Understanding Va speed has sometimes eluded me over the years. The stated object of Va speed is to prevent structural damage in turbulence, but this speed is not always printed in the POH of many older airplanes. Va speed also decreases as weight is reduced which is presumably to limit G forces. I don't fully understand this because wing stress decreases as gross weight is reduced. I guess that means more Gs could be pulled at lower gross weights, but actual stress is lower with less weight. I have never read a good explanation why this is as it is.
I look at maneuvering speed a little differently and it has given me good results over the years. This may be a little bit off topic so I apologize in advance.
I like to fly into small airstrips that sometimes require tight maneuvering with reference to the ground. So, what is the tightest turn i can make and what is the minimum airspeed that the turn can be completed safely?
To tighten your turn over the ground you need to be at the slowest airspeed possible while simultaneously pulling the most Gs available without stalling the wings. The turn will be substantially tighter if flaps are used, but I limit flap use to settings that are also rated for climb. In my mooney that is 15 degrees, or takeoff flaps. In my previous airplane, a Cherokee 140, that was 2/3 flaps, or the "steep climb" setting. In my Mooney POH there is a handy chart showing stall speeds at various bank angles and different flap settings. I didn't see one in the Cherokee POH so I generated my own.
Stall speed increases with the square root of load factor. Load factor increases with the reciprocal of bank angle cosine. So at 45 degrees bank angle V-stall = Sqrt(1/COS45°) * V-stall at 0°. Cosine 45° is 0.7071, the reciprocal is 1.4142, the Sqrt of that is 1.1892, so if the 0° stall speed is 60, the 45° stall speed is 71.35. If I was flying this airplane at 45° bank I would keep the airspeed indicator at or above 74 during the turn. If the bank angle is 60°, then the 60° stall would be 84.85, and I would keep my airspeed indicator above 88 during the turn.
In my Mooney the stall speed is 67mph no flaps and 64 mph with takeoff flaps. At 67 mph the 45° stall is 79.7 and the 60° stall is 94.8. I fly using 85 and 100 respectively. At 64 mph the 45°stall is 76.1 and the 60° stall is 90.5. I fly using 80 and 95 respectively. I use these numbers because they are easy to remember. All these numbers are well below Va of 130 or 120 when lightly loaded.
I also have a lift indicator that gives me further insight to prevent stalling the wings in a turn.
Hope this helps. One Miracle at a time.
