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Thinking of buying a PA38

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Thinking of buying a PA38

Old 17th Aug 2017, 01:35
  #21 (permalink)  
 
Join Date: Dec 2005
Location: Farm strip on the Fens in South Lincs
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There is a wing life extension mod available that will extend the wing's life by 7560 hrs.

Did someone say you needed a lot of runway to get a Tommy off the ground?

I used to fly into my friends strip in my Tommy and safely out again.

The strip was 260m long! OK, I would not attempt it if there less than 5 kts of wind straight down the strip and it would be lightly loaded but it could and can be done.

Most of my flying was farm strip flying and believe it or not a Tommy can make and ideal aircraft for that type of flying once you've mastered its idiosyncrasies.

So, to the OP, buy one, love it, fly it.
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Old 17th Aug 2017, 04:31
  #22 (permalink)  
 
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Originally Posted by rogcal View Post
There is a wing life extension mod available that will extend the wing's life by 7560 hrs.
That's useful, thanks. In my [incorrect] earlier post I'd recalled it as a lot less than that, (probably based on the various analyses I'd done at one time to determine if it was economic to do to one I was looking at - it wasn't!).

I used to fly into my friends strip in my Tommy and safely out again.
Perhaps worth mentioning to the OP that some did come out with 5" wheels, I'd be a bit more wary of strip conditions with them

FP.
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Old 17th Aug 2017, 06:09
  #23 (permalink)  
 
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Originally Posted by B2N2 View Post
Piper engineers should be shot for putting the fuel selector by your left ankle.
I prefer the left side location to Mooney's bottom location. In addition, Piper engineers were smart enough to use a valve with "Off-Left-Right" order, while Mooney is "Left-Off-Right". It's a life and death trick to switch tanks every time. So, you may not like Piper's approach, but it's possible to do worse.
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Old 17th Aug 2017, 09:13
  #24 (permalink)  
 
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To be fair the PA38 has the fuel selector central under the throttle and it requires two hands to select Off - allegedly this was requested by instructors during the aircraft design. I know other Pipers have the fuel selector down and left, and I am one of many who have accidentally selected Off in flight (thankfully I was at 4000 so plenty of time to work out what I'd done...)
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Old 17th Aug 2017, 10:50
  #25 (permalink)  
 
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Interested in this topic as someone who is looking to buy or join a group at some point, I've been a C152 person mainly with a few hours in a PA38 (I'd personally love a share in a Slingsby Firefly, so if anyone has one going spare......)

Can anyone explain why the low wing-hours for the Piper? I've not been able to get a decent explanation and it seems to be something that people quote a lot, but I'm not sure of the reason.
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Old 17th Aug 2017, 14:23
  #26 (permalink)  
 
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I learned to fly on PA38s and ended up owning one for a while. They're a very good aircraft and probably one of the cheapest group A certified aircraft you can get in the UK. I can't comment on the spinning characteristics, but I stalled in them plenty of times without any problems.


The wing spar life of 11,000 hrs can be extended significantly with a mod. In fact the guy who bought mine had this done and it's now up at 12,000 hours and still going strong. I'm not certain, but I think the mod can take it up to 16,000 hours, but it might depend on when it was done (i.e. how near to the 11,000 hrs it was).


There are two versions. The Mk1 has smaller wheels and seems a bit lighter and more sprightly. The Mk2 has larger 6" wheels which are a bit better on grass and loose surfaces, like sand. Worth considering if you intend to go somewhere interesting.


Even a high time example would be a safe enough bet if it's cheap enough. Fly it until it's time is up and recoup your money selling it for spares. Or use it as a greenhouse.
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Old 17th Aug 2017, 21:30
  #27 (permalink)  
 
Join Date: Mar 2017
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I would like to point out that I do know what I am talking about and I am in a position to comment. The various run of the mill American spam cans are relatively cheap to buy and operate. I have flown many Vans RV-8s and they have really nice handling and good cruise speeds but at 80,000 for a rough one they can't be considered on cost grounds and they are significantly harder to fly than a PA38. The Bristell NG-5 is a modern 2 seat Rotax powered 2 seater that would be more fun to fly sport aircraft. It would be slightly cheaper to operate since it burns 17 L/hr of mogas as opposed to 23 L/hr Avgas. The LAA permit aircraft will be much cheaper to maintain since you are allowed to do a lot of the work yourself under the supervision of your inspector. The downside is you will need the best part of 100,000 and you still need to build it. Many people go for a Jodel because they are cheap to buy, operate and maintain.
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Old 19th Aug 2017, 02:51
  #28 (permalink)  
 
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Originally Posted by hsw_ping View Post
Can anyone explain why the low wing-hours for the Piper? I've not been able to get a decent explanation and it seems to be something that people quote a lot, but I'm not sure of the reason.
Surprised no-one has answered before now - spar fatigue is the short answer. You could look up this site for the AD & further info.

