Mailing List lml@lancaironline.net Message #54343
From: Chris Zavatson <chris_zavatson@yahoo.com>
Sender: <marv@lancaironline.net>
Subject: Re: [LML] Re: Airspeed sensing switch /Auto gear extension / Engine out gear down Issue!
Date: Wed, 27 Jan 2010 18:52:48 -0500
To: <lml@lancaironline.net>
Terrence,
AOA will compensate for weight very nicely.  It doesn't know, however, what descent rate is being flown and how much energy is needed to arrest that sink.  The energy needed goes up as a squared function of the vertical speed.  In the 360, I span descent rates from 500 to 2,000 fpm on final (ILS to short approaches).  The energy thus varies by a factor ~16.  It is the higher end of the scale where one can get into trouble.  
The sink rate introduces a time element that the AOA indicator can't factor in.  Here is a thought experiment to illustrate the point.  In the flare you bump your AOA from your mark to just under stall.  This will generate a new, higher quantity of lift that will immediately begin to decay as the new added drag slows the plane.  Now, the higher your descent rate was, the longer that new, higher, but tapering lift must act on the plane to stop the sink (F=ma).  The question becomes:  Do you have enough time before you run out of speed?  If you do, then you stop the sink and all is good.  If not, you smack the runway at a high AOA but still descending.
 
Chris Zavatson
N91CZ
360std


From: Terrence O'Neill <troneill@charter.net>
To: lml@lancaironline.net
Sent: Wed, January 27, 2010 2:47:03 AM
Subject: [LML] Re: Airspeed sensing switch /Auto gear extension / Engine out gear down Issue!

Chris,
This problem of calculating the changing speed for different weights, etc., for keeping reserve energy,  is a good example of a problem that vanishes if you use an AOA.  Mine has a mark at the AOA that leaves a Cl reserve for flare.  I don't have any idea (or care)  what speed I'm at during final to flare... just keep the AOA where there's enough AOA lift left for flare.
Amen to no power, full flaps, gear down for flare. I try to get back to the power-off approaches my ex-WW II-pilot instructors taught me in Pprterfields ... but it's difficult or frustrating when the guy ahead doesn't take his interval upwind, or when someone announces he's coming straight-in after I'm already downwind.
Terrence N211AL L235/320 

On Jan 26, 2010, at 10:10 AM, Chris Zavatson wrote:

"Gear and flaps down too early with a dead engine will have disasterous consequences every time."
"That is a deadly mistake. Dragging gear and flaps, you need a steep nose down attitude to keep the airspeed at a safe speed. Unfortunately, in that regime you can not flair the airplane. When you try to flair without a little power these airplanes just slow down and hit the ground."
 
"When you try to flair without a little power these airplanes just slow down and hit the ground. The situation is much worse if you are heavy. "
=====================================
Full flaps, gear down and no power should lead to a greaser every time.  Our Lancairs have a broad range of drag and thus require a good understanding of energy management.
At issue here is how one defines 'safe speed'. 
As the approach angle steepens due to higher drag, so does the required speed (or energy) to round out and flair.  That higher energy is used to stop the descent and bring the vertical speed to zero.   One can even calculate the speed required as a function of descent angle to bring the plane to level flight in the flare at the same final speed.  In general, I prefer power off, steep approaches.  That way if the engine hick-ups, it is no big deal.  When heavy, it takes a few more knots for the same maneuver, but you can still grease the landing.  It is all about energy management.  One cannot stick to a fixed number on the airspeed indicator.  Doing so could have you coming up short of energy.  The broad range of drag available to us on these planes is truly an asset.
 
Chris Zavatson
N91CZ
360std




Subscribe (FEED) Subscribe (DIGEST) Subscribe (INDEX) Unsubscribe Mail to Listmaster