Lost

Not sure what crash is being referenced. Surely, not any of the recent ones.
And unless it is listed as probable cause, keep in mind it is only someone's theory.

lzakplt

Hey Lost, do you work for Cessna?

lzakplt

In court testimony it came out that the tail on the Caravan produces positive lift with the flaps up. It produces downforce with take off flaps, 20 degrees. So when removing t/o flaps tail force is switching from down to up. If the flaps come up at too low an airspeed, the wing can interfere with airflow over the tail at the point when the needed tail force is switching from down to up. The plane pitches up, the wings stall, and you have an accident.

Cubdriver

Izak, your theory isn't convincing to me. First of all, there is no reason for a tail stall at less than the critical angle of attack. The point of shifting from downlift to uplift is not by any means a likely time for a stall, it is the least likely time. If force exerted by the tail is transitioning from negative to positive, it probably is not a very large force to begin with and even an incorrectly designed stabilizer should be able to provide it. A change in downwash direction due to flap retraction would reduce the angle of attack of the stabilizer as the downwash angle will move toward a more agreeable, horizontal angle as the flaps go up.

If you were to say the tail did not stall but simply could not produce enough lift to satisfy load demands on it whether it be upward or downward, then that would more likely be when the flaps were down, not up. Flap transition should not introduce disturbance to the airflow. If so, how? It should improve the airflow as they retract. If on the other hand you said that putting the flaps down somehow degraded performance of the horizontal stabilizer it might be convincing, but I don't think you are saying that.

Another thing that comes to mind is a stable airplane has a negative d(CM)/d(CL) curve slope, which means the moments around the CG become more negative or nose down as the angle of attack is increased, and become less negative or nose up, even positive as the aoa of the airplane reduces. Although the aoa is reducing while the flaps retract, and moments on the CG are headed toward positive/destabilizing, it is also headed for a smaller magnitude at the same time. This is what is good about a negative slope d(CM)/d(CL) curve. As the airplane returns to a near zero aoa, it develops at the same time a small positive moment around the CG, which keeps it flying rather than starting a descent.

Just to clarify, negative stability curve is a good thing because it produces positive stability. Don't confuse +/- moments with +/- stability, that's a different thing. Negative moments are nose down moments.

185. You have an idea for a recovery technique based on speculative theory, a few hand calcs and zero flight test data. You propose to alter the operating procedures for Caravan and you may do what is required to avert catastrophe, but I would not recommend your proposed recovery technique if it does not appear in the Operating Procedures for the airplane. You would be second guessing carefully written and researched procedures made by authorized professionals, people who use comprehensive theory and testing to establish safe guidelines and have been through all the possible flight configurations and conditions. I know you have a lot of hours in Caravan, but they are not flight test hours. Perhaps you should contact Cessna and ask the flight test people what they think. I would do that before inventing my own procedures. They will probably just refer you to the POH, but they would probably listen to and discuss your theory with you. I admire you for having such an interest in the airplane.

Cubdriver

This is what I would like to know. I am not trying to be hard nosed but I have not heard a convincing theory yet. An unloaded stabilizer is quite possible and would be known to the designers. Such things are calculated as well as flight tested, there is no gray area. If they wanted an upload it would be designed for it. The argument that VGs are a band-aid solution to a poorly designed tail is too far-fetched.

lzakplt

Airflow separation may be a more accurate way to characterize what is happening to the tail than stall. This isn't a theory 185flier or I came up with. Sometimes Caravans crash for no apparent reason. The NTSB is not God, and their findings of probable cause are not always correct. (I think its just easier for them to fault improper deicing than to think that a plane with 25 plus years in service could have a serious, unknown problem.) I was briefed on an engineer's report several years ago detailing this phenomenon and it made sense to me.

The way I think it happens is the plane is climbing out and at too low an airspeed and the flaps are retracted. The angle of attack is high enough that when upforce is called for on the tail, the wing is effectively blocking it out. Airflow over the tail is normal until it needs to transition from negative to positive lift. Wings' downwash effects on a tail falling directly behind the wing may be more conducive to a tail with negative lift than to one with positive lift.

Picture someone at a low climb speed pushing flaps selector fully from 20 to zero, then diverting his attention elsewhere. If airflow separates over the tail when positive tail lift is required, and if right at that moment he continues to allow the flaps to retract, he is going to have a very bad day. All it would take to right this unfortunate situation (aside from not having retracted the flaps at too low an airspeed in the first place) would be to put the flap lever back down to where it was before the problem occurred. Then get some more airspeed, carefully, incrementally retract the flaps and continue the flight without incident.

Right at the moment when airflow separates over the tail the result is an uncommanded nose up. There is no Cessna recommended recovery technique for uncommanded nose up, so how is what 185flier and I recommend here in any way contravening Cessna?

185flier

iza is right
there is no stall, but a seperation of airflow over the elevator which does not allow sufficient uplift of the stab to control the nose

PenAir had an accident in 2001 and in the lawsuits that arose there were many depositions

PenAir deposed a cessna engineer who confirmed the tail had to carry an upload at certain weight and c/g loadings

they also deposed a test pilot who said that during testing he got into 4 unrecoverable stalls and had to deploy a chute to regain control

i was going to fly and see if i could duplicate but not after hearing that a cessna test pilot had been in unrecoverable situations

Cubdriver

So you are saying that downwash alone causes the flow separation. I doubt it especially in an airplane that has long coupled tail location and a horizontal tail that is only a few inches below the mounting height of the main wing. Two categories of causes of flow separation are common: surface contamination is one, and exceeding critical angle of attack is the other. Downwash is not one of them. And please do not go doing spins in an airplane that is not approved for spins, without a parachute, without a drag chute and loaded to the aft CG limit!

185flier

no i am not saying downwash just that there is a seperation and i wish cessna would tell us

don't worry i don't plan on trying to get an airflow seperation after hearing about the test pilots

lzakplt

Yes, what I'm describing is not common, and yes Caravans have been successfully completing most of their flights for a long time. If that's enough to make what I'm saying unbelievable to you, no worries. Fly safe.