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Chris makes good points. The concept of needing X knots excess in
order to flare is dead on. It scales with the square of the speed just as he
says.
The comments by Gary about the heights gained are correct due to
energy conversion seem correct. Gary’s discusssion about low drag in a
sailplane resulting in much better results than our Lancairs is not based on
science. The difference from one aircraft to another is roughly the speed you
need to change from the sink rate in the descent to zero. To a first approximation
the rest of the energy picture is the same.
This is an experiment you can go out and try. I do not see much
difference between my sailplane and my Lancair in altitude gained in a pull up.
From: Chris Zavatson
[mailto:chris_zavatson@yahoo.com]
Sent: Sunday, January 31, 2010 11:58 PM
To: lml@lancaironline.net
Subject: Re: [LML] Re: Engine out gear down Issue/The procedure! This
was a wake up call for me..
After
this discussion I coded some calculation done years ago when a similar thread
was discussed. The results show vertical and forward speed over time
given a number of inputs like weight, initial steady state descent rate,
initial approach speed, and a bunch of aircraft specific parameters like wing
area, aspect ratio,etc. One parameter that turned out to be a big player
was how much of the remaining lift available was used during the flare.
That is, hard do you pull back on the stick. 40% seemed to make the
landing profile from a steep descent look right in terms of duration of the
round-out based on observing typical landings. It is however a variable
that can make or break a landing. It stretches or compresses the time in
the flare significantly and thus allows more or less decay in air speed.
Induced drag changes were less pronounced than expected, especially when only
using 40% of the available margin to stall. This is reduced further in
ground effect.
A
typical steep (1,500 fpm) power off flare is showing a 15 kt decay in
airspeed whereas the transition from a 3 degree ILS (500 fpm) only cost 3
kts. The amount deacy is strongly tied to initial speed. At a
higher initial speed the decay is less than for the same approach at a lower
speed. For example a steep approach case entered at 80 kts lost
18 KIAS while the same profile entered at 90 KIAS lost only 9 kts.
That makes sense given energy being a squared function of speed. A
little more speed gets you a lot more energy.
From: Gary Casey <casey.gary@yahoo.com>
To: lml@lancaironline.net
Sent: Sun, January 31, 2010 6:57:44 AM
Subject: [LML] Re: Engine out gear down Issue/The procedure! This was a
wake up call for me..
I ran through some numbers regarding Michael's comments
below. Starting with the sailplane comment, he says that slowing from
135kts to 65 in a zoom can result in a gain of 800 feet. If all the
energy were converted to altitude(no drag) the gain would be 1,276 ft, so a
gain of 800 ft seems reasonable for a very low-drag airframe - 2/3 of the
energy can be converted to altitude. In the case mentioned of 120 kts to
75 kts the number comes out to a gain of 765 ft. How much of that is
eaten up by drag? Certainly compared to the sailplane, a lot. Would
the altitude gain be half? I doubt it. 1/4? Maybe, so as a
guess you could count on perhaps a 200 ft altitude gain. How much do you
need to flare? Depends on the descent rate. I calculated it based
on 2,000 ft/min - arresting that is equivalent to an altitude gain of 34.5 ft,
much lower than the 200 ft mentioned above. It would be good if someone
measured the actual no-power descent rate with gear and flaps down.
In summary, the math suggests that a no-power approach speed
of 120 kts should leave more than enough energy to flare. How much more?
I'll bet not a lot, but still more. I don't have an answer to the
question, but this is how the numbers work out. I have done a full-flap
no-power descent with my ES (gear down, of course :-) at 105 kts and the
descent rate was over 2,000 ft/min with a frighteningly high negative deck
angle. Flaring from that condition would be interesting, to say the
least. My conclusion is that Randy's warning is well founded.
I would, perhaps, disagree with the admonition that
retracting flaps when on final will result in a guaranteed disaster.
Certainly any change in configuration at the last second creates a high
work load, but at least in my airplane, the difference in behavior between 20
degrees and 40 degrees of flaps is 90% drag. So, if one were to think he
was high for the landing, added full flaps and then discovered he was now low,
I see no problem with then retracting the flaps to 10 or 20 degrees.
Assuming the speed were high (120?) the flaps could even be retracted all
the way without problem except for the pitch change required. And then
dropping the flaps during the flare is a good way to arrest the descent.
I'm a little reluctant to post this last paragraph as I have no
credentials (no military fighter jet experience, no instructor rating, no
multi-engine jet time, and no stays in Holiday Inn Express) except for a modest
understanding of the engineering principles involved.
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