Time to refresh our stall awareness. ALL Lancairs should have stall strips.

 

This is a paper written by Mark Kirchner, IVP owner, concerning stall strips.  I have them on my IVP.  It stalls straight ahead with a definite buffet and very docile.  I only do these at 10,000agl with a factory pilot on board.  There is no wing drop.  I don’t do power on stalls, accelerated stalls or spins.

MARK KIRCHNER NOTES ON STALL AND SPIN PREVENTION

Washington Lancair Builder's Meeting, February 2001

Objectives:

If stall characteristic provides a "wing-drop", recovery might require substantial altitude. Therefore, eliminate "wing-drop" characteristic.

If the aircraft wing loading is high (L-IVP is 32- 34 #/sq.ft.), even an incipient stall entry might require more altitude for recovery than is available during final approach. Therefore, provide adequate means to insure speed margin during final approach --- and inherent aircraft buffet warning if speed margin is violated and stall condition is approached.

If, during the banked turn to final approach, distractions due to weather, traffic, or whatever, result in the pilot pulling excess "g’s" during the turn, then the stall angle of attack can be reached while still maintaining the target approach speed. Of course, a combination of factors -- such as drifting below target approach speed due to the same distractions - can amplify this risk. Therefore, a means to provide a warning of the approach to a stall angle-of-attack -- independent of the maintenance of accepted speed margins - is desired. Again, the added warning of wing-buffet is desired.

Technical Approach:

Adding wing "stall strips" to the inboard leading edge of the wing works on both the "wing-drop" problem and the objective of developing wing-buffet warning.

Installing the Frantz-type AOA-instrument, along with judicious calibration, works on the objective of providing adequate stall angle-of-attack margins and normal approach speed margins.

WING STALL-STRIPS

Wing stall strips have been discussed at least three times in the Lancair Network News.

In Issue 33, Martin Hollman's original recommendation was described. It consisted of 16" strips, starting 24" out from the side-of-body. In Issue 36, Bob Young described a construction technique for adding these to the wing leading edge --- and at the same time, pointed out that Martin Hollman had revised his recommendation to 18" long strips.

In Issue 37, Peter Yates (from Australia) pointed out that the Australian regulatory offices would not certify his aircraft (even though a homebuilt) without meeting certain minimum stall qualities. Yates' aircraft, being the first of type in Australia, would set the standard for future L-IV certifications in that country. Yates ran a very interesting and extensive set of flight experiments using different lengths of stall strips. His objective was to make the minimum mod that would be accepted by the governmental agency. Also, since he noticed significantly higher touch-down flare stick forces with the stall strips, he wanted to keep the change to a minimum for that reason. (Note: I fail to notice significant flare stick-force differences with strips.) Another important factor influencing the Yates design is the right-wing-drop characteristic that is typical in the L-IV stall. Yates figured that it was more important to balance the stall by working on the left wing (by stalling it earlier) than to have identical stall strips on both wings. His final design had a stall strip only on the left wing, 8" long, starting 18" out from the side-of-body. His tests showed that the aircraft had a balanced wings-level stall, with reasonable buffet warning, and low landing flare stick-forces. The strips were triangular in cross-section with about a 9/16" base attached to the wing and 3/8" sides leading to the leading edge apex. The center-line of the strips met the leading edge of the wing about 0.2" below the leading edge vertical tangency plane intersection. (This is the vertical orientation originally suggested by Martin Hollman). The idea, of course, is that the apex of the stall strip be placed as closely as possible to the airfoil cruise stagnation point (so there is no cruise drag created), but still allowing stall initiation at high angles of attack.

Taking all of the above into account, I decided upon the following design for N776CM.

 Same vertical positioning as described above.

Strips on both wings; 10" long starting 18” out from the side-of-body.

(THIS IS WHAT I HAVE AND ADVOCATE---- Charlie K.)

I decided to go for both wings to help promote balanced stall regardless of power effects or unsymmetrical flight effects. I decided on 10" lengths because I wanted to minimize the size, but felt a couple of inches longer than Yates' successful design would be conservative. Also, I hadn't seemed to have the problem of landing flare stick forces that bothered Yates.