FWIW I learnt a little about aircraft structural fatigue from Nevil Shute's book No Highway, which was an interesting read for a youngster of my time.

At the time of writing this I believe the subject of structural fatigue was fairly speculative but as he was an engineer of some repute I suspect there was a degree of experiential thesis there. Certainly the story pre-dated the Comet issues, but not to a huge degree.

Sorry for the drift, the main thing is to ensure your [prospective] Tommy is up-to-date with all its AD's (there are others) and well in time as this wing spar mod shows..!

FP.
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Old 19th Aug 2017, 07:53
  #29 (permalink)  
 
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Metal fatigue has been understood pretty well since the investigations into some major railway disasters in the 1880s. What the Comet disasters taught us was about fatigue in pressurised aluminium structures - very much about the initiation mechanisms.

There isn't anything about the Tomahawk's mainspar life that wouldn't have been understood by a good aeronautical engineer in the 1930s - but it is what it is.

G
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Old 19th Aug 2017, 09:02
  #30 (permalink)  
 
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Anyone ever wonder why we called it the Traumahawk. Wobbly prop is one thing but wobbly empennage?
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Old 19th Aug 2017, 10:11
  #31 (permalink)  
 
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Originally Posted by Genghis the Engineer View Post
Metal fatigue has been understood pretty well since the investigations into some major railway disasters in the 1880s. What the Comet disasters taught us was about fatigue in pressurised aluminium structures - very much about the initiation mechanisms.
Quite right of course, the Comet issue was specific in that sense. I think the Comet-Shute relationship mix for me was because at the public enquiry the prosecuting(?) lawyer of the time cited No Highway at the outset of the proceedings - probably a bit of theatre to some extent but I guess it impressed me somewhat when I read that.

There isn't anything about the Tomahawk's mainspar life that wouldn't have been understood by a good aeronautical engineer in the 1930s - but it is what it is.
I agree that fatigue per se had been understood for some time but I'm not sure it was especially well understood in terms of aircraft at that time? There are many instances (Northwest 421 in 1948 [publication year of No Highway], and Aloha 243 forty years later, just to name a couple) that would suggest perhaps not.

At least with the PA-38 someone's thought about it beforehand and, looking at the AD, they've done so with a degree of science. Let's hope that works out in practice.

effortless: Anyone ever wonder why we called it the Traumahawk. Wobbly prop is one thing but wobbly empennage?
I shared my initial training between a DH-82 and a PA-38 and can well recall after my first stall/spin session with the PA-38 saying to the instructor 'how many people don't come back after that!'. So maybe its flight characteristics have just traumatised a good number along the way...

FP.
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Old 19th Aug 2017, 11:34
  #32 (permalink)  
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Airframes are intended to flex under load, and the limitations are very clear. When my buddy and I used to dogfight in our 150's, I was amazed to be chasing him around in a 3G turn (yes, I have a G meter), to see the sun light up the top of his wings - the wrinkles in the skin, tip to tip, were profound. Once on the ground again, a quick, nervous check, and everything looked perfectly normal.

Yes, the tail of a Tomahawk wobbles around a bit during a spin (which I did many times), and during flight in turbulence too. As the pilot, ask yourself: Why are you watching backward while you're flying forward? Don't worry about it!
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Old 19th Aug 2017, 17:23
  #33 (permalink)  
 
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Originally Posted by First_Principal View Post
Surprised no-one has answered before now - spar fatigue is the short answer. You could look up this site for the AD & further info.

FWIW I learnt a little about aircraft structural fatigue from Nevil Shute's book No Highway, which was an interesting read for a youngster of my time.

