engine failure on climbout
Guest
Posts: n/a
engine failure on climbout
I read this on the rec.aviation.student NG and found it an interesting discussion.
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I usually advocate climbing out, after takeoff in a lower-powered trainer
aircraft, at Vx. That's to get high enough, while still close enough to the
runway, to be able to turn around if the engine fails and get back to the
runway. But if the engine does fail, and if you're slow at getting the nose
down -- spending the proverbial 4 seconds wondering what that lack of noise
up front is -- there's a real danger of stalling.
A reviewer of my recent book Performance of Light Aircraft, Wayne Olson, who
was the head performance engineer at Edwards AFB before his retirement,
pointed this out to me and did a quick "back of the envelope" calculation of
how much the airplane will slow down in four seconds.
It turns out the equation of motion along the flight path can be integrated
exactly, so I did that and here are some representative figures. They are
for a Cessna 172 (160 hp with a 7557 prop), climbing out at Vx = 63 KCAS =
111.1 ft/sec true, with flaps 10 at 3300 feet density altitude, weighing
2400 pounds. This is the same as in the sample "turnaround" problem treated
in detail in the book on pages 296-303.
If the engine fails and the pilot maintains the same flight path (it won't,
exactly, so this is an approximation), the airplane will lose speed, in four
seconds, down to 49.4 KCAS. Stall speed (wings level) is 48.5 KCAS, so he or
she only has one knot to spare. So, as they say in the birthing suite,
"Push!" (But not that hard.)
The airplane decelerates because of drag (initially about 240 lbf, 57% of
the total) and also because of the inclined flight path (about 178 pounds,
43% of the total). But the surprising thing, to me, was that drag INCREASES
during the slowdown. That surprised me because I initially reasoned that
dissipation should be getting less since speed is slacking off, but I was
wrong. That old induced drag will get you if you don't watch out! Here are
the sizes of the parasite and induced drag components, in lbf, at the two
(initial and final) airspeeds:
At Vi = 111.1 ft/sec, Dp = 90.3 and Di = 149.1 for total D = 239.4;
at Vf = 87.6 ft/sec, Dp = 56.1 and Di = 239.8 for total D = 295.9.
So drag has gone UP 57 lbf during the 4 seconds.
If you're climbing out at Vx and it gets quiet up there, push.
John.
--
John T. Lowry, PhD
Flight Physics; Box 20919; Billings MT 59104
Voice: (406) 248-2606
Web: http://www.mcn.net/~jlowry
[email protected]
___________________________
.
------------------
"..You must ensure you don't cross his nose, give him a shot at you. That is critical!" Malan
____________________________________________
I usually advocate climbing out, after takeoff in a lower-powered trainer
aircraft, at Vx. That's to get high enough, while still close enough to the
runway, to be able to turn around if the engine fails and get back to the
runway. But if the engine does fail, and if you're slow at getting the nose
down -- spending the proverbial 4 seconds wondering what that lack of noise
up front is -- there's a real danger of stalling.
A reviewer of my recent book Performance of Light Aircraft, Wayne Olson, who
was the head performance engineer at Edwards AFB before his retirement,
pointed this out to me and did a quick "back of the envelope" calculation of
how much the airplane will slow down in four seconds.
It turns out the equation of motion along the flight path can be integrated
exactly, so I did that and here are some representative figures. They are
for a Cessna 172 (160 hp with a 7557 prop), climbing out at Vx = 63 KCAS =
111.1 ft/sec true, with flaps 10 at 3300 feet density altitude, weighing
2400 pounds. This is the same as in the sample "turnaround" problem treated
in detail in the book on pages 296-303.
If the engine fails and the pilot maintains the same flight path (it won't,
exactly, so this is an approximation), the airplane will lose speed, in four
seconds, down to 49.4 KCAS. Stall speed (wings level) is 48.5 KCAS, so he or
she only has one knot to spare. So, as they say in the birthing suite,
"Push!" (But not that hard.)
The airplane decelerates because of drag (initially about 240 lbf, 57% of
the total) and also because of the inclined flight path (about 178 pounds,
43% of the total). But the surprising thing, to me, was that drag INCREASES
during the slowdown. That surprised me because I initially reasoned that
dissipation should be getting less since speed is slacking off, but I was
wrong. That old induced drag will get you if you don't watch out! Here are
the sizes of the parasite and induced drag components, in lbf, at the two
(initial and final) airspeeds:
At Vi = 111.1 ft/sec, Dp = 90.3 and Di = 149.1 for total D = 239.4;
at Vf = 87.6 ft/sec, Dp = 56.1 and Di = 239.8 for total D = 295.9.
So drag has gone UP 57 lbf during the 4 seconds.
If you're climbing out at Vx and it gets quiet up there, push.
John.
--
John T. Lowry, PhD
Flight Physics; Box 20919; Billings MT 59104
Voice: (406) 248-2606
Web: http://www.mcn.net/~jlowry
[email protected]
___________________________
.
------------------
"..You must ensure you don't cross his nose, give him a shot at you. That is critical!" Malan
Guest
Posts: n/a
When I'm clearing a new type, I always make a simulated engine failure in the steepest permitted climb a part of the tests - I have on one occasion imposed a placard steepest permitted climb attitude because of this very issue, and know of other people who have done the same.
I've also tried to model this, my estimate of the deceleration rate following a sudden engine failure whilst trying to maintain level flight (not a climb) was dependent on a fairly complicated formula that I'd be glad to email to you if you ask, but came out about 1.5 - 3.5 kn/s depending upon the drag and inertia (say 2 kn/s for a typical light aircraft).
Having said that, when I've tried to validate my formula by flight testing it, I get actual values about half that - say around 1 kn/s for a light aircraft - probably 3 times than that mind you in a climb (I haven't got any precise numbers in that case). This would seem to agree with your values.
I've also tried to model this, my estimate of the deceleration rate following a sudden engine failure whilst trying to maintain level flight (not a climb) was dependent on a fairly complicated formula that I'd be glad to email to you if you ask, but came out about 1.5 - 3.5 kn/s depending upon the drag and inertia (say 2 kn/s for a typical light aircraft).
Having said that, when I've tried to validate my formula by flight testing it, I get actual values about half that - say around 1 kn/s for a light aircraft - probably 3 times than that mind you in a climb (I haven't got any precise numbers in that case). This would seem to agree with your values.
