Rob de Bie Posted June 24, 2015 Share Posted June 24, 2015 Over the years I read a lot about the problem of converting an A-37 model into a T-37. And so far I have not found a single model on the interwebs that showed a properly converted T-37, no-one tackles the nacelle. That triggered buying some T-37 references and Academy's 1/72 A-37. I now see how much of the nacelle has to be changed, but it does not appear to be an impossible job. Using lots of photos I'm trying to figure out where the J69 was positioned in the nacelle, to understand the aligment of the exhaust in terms of toe in / out and pitch angle. The red turbine stripe tells me roughly where the J69 is positioned, fairly far aft. But there's one thing I don't understand yet: it appears the engine is too wide to fit. The nacelle width is defined by the fuselage side and the nacelle's outboard rib. I found a photo that shows that the outboard rib on the nacelle is flat, and my main question is whether the fuselage side is also flat, or maybe bulged inwards to accomodate the engine. I cannot find photos showing this, so I hope that there are some former T37 mechanics can answer this question. Rob Quote Link to post Share on other sites
Rob de Bie Posted June 24, 2015 Author Share Posted June 24, 2015 (edited) Here's a first version of a drawing using a T-37 line drawing (from a German F40 booklet) and a photo of a J69, that I scaled on the basis of the turbine diameter. I positioned it such that the turbine warning stripe concided with the turbine rotor. And I added a rough sketch of the starter bullet. For the lateral position, I used the nacelle's outboard rib, and that leaves far too little room in the inboard side. So I guess there was a cavity or inward bulge on the fuselage side? If so, how was the engine removed - by pulling it aft maybe? Any comments are appreciated! Rob Edited June 24, 2015 by Rob de Bie Quote Link to post Share on other sites
boom175 Posted June 24, 2015 Share Posted June 24, 2015 You removed panels on the top and bottom. Hooked a small hoist on top, put a cradle on the bottom and lowered the engine in to the cradle. Quote Link to post Share on other sites
Cubs2jets Posted June 24, 2015 Share Posted June 24, 2015 Apparently the installation of the J-69 engine in a T-37 is still classified Top Secret. :blink: I could not find a single image of the "inner workings" on the inter-web. C2j Quote Link to post Share on other sites
Rob de Bie Posted June 24, 2015 Author Share Posted June 24, 2015 Boom175, Cubs2jets, thanks for your replies. It's interesting to know that the engine dropped out from the bottom. Indeed the hatches on the lower side are much wider than those on the top side. And hurray, it indeed fits between hatch edges on the lower side: I have to assume that the fuselage sides ran along these lines too. I finally found a photo showing some of that, but it's difficult to see. The inboard side of the nacelle appear to be flat, no inward bulge or anything: I think I see that the flat part curves outboard on the top side. All in all that fuselage shape is pretty complex in the nacelle area! I would dearly love to see an assembly line photo, or a line drawing from a Structural Repair Manual or similar. Rob Quote Link to post Share on other sites
Rob de Bie Posted June 24, 2015 Author Share Posted June 24, 2015 I found some more proof that the fuselage structure has large flat panels on the side to make room for the engines. Look at the large triangular 'boxes' on either side of the ejection seats. My guess is that this is the load-carrying structure. If that's true, then the fuselage structure tapers behind the seats to a fairly narrow but high box structure, rather different from the external shape. It would also explain why the fuselage tank is relatively narrow. That's shown in the Famous Aircraft of the World booklet on page 14, but I did not understand that until now. Rob Quote Link to post Share on other sites
Rob de Bie Posted June 24, 2015 Author Share Posted June 24, 2015 (edited) Sorry, double post Edited June 24, 2015 by Rob de Bie Quote Link to post Share on other sites
Jennings Posted June 24, 2015 Share Posted June 24, 2015 Apparently the installation of the J-69 engine in a T-37 is still classified Top Secret. If we told you, we'd be forced to kill you... Shhhhhh! Quote Link to post Share on other sites
SERNAK Posted June 25, 2015 Share Posted June 25, 2015 I would dearly love to see an assembly line photo, or a line drawing from a Structural Repair Manual or similar. Rob Hi Rob, It would be difficult to find such a manual, especially since the aircraft has been withdrawn from service for the past two decades (more or less). I think a friend of mine once did the conversion you wanna do, that is, from an A-37 to a T-37 but, I don't think he had paid so much attention to the position of the J69s Quote Link to post Share on other sites
Jennings Posted June 25, 2015 Share Posted June 25, 2015 The Air Force Museum has the manuals, but the problem is it is their stated policy not to cooperate with anyone related in any way to the model kit hobby or industry. Quote Link to post Share on other sites
mrvark Posted June 25, 2015 Share Posted June 25, 2015 I lightened the shadows of your first pic, if that helps. Can lighten more if needed. Also, here's a shot I took at AMARC a number of years ago that provides a pretty good look at the cross section: Quote Link to post Share on other sites
