Stalls and Thoughts

On Mar 16, 8:54 am, "Bob F." wrote:
Minor point. In aircraft engineering, you can interpolate but never
extrapolate, as the saying goes. IOW, given two data points, it's
acceptable to find a third in the middle someplace (interpolation), but
never go beyond or outside the graph numbers (extrapolation). You should
not make any predictions about what's out there. That's test pilot area.


Good point!

The A36 POH provides airspeeds for best glide (110 KIAS) and
maneuvering (chandelles and lazy 8s @ 120 KIAS).

The POH does not specify if this is at gross or some weight less than
gross, so I use these speeds at all times -- empty or not.

Dan Mc

I think "dragging it in" refers to the practice of setting up a
landing configuration far from the touchdown point, and then applying
lots of power to overcome the high drag configuration to make the
runway.

My experience with the term was this: during dual as a student pilot I
screwed up my angle of descent on a short field landing attempt. I was
using the a touch down point marking, well into the runway, as the
spot where I was supposed to put down. My instructor said, "You'll
have to drag it in to not land short of your aiming point."

Whereupon added power (not full), shallowed the descent angle to
almost level, and "dragged it in" to the touchdown point marker.

It was a craptastic landing, and illustrated much of what has been
discussed.

wrote in
:

On Mar 15, 7:57*pm, Bertie the Bunyip wrote:
"Bob F." wrote
innewsradndOZJqc580HanZ2dnUVZ_qOk
:

Yes, you learn a lot about Dutch rolls real quick. *...and I did
experience a elevator hydraulic cylinder stall once in a 707. *That
was an experience.

Ouch. I didn't know they had hydraulics on the elevator. I thought
the 707
*
was all tab control except the rudder.
I had a pitch problem in an A300 at about FL190 once. That was pretty
exciting, but since it was the automatics that caused it we were able
to disconnnect and get it all back under control. Scared the crap out
of us. We thought we had something on the airframe come loose and
cause the pitch

problems. Nothing else made sense until we got down and maintenence
diagnosed the problem. We got a mach buffet recovering (2.5 G) but of
course that part of it wasnt too dramatic because of the relatively
low altitude. It would have been a different story at 330 ( the 'Bus
was not good at altitude)

Bertie

2.5 G ... that would be something new in the passenger cabin. Get any
screams or spilt coke?


Nope, no pax on that flight.

the airplane had just come off a heavy check. A 'D' Check which is nigh
on a complete rebuild. It was a cluster**** of certification, paperwork
and miscomunication that caused the problems. For one thing the airplane
had had a complete refit of all it's control surface bearings, and i had
known that which is what made me think we had airframe damage. The guy
who had tested it after the rebuild had discovered that the stal warning
system and alpha floor protection ( automatics that engage the
autothrottles and apply power regardless of what you want) was firing
off too early.
All this hapened to us on this flight and then some. The power came on
and the nose pitched up fairly quickly ( because of the underslung
engines) the pitch trim failed simultaneously which allowed the nose to
come up even more. The FO was flying but I took it and disconnected the
autoflight and put the nose down. Out intercept alt of FL 190 was coming
up quick so I pulled ( doing over 260 indicated) and the nose came up
very quickly due to the stab trim being wildly out for the speed. That's
when we got the buffet. Analysis of the FDR after determined the G and
flight profile. Airbus tried to blame me but actually caught hell for
inadequate flight testing both during certifiaction and failure to
rectify reported faults from the D check flight test.

Speaking of g and jets, what kind of g loads can airliners like 707,
727, 737, and then the really big ones take?

Certified limits are +2.5 , -1.0 clean and +2.0 and 0.0 with flaps out.
I believe it's the smae for all jet transoprts. They'll come apart after
that. Scary, eh?
The airplane needed to be checked for damage after this exposure, but
there was none.


