hoooooolllllyyyyyyyy s**t......

[DeltaTango]

It had been a long and satisfying flight.
Beautifull weather, and not so much traffic to report. We were comming back from a 2.5 hour flight which was devoted to a radio stations listenners on a need to know basis about the various traffic jams up and down the coast.

It was the last of 2 trips I had to join, in order to be scheduled to fly the highly complex sortie with the mission of reporting traffic jams.

Anyway, 10 miles out from home, the pilot suddenly started to tighten his harness and make some safety turns after which he turned and asked:
"How do you feel about spins?''

I quickly tightened myself as best as I could to the seat and replied very calmly:
"F....f.....find w...with me......"

So he went on to a couple of half spins, I remark this since they looked nothing like any stalls I have ever performed, and we turned for home.

My doubts about the maneouver has opened new questions, one of which are: What exactly happenes during a spin???!?!?

Please fill in the missing bits this clouse:
One wing stalls and then_____________because_____________and the result is_________________.


Thank you
DT

[willbav8r]

Erm, I think you'll find BOTH wings are stalled.....

[mad_jock]

Depends what tye of plane you are in. But both wingsare always stalled.

Some are more docile than others. A cessna C152 has to be worked to keep it in and will usuall recover itself without the shove forward the POH recommends. Now the PA38 with half tanks on board will be a different kettle of fish. The rotation will be 3-4 times faster than the cessna and you will loose vast amounts of height very quickly. But if you follow the POH it does come out at the bottom. Typically for a 3000ft height loss it will be 3-4 rotations depending how much fuel you have on board in about 15 seconds. It also sounds scary as hell because of the stalled wings there are huge vortexs shedding off the wing which because it has a high T bounce off it causing the whole thing to shake, the control runs up the tail bang off the skin and cause a hell of a racket. Which when combined with the wing drop cause people to call it a Terrorhawk. Personally I luved instructing in them and a student who learn in a tommy found cross training onto other types a breeze.

MJ

[B2N2]

All about flying

Check out this link http://www.av8n.com/how/#contents
item #18 deals with spins but lots of good info on this one...
Have fun..

[Thumpango]

PA38 in a spin

Mad_Jock,

there are lots of stories about PA38 spins, but just how badly do you need to treat a traumahawk before it gets upset and bites?

I really like the PA38 and have grown to love it but this area does worry me a little - well, more than a little actually!

[mad_jock]

The CAA made them put strips on the leading edge which gives you buffet now so the stall isn't quite as instant as it used to be.

Basically if you don't control the wing drop on the stall it will flick you into a fully developed spin. But it will come out again by its self. But it happens that fast its very likely that you will go through Vne.

I have managed to get it to flick through into a spin using roll to correct wing drop. But it requirers such a hurrendous attitude I don't think your average PPL would even come close to getting it into that part of the envelope.

The reason why most FI's don't like Spinning the tomahawk is that most FI training is done on a C152 or the like which dosn't really spin properly and you have to work to get it into a fully developed spin. The tommahawk is much faster and if they don't know about carrying low fuel will come as a bit of a shock.

It also takes a while to be able to patter the spin properly. You usually have to do a couple first with the student before pattering it because of the amount of swearing coming from the LHS.

MJ

[B2N2]

Spinning a Pa38

Have been at least 12 fatal stall/spin accidents in Traumahawks.
Most of them on skill tests/checkrides with experienced pilots/examiners usually for initial FI rides.
One examiner managed to recover at very low altitude and told a horrible story of the yoke being stuck in the full back position.
They later figured out it was a design error in the yoke/panel configuration.(AD issued)
Also the Pa38 was designed to be assembled by an unskilled labour force (to keep prod. cost down) and the prototypes used for certification had additional strengthening in the aft fuselage and wings that was omitted in the production versions.
There's a rumour that the Piper Chief designer at that time sent a memo to Piper HQ to recall all Pa38's and shred em.
You'll never see me flying one..imo it's a low wing T-tail 152, so they got everything wrong...lol

[wobblyprop]

I learnt in a tomahawk and remember the instructor saying there was an AD to strengthen the tail if you intentionally wanted to go spinning it. Presumably a 4 point harness would be nice, too.

tomahawk scared the life out of me the first time i stalled it.

[mad_jock]

I think most have the tail mod now for the cracking problem.

