Vertical stablizer

[Pugilistic Animus]

411A you were correct all along---just, I guess, not erudite enough for pprune

PA

[Brian Abraham]

Is there indeed any evidence of a 'mini' vortex on a straight wing?

Certainly is BOAC, on possibly the most widely used airfoil of all time, the NACA 230 series which combined high lift, low drag, mild pitching moments (meaning low trim drag), and even a bit of laminar flow on its lower surface. It was designed in 1935 by NACA Langley researcher Eastman Jacobs. Used on the Argosy, Aerocommander Shrike, Cessna Citation and some of their piston twins, Beechcraft twins and singles, Auster, Lancaster, Shack, Lincoln, 748, P-39, Provost/Strikemaster, DC-4, DC-6, DC-7, RV series of home builts, P-38 and Lockheed Constellation - a few of the many. At high angles of attack it develops a standing vortex span wise on the upper surface, making the airflow believe the airfoil has greater camber. At the stall the vortex collapses suddenly giving the sharp stall for which the series is famous. The reason behind this characteristic is that the 230 series has the maximum camber location far forward and hence stalls from the leading edge. A more desirable gentle stall is obtained with the maximum camber location further aft as on the NACA 24-, 44- and 6- series.

The strong down*wash over the centre sections of a swept wing effectively reduces the angle of attack in that region. It also delays flow separation until a very high angle of attack is reached. For a given value of CL, the higher angle of attack for the swept wing incurs problems of visibility at low speeds, particularly on the approach to a landing.

Fig 25 Comparison of Straight and Swept Wing CL Curves



Vortex Lift

Due to the spillage of air around the wing tip caused by the mixing of the relatively high pressure air below the wing and the relatively low pressure air above the wing, a vortex is formed. The strength of this vortex varies as the angle of attack. Normally this vortex is left behind but on certain aircraft it is used to produce lift.

On slender delta aircraft at subsonic speeds the vortex is arranged so that it lies on top of the wing and has its origin at or near the wing root. As the core of a vortex is a region of low pressure, there is a vortex core lying along the top of the slender delta wing (Fig 34) creating a greater pressure change between the top and bottom surface of the wing than there would be if the vortices were attached to the wing tips. The vortex system also delays separation from the rear of the wing.

Fig 34 Slender Delta Aerofoil



The stalling angle for this type of aerofoil can be very large (40º or more) but the lift is accompanied by very high drag and a low lift/drag ratio. Also any irregularity in the vortex formation will cause adverse stability problems. However in moderate angles of attack the value of C is increased as shown in Fig 35.


Fig 35 Effect of Vortices on CL



Aspect Ratio and Stalling Angle

A stall occurs when the effective angle of attack reaches the critical angle. Induced downwash reduces the effective angle of attack of a wing. Since induced drag is inversely proportional to aspect ratio it follows that a low aspect ratio wing will have high induced drag, high induced downwash and a reduced effective angle of attack. The low aspect ratio wing therefore has a higher stalling angle of attack than a wing of high aspect ratio.

The reduced effective angle of attack of very low aspect ratio wings can delay the stall considerably. Some delta wings have no measurable stalling angle up to 40º or more inclination to the flight path. At this sort of angle the drag is so high that the flight path is usually inclined downwards at a steep angle to the horizontal. Apart from a rapid rate of descent, and possible loss of stability and control, such aircraft may have a shallow attitude to the horizon and this can be deceptive. The condition is called the super stall or deepstall, although the wing may be far from a true stall and still be generating appreciable lift.

[BOAC]

Excellent finding, Brian - thank you. I did not know about the straight wing vortex. Amazing that the Auster and Connie had that.

I assume the BAC Lightning produced significant l. e. vortices since a lot of manoeuvring in that a/c was in varying degrees of buffet, including the finals turn in the circuit and the well-known 'rotation' take-off which used to go right throught the stalling angle (and still fly!). The secret was to have no yaw................

[411A]

DC-4, DC-6, DC-7...

An interesting item about the wing used on these series of aircraft...the airfoil section was not changed much, only the washout required for the later models...and the span for the -7C model.
My dad was engineering project manager on the DC6 and DC7 programs, and he mentioned Douglas tried various types of wing tip treatments, to try to reduce drag further...tip tanks, fences, small winglets...and nothing worked better then the original tip design.