At the time of writing this I believe the subject of structural fatigue was fairly speculative but as he was an engineer of some repute I suspect there was a degree of experiential thesis there. Certainly the story pre-dated the Comet issues, but not to a huge degree.

Sorry for the drift, the main thing is to ensure your [prospective] Tommy is up-to-date with all its AD's (there are others) and well in time as this wing spar mod shows..!

FP.
Ahhh thank you I do just wonder why, given all we know about metal stresses, loads, fatigue etc, that this is still a problem? Hmm.

Originally Posted by Pilot DAR View Post
Airframes are intended to flex under load, and the limitations are very clear. When my buddy and I used to dogfight in our 150's, I was amazed to be chasing him around in a 3G turn (yes, I have a G meter), to see the sun light up the top of his wings - the wrinkles in the skin, tip to tip, were profound. Once on the ground again, a quick, nervous check, and everything looked perfectly normal.

Yes, the tail of a Tomahawk wobbles around a bit during a spin (which I did many times), and during flight in turbulence too. As the pilot, ask yourself: Why are you watching backward while you're flying forward? Don't worry about it!
I can definitely vouch for the flexibility of airframes... Aeros in a rented C152 which regularly made the door pop open mid-flight. Yay(!) Airframe was twisting enough that the latch would just pop right out of the lock... of course could never reproduce on the ground. I believe they had to take one of the maintenance guys up for a couple of loops to show him the problem The plane was well-known for it. Also it's slightly lop-sided yoke position. Ahh the little foibles we come to know and love...!
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Old 20th Aug 2017, 14:23
  #34 (permalink)  
 
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Originally Posted by Pilot DAR View Post
Airframes are intended to flex under load, and the limitations are very clear. When my buddy and I used to dogfight in our 150's, I was amazed to be chasing him around in a 3G turn (yes, I have a G meter), to see the sun light up the top of his wings - the wrinkles in the skin, tip to tip, were profound. Once on the ground again, a quick, nervous check, and everything looked perfectly normal.

Yes, the tail of a Tomahawk wobbles around a bit during a spin (which I did many times), and during flight in turbulence too. As the pilot, ask yourself: Why are you watching backward while you're flying forward? Don't worry about it!
I'd second that - flexibility is a major part of how aircraft structures handle loads - a very rigid structure will suffer much higher transient stresses under variable load than a flexible one.

After all, think what it would do to your spine if the undercarriage and tyres were absolutely rigid, instead of flexible, during landing.

G
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Old 20th Aug 2017, 15:07
  #35 (permalink)  
 
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Originally Posted by First_Principal View Post
Surprised no-one has answered before now - spar fatigue is the short answer. You could look up this site for the AD & further info.

But why does a PA38 have a specified fatigue life while a C152 or PA28 doesn't?
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Old 20th Aug 2017, 15:54
  #36 (permalink)  
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But why does a PA38 have a specified fatigue life while a C152 or PA28 doesn't?
Probably for the simple reason that objectively, based upon service experience and condition, it was found to be necessary. Cessna wings are so remarkably over built, and simple in structure and inspectability, that fatigue seems never to have been a concern. I used to fly a Cessna 207 with more than 19,000 airframe hours, which was a perfectly fit and fine plane.

PA-28s were the subject of and AD getting on 30 years now, which required wing spar attachment inspection. I understand that the basis for issuance of that AD was statistically skewed. The resulting inspection effort was widely agreed to be doing more damage, than the "no damage" being reported by the first inspections. I believe that the AD was withdrawn.

Different aircraft types have different structural design philosophies, so you simply have to understand that which applies to your aircraft, and manage it accordingly....
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Old 20th Aug 2017, 16:05
  #37 (permalink)  
 
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So far as I have ever been able to tell, different engineering design philosophies.

Two different approaches are "life by inspection" and "fixed life", both are regularly used on structurally critical components on aircraft. In global terms, the cost of the PA38 mainspar mod is, after all, pretty cheap - it's just a lot of money compared to the second value of a PA38: but not even compared to what a PA38 was new. Think of the happy owners of de Havilland Twin Otters, where the entire wing is lifed and has to be replaced !

