Ta 152

[dtango]

I just got into a nasty tail first stall. Aft tank was empty, and Combat Trim was off. I put elevator trim all the way down and attempted recovery. I lost 5,000 feet and never regained control.

Other AH airplanes besides the Ta-152 suffer from nearly unrecoverable deep stalls.

[Tupac]

Other AH airplanes besides the Ta-152 suffer from nearly unrecoverable deep stalls.

not with the frequency they occur in the 152.

[dtango]

not with the frequency they occur in the 152.

Why is this a problem and how?

[dtango]

I very much dislike answering people who claim something is wrong yet don't reasonably test the possibilities that it could be right.  But hoping to reduce further agony and torment for all involved let's cut to the chase.

Why does the Ta-152 exhibit unforgiving handling qualities leading to departed flight?  Already noted by others in this thread, the most likely answer hasn't changed from previous years (here and here):  ADVERSE YAW.  

Films & flight tests of the Ta-152's flying qualities reveal that under certain circumstances it will yaw and turn itself broadside into the wind and out of controlled flight.  This can occur suddenly.



The above is a 3-frame sequence of departure characteristic of all the films and flight tests I've seen.  (Yes Krusty, this is from your recent film.)  Why does the Ta-152 do this and under what conditions?  Is this consistent with aerodynamics?  We'll need to dig deeper than the topic has been discussed in the past for further understanding.  I'll have to leave it at that for the moment to give me some time to organize the explanation.

[Tupac]

The Ta152 definitely has LOTS of adverse yaw, but its not in keeping with a sudden departure of controlled flight, followed by an unrecoverable stall. I've never heard of a plane stall and crash because of adverse yaw.

I would compare what happens at the beginning of the stall to a ground loop, because its almost like the tail slides out from under you, and once it starts there is no stopping it.

[moot]

its almost like the tail slides out from under you, and once it starts there is no stopping it.

There is, just keep it in check with the rudder.

[dtango]

The Ta152 definitely has LOTS of adverse yaw, but its not in keeping with a sudden departure of controlled flight, followed by an unrecoverable stall. I've never heard of a plane stall and crash because of adverse yaw.

The question to ask is if it's possible.  The answer is yes.



Directional divergence can happen in the real world.  Our Ta-152 exhibits directional divergence characteristics under specific conditions.  "Directional divergence can be 'triggered' or aggravated by lateral control inputs if these control inputs generate significant yawing moments" (USNTPS FTM 103) .  This is what's happening with the Ta-152.  Thus the yawing moments are key to understanding what could be happening.

Aircraft Yawing Moments
What causes an airplane to yaw?  Summation of aerodynamic and inertial forces about the cg determines how an airplane pitches, yaws, & rolls.  About the yaw axis the moments consists of a bunch of coefficients.  Avoiding tequila shots and Tylenols the brain needs to absorb the gory math here's a nice (but not quite complete) diagram that mostly sums the main contributors to yaw.



What's of particular interest to us is what's labeled "adverse yaw".  More completely we'll refer to it as yaw due to rolling.  An airplane's roll and yaw are coupled for various reasons.  If uncoordinated with rudder, when an airplane rolls it naturally yaws, usually in the opposite direction of the roll.  This is known as adverse yaw.  I'll leave it to the reader to read up on the intuition behind why this happens.

Dynamic Factors of Yaw Due to Rolling
What we want to do is unpack the variables behind it.  Fortunately for us the propeller heads at NACA given us an equation to develop understanding.  Here it is:



where Cl=lift coefficient, p=roll rate, b=wingspan, V=airspeed.

The first observation that's vital to make is yawing moment due to rolling like all things aerodynamic is not a STATIC value.  It's DYNAMIC, meaning it increases or decreases as flight conditions change.  With the Ta-152 for it to yaw out of flight means that the yawing moment increases to a point that it overcomes other yawing variables that could balance it out so we're interested in what increases yaw due to rolling.

The obvious variable for the Ta-152 is it's wingspan.  Yaw due to rolling increases with an aircraft's wingspan.  But this is a static value and we can't stop here.  Adverse yaw also dynamically increases with increasing lift coefficient and roll rate, and with decreasing airspeed.  Thus adverse yaw increases when an airplane performs maximum maneuvering (generating maximum lift and roll) at slower airspeeds, the more the maneuvering and the lower the airspeed, the greater the adverse yaw.  

Max turning at high Cl, max rolling rates, slower speeds describe a broad range of maneuver fighting situations in AH.  The Ta-152's longer wingspan exacerbates this.  It's in exactly these situations a pilot needs to be more concerned to deal with the dangers.

This explains why Combat Trim increases adverse yaw as airspeed bleeds because elevator trim increases automatically which raises Cl to trim the plane for lower airspeeds.  That's why flight tests with CT off the Ta-152 is not as adverse yaw twitchy compared to CT on because elevator stick input with CT on at slower airspeeds is at a higher initial Cl.  Cross controlling also makes it worse.  Opposite rudder input from roll in a barrel roll maneuver exacerbates the adverse yaw.

