lml@lancaironline.net Mailing List Archive

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.

Chris Zavatson

N91CZ

360std

www.N91CZ.com

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.

Gary