Rob de Bie Posted June 25, 2015 Author Share Posted June 25, 2015 (edited) Jennings, as you most likely know, there are several sites that offer T-37 flight manuals in PDF format. In my experience they are not very useful for modeling purposes. Www.flight-manuals-on-cd.com offers a CD with four manuals, including 'Handbook of Maintenance Instructions for the T-37A'. Would that be useful for modeling? A bit more searching shows that Essco Aircraft sells the Cessna T-37B, C Aircraft Structural Repair. I have a structural repair manual of the F-104, and it contains a lot of good stuff for modeling. Having said that, I think my main question regarding the J69 installation in the T-37 nacelle has been solved, mostly. Enough at least to estimate the position and angles for a tiny 1/72 scale model. I guess I have to start modifying and sculpting the nacelles now. Rob Edited June 25, 2015 by Rob de Bie Quote Link to post Share on other sites
Rob de Bie Posted June 25, 2015 Author Share Posted June 25, 2015 I lightened the shadows of your first pic, if that helps. Can lighten more if needed.Also, here's a shot I took at AMARC a number of years ago that provides a pretty good look at the cross section: Jim, thanks for the lightened photo, I hadn't done that yet, and it reveals more detail in the nacelle. Your cross section photo reminded me why I got stuck initially: I had seen this Cessna drawing that suggests that the contour of the fuselage continues through the nacelle. Whereas in reality it is pinched considerably, with flat sides, as can be seen in the AMARC photo. Rob Quote Link to post Share on other sites
Ben Brown Posted June 25, 2015 Share Posted June 25, 2015 (edited) Rob, I got the paper version of that drawing from Cessna many years ago. To get from an A-37 to a T-37, the entire underside of the nacelles will have to be changed. The A-37 was much deeper. I don't think the topside was very different between the two jets, at least from a modeling perspective. If you want a larger version of it, PM your email to me and I'll send it to you. Ben Edited June 25, 2015 by Ben Brown Quote Link to post Share on other sites
BillS Posted June 26, 2015 Share Posted June 26, 2015 My first active duty assignment in the USAF was as a maintenance officer on T-37/8s at Reese AFB from 1978-81. I can say with certainty that your plan form depiction of the J-69 is way undersized. As you know,the J-69 was a centrifugal flow gas turbine. It was fat; I'll bet at least a yard in diameter at the widest point. The starter/generator fairing extended nearly to the intake lip. The exhaust duct was longer too. I feel certain that the external shape and cross section of the original tweet wasn't altered. The J-85 was diminutive compared to the J-69 so the engine bay was plenty big to accommodate the tiny J-85. I'm certain external features of the panels were different to accommodate the J-85's accessories and the intake FOD screen present on the A-37. The Tweet exhaust also had a "thrust attenuator" on the exhaust blast panel not present on the attack version. Of course the landing gear was different with bigger tires on the nose and mains which meant bulged gear doors etc. I'll bet dollars to donuts the basic airframe external shape was no different between versions. Oh yeah, the J-69 came out of the top of the engine bay, not bottom. there was no ground clearance to roll the fat J-69 out from underneath. Heck, the thing sat so low, mechanics had those "creeper"thingys to lay on to facilitate getting underneath. The Tweet was a nice airplane: underpowered but fun to fly and simple to take care of. The Air Force got it's monies worth out of it. Wish someone would do a nice 48th scale model. In it's life it carried many interesting paint schemes so model builders could have a hay day. Quote Link to post Share on other sites
Rob de Bie Posted June 26, 2015 Author Share Posted June 26, 2015 I got the paper version of that drawing from Cessna many years ago. To get from an A-37 to a T-37, the entire underside of the nacelles will have to be changed. The A-37 was much deeper. I don't think the topside was very different between the two jets, at least from a modeling perspective. If you want a larger version of it, PM your email to me and I'll send it to you. Ben, thanks for the kind offer, but for the moment I'm fine. I have a 1200 pixel version of the drawing, waiting to be processed to compare it to the other drawings that I found earlier. It now seems that those drawings (from F40 and Famous Aircraft of the World) have the root wing chord quite wrong, too large. Rob Quote Link to post Share on other sites
Rob de Bie Posted June 26, 2015 Author Share Posted June 26, 2015 My first active duty assignment in the USAF was as a maintenance officer on T-37/8s at Reese AFB from 1978-81. I can say with certainty that your plan form depiction of the J-69 is way undersized. As you know,the J-69 was a centrifugal flow gas turbine. It was fat; I'll bet at least a yard in diameter at the widest point. The starter/generator fairing extended nearly to the intake lip. The exhaust duct was longer too. I feel certain that the external shape and cross section of the original tweet wasn't altered. The J-85 was diminutive compared to the J-69 so the engine bay was plenty big to accommodate the tiny J-85. I'm certain external features of the panels were different to accommodate the J-85's accessories and the intake FOD screen present on the A-37. The Tweet exhaust also had a "thrust attenuator" on the exhaust blast panel not present on the attack