For our Ta-152 maximum maneuvering with rolling at slowing airspeeds without proper coordination appears to create enough yaw due to rolling to cause it to tail skid out of control.  This is however easily avoided by either rudder coordination to offset and or pushing elevator down to reduce Cl while in a roll.


Real Life Ta-152 Flight Tests
From the aerodynamics it seems explainable why our Ta-152 can depart out of controlled flight from yaw due to rolling.  But what about real life flight testing of the flying qualities of the Ta-152?  According to Dietmar Harmann, he summarizes in English the findings from 1944 Rechlin flight tests of the 152-H as thus (thank you moot for the citation):



Notice the statement "Stability about the vertical axis weak.  Aircraft has a certain tendency to skid."  This seems to describe the weak lateral stability that can lead to directional divergence I mentioned at the start of this post.

We could discuss further details but I'm plum out of time and energy .

[Tupac]

Wow.....that is pretty encompassing......I guess it is ok

[Babalonian]

Tupac, during your stalls what are you doing with your throttle during this time?  Trim is of almost no use in the recovery.  You will experience a "teetering" motion while stalling backwards, step 1 would be to try and get that teetering rythym and start pushing down and pulling up with it on the stick, step 2 is then to "bounce" your throttle between 20% and 60% along with this teetering action you're also now going along with on your stick.  If that isn't enough or doing it, dump flaps and drop gear while keeping up step 1 and 2 (be careful with the gear though, once you nose down, imediatley gear up or you'll likely sheer them off).



Still working offline on my filming and "presentation" now as I'm starting to call it.  Looks to be I'm concentrating on aileron induced adverce yaw effects in combination with prop torque, and it's so far been very interesting (I'm learning a bit more) and I'm inducing some of the stalls I've had some of my greatest concern with. 

At this point, looking at the films I have so far, if you induce an adverce yaw with your ailerons while attempting a quick left roll with full throttle and even a nose-down angle at an extreme speed (~15-20 degrees, 450 mph +), you can not recover from it _just_ with your rudder if your first reaction to the sliding yaw is to fight it with your rudder, it continues to depart and eventualy lands you in a nasty stall.  If you cut throttle and keep up the rudder work, you can recover from it, but not without loosing a bit of speed.  In contrast, attempt this same settup but with a right roll (against the torque), and a little rudder slapping is all you need to recover, without loosing much in speed/E in comparison to the left roll.

Still trying to find the "goose" with my goose hunt though, with a nose down and full throttle and high speed, I feel like maybe the torque is being more overpowering that it should... it could be the rudder isn't comanding enoug hauthrotiy to counter the departure though, but that is contradictory to it being quite adequate in authority when countering a departure induced by a strong right aileron roll in the same situation.  Then again the problem could be that there isn't one at all, "I feel like" a fast roll during a high-speed nose-down dive with full throttle shouldn't so easily induce a yaw instability of such heavy-handed proportions when you roll to the left, but that's just it, it's a hunch that is being rather difficult to disprove or prove by anything concrete I can find, so maybe someone else here has something on it that can help me out?


^ is good but typical of any data and information out there that I can find.  Not detailed _enough_, and I'm starting to conclude that it might just be the way things are gonna be with it being a 152 afterall.

[Babalonian]

I very much dislike answering people who claim something is wrong yet don't reasonably test the possibilities that it could be right.  But hoping to reduce further agony and torment for all involved let's cut to the chase.

Why does the Ta-152 exhibit unforgiving handling qualities leading to departed flight?  Already noted by others in this thread, the most likely answer hasn't changed from previous years (here and here):  ADVERSE YAW.  

Films & flight tests of the Ta-152's flying qualities reveal that under certain circumstances it will yaw and turn itself broadside into the wind and out of controlled flight.  This can occur suddenly.

(Image removed from quote.)

The above is a 3-frame sequence of departure characteristic of all the films and flight tests I've seen.  (Yes Krusty, this is from your recent film.)  Why does the Ta-152 do this and under what conditions?  Is this consistent with aerodynamics?  We'll need to dig deeper than the topic has been discussed in the past for further understanding.  I'll have to leave it at that for the moment to give me some time to organize the explanation.

Just noticed, and it may be that the resolution isn't good enough, but in that image it looks like the rudder is being worked to purposely induce the yaw/stall, not fight it.  I hope it was intended for demonstartion purposes and not as an example of any potential problem/error.

[Stoney]

Just noticed, and it may be that the resolution isn't good enough, but in that image it looks like the rudder is being worked to purposely induce the yaw/stall, not fight it.  I hope it was intended for demonstartion purposes and not as an example of any potential problem/error.

Those pictures were from Krusty's original film.  Tango said that.

