185flier

85kt Sf=0 Balance
Wing lift up + 7709. lb
Tail lift up + 146.1 lb
W = 7855.1




7855.1
LW LT
0.4058 19.34'


7,855.1 * 0.4058 3,187.6 Tail up
302.3
2,885.3 149.2 lb up



7,705.9 * 0.4058 3,127.1
302.3
2,824.8 146.1 lb up



7,709.0 * 0.4058 3,128.3
302.3
2,826.0 146.1 lb up




7,709.0 146.1

≅ M
7855.1 ≅ F

185flier

Balance


7855.1
9370.5 19.34'

w




7,855.1 * 0.4058 3,187.6
9,370.5
(6,182.9) -319.7 Down

8,174.8 * 0.4058 3,317.3
9,370.5
(6,053.2) -309.7 Down

8,164.8 * 0.4058 3,313.3
9,370.5
(6,057.2) -313.2 Down

8,168.3 * 0.4058 3,314.7
9,370.5
(6,055.8) -131.1 Down

8,168.2 * 0.4058 3,314.7
9,370.5
(6,055.8) -313.1 Down






8,168.2 313.1


7855.1

185flier

the previous two posts are mathematical formulas that show for the type of airfoils on a caravan what the wing lifts at 0 flaps and at 20 degrees flap
the weight and balance inf are from a caravan that crashed in Dillingham Alaska in Oct of 2001`
an aerodynamic engineer did this

an eye witness described the accident as a normal t/o and at about 700 agl the nose pitched up followed by a left turn and then the aircraft fell to the ground

what happened was an airflow seperation from the elevator (that is why v/g's are full span on the rear of the stab for better elevator control with an uploaded stab) which caused the pitch up and if you were to push control in that calls for the stab for more lift which it can't do and as the nose continues to pitch up the main wings stall and down you go with no way to recover conventionally.

you can recover by deploying the flaps and creating a download and gain control of pitch

185flier

if you read the report it says ice was a contributer
we de-iced the aircraft twice that morning with close to 200 gallons of fluid
similar to the one in ohio
we had 18 eyewitnessess inspect the a/c within minutes of impact and there was no ice on our plane

Lost

Those numbers really do not mean too much without a foundation of where exactly they came from.
There are many factors, i.e., is this in level unaccelerated flight? CG location (I would hazard a guess that it is outside the limits aft)? Who came up with the formula? I only see output there? Wind tunnel results?

All I really know is that there would be negative dynamic stability if the CG is between the wing and the elevator. The wing produces a forward pitching moment, and with the the CG in front of the lift, the stab produces the restoring torque (read: down force) to counter act it. If you were to pitch the aircraft over and increase airspeed, the stab generates more down force and restores things back to where they were, after a few oscillations. That is dynamic stability. Works the other way also. Slow down, and some of that restoring torque attenuates, the nose drops, and goes back to "trim" speed. Nothing new here. All tried and true.
Now, go and re-arrange things and make the stab carry some of the lift function, and dynamic stability goes away. That is why we would have a wild (fatal) ride if we load much past the rear CG limit. Increase airspeed, and you get even more lift in the tail, and the nose pushes even further towards the houses.

Cubdriver

185, sorry but your data is too disorganized to follow. Perhaps you could organize it with labels for what the key quantities are and post it online with a link here.

As for the argument there may be upward tail load I would not dispute the possibility of that. But if you are arguing tail stall may occur then you have not shown that to be the case. To do so I think you would need to show the tail was beyond its critical angle of attack. Flight data is what really tells the story and hand calcs do not. I don't think you have that data, and it is not public.

As for the theoretical question can an aircraft be statically stable and have a CG behind the center of lift on the main wing, it can. The tail would have to be uploaded to compensate and as long as the overall center of lift for the two put together is behind the CG it is statically stable. If the CG is so far back the center of lift is equal to the CG location then it is neutrally stable. Unstable and neutrally stable CG locations are not approved for Part 23 aircraft.

There is a nice link that explains this idea here .

lzakplt

The first part of the above quote stands to reason, the second part you need to rethink, I think.

Picture a lifting tail, with a positive angle of attack in level cruise flight at say 150 KIAS. Now push the nose forward and retrim to 170 KIAS. How has the tail's angle of attack to the relative wind changed? Its less. Less angle of attack means less lift, more or less.

In the other thread you asked what, in my Caravan world view, happens to the tail airflow when tail aerodynamic forces shift from down to up? I think under the wrong conditions (too slow) the airflow over the tail fails to provide the required up lift when called for and the plane falls out of the sky. That, I posit, is why conventional aircraft design employs downforce only on the tail.

The most important thing to take away from all this is what 185flier already wrote. If you are retracting flaps in the Caravan and weird things happen, put them back to where they were when the plane was still flying agreeably.

Lost

Iza, nope. I am talking about dynamic stability, what we need to have a nice stable a/c.
In the example above, you have it trimmed for 150 knots. Then without trimming, push the stick to 170. With the cg behind the cl, it will diverge and start accelerating well past the 170. Until parts start coming off, etc. With positive stability it would return (sooner or later) to 150. But only with the tail providing a down force.

Ever fly a plane with the cg further back than was comfortable? Gets all loose right? Well, imagine it even further back. It becomes impossible. That is what it would be like with the stab providing lift. You would get some great efficiency on your way though.

185flier

it doesn't matter if up or down force it's how the plane reacts
in a tailplane stall with a download the nose pitches down usually on approach in ice that is what we train for
in an upload it is simply opposite imagine flying upside down
the airplane flies and usually the factor of length of flight has to do with the engine not the stabilising factors
cessna says and they are right
" it does not matter to the pilot if it is uploaded or downloaded, when you push forwards the nose goes down, when you pull up the nose goes up "

they are right
it is only during stall recovery that it matters

many people think me a wacko but just remember carrie extra speed it does not hurt and if during flap retraction you get a nose up pitch redeploy the flaps

even if you do not believe me doesn't that make sense

DamonMeyer

True - the Cherokee line is noticeably more efficient but squirrelly in pitch with an aft cg loading. I've flown a Warrior near aft cg limit; with the all-flying stabilator, trimmed it up for level flight - you get your best cruise speed, but induce a pitch oscillation and the recovery is very anemic compared to disturbing pitch from a fwd cg plane trimmed up for level flight.

In this Caravan crash, what are people positing as the reason for the flow separation at the tail, if not ice?