1. Had not allowed sufficient margin for the gusty wind
conditions
2. Trees and hangers that were on the upwind side of my touch
down point may have perturbed or blocked the flow so that when I got
within their shadow (leeward side),where I lost
considerable airspeed very quickly
3. I should have paid more attention to other pilots who have
reported "down drafts" in that area (including one who reminded me today,
that he had touched down 20 ft short of the runway on that same approach
with his wife on board (for her first flight with him) and he claimed he still
had above stall airspeed when he hit the grass. But, hey I had been flying
out that airport for 4 years and never had encountered one (well, until Feb 12
that is {:>))
4. Every once in a while everything stacks up
NOT-IN-YOUR-FAVOR!
Monster Prop Air Brake
Well, I will add one more possibility (perhaps the most likely). I
found out today by experimentation that with the big monster prop (76,88)
that if I pull the power back sufficiently, the airspeed decays very
rapidly. Let me explain - It appears that so long as I have the prop
turning over fast enough so that it is providing a "no drag" to the air flow -
everything is just fine. However if I lower the power such that the prop
is no longer a "no drag" component air speed decays much faster than with my old
prop.
For instance if I am doing 89-90 mph (as if on final) with the engine
turning around 2800-3200 rpm everything is just fine. However, if (as I
recall doing on that hard landing) I retard the throttle to around 2000 rpm
(which I did that day as I was slightly over shooting my touch down point) , the
air speed decreases rapidly and the rate of descent goes up considerably.
Never had that type of thing happen with the smaller prop or at least not to any
noticeable extent.
I speculate that the combination of the larger disk area (25%)
of the 76 vs the 68 inch prop dia and the 2.85:1 gear box offering more
resistance to the airstream turning the prop (compared to the 2.17). So if
the prop is turning at least fast enough so that is not a drag,
there is no noticeable effect, but once the engine is retarded to where it
becomes a drag component to the air stream - it apparently be comes a "BIG" drag
component.
So don't know whether this theory holds water, but the effect is there and
I found today that keep the engine at 3200 rpm resulted in a considerably
difference. Any ideas from you prop guys??
Ed