[quote author=Colonel Sanders link=topic=4060.msg10920#msg10920 date=1472608894]The faster you fly the second half of the inside
loop, the less time you have to fall and the higher
you are at the exit.
Every experienced aerobatic pilot knows this
from experience, even if the exact theoretical
explanation might elude them.
The optimal flight condition for this is actually the
top left corner of the Vg diagram: Clmax at max
structural G. You're pulling lots of G, producing
lots of lift but also enormous of drag. To oppose
that drag and maintain speed, you need full throttle.[/quote]
Pulling lots of G ... Now i get it. Would less energy also mean a lot less G at exit ?
Any male pilot pushing sixty is going to wanna make sure to get that bladder good and empty for "the second half of the inside loop" (a whole science on it's own). What's the norm there .. going from zero/neg G inverted immediately to 2 maybe 3?
There's some video about a Pitts accident which shows the low energy at the topgate very clearly ... an older Yugoslavian .. about 7-8 years ago /listed in the ASN database. It's plain to see there, using a minimum energy estimate for the loop allows it to be vulnerable sooner to any smaller/unforseen negative performance factor; and it's clear as per the Col's description (exceptionally informative read) is then never an issue with ample energy at the correct top/inverted and full/excess power thru the extra drag / G's of exit.
Marcus Paine
-
Colonel
- Posts: 3450
- Joined: Wed Apr 29, 2015 10:31 am
It's actually perfectly ok to have very low airspeed
at the top of a loop (inverted) as long as you know
what you are doing.
At a loop entry, you could pull +8G, or +4G. The
loop with the lighter G is going to be bigger, so you
are going to convert more airspeed to altitude and
be going slower, over the top of a really big loop.
In fact, this is one way to screw up a loop - not pull
enough G at the entry, and try to fly a HUGE loop
that your aircraft simply isn't capable of.
Or, you could pull too much G, incur too much drag,
and kill your energy that way.
Anyways. Let's say you are almost stopped at the
top of a very big loop - indicating say 10 mph. But
your stall warning isn't on - why is that? Even though
you are inverted, you have a positive angle of attack,
so your flight instructor will tell you that you should
be stalled, because you are below Vs.
However, your flight instructor doesn't have a clue.
Vs will only attain Clmax under certain conditions,
including +1G. You are not pulling +1G at the
top of a loop. The G is very light and slightly
positive, if you do it right.
I ask people all the time: when your aircraft is
parked on the ramp, is the stall warning on all
the time? It ought to be, because you are less
than Vs, right?
But the wing isn't doing any work - the landing
gear is holding the airplane up, parked on the
ramp. Not the wings.
Similarly, going slowly over the top of a loop,
the wings aren't doing any work. You have no
AOA. If you try a newbie stunt and put some
AOA on, at very slow airspeed, you will instantly
stall and incur huge drag as the Cd ramps up.
What pilots need to realize is that as they go
slower, they cannot ask much of the wing. But
when they go fast, they can ask plenty of the
wing and generate lots of G.
This relationship between speed and G is not
well understood.
The Kicker: at slow airspeed, the tail starts
to wag the dog. Specifically, the prop can
overpower the flight controls and make very
bad things happen. When going slowly, get
the power off, to avoid torquing off into an
inverted spin, which I suspect is what happened
at Cold Lake. I've done it more times than
you can imagine. It's no big deal if you are
comfortable with inverted spins at very low
altitude, which is pretty much what happens
at the end of every tumble. Warbird guys
don't do that much.
Again, despite what you might hear, I vastly
prefer to be in an inverted spin vs an upright
spin at low altitude. I can recover very quickly
from just about any inverted spin, and that
certainly isn't the case in an upright spin.
Why is that?
Hint: look at this rudder:
[img width=500 height=374]http://www.pilotfriend.com/aircraft%20p ... s10/18.jpg[/img]
at the top of a loop (inverted) as long as you know
what you are doing.
At a loop entry, you could pull +8G, or +4G. The
loop with the lighter G is going to be bigger, so you
are going to convert more airspeed to altitude and
be going slower, over the top of a really big loop.
In fact, this is one way to screw up a loop - not pull
enough G at the entry, and try to fly a HUGE loop
that your aircraft simply isn't capable of.
Or, you could pull too much G, incur too much drag,
and kill your energy that way.
Anyways. Let's say you are almost stopped at the
top of a very big loop - indicating say 10 mph. But
your stall warning isn't on - why is that? Even though
you are inverted, you have a positive angle of attack,
so your flight instructor will tell you that you should
be stalled, because you are below Vs.
However, your flight instructor doesn't have a clue.
Vs will only attain Clmax under certain conditions,
including +1G. You are not pulling +1G at the
top of a loop. The G is very light and slightly
positive, if you do it right.
I ask people all the time: when your aircraft is
parked on the ramp, is the stall warning on all
the time? It ought to be, because you are less
than Vs, right?
But the wing isn't doing any work - the landing
gear is holding the airplane up, parked on the
ramp. Not the wings.
Similarly, going slowly over the top of a loop,
the wings aren't doing any work. You have no
AOA. If you try a newbie stunt and put some
AOA on, at very slow airspeed, you will instantly
stall and incur huge drag as the Cd ramps up.
What pilots need to realize is that as they go
slower, they cannot ask much of the wing. But
when they go fast, they can ask plenty of the
wing and generate lots of G.
This relationship between speed and G is not
well understood.
The Kicker: at slow airspeed, the tail starts
to wag the dog. Specifically, the prop can
overpower the flight controls and make very
bad things happen. When going slowly, get
the power off, to avoid torquing off into an
inverted spin, which I suspect is what happened
at Cold Lake. I've done it more times than
you can imagine. It's no big deal if you are
comfortable with inverted spins at very low
altitude, which is pretty much what happens
at the end of every tumble. Warbird guys
don't do that much.
Again, despite what you might hear, I vastly
prefer to be in an inverted spin vs an upright
spin at low altitude. I can recover very quickly
from just about any inverted spin, and that
certainly isn't the case in an upright spin.
Why is that?
Hint: look at this rudder:
[img width=500 height=374]http://www.pilotfriend.com/aircraft%20p ... s10/18.jpg[/img]
-
David MacRay
- Posts: 1259
- Joined: Wed Jun 03, 2015 3:00 pm
I definately partially blame the Colonal for my learning the difference between a stalled wing and one that is going too slow to generate enough lift (during landing) or a wing with zero air flowing generating no lift.
He actually took the time to discuss it on AvCanada. Quite a long time since text can be more difficult to communicate in.
He actually took the time to discuss it on AvCanada. Quite a long time since text can be more difficult to communicate in.