Bertie

Dudley Henriques wrote in
:

Bob F. wrote:
Ok Dudley,

Let's get into an area of your expertise. I've always wondered this.
On a P-38, when they first came out the Allison's turned counter
rotating (top blade towards cabin, as you would expect). But after
the first 12 or so proto's and for the rest of production, they
interchanged the engines. That is each engine's top blade rotated
away from the aircraft. Why? Serious, I really want to know.

Well Bob, I hope it's a friendly request and I'll take it this way.

To be absolutely truthful with you, P38 design and development really
isn't in my area of expertise but I'll give it a shot based loosely on
something an old friend Jeff Ethell once mentioned to me about the
engines. As you now they were Allison V1710's and thy were handed
inward in the XP38. The prop swing was indeed changed I believe in th
second run or even back as far as the YP38.
I'll admit it always puzzled me as well. I knew the Brits didn't like
the handed engines and even our side had maintanence issues. For many
years I thought it might have had something to do with the flow
direction linkages on the turbochargers but I believe the change was
made after gunnery testing indicated the change would improve the
airplane's stability in the gun firing equation.
Not absolutely certain, but I believe I recall Jeff mentioning it in
this context.


It was due to buffeting of the tailplane during high speed flight. The
flow from the props was supposed to be the problem and they decided to
try and they decided to try swapping the eninges around to see if it
made any difference. It did , so they they just left it like that.
As for the brits not liking it. I'd have no doubt about that. They
suffer more than any country I know of from "not invented here"
syndrome. If they didn't like 'em they didn't have to take 'em. they
could have continued along with their cute little Ansons and
Beaufighters.


Bertie

Dudley Henriques wrote in
:

Bob F. wrote:
Thanks for the response. I heard different stories but nothing that
made sense to me. The closest partial story had something to do with
the props slipstream on the stabilizer (H or V, I don't know) gave
them better performance (whatever that means) in high speed, high
powered dives. I could never get a complete story. I even talked
with 3 of the last pilots that shot down Adm. Yamamoto, when they
gave a talk at the Boeing Museum about 20 years ago. They had no
idea either.

The slipstream I believe could very well have been a factor although
I've never seen the Schlieren photography from the tunnel tests.
Apparently the direction of the slip stream spiral hitting the
vertical stabilizer from the inward props was causing issues, most
likely from any asymmetricals or differentials in the throttle
settings during gunnery. The guns solution requires a center ball or
there's a high degree of trajectory shift .
Tony LeVier would have been the guy to settle up on this issue. He and
Kelsey did most of the tests on the 38. I met him during the L1011
program. Great guy.



and he usd to post right here! Yeah, if anyone would know it would have
been the guy who test flew it!


I wouldn't be surprised if he had posted something about it over the years.
I know I have the answer written down somewhere, but where....


Bertie

Peter Clark wrote in
:

On Sat, 15 Mar 2008 19:39:13 -0400, Dudley Henriques
wrote:


I believe you are repeating wht I have said. I said that "dragging it
in" generally refers to flying the approach in the area of reverse
command or if you will behind the power curve. This is absolutely
correct. Coffin corner is the area behind the curve where sink rate
can't be stopped with power but requires reduction in angle of attack.
For a perfect example of an aircraft in coffin corner, see the Edwards
AFB accident involving a young AF pilot who got his F100 so deep into
coffin corner behind the curve he couldn't recover the airplane; not
enough air under him to reduce the angle of attack. He applied full
burner but couldn't fly it out on power alone. Reduction of angle of
attack was what he needed and he didn't have the room. THIS is the
definition of coffin corner and it most certainly IS in the area of
reverse command.

I thought coffin corner was the point where if you go slower you stall
and if you go faster you hit critical mach number?

Kind of , but the bottom side isn;t exactly a stall, it's also a mach
buffet. the main distinction being it happens at a higher than normally
indicated airspeed, and more crucially, a lesser angle of attack. The net
effect is the same, but it's important to distinguish between the two since
the picture when it happens is substantially different.

Just in case any of you guys are thinking of a VLJ.