version. Of course the landing gear was different with bigger tires on the nose and mains which meant bulged gear doors etc. I'll bet dollars to donuts the basic airframe external shape was no different between versions. Oh yeah, the J-69 came out of the top of the engine bay, not bottom. there was no ground clearance to roll the fat J-69 out from underneath. Heck, the thing sat so low, mechanics had those "creeper"thingys to lay on to facilitate getting underneath. The Tweet was a nice airplane: underpowered but fun to fly and simple to take care of. The Air Force got it's monies worth out of it. Wish someone would do a nice 48th scale model. In it's life it carried many interesting paint schemes so model builders could have a hay day. Hello Bill, thanks for responding to my question. You made me look into the J69 dimensions, and it got confusing really fast. Jane's quotes 22.3" / 566 mm for the width of the J69T-25 of the T-37B, which is the value that I used for my analysis. However, most web pages quote 24.9" / 632 mm, 11% more. In the fabulous Flight archive (link) I found an issue with a side view drawing of the J69T-9 of the T-37A, quoting 22.3" too, but cannot match that size with the quoted length. I think the width/diameter confusion could have to do with a mounting ring that is attached to the turbine section on some versions, but absent on other versions it seems. Here's an undetermined version, the one I used in my drawing, without a mounting ring: I realised that I made the starter/generator fairing too short, but I cannot not figure out how to move the engine forward. Unless the turbine warning stripe is misleading me. I did find out that the root wing chord of my (red) line drawing is incorrect, maybe there the problems lies. Back to the drawing board! Rob Quote Link to post Share on other sites
BillS Posted June 27, 2015 Share Posted June 27, 2015 Golly the thing looks tiny doesn't it? I'm going from memory and since I love facts, we gotta roll with the dimensions you state. After responding to the original article here, I started looking at pics on the net. The starter generator was also kind of offset to the inboard it looks like as well, not centered in the intake. One thing I do remember for sure was how crazy loud the little J-69 was; i mean a high pitched, deafening squeal that would wake the dead! Although not visible in most images, it also left a soot trail all over the back of the tweet's fuselage necessitating the use of this brownish solution wiped on to the aft section to make cleaning easier. Quote Link to post Share on other sites
Chris L Posted June 27, 2015 Share Posted June 27, 2015 Over the years I read a lot about the problem of converting an A-37 model into a T-37. And so far I have not found a single model on the interwebs that showed a properly converted T-37, no-one tackles the nacelle. That triggered buying some T-37 references and Academy's 1/72 A-37. I now see how much of the nacelle has to be changed, but it does not appear to be an impossible job. Using lots of photos I'm trying to figure out where the J69 was positioned in the nacelle, to understand the aligment of the exhaust in terms of toe in / out and pitch angle. The red turbine stripe tells me roughly where the J69 is positioned, fairly far aft. But there's one thing I don't understand yet: it appears the engine is too wide to fit. The nacelle width is defined by the fuselage side and the nacelle's outboard rib. I found a photo that shows that the outboard rib on the nacelle is flat, and my main question is whether the fuselage side is also flat, or maybe bulged inwards to accomodate the engine. I cannot find photos showing this, so I hope that there are some former T37 mechanics can answer this question. Rob Rob, I had some general information I was trying to share with you but your mail box would not accept it. Regards, Christian Quote Link to post Share on other sites
Rob de Bie Posted June 27, 2015 Author Share Posted June 27, 2015 Rob, I had some general information I was trying to share with you but your mail box would not accept it. Hello Christian, I'm curious now! :-) Can you try my regular e-mail address: robdebie at xs4all.nl? Thanks in advance! Rob Quote Link to post Share on other sites
Rob de Bie Posted June 27, 2015 Author Share Posted June 27, 2015 Golly the thing looks tiny doesn't it? I'm going from memory and since I love facts, we gotta roll with the dimensions you state. After responding to the original article here, I started looking at pics on the net. The starter generator was also kind of offset to the inboard it looks like as well, not centered in the intake. One thing I do remember for sure was how crazy loud the little J-69 was; i mean a high pitched, deafening squeal that would wake the dead! Although not visible in most images, it also left a soot trail all over the back of the tweet's fuselage necessitating the use of this brownish solution wiped on to the aft section to make cleaning easier. Hello Bill, your're right about that starter being rather offset to the inboard. This is the best photo I found so far: The photo is part of a nice photo set by K. Panitsidis and M. Andreou. Rob Quote Link to post Share on other sites
BillS Posted June 30, 2015 Share Posted June 30, 2015 Nice little photo gallery Rob. Good luck with your project. Maybe this thread you started will spur on a manufacturer to make a proper tweety bird! Quote Link to post Share on other sites
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