G
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Old 20th Aug 2017, 17:08
  #38 (permalink)  
 
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Mildly off topic, but how does the Beechcraft Skipper compare?
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Old 20th Aug 2017, 17:56
  #39 (permalink)  
 
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Originally Posted by Genghis the Engineer View Post
I'd second that - flexibility is a major part of how aircraft structures handle loads - a very rigid structure will suffer much higher transient stresses under variable load than a flexible one.
If you ever get the chance to be on top of a church tower while the bells are ringing you'll find it flexes. Rather alarming the first time...
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Old 21st Aug 2017, 00:20
  #40 (permalink)  
 
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Originally Posted by Dr Jekyll View Post
But why does a PA38 have a specified fatigue life while a C152 or PA28 doesn't?
I recall some controversy over the PA-38 at or after certification in which there was some suggestion that the pre-production aircraft differed from those actually produced. I know there was some issues with stall strips that should be retro-fitted, but what I'm thinking of related to changes in design that could have an effect on fatigue of the main spar.

Without going into a lengthy discussion on fatigue and its various components (there are plenty of learned articles on line) I think the following makes interesting reading:

"Research into the PA-38's design, certification, and manufacture revealed the design originally started out with a conventional tail, i.e., a low mounted horizontal stabilizer/elevator rather than the "T" tail it was certified with. The prototype was designed, built, and test flown during late 1969, 1970 and into 1971. For reasons not defined, the design was placed into storage at the manufacturer for about four years. The PA-38 prototype was taken out of storage and reconfigured with a "T" tail. One of the two original design engineers remained with the company and worked on the resurrected prototype's development.

Originally, the wing was designed using the NASA GAW-1 airfoil and 11 full ribs from wingtip to wing root on each side. This wing had a "U" shaped channel main and secondary spar from wing root to wing tip. The two-piece main spar was joined next to the aileron/flap junction of the wing. Both original design engineers said the PA-38 prototype had been built with a rigid wing structure. One of the design engineers said this type structure is necessary when using the GAW-1 airfoil.

The certificated PA-38 had its main wing spar made from flat aluminum stock. The "U" shaped secondary spar was retained. Lightening holes were cut out along the main spar's outboard half. The main spar had span length "L" shaped extrusions attached to its top and bottom.

The reconfigured airplane retained the same airfoil but had its wing rib numbers reduced to four full sized ribs and four nose ribs per wing. Wing surface shaping extrusions replaced the ribs that were eliminated from the prototype. The extrusions were positioned between the main and secondary spars. There were no vertical braces between the top and bottom extrusions. The original design engineers were asked why the design had been changed. Both said it was their opinion that the airplane's structure had been simplified for manufacturing purposes after it left their design shop. The design engineer said that removing wing ribs and changing the spar design would make the wing less rigid, i.e., "soften" the wing. According to this engineer, the wing's softened structure would not enhance the wing's stall and spin characteristics. He said the softened wing structure could change the airfoil shape, making the wing a new and unknown commodity in stalls and spins. He said he had inspected a PA-38 wing and found it to be very soft, and able to be torsionally twisted without substantial effort.

FAA Service Difficulty Reports related to the wing were examined for the period between 1986 and April, 1995. Fifteen reports showed loose rivets in the wing, bent aft spar attach fitting, and undertorqued wing spar attachment plate bolts. Before sending the airplane to the production design shop, the remaining design engineer stated the airplane had problems with "A very strong rolloff, I think, to the left." He said he designed a wing root glove that was very effective "...in terms of cooling off the stall characteristics." He also revealed the airflow next to the wing root, aft fuselage, and tailcone was not adhering as it should. He said the purpose of the cuff was to improve the airflow along the aft fuselage and give the airplane better stall characteristics. The company production design shop engineers removed the cuffs from the airplane after they had received it from the design shop. The airplane received its FAA type certificate without the leading edge cuffs."


While the above concentrated on flight characteristics [it came about as a result of an accident in 1994] it seems clear to me that there were substantial changes to the structural design that could potentially and negatively affect the wing over time. Moreover there appear to be reported issues that lend support to this view.

In the AD I referenced it was clear that the remaining spar time, post-mod, depended upon pre-mod hours, which are I suppose sufficiently proximate for cycle time (a requirement for cumulative damage/fatigue). Although I've no hard evidence to point to (and don't have time to look further) I don't think it's too long a bow to draw that the fatigue life results from Piper reacting to real issues discovered with the production wing, and that the mod extending spar life dovetails into this and known factors affecting fatigue.

FP.
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