Bertie

That's what I heard before. Makes you wonder. Who would have thought of
that? "Oh, buffeting, let' s swap the engines and see if that works."
More likely story is they accidentally installed the engines wrong and
someone said, "Hey, this thing performs better this way". You can see I have
a lot of confidence in American ingenuity.

--
BobF.
"Bertie the Bunyip" wrote in message
.. .
Dudley Henriques wrote in
:

Bob F. wrote:
Ok Dudley,

Let's get into an area of your expertise. I've always wondered this.
On a P-38, when they first came out the Allison's turned counter
rotating (top blade towards cabin, as you would expect). But after
the first 12 or so proto's and for the rest of production, they
interchanged the engines. That is each engine's top blade rotated
away from the aircraft. Why? Serious, I really want to know.

Well Bob, I hope it's a friendly request and I'll take it this way.

To be absolutely truthful with you, P38 design and development really
isn't in my area of expertise but I'll give it a shot based loosely on
something an old friend Jeff Ethell once mentioned to me about the
engines. As you now they were Allison V1710's and thy were handed
inward in the XP38. The prop swing was indeed changed I believe in th
second run or even back as far as the YP38.
I'll admit it always puzzled me as well. I knew the Brits didn't like
the handed engines and even our side had maintanence issues. For many
years I thought it might have had something to do with the flow
direction linkages on the turbochargers but I believe the change was
made after gunnery testing indicated the change would improve the
airplane's stability in the gun firing equation.
Not absolutely certain, but I believe I recall Jeff mentioning it in
this context.


It was due to buffeting of the tailplane during high speed flight. The
flow from the props was supposed to be the problem and they decided to
try and they decided to try swapping the eninges around to see if it
made any difference. It did , so they they just left it like that.
As for the brits not liking it. I'd have no doubt about that. They
suffer more than any country I know of from "not invented here"
syndrome. If they didn't like 'em they didn't have to take 'em. they
could have continued along with their cute little Ansons and
Beaufighters.


Bertie

"Bob F." wrote in
:

Right, but I'll think of another entertaining Q. I'm sure you'll hit
one. As Tony said, "We're just tawkin here".


Came across this looking for info on the P-38s problem.
This makes some kind of sense since the flwo fom the props would modify the
alpha the stab would experience and exacerbate tailplane mach problems.

http://www.princeton.edu/~stengel/MAE331Lecture13.pdf

Down lower on the page they discuss mach tuck and the info is 100%
accurate. What isn;'t mentioned is that the stab will also buffet and
anything contributing to the acceleration of air due to high alpha on the
tailplane will bring a buffet on at a lower airspeed. No tailplane
authority equals a pitch down moment equals more speed equals more buffet
equals .... you see where this is going..



Bertie

"Bob F." wrote in
:

That's what I heard before. Makes you wonder. Who would have thought
of that? "Oh, buffeting, let' s swap the engines and see if that
works." More likely story is they accidentally installed the engines
wrong and someone said, "Hey, this thing performs better this way".
You can see I have a lot of confidence in American ingenuity.


No, it was a deliberate move. They had some idea of what was going on even
then. Lippsich's stuff was well known worldwide even if the germans were
keeping the best stuff for themselves and contaptions like millers "Time
Flies" and the rapidly advancing experimental fighters were giving data
about what compressibility and buffet were all about. I know i have some
reliable info on it somewhere.

Bertie

"Bob F." wrote in
:

That's what I heard before. Makes you wonder. Who would have thought
of that? "Oh, buffeting, let' s swap the engines and see if that
works." More likely story is they accidentally installed the engines
wrong and someone said, "Hey, this thing performs better this way".
You can see I have a lot of confidence in American ingenuity.


Should have looked here first

http://en.wikipedia.org/wiki/P-38_Lightning


This seems to be a pretty accurate account of the teething problems the
airplane had and the remedies they used. i'm pretty sure the prop rotation
was part of the buffet solution, but this article seems to indicate
otherwise.


bertie