Stall Control

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Transcript of Stall Control

  • Tony Elliott 05/2008 Page 1

    Wings, Wings, Wings Stall Control with camber changes

    Martin Simmons has done an excellent job of describing many aspects of lifting systems and the numerous details

    that need to be considered to achieve efficient and well designed solutions. The key concept to note however, is

    that there are MANY variables that need to be considered when designing lifting surfaces. This short paper is

    written to expand on the details written in Martin Simmons Model Aircraft Aerodynamics publication as it relates

    to stall control by camber changes.

    One has to remember that you cannot design a lifting system to meet an infinite variety of flight conditions. It

    simply is not feasible with current technology. Many attempts have been made over many years to develop

    variable geometry wing surfaces specifically to enhance a limited flight envelope but the solutions are complex,

    expensive and invariably heavy. The key to remember in the details that follow is that we are attempting to design

    a wing system that meets a specific flight envelope one that has a known and controllable slow speed stall point

    and a known, designed top speed with a never to exceed speed.

    So, lets look at a simple solution for tip stall control through camber and physical twist variation in a tapered

    wing. Note: in the discussions that follow, one core assumption is made. I have assumed the same as Martin

    Simmons in that an Elliptical lift distribution exists across the span of the wing. This is a key assumption.

    I can personally see why one might assume that a wash-in might be required with a higher cambered tip airfoil

    based on the aerodynamic zero lift angle of attack parameter. It may seem intuitive that this might be the case IF

    your point of reference is the zero lift angles of attack ONLY. However, as noted above, there are other factors

    that need to be considered to understand why this would be the wrong course of action. Adding a higher

    cambered airfoil to the wing tip and then introducing wash-in would cause significant flight issues with a

    considerably smaller flight envelope and significant tip stalling characteristics.

    To understand the concept of increased camber at the wing tip with WASH-OUT introduced to counter

    aerodynamic wash-in one must review the lift curves of a specific family of airfoils with varying cambers. This is

    shown in Martin Simmons book as Fig 7.4. You will notice that the lift curves move to the left and upwards on the

    graph as camber increases. What is also apparent is the increasingly negative aerodynamic zero-lift angles of

    attack as the camber increases. However, what is NOT so apparent is the reduction in geometric (physical) angle

    of the stall angle that occurs as the camber increases. This is shown in Martin Simmons diagram (7.4) where the

    coefficient of lift max intersects with the angle of attack horizontal axis in the middle of the graph (notated by

    N.B. geometric stall angles).

    Lets summarize those three points:

    1). As camber increases, the lift curves move left and upwards on the lift/angle of attack graph

    2). As camber increases, the zero-lift angle of attack increases negatively

    3). As camber increases, the physical angle of attack at which the airfoil stalls decreases

    So lets see what all this means:

  • Tony Elliott 05/2008 Page 2

    Example 1: Reducing camber at the wing tip no wash-in or wash-out added

    Lets look at a wing that is designed with a highly cambered airfoil at the root and a low cambered airfoil at the

    tip. IF the wing is built with the chord lines flat on the building board, using the two left-most lift curves from the

    diagram, you will see that the root will stall before the tip as the angle of attack of the complete wing is increased

    (upper right hand side of the diagram where Cl max occurs). This is good when it comes to eliminating tip stall at

    slow speeds.

    Now, as we look at these same two lift curves, lets assume we point the nose of the aircraft down and reduce the

    angle of attack of the whole wing. As we do this, we move down the lift curves to the lower left quadrant of the

    lift curve diagram noted as negative or inverted stall in the Martin Simmons diagram 7.4. Here you will see the

    lift curve for the moderately cambered tip airfoil reach the negative lift point BEFORE the strongly cambered root

    airfoil. What this means is that at high speed (or perhaps better defined, low angles of attack), the tip actually

    starts to lift downwards while the root is lifting upwards. This is not a good thing for numerous reasons including

    increased vortex drag at the tip, numerous structural considerations as well as a now well defined upper speed

    limit.

