the beam longer on the right side so that the width projected onto the flow direction is the same. $\endgroup$ – Sanchises Aug 30 '20 at 13:57
3
$\begingroup$

Sweeping back a wing of fixed spar length and area does decrease form drag. However due to both its lower aspect ratio (span/chord) and sideways flow effects, it increases induced drag and decreases lift, so that the net L/D ratio or efficiency of the wing falls.

Whether the absolute amount of drag goes up or down depends on other factors, such as wing loading and angle of incidence. In most situations drag would be higher. Where it is less, other design optimisations such as reduced spar length would offer greater improvement.

$\endgroup$
8
  • $\begingroup$ Just keep in mind that in my case,wings are not produce same lift force.Sweep back wing produce smaller lift,because area and spar length are same in both case. $\endgroup$ – member2017 Aug 30 '20 at 16:53
  • $\begingroup$ @member2017 There are a lot of parameters you need to clarify before exact comparisons can be made, wing loading, spar length, chord, angle of incidence, airfoil profile and aircraft weight being just a few that spring to mind. $\endgroup$ – Guy Inchbald Aug 30 '20 at 18:16
  • 1
    $\begingroup$ @member2017, comparing wings producing different lift is mostly meaningless. The wing needs to produce lift to balance the weight of the aircraft, so for a meaningful comparison the wing should be producing the same lift. It may do so at different lift coefficient and/or different angle of attack, and may be able to produce different lift at its critical angle of attack, but the points you compare should generally be for the same lift. The bird does not become lighter when it sweeps its wings for faster flight or heavier when it stretches them out for soaring. $\endgroup$ – Jan Hudec Aug 30 '20 at 20:53
  • $\begingroup$ @JanHudec I agree with you,but I need this comparation for some other purpose.I just want to know if swept wing will has less lift and less total drag compare to straight wing? $\endgroup$ – member2017 Aug 30 '20 at 21:02
  • $\begingroup$ @member2017 then you have to specify how you match the flow conditions. $\endgroup$ – Jan Hudec Aug 30 '20 at 21:07
1
$\begingroup$

The main issue is that sweep does not benefit low speed and sweep adds dangerous behaviors. Wing sweep is needed only for the trans-sonic speed range, slower than mach .65 or faster than mach 1.5 and they do not add benefit.

$\endgroup$
1
  • $\begingroup$ If sweep is sufficient, it reduces drag even upwards of Mach 1.5. As long as the leading edge is subsonic, drag is substantially lower. $\endgroup$ – Peter Kämpf Sep 2 '20 at 15:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.