Finally Got a Partial Answer

[VictorBravo]

I finally got a partial answer to the issues I have been agonizing over for weeks. This should be the final word on the Harer STC spar modification questions. Actually it shouldn't really be the final word, but the few people who should have the final word have not made any official or unofficial ruling on it... so for now I guess I am the authority on this issue! (I asked for everyone's help on this and I got few replies)

One fellow on this forum owns an F-19 and was kind enough to reply to a private message. Here is his (edited) reply:


"Lucky you wrote when you did as I am just ready to close up my last wing.

I just went out and measured my spars which are stock spars from a 1951 Model 19 manufactured in Conway, Pennsylvania (1,500# gross).

The front spar is 3/4". There are 1/8" doublers at the strut attach point making the total thickness 1" at that point. I can't see any 1/32" doublers or triplers. "


So a factory approved 1500 pound legal airplane was built in 1951 with spars that were 3/4" spruce and strut attach doublers that are 1/8", yielding a total spar width of exactly one inch. The Harer STC clearly says "no changes" to the spar at that location, because a standard Taylorcraft spar SHOULD ALSO be 3/4 plus two 1/8 doublers. Makes sense so far.

The other Gilberti / Harer drawing shows that the wing spar/strut fitting is wider than one inch (1 and 1/16). There is no explanation of this, but again if a 1500 pound gross airplane was certified with the strut fittings one inch apart instead of wider, then it is clear that modifying an older T-craft to the Model 19 specs can be done legally and safely with the spars one inch wide and the fittings one inch apart, not 1 and 1/16.

There is at least one other Taylorcraft recently upgraded in England that I believe has 13/16" wide spars. The two 1/8" doublers would yield a total width of 1 and 1/16 inches in that case. So in that case, Gilberti/Harer's drawing of welding the fittings wider than one inch will yield a fitting that matches this airplane's spars.

HOWEVER I have not seen any information from any person that shows 13/16" is an approved wing spar dimension on any 1940's or 1950's Taylorcraft model. Unless someone can show that 13/16" spar stock was used by the factory on model B airplanes, I can only guess that any airplane with 13/16" spars does not meet the original design specs and it should not have been used by Gilberti as the basis for his strut fitting drawing.

There is a report of a Taylorcraft factory drawing from 1947 showing two very thin 1/32" plywood doublers (triplers in this case) that go between the strut fittings and the original 1/8" plywood doublers. Although I do not doubt that a drawing like this exists, I also have not seen or heard of any specific structural strength/safety reason that these extra pieces are required ... or when they are required... or which airplanes they are required on... or why there is no direct mention of them in the STC. In fact, there are three mystery lines on the Gilberti STC drawing that match the reported dimensions of these thin triplers, however there is no mention of them and no nomenclature or parts callout.

And therefore I must come to the only conclusion that makes sense from the limited information I have. If these mysterious 1/32" thin doublers do exist, they exist as non-structural shims to compensate for some variation in the thickness of the spars used at the Taylorcraft factory. I must also come to the conclusion that Jack Gilberti happened to choose a NON-standard set of extra-thick B or BC or BC-12 spars on which to base his drawing (G-110) of the upgraded strut fitting, and that when he figured out he had measured the wrong size spars he hastily came up with a set of 1/32" shims rather than having to get his original drawing re-approved by the FAA.

Based on the existence of the 1951 Model 19 (with an approved gross weight of 1500 pounds and a base spar dimension of 3/4" with only two 1/8" doublers at the strut attach), I feel confident to say with authority that (when upgrading to the 85 horsepower Harer STC) it is appropriate to weld the strut fitting straps together to match WHICHEVER spar thickness you have, so long as the spar is at least 3/4" spruce plus at least two 1/8" birch plywood doublers (the stock model B and BC and BC-12 spar).

It is NOT necessary to weld the fittings 1 and 1/16" apart if your spars are only one inch wide at the doublers. It is NOT necessary to put additional thin plywood pieces onto a spar that is already strong enough for 1500 pounds, just so you can use the wide version of the strut fitting on drawing G-110.

Adjust the fitting width to match your spars, don't screw with the spars to match an incorrect drawing of the fitting! It was a mistake or lack of attention to detail for Gilberti and Harer to offer an STC that was not fully thought out, or without all of the details worked out.