    The aircraft designed in this way may handle fairly well at slow speeds (high angles of attack) but will be severely

    limited in high speed (low angles of attack).

  • Tony Elliott 05/2008 Page 3

    Summary 1: Reducing camber at the tips allows us to control low speed tip stall but unfortunately causes the

    wing tips to lift downwards at higher speeds net result is a very narrow flight speed envelope.

    One can counteract the low angle of attack (high speed) downward lifting tips by introducing wash-in (raising the

    LE of the tip airfoil) so that it stalls after the root airfoil at high speeds but this would then cause the tips to stall

    before the root at slow speeds!! By adding wash-in to resolve the high speed issue at the tip, you counteract the

    advantage gained at slow speeds at the tip! There is no real good solution to be gained by reducing tip camber.

    Summary 2: Adding wash-in to counteract downward tip lift at high speed causes low speed tip stalls to occur

    Now lets look at the opposite scenario low camber at root, high camber at tip.

    Example 2: Increasing Camber at the wing tip no wash-in or wash-out added.

    By having the increased cambered airfoil at the tip and the lower cambered airfoil at the root, you will see from

    the upper right quadrant of the lift diagrams (fig 2 below) that at high angles of attack (slow speed) the tips will

    stall before the root which is NOT good. If you carefully look at the lift curves, specifically at their stall points in

    this upper right quadrant, the higher cambered airfoils at the tip DEMONSTRATES wash-in specifically

    aerodynamic wash-in (not physical or geometric wash-in) by the fact that the tips will stall before the roots at low

    speed.

    Now, If you review the lower left quadrant of the graph (low angles of attack or high speed), the root airfoil

    approaches the negative lift point BEFORE the tip which is good since the root is likely to stall before the tip.

    Summary 1: increasing camber at tips causes tip stall at low speed but at high speeds, the tips still lift as they

    should do.

    With reference to figure 2, the reader may now see a method of correcting the low speed tip stall caused by the

    higher cambered airfoil at the tip. If we now move the left most lift curve (the lift curve for the tip) to the right in

    the diagram (see Fig 3 below), we can move the low speed stall point much closer to the root low speed stall

    point. In fact, we need to move it just past the root airfoil stall point so that the root stalls before the tip at low

    speeds. By moving the highly cambered airfoil lift curve to the right, we also reduce the difference between the

    zero lift angles of attack in the lower left quadrant. Note that for a family of varying cambered airfoils, the

    differential between the low speed stall points in that family is smaller than the differential between the zero-lift

    angles of attack. Therefore, we can move the curve to the right by an amount that causes the stall to occur at the

    tip at a higher angle of attack than at the root hence controlling low speed tip stall!

    Summary 2: By adding wash-out equal to the zero-lift angle difference between root and tip airfoil, we control

    tip stalling by moving the tip airfoil stall point PAST the root airfoil stall point PLUS we control high speed tip

    airfoil lift over a much larger flight speed envelope.

    The process of increasing camber at the tip and then moving the tip lift curve to the right by adding wash-out

    really opens up the flight envelope of a flying surface. The process almost eliminates tip stalling while increasing

    the high speed flight range because the complete wing has the same zero-lift angle of attack (tips do not lift

    downwards at high speeds).

  • Tony Elliott 05/2008 Page 4

  • Tony Elliott 05/2008 Page 5

    Still not clear? Lets view it from a different angle (no pun intended). Lets look at one lift curve say the highly

    cambered lift curve. This curve represents the coefficient of lift as it changes with respect to angle of attack. Our

    goal for controlling tip stall is to move the low speed stall point of the tip airfoil to a larger angle of attack than the

    low speed stall point angle of attack of the root airfoil. From figure 2 we see that the stall point of the tip occurs

    BEFORE the root. In other words, the tip airfoil has already reached a POSITIVE, NOSE UP angle of attack to cause

    it to stall. We need to pull the nose of the tip airfoil downwards by a distance that lets the root airfoil stall first.

    Pulling the nose of an airfoil down means that you raise the TE of the airfoil wash-out! Moving the li