Also its manditory to have 4 point harness if you spin them.

I believe a student managed to lodge themselves forward across the instructors lap which must have been fun at the time.

They are a good trainer, plenty of room, instructor can get at the fuel selector easly. Quite responsive, huge rudder.

Worst thing about them is that awful trimmer.

MJ

[BigEndBob]

When demonstarting the effect of rudder the Tomahawk pitches down before any significant yaw sets in.....very strange.
The Tomahawk always seemed like a nice aeroplane missing a wide chord wing and low tail.
Flying clubs love them because they are cheap ....and so are Ladas.
These days i bet there are very few Tomies that haven,t had their front end rebuilt!

[mad_jock]

When demonstarting the effect of rudder the Tomahawk pitches down before any significant yaw sets in

I think it could be because it has such a large rudder. When you put the rudder in there is a increase in drag which causes the pitch change.

The only other thing i can think of is the trimmer being that daft spring tension device, its not actually a proper trimmer. By altering the airflow round the back end you don't get enough tension on it.

I must admit i have to make a mental note about side slipping when flying a PA28 after the PA38.

MJ

[Send Clowns]

Second best advice I heard about the PA-38 was "never look at the tail if you're spinning the Tomahawk" (given by a man who had been an instructor for 50 years). Best advice was "never fly a Tomahawk". I took both

A description of a spin really relies on a demonstration with an aircraft model. In essence it is a "flight" condition with wings stalled and yaw. In a developed spin there is yaw, roll (in the same direction as yaw for an erect spin) and pitch at the same time. In many aircraft these combine to a stable motion about an axis close to one wing tip combined with rapid height loss (I seem to remember up to 11,000 feet per minute quoted for the Bulldog, though I may be mistaken). Other aircraft such as the Cessna 152 never really become stable and "nod" through the spin, which I find more uncomfortable although these recover more easily. The unstable spin and easy recovery are connected.

[Keygrip]

Interesting reading - the history of the terrorhawk - used to fly 'em out of Manchester and agree with all the tail wagging stories. Scared the c**p out of me during the instructor course - I don't believe the tail of any aircraft was ever designed to do what I saw it doing whilst spinning.

What I've been curious about (since reading the first post on this thread) is why was an aircraft - on what would appear to be a public transport flight, carrying a passenger - spinning?

[mad_jock]

Its better it flexs than it dosn't. Stiff tail equals high stress.

Well looking at the posters country I would have though that spinning wasn't half as dangerous as driving home afterwards.

Some instructors love them some hate them. I always liked them. Plenty of room proberly being honest being the main factor. Linked with low wing for teaching circuits and a sprung steel gear which as long as you keep the nose wheel out of the way the student can drop onto from 10ft with only there pride damaged.

Yes they are cheap, if the instructor enjoys teaching in them. If the instructor doesn't like them its a bit unfair on the student because the instructor will tend to get a bit snatchy with the controls at any sign of low airspeed. I have had a few inherited students from an instructor like this and discovered that the instructor had refused to do a full clean stall due to them ****ing themselves the first time they tried one. And had installed fear into the student about the handling aspects (the student though a 35-40 deg turn was steep)

MJ

[Finals19]

Here's a bit on the PA38 I managed to read up on:

.....the original PA38 prototype was markedly different from the production model - it had eleven ribs per wing (therefore a stiff wing) and a vertical tail offset by 2.5deg, and also the engine thrust line offset. The final test report gave this build the thumbs up as a excellent training aircraft. Piper observed that a prerequisite for this was that the wing used had to be stiff (GAW1 type wing). However when the production model came out, all but four of the wing ribs were removed, and there was no off set tail fin or engine thrust line.

The more flexible wing initially caused nasty stall characteristics and snap spin characteristics. Even after stall strips were added things only improved to a point.

There is also an apparent issue with W&B limitations. Near MTOW there is a possible issue with aft out of limits c of g when the seats are shifted to there rearmost points.......

I think I will stick to 152's which are very easy to spin and even easier to recover....

[Send Clowns]

It's better it flexs than it doesn't. Stiff tail equals high stress

The problem comes with metal fatigue and with breaking strains! Also with confidence. On a more serious note the aerodynamics of any surface can be altered by its movement. You must know about control reversal with wing deformation. Not sure what would happen in a spin with the tail waving all over the place!

