>It seems lower AR requires more pitch to maintain speed and
>control deflection, and is less prone to windup.
>
>Higher AR requires less pitch, less control deflection, and
>is more prone to windup (it seems to make more energy from
>the wind).
>
>Higher AR definately makes more lift.
HI guys,
At the risk of sounding like Al, you guys need to read some of the stuff written (by myself among others) about aspect ratio.
Doug's discoveries, as Brett says, aren't really discoveries at all but rather a re-expression of why higher ARs can be a double edged sword; a subject that has been discussed in pretty much all of my articles as well in well informed scholarly publications which have a great deal more merit than do my infantile fumblings over aerodynamics.
Lift efficiency can be simply described as a ratio of the amount of lift produced compared to the amount of drag developed in the process. In the aviation world this is decribed as L/D or lift over drag.
For all the volumes of discussion we've seen on this forum about the importance of airfoils and the alleged superiority of one versus another, all remotely acceptable airfoils must really take a back seat to the effects of aspect ratio in terms of L/D.
The extremes are, again, the easiest way to illustrate the phenomenom. Compare the aspect ratios and glide performance of low (comparative) speed high performance sail planes and the high speed of delta winged jet fighters...then think of a stunt ship as something in between the extremes.
The mission of the sailplane is to gain the absolute maximum duration from a given amount of stored energy and speed is a non-issue. The delta winged fighter's mission, on the other hand, is to fly just as fast as it can and it carries its own supply of energy in the form of tons of kerosene. Neither aircraft can pretend to perform the mission of the other even though they are at the pinnacle of performance for their given mission. This is true even if they have roughly the same wing area and not totally dissimilar airfoils(not an unlikely possibility)
Aside from the energy source, the primary reason for this vast difference in capability of the two airframes is the aspect ratio of the wing. The sailplane hasn't enough energy available to drive its high aspect ratio wing to even a fraction of the speed at which the delta fighter excels (to say nothing of such things as structural integrity, etc}. The fighter, on the other hand, at speeds common to the sailplane will literally fall out of the air because what little lift can be developed at such a speed comes at the cost of so much drag that it will fall faster than it moves forward. (In reality, it will probably stall at such speeds because the angle of attack necessary to even attempt to create the lift required will exceed critical A of A and it won't actually be flying at all)
General statements can be made about lifting surface planforms (shape from the top view) that enlighten us in that vast realm between sailplanes and fighers. To wit:
All other facets being the same (i.e. airfoil, air density, air speed) high aspect ratio wings create large amounts of lift change with small and/or modest angle of attack changes at a given airspeed. While doing so they produce only a modest amount of additional drag. Drag produced as a byproduct of lift is known as "induced" drag.
Low aspect ratio wings of the same area as above require much greater angle of attack changes to produce the same amount of lift achievable by the high A/R at the lower A of A. The drag increase will be dramatically greater and the difference proportional to the difference in Aspect Ratio.
Both equal area wings are capable of roughly the same maximum lift potential because the lift properties of the surface are mostly dependent on the area and efficiency of the airfoil (coefficient of lift). This is not totally accurate for reasons doing with technicalities that could be discussed separately, but for our discussion only cloud the comparison; reynolds numbers and spanwise flow, etc.
The high A/R suface will produce this maximum lift at a comparatively low A of A and will, of course, stall at only slightly greater A of A.
The low A/R wing will produce this lift at a very high A of A (ponder the pictures you've seen of the Concorde on final approach and then think of a sailplane trying to emulate that pitch angle!)
Once again, the induced drag of the low A/R surface will be monumentally greater than for the sailplane type of wing and will require tons of energy to be expended in order to maintain the airspeed required to produce the lift the sailplane can generate by the simply utilizing its own mass and kinetic energy.
Stunt ships fall into a broad category of moderate aspect ratios generally a bit on either side of five to one. An aspect ratio of four to one will produce an stunter which will slow noticably in corners unless (as Doug points out) there is some mechanism to moderate that loss of airspeed, i.e. the 4-2-4 break, for instance. Wing loading will be a very big deal to such a stunter, especially if the power source is modest.
An A/R of six to one or greater, on the other hand, will produce a stunt ship which is much more amenable to heavier wing loadings because it creates a lot less drag in producing the lift necessary for corners of a given radius. It will require less A of A change to produce the lift and will be less dependent on flaps or high lift devices of other sorts.
I doesn't come without a down side, however! Because lift required for a given corner radius is more or less constant (a one g difference between "going up" corners and "coming down" ones), anything that changes the lift produced by the wing will alter the track of the high A/R ship more than one of the same area with a lower A/R. Because we fly in a tethered environment, when the wind blows our "airspeed" is constantly in flux as we fly into, out of, and across the wind direction.
Because lift changes as the square of airspeed, any changes in airspeed occassioned by our tethered environment and the wind will alter the lift produced. Thus, the tendency for loops to tighten up and flight paths to alter seemingly at random (but actually predictably) and manevuers like loops to wind up faster and faster.
For these reasons a higher aspect ratio will behave better with a power source that attempts to moderate changes in airspeed...the single speed piped run being ideal and the four stroke potentially even more so. A 4-2-4 run will complicate and exacerbate the problems to a point that they probably can't be solved to a great enough degree to reach a competitive platform in unkind wind conditions.