If anyone knows different, then NOW would be the time to get off your ass and say something. Since nobody had addressed this question before, and very few people got involved in the discussion when I raised the issue, then (even though I am not claiming to be the guru about this) I suspect that this will become the final word when future Gilberti / Harer STC questions come up.

I am flabbergasted that nobody has gotten to the bottom of this question years ago, and I'm thoroughly disappointed that my two discussion threads on this were largely ignored. As a service to my fellow Taylorcraft enthusiasts, I am posting the above opinions and suggestions so future STC upgraders have what I consider to be the logical interpretation of a whole lot of conflicting and confusing information.

Bill Berle

[Yellow Duck]

Re: Finally Got a Partial Answer

Very good Bill. All tho, I am not planning on doing anything now, I may do so as soon as I can find a project.

I would like to have a "Sport Legal" T-Craft, but with the 85hp with the 0-200 Crank A-La-Dream Machine.

Have the "Yellow Duck" Flying, so ready to start. "TRIBE, KEEP ME IN MIND IF YOU EITHER HAVE SOMETHING OR KNOW OF ONE."

[Don Eide]

Re: Finally Got a Partial Answer

Bill,

Final word? I doubt it.

Those mystery lines of yours are not a mystery to me. They clearly show the 1/32 birch plywood reinforcement on the front face of the spar and are also shown on the top view. Apparently Gilberti's spars already had this Taylorcraft reinforcement # 2569 installed so he stated no-changes. I know a DAR (Airworthyness Rep) who is also a DER (Engineering Rep) and the same guy I got Drawing #2569 from, says they are required. Drawing G-110 VERY clearly states that the spacing is 1 1/16 inches wide. IMHO if you don't make the Strut Fitting 1 1/16 inches wide, than your plane will be in noncompliance with the STC.

Just my opinion I know.
But mine will be 1 1/16 wide.

Don

[Robert Lees]

Re: Finally Got a Partial Answer

Don, you are about to receive some level of confirmation of your opinion, in that there were 1/32 additional doublers fitted. But I must add that some spars, including mine, I think (1946 BC12D), that were slightly thicker...it's a bit difficult to measure mine with all the varnish:

Here is my photo of the front strut attach at the spar of our 1989 (yes, 1989)F22. Sorry about the poor view; it's taken through the inspection hole.

My measurements make the front spar 3/4 thick, with 1/8 doublers. The red arrow highlights an additional doubler, approx 1/32 thick (i.e. wafer-thin). It appears very similar to the Harer drawing to which both Don and Bill refer.

This 1989 F22 aircraft is approved for 1750lb gross weight, both here in the UK and the USA.

Amazing, since it appears the spar is the same thickness as the 1939 B models of 1150lb gross.

Rob

[VictorBravo]

Re: Finally Got a Partial Answer

>>Final word? I doubt it.

Perhaps. But considering how difficult it has been to get any sort of official or authoritative answer from anyone in a position to give it, it may well be.

>>Those mystery lines of yours are not a mystery to me.

The reason I use the word mystery is that there is nothing in the STC package that shows what they are. Period. In the STC there is a description and a drawing for everyhting from battery ground lugs to the pilot's shoelaces, but when it comes to a potentially important piece of primary structure, there is no clear explanation of what the lines are. There is also only one of these doublers shown in the top view, which confuses things even more.

>>Apparently Gilberti's spars already had this Taylorcraft reinforcement # 2569 installed so he stated no-changes.

Then Gilberti screwed everyone who ever would do his upgrade. He could also have had a spar that was made of hickory wood, three inches wide, that DID NOT match what the standard Taylorcraft spars were, and based his entire STC on that. How does that help the hundreds of STC buyers with standard 3/4" spars with 1/8" doublers?

Considering that Gilberti was the official factory engineer, who knew exactly what spars were put in what airplanes, you'd think he would have addressed it in the STC. Instead, according to you, he just WRONGFULLY ASSUMED that everyone else had the same spars as he did, which were DIFFERENT than the spars Taylorcraft put in BC-12D's, and wrote "no changes".

>> I know a DAR (Airworthyness Rep) who is also a DER (Engineering Rep) and the same guy I got Drawing #2569 from, says they are required.