I found both the buffet speed margin and the strength of the buffet to vary with power setting and flap setting. A range of approximately 3 knots to 6 knots buffet speed margin was experienced. The initial stall break is balanced without wing drop, although I have never held the airplane into the stall beyond the initial break to test the characteristics beyond. I do not recommend that the airplane be tested beyond initial stall break.

The increase in stall speed due to the stall strips was small and difficult to measure -- so I would guess 1 to 2 knots is close to right. The same is true with respect to the effect upon cruise speed at a given power. It was difficult to measure any change in cruise; this implies that the vertical location of the stall strip at the leading edge was close to optimum.

Finally, I would advise that any testing ( even simple approach-to-stall testing) be conducted at 10,000 feet AGL or higher.

FRANTZ AOA (Angle-of-Attack) INSTRUMENT

There are two major benefits for using an AOA instrument to provide stall margin compared to simply using airspeed. First, at one "g" flight, the stall airspeed varies with the square-root of aircraft weight; therefore, if a constant (average) target landing-approach airspeed is used, it is only an approximation for very heavy or very light weights. Secondly, at a given airspeed (which may be chosen to provide adequate stall margin at one "g"), the margin diminishes rapidly in a turn -- without any indication to the pilot as to the extent.

The AOA instrument allows one to index the desired margins with respect to wing angle-of-attack, and the margins stay the same regardless of weight. And, for a given airspeed, the "g-effect" is automatically included in the display (and aural warnings). One can select different margins for different flap settings. For example, my landing margins are used only for full flaps. For partial flaps, I have larger margins because I feel I do not need "tight" margins if I am not on final landing approach. I have experimented quite a bit with different calibrations (margins) for my instrument. Here is what I have the instrument set up for at the present time. The example numbers are for a typical landing weight of 2900 Ibs. (2250 Ib. W.E. + 2 men + 45-50 gal. fuel). For other landing weights the airspeeds would vary as the square root of the weight (for the same AOA reading).

At 12" Man. Press. Full flaps:

2900 lbs. weight, gear down.

Target Vapp.    1g Aural Warning        1g Light Buffet         1g Stall Break

99 knots        84 knots                70 knots                65 knots

(1.52 Vs)       (1.29Vs)                (1.07 Vs)               (1.0 Vs)

Several comments:

Those who have experience flying heavy transports may know that their published approach speeds are 1.3Vs (compared to my 1.52Vs, above). However, the factors that govern the requirements for margins tend to favor equivalent incremental speed margins rather than equivalent speed ratio margins. Thus, the lower speed aircraft needs a higher speed ratio margin than the higher speed aircraft.

The 99 knot approach speed (for the average landing weight shown) is essentially the same as the 100 knot approach speed advocated in training at Redmond.

For a light landing weight of 2600 lbs, the Vapp would be 94 knots; for a heavy landing weight of 3200 lbs, the Vapp. would be 104 knots.

        At the target approach speed, the Aural Warning would sound at 1.38 g.

The AOA instrument has one other feature that should be noted. It has an Aural Warning at an absolute speed (not angle-of-attack) for "Gear Down". This can be set at any speed. I have mine set at 110 knots. I fly a target speed of 120 knots on instrument landing patterns (15 in. 2500 RPM, Half-flaps gear-up). If I drift down to 110 knots, I get a gear-down aural warning --- which, since I am slow, means I have probably crept up in altitude from my target pattern altitude.

My theory on why the aircraft has a right-wing-drop at initial stall break: The propwash provides an incremental upwash on the left wing and downwash on the right wing. However, this is concentrated primarily on the inboard part of each wing. In order to balance this, a small amount of down-aileron on the right (and up-aileron on the left) is flown. Although a down-aileron wing section (like a flapped wing) stalls at a higher total lift, it does so at a lower wing section angle of attack than does an up-aileron wing section. Thus the right wing stalls first, providing the wing drop.

Carl Cadwell

 

I second this.

Charlie K.

 

 

 

 

Charlie K.