How long can a Cherokee stay aloft
Dave St.Clair
would stay airborn after full engine failure. I haven't done any
practicing in this area since my student days (all of a year ago :-))
and that was in a later model Warrior II.
Let's assume about 7500' AGL, best glide speed and no wind.
My wife asked me this and I really didn't have a good guess. As
I'm a low time pilot, I'm reluctant to do an empirical study.
thanks,
dave
HOWARD PETRI
and am a glider pilot who worries about such things. The general consensus
is that a power plane has a best L/D of 10 to 1; ie it glides forward
10 ft for every 1 ft of altitude loss. I am going to guess that
the best L/D speed of the 180 is 80 mph. From an altitude of 7500 ft
you should be able to glide 75,000 ft before you hit the ground, ie
14.2 mi. Using the old standby formula Time = Distance/Rate and
plugging in the numbers you get 10.8 min of time aloft. Of course
this is just an approximation, sinking air and not holding the best
L/D speed can change everything. This is a reasonable ball park
figure to start with though.
Howard
Pegasus (EJ)
Michael Corvin
|> Does anyone have a guess how long a Cherokee 180 (180E to be exact)
|> would stay airborn after full engine failure. I haven't done any
|> practicing in this area since my student days (all of a year ago :-))
|> and that was in a later model Warrior II.
|>
|> Let's assume about 7500' AGL, best glide speed and no wind.
|>
This is easy to figure out, given the data in the emergency section in
your POH. Working from memory (SHOULD be correct) for a Cessnoid 172:
Best glide: 65kts
Best glide ratio (from plot of height vs. glide range): about 9:1
Condition: windmilling prop, no flaps
calculate:
1. GLIDE SLOPE: arctan(1/9)= 6.34 deg
2. RATE OF DESCENT: 65kts * (6080[ft/nm]) * (1/60[hr/min]) * (1/9) = 732 fpm
[note: 1/9=approximately sin(6.34) [= approximately tan(6.34)]]
From 7500ft AGL:
3: GLIDE RANGE: 9 * (7500[ft]) / (6080[ft/nm]) = 11 nm
4. GLIDE TIME: (7500[ft]) / ([732[fpm]) = 10.3 minutes
CAVEATS:
a) The POH 9:1 glide ratio is optimum, achieved with a clean, perfectly rigged
airplane and perfect pilot technique (trim, trim, trim!!) in still air.
The actual will probably be less (8:1 is probably conservative enough).
b) The calculation gives no margin for landing. In practice (as in gliders)
The landing spot should be decided on by 1000ft AGL and one should be
setting up a regular pattern for it (well, in gliders we can start the
pattern lower due to the better glide ratio). At 1000ft you probably will
be on the ground in barely more than a minute. Subtract 1000ft from the
starting height to give a glide range to the landing site of 9.6 nm
and 8.9 minutes.
c) Wind will significantly affect the glide range by changing the
ground speed but will not change the glide time. The speed to achieve
best range will increase above best glide when going against a headwind.
The speed for best range with a tailwind will be slightly lower than
best glide but not less than the airspeed for minimum sink (which is
typically not too much less than best glide - ie stick to best glide).
c) Dave, you must reside in shallower lands... around here 7500ft AGL would
be around 14,000ft MSL :-) and few folks get that high in a straight 172
or a Cherokee ;-).
If it's a boomin' thermal day, or you catch ridge or wave lift one could
conceivably extend the glide - but this is probably _highly_ inadvisable in
an emergency situation. (I've actually thermalled a C-152 in the summer
here in Denver, climbing at almost 300fpm in a 45deg bank and throttled back
to about 1400rpm... one mother thermal!.. centering was tricky since the
VSI has so much lag compared to a variometer.)
BTW: You can check the glide ratio for your airplane by rearranging the
calculation in (2). While flying at a safe altitude in still air, trim
to the best glide speed (at idle, don't forget carb heat if conditions &
procedures warrant), let things stabilize, and note the rate of descent.
Then:
N =(approx.) (6080/60) * (GLIDE SPEED[kts]) / (DESCENT[fpm])
GLIDE RATIO = 1:N
(The approximation is again because I'm not using the trig function since
the glide angle is small...) Note that with a dead engine the ratio will
probably decrease (get worse) noticeably compared to an idling engine.
If these calculations confuse, draw a "velocity" triangle. The airplane
glide speed is the hypotenuse (a practice illegal in 15 states...) and
the other sides are the rate of descent and the horizontal velocities.
Hope this helps. There will be a pop quiz tomorrow...
(sheesh, am I getting long-winded in my old age!...)
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Michael Corvin PP-ASEL, PP-G zw...@starfighter.den.mmc.com
just another spaced rocket scientist at Martin Marietta Astronautics Group
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=============== My views, not Martin Marietta's ========================
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Ove Kristian Pettersen
I did my ground-training on a 181. As far as I can recall the speed for best L/D is about 87 mph.
But if the point is to stay airborn as long as possible, 87 is not the speed to
go for.
87 gives you the possibility to glide the longest distance.
If one lower the speed a bit (I cant recall how much) till you end up at speed
for minimum sink, you will stay airborn longer than if you settle for 87 mph.
Speed for minimum sink is nearly never used on an engined plane, but with
a glider it is used very often.
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ovekp