What information does he base that opinion on? There is NO specific drawing of thin wood doublers #2569 included in the STC package. There is no real discussion of them in the STC anywhere. What makes you or the DAR think that this drawing nobody else has seen applies to this situation? Why does your DAR think that after Gilberti made all the other myriad mistakes and oversights that his assumption about spar doublers and measurements of the fittings were correct?

>> Drawing G-110 VERY clearly states that the spacing is 1 1/16 inches wide.

Yes, it does. But it does not say why and it does not remotely address the confusion he would be causing in the minds of his customers. This I believe was a mistake on Gilberti's part. Either Gilberti was looking at a non-standard set of spars when he added up those fractional sizes, or he made really bad assumptions about which parts went into which airplanes, OR he just thought it would be easier for HIM to make people add non-structural shims rather than going back to the FAA and admitting he had made a mistake.

> >IMHO if you don't make the Strut Fitting 1 1/16 inches wide, than your plane will be in noncompliance with the STC.

Here's a very serious question for your DAR:

It appears that the 1951 Taylorcraft Model 19 came without these extra doublers. It was certified to Utility Category flight loads at 1500 pounds I believe. The CAA / FAA says it is safe and legal at 1500 pounds, without the 2569 doublers. The strut fittings and nearby spar are one inch wide......

QUESTION: Is this a safe airplane or not?

QUESTION: Is this, or is it not the exact airplane (Model 19) that the STC allows you to re-create from an earlier B model?

Here's another fun question I was thinking about... if the fittings are welded further apart, the upper strut end will be able to slide back and forth a little. This will effectively allow your wing to twist back and forth a little, changing the washout at random, and making a very frightening clunking noise in turbulence. Now THAT is a serious structural problem. Since the STC says nothing about that, can we assume it is safe even though ANY DAR/DER/FSDO/IA/owner-operator would not fly it that way?

I think I'd lke your DAR to address those three questions. Come to think of it, I'd like for Forrest to address them too because he has the original papers and engineering info to once and for all come up with a defnitiive answer for what Gilberti did and why, and to de-cipher the questions brought up by the STC.

Why he has not done this three weeks ago is a bigger mystery than the lines on the drawing.

What is interesting is that the F-22 Rob Lees photographed seemingly has the 2569 doublers installed. This could mean that IF the doublers actually are structural parts instead of just shims, they allow the gross weight to go from 1500 to 1750 pounds.

[Robert Lees]

Re: Finally Got a Partial Answer

I seriosly doubt them as structural, Bill, they are VERY thin. They seriously look like packers to me...they are only a little wider than the steel strut fittings themselves.

Rob

[mikerice]

Re: Finally Got a Partial Answer

I looked at at least 12 Taylorcraft wings that are open and all later model wings (stamped ribs) had the 1/32" second doubler. I was looking at F22 and/or F21 wings for sure (flaps and large gas tank), and probably a model 19 or F19 (no flaps and 6 gal wing tank), possibly a BC12D.

It was obvious that the factory did build the later wings with a 1/8" and a 1/32" doubler at the forward spar strut attach point. I could not determine when the second doubler appeared, but a pre-WWII wing did not have it, which makes sense.

Based on the timing (date) of the original Gilberti STC, I would say the gross weight increase from a BC12D to a Model 19 was probably the point the second doubler was added. I do not know why the 1/32" doubler is added, but there is a obviously a good reason as all later aircraft had it.

This is based on a quick visual inspection without reference to aircraft model number, or date of mfg. Unfortunately, I do not have time to do more in depth research at this time.

Mike Rice
Aerolearn

[Jim Herpst]

Re: Finally Got a Partial Answer

I have looked at an 85 Hp Taylorcraft BC12D wing and did not see em.

I believe Victor has closed the book on this issue and those building will have to follow common sence here.

Then again......
Forrest??????? how are your drawers working these days ? Can you still look in em?? tehee hee

Builder beware till then. Me I am going to comply with the non electrical mod on my pre war and no I aint puttin in any stinkin1/32 plates !!

Best Regards yall

[VictorBravo]

Re: Finally Got a Partial Answer

Thanks for the investigation guys! Now that I have pissed just about everyone off, I would like to say that the crux of the issue (to me) is now what the function of those 1/32" doublers really is. If several post-war airplanes have them and several do not, then this seems to be an issue that needs to be figured out whether someone is doing the STC or not.

I believe that Don has a factory drawing of the doublers. I would bet that the three "mystery" lines on the STC drawing are indeed these doublers. That makes sense, although I am irritated that there is no supporting info for them on the STC.

