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Scott, I think the difference when talking about this might be in the assumption of reference axes. I'm using the flight path for the creation of the lift, drag, thrust and gravity directions, but you could use absolute vertical and horizontal axes as well - the numbers come out the same. So in a descent the axis is tilted from horizontal by the angle of the descent (a "zero degree descent" would then be level flight). So the drag is slightly in the up direction, overcoming a tiny bit of gravity. The statement someone made that lift (or more accurately "wing load factor", if you will) is less than 1 G during a descent is technically correct, but practically it is close enough to 1 G to make the discussion moot. For wide deviations from level flight, such as during aerobatics or even
the no-power turn back to the airport, the assumption that descent angle doesn't affect the load factor is less accurate.
My comment about doing the turn referencing instruments is one borne of inexperience in the conditions of high bank angles, no power and close to ground. I just don't know how one (me, at least) could perform the turn-back accurately referencing outside the window. For instance, what does the ground look like during 60-degree no-power bank? How does prevailing wind affect the view? I know the instruments will look the same as when I practice that at altitude.
I'm afraid I lost your spreadsheet, but I have created one in the past that I'm sure says about the same thing. The accuracy of mine decreases at extreme bank angles (over 60), so I don't know whether or not there is a true optimum bank angle. Regardless, I'm sure it is higher than a typical pilot will be able to accurately control. So the target then becomes the highest bank angle that will allow the pilot to maintain a "safe" margin above stall. This discussion has been very useful to me, as are most of the inputs to the List. Gary
Gary,
Please go back and re-read what I wrote. Where did you find the
formulation that the reduction in wing load is related to the cosine of the descent
angle in a wings level descent at some arbitrary speed? After all, then
the "cosine" of a zero degree descent should be 1.
Remember that in a banked turn there are two components, wing loading
(perpendicular to the aircraft) and vertical due to gravity. Do not be
confused by a term like G as it
is only used as a substitute for the weight of the
airplane.
Please plug some numbers in the spread sheet. Note that as turns become
more shallow the turn rate slows, the aircraft travels a greater distance
and there is a greater loss of altitude. If you are below a critical
altitude, a 30 degree banked turn will not get you back.
Instrument reference is best used for engine failures at night or that
risky takeoff in 0-0 conditions.
Good Luck,
Scott
In a message dated 6/6/2013 3:44:42 P.M. Central Daylight Time,
casey.gary@yahoo.com writes:
Terrence, Scott and
George are all correct - sort of. George said in a
steady-state descent the wing is still supporting 1 G, but Scott says it is
supporting less than 1 G because of the descent. For a typical descent of
500 ft/min at a speed of 180 statute miles per hour the wing is supporting
99.95 percent of the weight (the cosine of the descent angle, which is 1.8
degrees). So for all normal climbs and descents George is essentially
correct. Of course, for a vertical climb or descent the wing supports nothing.
For a higher descent angle, such as for turning back with no engine, is
the descent angle significant enough to change the stall speed? I haven't
run the numbers, but I suspect it is a very small factor compared to the
increase in lift required for the
bank.
Here's the technique I think is theoretically correct, and one that I have
practiced. As Terrence said, "Angle, angle, angle." When power failure is
first perceived, simultaneously roll into a steep bank while keeping the
AOA at the optimum value with back pressure on the stick. Initially, that
will require forward stick movement - remember, just because the plane is
banked doesn't mean the G force goes up. Now as the airspeed increases,
increase back pressure to hold the same AOA. With an AOA-indicator-equipped
plane you only control 2 things - the bank angle and AOA. When you are
again pointed at the runway (at an angle, but don't be picky) immediately level
the wings. What bank angle? I haven't run the numbers, but as Dave has
said, it is a
steep angle. The steeper the angle the more difficult the
maneuver, so I have picked 45 degrees as my personal target. 60, 70, or even
more might be the theoretical optimum, but that requires more skill than I
think I would have in a crisis situation. The completion of the turn could
happen quite close to the ground, but the extra speed required for the turn
will be used to arrest the rapid descent and return to the "normal" glide
speed (remember to hold the AOA after the wings are level).
What to do if your plane is not AOA-indicator-equipped? The maneuver is
still the same, but you have to control G loading as a function of airspeed.
Of course, you likely don't have a G meter either, so you have to use
your own derriere for that purpose. The mental gymnastics get to be
a real
challenge and I suspect that very few pilots would be able to accurately
control AOA during the maneuver. The result is that the bank angle has to be
reduced to maintain some degree of accuracy in AOA. I would guess that 30
degrees bank might be a good target for most non test pilots. If you get
the AOA too high you will certainly arrive at crash scene much sooner - too
low and you will lose more altitude than necessary.
I'm sure that the maneuver can best be performed in reference solely to
instruments, as the view of the ground, close-up, oddly angled and rapidly
rotating would be a huge distraction. Practicing at altitude doesn't really
prepare one for that. However, it does prepare you to concentrate on the
instruments, and that might help. In
principle, the turn is exactly
similar (my favorite words) to the Chandelle performed for the Commercial ticket,
except done without power.
Just my 2 cents worth,
Gary Casey
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