Originally Posted by
Pilot DAR
Oh, by the way, from that steeper descent angle, it will require a greater acceleration upward (G) than normal to flare. Slightly greater G requirement means that stall speed goes up when you pull. Cessna test pilots have learned the same lesson I've learned during flight testing, that the ten knot excess speed above stall on approach is just not enough.
A steeper descent angle does not require a greater acceleration to flare. The guy is talking about a steeper descent resulting from lower airspeed to begin with and I'm quite sure about this because it's in all the textbooks - centripetal acceleration = v²/r. Therefore, load factor in the flare is closely approximated with the formula 1 + v²/rg. The load factor reduces as a function of v². Or, for the same load factor you can flare lower with a tighter arc.
Note, I am not saying the margin has not been reduced. What I am saying is your explanation is wrong. It does not follow that the steeper descent angle requires “greater upward G”. That is not the problem. The problem is you have less kinetic energy available for the flare, which becomes a limit in the power off case. As the angles are quite small, the energy you are going to use up can be approximated with D x r x approach gradient, where D is the average drag. So increasing the gradient obviously eats into the energy margin. Your load factor results in a shorter radius (and this wins out over the extra induced drag) so you can save some energy (within the limit of CLmax) by increasing load factor. So in one sense, the steeper angle may lead you to increase load factor – assuming you have a margin to begin with - but that is to shorten the radius to save energy. Nonetheless, in the scenario given, of reduced airspeed, the steeper descent angle does not “require greater acceleration upward”. This stands to reason when you simply consider the fact that you have a lower vertical velocity to start with.
FWIW I think 172 driver's suggestion of reducing the airspeed below best glide is a subtle and efficient way of controlling the glide ratio in the engine out scenario. It's one option to get back to the nominal glide path. I would not be messing about with it on short final, nobody is saying is saying you should persist with some low airspeed like 1.1 Vso right into the flare.