However, one person reports that his 1500 pound gross F-19 does not have them, and the spars are the same as my 1940. Another person reports that his 1946 airplane does not have them, but his main spar is thicker than the rest of ours so the wide fitting is correct for him. Now another fellow says he looked at 12 airplanes and most of them had the thin doublers.

My question at this juncture is three-fold:

1. Are the 1/32" doublers for strength or are they shims ("packers" in the UK it seems) to make the later strut fittings fit?
2. Why do some planes of different eras have these doublers and some do not?
3. Could the reason for all this be that the extra width of the strut fitting itself added some strength in some strange way?

I'd love to have a real aero engineer address this, but it would appear to me that adding such a thin doubler on top of the existing thick doublers would not add a whole lot of strength. If they needed more spar strength at that point, they could have easily made the existing shape doublers out of thicker plywood, say 3/16 or 1/4. Or more likely they would have extended the thick doublers further inboard and outboard to spread the loads out over more area.

Any old broken down model airplane builder will agree that it is VERY bad to have any looseness in the upper end of the strut. This is an unsafe condition and could very easily lead to flutter or serious structure problems. Gilberti and Harer have not had anything to say about that, and believe me any looseness at that location is potentially life-threatening. It's the most basic physics that even a non-engineer like me can understand. It REMOVES the rigidity of the truss formed by the spar, fuselage upright, and strut.

I dare say that knowing if the 1/32" doublers add any real-world strength will determine whether it is correct to install them when upgrading an assembled airplane. In order for me to be willing to put them in, some engineer (or former T-craft test pilot... ahem...) would have to show me that my airplane will fall out of the sky without them.

If that is actually true, and the doublers are really needed for more strength, then I would consider putting them in.

Either way there was a major oversight on the part of the STC designer for not addressing this.

Please, anyone and everyone who can, please look in your wings, measure your main spar thickness, see whether the wafer-thin doublers are there, and post your findings.

[Jim Herpst]

Re: Finally Got a Partial Answer

Vic,
Who is pissed off ? Do not worry about this. You are only trying to get at the truth. Also Forrest always says he has a thick hide and hes not going to be upset eithor.. right ol buddy? Hell I have thrown more than my share of darts at him he has not yelped yet..... ( Just don't mess with his hound)

Now AND AGAIN I looked at a factory BC12-D 85 that had 3/4 inch spars. Perhaps someone changed em but they DID NOT have the 1/32 plates.

Now what we need is that word from Forrest in my way of thinkin to seal the deal ! If that does not happen then we need an engineer to render his opinion. Sorry but an A& P is not qualified to provide much authority on a matter like this IN my oh so humble opinion.

Best regards yall

[VictorBravo]

Re: Finally Got a Partial Answer

Thanks Jim.

I try not to piss Forrest off too badly. First, he knows a lot about T-crafts and is a great resource. Second, my mommy told me to never piss off the police...they can get a little cranky.

When I get my teeth into some little mystery like this one, I can get a bit obssessive/compulsive. And Prozac is not allowed for pilots ! But I truly swear on a stack of old sectional charts that I wasn't tryign to be a pain in anyone's empennage. It's just that this issue is for real, and whatever the answer is we all should know once and for all.

At this stage I just want to have the questions answered and find out what was really the situation back then, from both an engineering perspective and whatever else was going on "behind the scenes".

I have two or three T-craft guys who are going to measure their spars for thickness (and see whether they have you know what).

Just for grins, I think I'm going to send Don some real thin plywood in a gift box. I've probably ruffled his feathers a little too far, but it was truly not my intent.

[Don Eide]

Re: Finally Got a Partial Answer

Bill,

Your thinking of sending me a gift box??
How can one not be happy at a free gift.
I'm not mad at you or anyone.
This was just a good change of interpretation.
I have two Taylorcrafts to rebuild.
The first one will have the 1/32 plywood.
The second one I'm still thinking about, but thinking I'll try to build it as light as possible, which means no extra parts or beefup.

Don

[Victor Gisclair]

Re: Finally Got a Partial Answer

How abt. Forest, I have not see a post of his for quite some time. I hope he and his family are OK.

[Robert Lees]

Re: Finally Got a Partial Answer

Saw Forrest two weeks ago, he was just fine then.