[mad_jock]

H'mm breaking strains no aeroplane operates materials inside the plastic area of the stress strain curve to my knowledge.

Strain is related to stress via youngs moduli in the linear region of the stress strian curve so a breaking strain is really a buckshee punters way of defining breaking stress which again is a buckshee term. You have Yield Stress where the metal goes plastic and deforms, Ultimate Stress (sometimes is less than Yield stress but huge plastic strains have occured) when the whole lot fails and metal departs from each other under a plastic (ductile) failure, and Critical crack intensity when there is a crack and the whole lot goes with a bang.

In relation to the S-N curve for fatigue its highly non linear at the high stress levels. As the stress levels decrease the number of cycles before fatigue increases by factors of 10 quite quickly. From memory at 75% of Yield stress you can get 1e4 cycles out of steel and at 25% 1e8.

There for if you make the tail stiff there is high stress so fatigue becomes a problem along with critcal crack intensity which was why they stuck the plates in the tail to stop the cracking problem. But this wasn't to stop the flexing in the tortional sense which everyone can see and dosn't like. It was at the root of the tail where the control runs go up the front of the tail Flexible tail means low stress so fatigue is not a factor.

The only thing which isn't stalled is the rudder hence it is the only control that works. This can be seen if you spin an aircraft and while recovering shove the nose forward. When the tail starts flying again you will get a nasty neg G bunt and other nasty complications that the test pilots won't even try. So until you come out of the spin the controls apart from the rudder arn't effective so can't be reversed and after the spin the wings are unstalled so no more vortex buffet so no more flexing.

MJ

[Send Clowns]

But over time with a metal fin the breaking strain reduces, due to metal fatigue. I did indicate that this was not entirely serious, although it is not confidance-inducing. However if the fin twists then the rudder must become less effective, even if it does not go far enough to reverse.

[mad_jock]

What do you mean by strain? Strain is defined by change in length divided by the original length. (delta)L/L

The tail flexs in torsion, the top appears to move alot, but does it?

Yes it will move a little but what people actually see it the ends of the elevators deflecting up and down which are moving up and down by less than 10cms which I suspect is acutally very little movement in torsion on the actual emphange which is the bit thats doing the twisting. Its the arm away from the emphange which causes what appears to be large movement. Which when combined with the high T's inertial being so far away from the center of rotation makes for what looks like large deflections.

As soon as the rudder is put under load to correct the spin the whole lot will stop due to the load being more than the load being applied by the vortexs. This load will be factors in magnitude above that being created by the vortexs. And it will be the same as if the plane was a low T. The amount of twist that occurs will again be the same as the low T so control reversal of the rudder will be just as unlikely as in any other part of the flight envelope or any other type.

So with out doing any calcs on it I would reckon the actual maximum stress created by the torsional twisting of the tail in the spin is proberly just past the static port on the emphange. The place where it occurs is dependant on the section taper of the emphange. And is proberly lower than in a side slip.

And to note you cannot tell anything about the fatigue state of a structure by just looking at a displacement. Its the stress level which this deflection induces when analysed in comparision to a S-N (which stands for Stress-Number of cycles) material curve. Linked in with the history for that bit of metal. 1 tail scrape would take years off the life of the structure compared to spinning once a day everyday of the aircrafts life.

MJ

[Send Clowns]

Jock

Since there is no overall stretching in this case strain is the amount of movement. This still allows the concept of a breaking strain, which will reduce with fatigue.

In case you hadn't noticed (such as the hint that I said it directly) the comments on the tail actually breaking were not intended to be taken especially seriously. One is forced in aviation to trust the people who certify an aircraft that it will not fall apart. The point about the tail moving so much in that area is one of confidance - it does not increase with large movements. I also did not say anything could be told about fatigue life by movement. What I said was that fatigue can be caused by repeated movement well below the initial breaking strain, which it can. You only have to twist a paperclip to prove it. A tailscrape will of course make no difference to the fin, although it will cause damage in the tail boom.

More seriously if you are suggesting that the loads twisting the tail are much less than the normal loads imposed by control inputs

As soon as the rudder is put under load to correct the spin the whole lot will stop due to the load being more than the load being applied by the vortexs [sic]

then I would suggest that the twisting under the (greater) loads caused by the rudder displacement will be greater. Of course control reversal is caused not by cyclic flexing but by the twisting caused by control deflection.