TH-55A teething

[Barannfin]

I just finished reading chickenhawk for the second time (first time after actually flying helicopters). I noticed something in the end of the book about the TH-55s when they were first introduced.

Seems they had a problem of tucking nose down when they practiced auto's. The a/c would nose over and the controls would have no effect. Control would be regained by pushing forward and somehow regaining control of the rotor system.

It sounds alot like negative-g on teetering systems, but not quite.

I have never even heard of this, so I was wondering if some people who do know could enlighten me. What exactly is going on during this type of loss of control? Also is this still a problem with that airframe?

thanks in advance.

by the way I highly recommend the book, has some amazing stories of what those hueys can do. (although about 90% of it I imagine would technically be illegal)

[Barannfin]

cmon guys, somebody must know what he was talking about in the book. :confused:

do any other 3 blade articulated helicopters have this problem??

[bigruss]

apparently thats what the aerofoil shaped thingo over the top of the cabin is all about. they had to put it on to somehow help with the airflow over the tailrotor i think. works alright now

[zachUK]

Sometimes on the 300, the nose really does drop dramatically when you lower the collective entering auto ... other times not as much. Not sure what conditions excaberate the tendency ... perhaps wind direction?

[Winnie]

Stepping on the wrong pedal will get your nose to "tuck" under! That is for sure, since one of my students showed me that once!
Otherwise, apparently, the Upper Canopy Slat as it is called, is to smooth out the airflow to the tail rotor, and not so much about the nose tucking under. The nose on any helicopter I believe will pitch down during entry into an autorotation, also true in H269's and this is corrected by applying aft cyclic. Hence when I teach my students autorotations the littany is: DOWN-AFT-RIGHT for down collective, aft cyclic and right pedal to get the auto going. simulteneously of course (opposite on those machine with the blades turning the wrong way:= ). Hope this helps somewhat.

By the way, if you want, go to www.ntsb.gov and search under accident investigation to find how many H269 accidents there are. not very many if you compare to some of the other light machines!

[Devil 49]

It's been forever since I read "Chickenhawk", but I was a 55 IP at Wolters. Much of what Mason wrote about is unique to his experience.
My guess is that Winnie is on the right track-grossly out of trim autorotations could be the basis of the books' highly dramatised events. If my my memory serves me correctly, one demonstration in my training as an instructor went-
Power chop
NO (repeat NO, as in "none") trim correction
Aircraft yaws left and pitches down dramatically
Instructor adds a little more left pedal to complete demo
Pitch down increases to what seems vertical incidence
Cyclic -ineffective-
The planet starts getting big, real fast
Right pedal until aircraft in trim, and control returns.

If you're flying low level and/or never make the connection to correct the yaw with right pedal...

[Barannfin]

OK so my understanding is in this situation you bring it back in trim and the cyclic should become effectie again. I imagine that at this point youd already be dead in the R-22.

Ok so were getting somewhere, but I still dont understand why the cyclic becomes ineffective here?

Also is the pitchdown a result of the lift of the rotor system suddenly evaporating, or is it from some wierd connection with the out of trim flight? Does the pitchdown occur when there is no cyclic or pedal adjustment or can it happen if aft cyclic is applied but no trim correction?

Im sorry to be asking so many questions but I am at college and I stupidly left my basic aerodynamics books at home.

[Winnie]

BARANFINN:
The answer to your question as far as I know is this:
When you lower the collective, the angle of attack changes differently on the advancing and on the retreating side, thus causing the nose to drop (hence aft cyclic in the entry of an auto), if you should step on the wrong pedal (Out of trim) this brings the nose further down. I don't know if the cyclic is totally ineffective, but like in a plane, adverse yaw is not to your advantage. So to correct the proble, correct the trim, then use aft cyclic to regain a level attitude. With regards to the R-22, well I only have 11 hours in it, but I would Hazard a guess that you may be correct there.

[Devil 49]

Can't speak with any authority from the single demo, but consider that the canted horizontal stab is relatively large and at the end of a long arm. The helo was effectively yawing into the horizontal stab and descending rapidly, increasing angle of attack on the aforementioned surface and increasing effect. There are limits on the amount of authority (Raw force) of any cyclic control system.

[ATPMBA]

This question has come up last year. SASless gave me a response which explains the situation.


Re: TH55 Tuck Under Habit.

quote:


ATPMBA wrote on 17th July 2002 20:43:
Please tell me more about this tuck under habit. How do you get into it ? Full left pedal at the start of the autorotation ? I've been flying S300C's for the last four years and this is the first of heard of this. Is that canopy slat suppose to fix the problem ?

Thanks.



My information is dated....however your statement is correct. The slat overhead the canopy was a fix to help correct the tendency for that little rascal to tuck its nose upon entering autorotation. That was really exaggerated if the student got confused with his tootsies and tromped on the left pedal. At one time the TH-55A was actually flown without the slat and its handling qualities were noticeable the worse for it in those maneuvers. I also think the tucking desire was also enhanced by low rotor rpm as well.....

My real memories of flying the aircraft was that it was very sensitive in pitch and yaw and thus either way, a tucking moment could be induced.....excessive yaw will give a rolling and nose down pitching as well as just being sensitive in pitch. The slat acts as "drag" and would tend to hold the nose up as I recall.

Go to verticalreference.com and ask for some old time Fort Rucker Flight Instructors about this.....some of them who flew the aircraft there at Rucker might be able to better answer your question.