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
From: Michael Newman <mnewman@dragonnorth.com>
Subject: [LML] Re: Engine out gear down Issue/The procedure! This was a wake up call for me...
To: lml@lancaironline.net
Date: Wednesday, January 27, 2010, 4:52 PM
I am a LOBO instructor. I own and fly a Lancair IV-P.
I am also a sailplane instructor.
This discussion reminds me of one in the sailplane community talking about deploying full spoilers and being able to flare properly. It was thoroughly debunked there as it should be here.
The issue is carrying enough energy in the form of airspeed to arrest the rate of descent without going below the stall speed.
There is no
question in my mind that the IV-P with gear down, flaps down, engine stopped and spoilers fully extended can carry enough energy to arrest the rate of descent (flare). 120 knots will be more than enough. I have landed my IV-P with full spoilers, gear and flaps down. 95 knots is plenty of airspeed to arrest the descent in this configuration with the engine as pulled back as I can get it. I doubt a truly dead engine will add as much more drag as the full spoilers.
Gliding at 120 knots
with a stall speed is in the range of 75 knots is a differential of 45 knots. This is enough to climb a few hundred feet much less arrest the rate of descent. Work out the physics and you can see how much altitude you gain for converting this much horizontal speed to vertical speed. In sailplanes we expect about an 800 foot altitude gain in a pull up from 135 knots to 60 knots. Possibly a surprise to people here is that a Lancair will not be much different. It is energy conversion not drag that matters.