Swept Wing Theory
 

Hi can anyone help on this? I have always since my ATPL training struggled with this one.

I was taught that for faster airspeeds swept wings can help delay onset of Mcrit by effectively "fooling" the air into thinking that it was travelling slower. I always visualised this as the wing cross section being made thinner by virtue of sweepback (thickness to chord ratio lower) as if taking a slice out of the wing along an axis parrallel to the logitudinal. In this case obviously the air is less accelerated owing to lower effective camber, so the flow is slower than it otherwise would be (with the consequence that the aerofoil produces lower lift at all speeds - necessitating amongst other things higher angle of attacks).

If this is correct, would it therefore not be more correct to say that this effect itself is not the prime reason for sweepback? Afterall, would not a straight wing with the same thickness/chord ratio and camber produce the same effect in terms of speed of air across the aerofoil?

Would it not be better to say that sweepback in isolation has the main effect of providing greater streamlining when viewing an aeroplane from above, and leave it at that?

On a seperate note, why do swept wing aircraft fly with higher nose up attitudes (eg on an ILS)? Is it because angle of attack has to be higher owing to lower cross sectional camber, or is it to do SPECIFICALLY with sweepback ie wing tips being further behind roots)?

Thanks - I hope I have put forward what I am trying to say in an intelligible manner

Landmak

Take a straight non tapered wing and saw through it at 90 deg to the leading edge.

Now cut through at the sweep angle of your choice.

The two cross sections will have the same thickness but the one with sweep has a longer chord

Hence the thickness chord ratio is less on the swept wing.

Thickness chord ratio determines the speed at which shockwaves form.

good thinking
 

take the wind tunnel out of it. A 200 mph plane does not fly in 200 mph air. Air has mass and inertia. When air is blown through a wind tunnel, the effects on airfoils are different than in a real situation. For all practical purposes, air is still with very little inertia. What really happens with sweep is theory. It has so far been too expensive to find out what really is going on. The books are full of theory. Go explore and find out what you can do. If you believe the books, then nothing can be improved. But we know it all can be improved. Keep on thinking, but make some real tests too and not in a wind tunnel, but with real aircraft.

landmark1234 appears to have the idea but is struggling with the behind the scenes "why ?".

The other part of the equation is wing strength. In its simplest form, that depends on some sort of spar structure to hold the wing together under load. A spar is not dissimilar, in principle, to a bridge beam. Depth has a very much higher influence on strength than, say, width.

First, we need the wing thickness to hide the spar bits which stop the wing falling apart.

Then, we cast around for ways to moderate the undesirable effects of the wing thickness on airflow.

Hence - wingsweep to take advantage of thickness:chord ratio determines the speed at which shockwaves form

why do swept wing aircraft fly with higher nose up attitudes ?

Faster aircraft wings are designed to work best at high speed cruise. However, those aircraft still have to land and takeoff within sensible speed ranges. To get the lift needed at the lower speeds, they employ higher incidence (usually along with LE devices to permit those higher angles) .. hence comparatively high pitch angles on takeoff (especially) and approach and landing.

thanks guys for all your replies -
 

I guess I am more interested in the theory than the practise....

@ Broomstick Flier
 

The WMcrit will not be changed but as the wing is swept, the wind will now be sensed as two vectors, one perpendicular to the leading edge and one parallel. In order to the perpendicular vector reach Mcrit the relative wind must be higher than it, so the aircraft can fly faster. How much faster?


Can you explain this in simpler terms ? Thanks

I think broomstick's original post is about as simple as you can get it. I understood it and I have only an O level in sums.

Landmark1234,

Thank you for raising this topic. I have been doing some studying on this, understanding the basic principles post ATPL but wanted a firmer understanding of the topic. Many of the explanations i have found are lacking any substance or useful explanation.

For example regarding velocity vectors (Normal v Effective) on a swept wing and how they act in relation to the wings cord, "Ace the technical pilot interview" states : "in effect the wing is persuaded to believe it is flying slower than it is" :confused:. I find explanation like this particularly annoying and unhelpful.

I found a useful diagram that helped me to understand the difference in tickness:cord ratio between a straight and swept wing, the link is below.

Swept and unswept airfoils

The diagram simply shows what you assumed and John Farley stated in an earlier post. Sweeping the wing simply extends the cord length whilst maintaining the wings thickness thus thickness:cord ratio decreases. This is visible when a cross section is viewed parallel to the aircrafts longitudinal axis.

Don't forget as stated below there are other advantages of sweeping a wing than a greater Mcrit.

- Greater lateral stability (Just look at a hang glider)
- Higher wing loading, therefore less effected by turbulence
- Greater wing thickness for equivalent Mcrit allowing for stronger structures carrying more fuel and equipment.

These advantages come with many disadvantages that we all know, I.e. Poor low speed handling and wingtip stall tendency due to spanwise flow. I guess you win some you loose some. I look forward to reading some more explanations and enhancing my own knowledge.

Hope this helped in a small way.

J

I'm with Broomstick. If the effect of sweep were just the reduction of T/C ratio in the longitudinal cross-section, then why not have a straight wing with that reduced T/C?

The higher nose angle on approach is due to the leading edge slats or droop.

The wing angle of incidence is designed to give minimum fuselage drag in cruise.

Changing the wing camber only using trailing edge flaps gives a "flat" fuselage incidence on approach - a la Fokker 100 (and light aircraft).

Adding a leading edge high lift device in effect reduces the incidence of the wing in relation to the fuselage.

Therefore the fuselage needs to be pitched up to have the wing at the optimum AoA on approach.

Quite a lot in here..

Swept wing - Wikipedia, the free encyclopedia