Still trying to find the "goose" with my goose hunt though...I'm starting to conclude that it might just be the way things are gonna be with it being a 152 afterall.

 

[dtango]

Just noticed, and it may be that the resolution isn't good enough, but in that image it looks like the rudder is being worked to purposely induce the yaw/stall, not fight it.  I hope it was intended for demonstartion purposes and not as an example of any potential problem/error.

The 3 frame pic is from Krusty?s film.  Primary control inputs don't show up on film so there's no telling what his rudder input is.  Sorry, I have no idea why this matters.  As I stated above cross controlling makes adverse yaw worse.

 

I feel like maybe the torque is being more overpowering that it should... it could be the rudder isn't comanding enoug hauthrotiy to counter the departure though, but that is contradictory to it being quite adequate in authority when countering a departure induced by a strong right aileron roll in the same situation.

  
Let's see...  The Ta-152's prop rotates clockwise thus torque rolls the 152 quicker left than right.  In a left roll the torque produced by the engine (running at a frick'in 2100 HP spinning a nearly 12" metal propeller) increases left roll rate (p)  which increases adverse yawing moment.  In a right roll the engine torque decreases right roll rate (p) which decreases adverse yawing moment.  

Please tell me, why would I expect adverse yawing moment to be the same in a left roll as in a right?

Still trying to find the "goose" with my goose hunt though...

Well, at least you stopped chasing the CG wild goose .

[Lepape2]

[...]
What's of particular interest to us is what's labeled "adverse yaw".  More completely we'll refer to it as yaw due to rolling.  An airplane's roll and yaw are coupled for various reasons.  If uncoordinated with rudder, when an airplane rolls it naturally yaws, usually in the opposite direction of the roll.  This is known as adverse yaw. I'll leave it to the reader to read up on the intuition behind why this happens.
[...]

As the plane rolls left, the right airleron goes down and provides more lift on the right wing tip than the left. This increases drag on an over sized wingspan thus making the plane yaw to the right profusely. If a left roll is executed at the same time as a tight turn, the "propeller gyroscopic" as like to to call the "prop gyro precession" will also provide right yaw due to the elevator pushing the nose upward. Chopping off the throttle will also increase the chances of an unrecoverable tail skid because the airflow generated by the prop over the tail to keep it stable at slow speeds becomes obsolete even if it causes the RPM to be quite slower.

EXCELLENT post by the way! This is a must read for many players here wishing to understand more about this mysterious and unknown part of aircraft dynamics.  

[...]
Please tell me, why would I expect adverse yawing moment to be the same in a left roll as in a right?
[...]

Its just because the adverse yawing moment is not dependent on prop generated forces but only on aerodynamic forces. Thus a CW or CCW prop will have the very same (almost) adverse yawing characteristics for the same airframe no matter if you roll left or right.


Very interesting topic!

[dtango]

Good addition of gyroscopic effects lepape  .  Any pitch rates would effect yawing moments (as can be seen in the Cn diagram posted).  I left it out of the discussion along with other things like inertial moments, p-factor etc. to keep it simpler.

Its just because the adverse yawing moment is not dependent on prop generated forces but only on aerodynamic forces. Thus a CW or CCW prop will have the very same (almost) adverse yawing characteristics for the same airframe no matter if you roll left or right.

Well actually as I was trying to help Babalonian understand engine torque does have an effect and the effect is aerodynamic, through Cn-p.  How?  If we look at yawing moments they can be broken into the following coefficients:



Cn-p (yaw due to rolling) and Cn-delta-a (yaw due to aileron deflection) are two components of adverse yaw.  

Cn-delta-a is what you're referring to in your description of adverse yaw.  Due to aileron deflection the upgoing wing produces more lift and induced drag which yaws the plane opposite roll.  The following is a nice conceptual depiction of it.



However Cn-p is also a contributing factor as well - yaw due to roll rate.  Roll rate changes the local section aoa which causes the lift vector to be tilted which induces yaw opposite roll.  Here's a pic that demonstrates this:



The greater the roll rate the greater the lift vector tilt and the greater the yaw.  Engine torque creates a rolling moment opposite the direction of prop spin.  For a CW prop this is a left rolling moment.  In a left roll, the engine torque adds to the left rolling moment increasing the rolling rate.  The increased left roll rate increases tilting of local lift and bingo, more adverse yaw.

BTW, thanks for the kind words.

[Lepape2]

Yes, I didn't really talk about the relative wind direction changing the AOA of each wing in a roll. As I remembered vaguely (thus unsure and not posted), I thought the descending wing would have had a higher Angle Of Attack and induce more drag but that didn't fit at all and was a mistake because it would yaw in opposite direction than reality (unless you suppress the effect of aileron deflection drag). The tilting of the lift vector backward on the right wing in a left roll better explains the 2nd phenomenon (Cn-p) you are talking about - in the TA152 case, enhanced by a superior roll rate induced by engine torque (better felt at slow speeds)).