Scaled-down T-28

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Matt G.

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I guess it's finally time for me to start a thread for this thing. I thought I'd keep it more under wraps until this project became more well-defined, but after spending the better part of a day reading various threads on here to learn useful things, I now think it might be better to get input from those far more knowledgeable than me so I don't go too far down the wrong path. While I have two degrees in Aerospace Engineering and I currently work as a Stress engineer in the field, I've just begun my career, and I have a lot to learn. I've been slowly collecting design books covering a range of Aero and Mechanical Engineering topics to assist me with the process. Enough about me, more on the plane....

I've always liked warbirds and radial engines, but I know I won't likely ever be rich enough to own a P-51, F4U, or even a T-28. I also want a somewhat practical cross-country airplane. Wanting the ability to carry a second person eliminates quite a few potential candidates. For some reason, I've always liked the looks of the T-28 since I first saw one at an airshow when I was little. I could very well be the only one that feels this way, as I've never heard of anyone making a scaled-down T-28. My overall requirements are more or less as follows:

  • Non-cramped accommodation for two tall people (I'm 6'3", and consequently don't fit well in quite a few planes)
  • Resemble T-28 in looks, and use the Rotec R3600 radial engine
  • Easy to fly and safe handling qualities
  • ~600ish statute mile range with VFR reserves
  • 150+mph TAS at cruise
  • Design for crashworthiness
  • Stall speed less than 60 mph in landing configuration
  • 2000' takeoff distance to clear a 50' obstacle at 5000' DA (might be a rather lofty goal)
  • aluminum semi-monocoque construction
  • ~50lb of baggage (not a clue where that's going to fit, at the moment)
  • Probably a few more things I can't think of at the moment
Having just started this project, I've yet to even settle on a scale factor; I'm trying to decide between 65%, 70%, and 75%. Right now, it looks like 70% is the best compromise. At that scale factor, I'd like to see a roughly 1200lb empty weight and 1800-2000lb gross weight at similar overall dimensions to a Cessna 152. Not having designed a small aircraft outside of an academic exercise while in school, I have no idea if I stand a chance at hitting that combination of requirements, so any input from those with experience would be much appreciated.

As soon as I can get ahold of some good drawings to use to make my OML loft, I'll settle on a scale factor and start moving forward with laying out the location of various systems and attempting to see if its possible to make the CG wind up where it needs to be, which means I first need to open up my copy of Perkins and Hage and see if I'll even have a reasonable CG range. The horizontal and vertical tail volumes look ok for a GA aircraft, so I don't see any immediate problems with making this work. I also want to change the airfoil from the 64A-215 the real thing has to something more appropriate for a GA aircraft. I suspect that airfoil may have been chosen to make the T-28 fly more like the jets of the day that pilots would transition into. Or I could be way off base there. At any rate, there are probably better airfoils to use. Anyone have airfoil family suggestions? I think I'd like to stick with 15% thickness.

This is going to be a VERY long-term project for me, most likely. I'm just starting my career, so I don't currently have the facilities or tools to construct an aircraft. I also need to learn more about building with sheetmetal. Hopefully by the time I get this designed I'll actually have everything else in place to build it. Looking further ahead to the future, I'm wondering what ridiculous sum of money it'll cost to insure this thing after it is completed. Surely someone here with a one-off design can help prepare me for what to expect when I reach that stage.

I'll post more updates as I proceed, and I welcome any and all constructive feedback!
 

Jay Kempf

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I don't know what it is about the T-28. It was probably one of the first model airplanes I ever built a LONG time ago. To me it is the archetype for all airplanes. It looks more like an airplane than any other airplane out there. It was built to military needs with a large engine and huge amounts of volume. I would think that a scaled down version carefully done would be an amazing airplane. Don't skimp on size just because you are looking at a SCALE factor. The tendency when scaling is to want the smallest scale you can get away with. There really isn't much difference between 70 and 75% in terms of a one off as most of your problem is hours of design and labor. Materials are not really the concern. Scaling to a known engine and fitting the large lumps in is the way to start. And just putting two pilot profiles on a 2D drawing can be a gotcha. Mock up the cockpits with some plywood and major cross sections cut out in negative to make sure you haven't violated any unforeseen corners. My two favorite tools are MS Excel and Solidworks. You can do it with free 2D software with a lot of cross sections and projected views but the 3D tools get you to visualize problems much faster. MS Excel isn't the only way to do calcs but it is just so easy to use and play what-ifs in.
 

cjensen

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This is very cool Matt...I'm quite interested to see how this progresses! The T-28 is one of my fav's as well...:)
 

djschwartz

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Overall your target sounds reasonable. Using the RV-8A as a comparison (just because it's a known quantity, not trying to imply you should start with one), your target empty weight is just bit higher than the RV-8 which makes sense if you're going to have retractable gear as did the real T-28. The Rotec 3600 is in about the same weight range as the Lyc's typically used on the RV-8. Differences will be in the details like exhaust system, baffling, cowling, etc. I would expect that to be a bit heavier than for the 4-cylinder Lyc, but not much.

I think the obstacle clearance target may be a bit optimistic especially without a constant speed prop. And I don't know if one is available for the Rotec. Of course, a C/S prop adds more weight, too. But RV-8's do fly OK on 150 HP with a fixed pitch prop though most builders put more power on them for more exciting performance.

North American began putting laminar flow airfoils on aircraft with the P-51 and these were common on many high speed subsonic aircraft of the late 40's and 50's. I think such an airfoil will give you little speed advantage for the target performance level you're aiming at and it could adversely affect the takeoff and climb performance. If you want to go with a well known airfoil I'd recommend a 230 series airfoil. My Stephens Acro has a wing similar in plan form to what you'd end up with, though it is more severely tapered, and it uses a 23012. Van's designs up through the RV-8 use a 230 series airfoil of about 13.5% thickness (I think?). The 23015 is also a good airfoil. You might gain a bit of efficiency tapering the thickness down to 12% at the tip. The Extra series aerobatic aircraft use a 15% thick airfoil at the root and 12% at the tip. It's a symmetrical aerobatic airfoil of course. There are newer airfoils too. The RV-9, 10, and 12 use some of these. Unfortunately, many of these are proprietary and getting accurate data on them can be difficult.
 

snaildrake

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OK, time for visual points of reference - save us all cycles on Google :). I'm more familiar with the T-6 Texan, so this has been instructive. -Dan

3-view:
t28trojan3view.jpg

Army T-28C:
T28army.jpg
 

Matt G.

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Thanks for the replies, guys. Here's the picture of the Rotec I posted in the 3D CAD thread...this is about all I've accomplished so far in this area. I may attempt the fuselage again for the 4th time...
Rotec_R3600_Inventor.jpg

More things that are on my mind that I haven't had time to sort out yet (and that I probably don't need to yet) include:

How that two-piece canopy works; I have a diagram of the cable routing, but I think I'm going to need to build a model to see how it works.
Simple, robust retractable landing gear; I initially wanted to make air/oil oleo struts, but now it seems I could be successful with either a spring or rubber discs inside. I also would like to simplify the retraction linkage over the real thing's design. It looks rather complex. I also have absolutely NO idea if I would be better off with manual, electric, or hydraulic retraction. If it makes any difference, I'm currently considering manual flaps for simplicity.

Jay-

You've pretty much verified my current train of thought. This will be an excel airplane for the most part until I get to the detail design phase. I do intend to build a cabin mockup once I get a little further so I can figure out the best design and placement of controls for comfort. I've already found with other projects that stuff that 'looks right' in CAD is awkward in real life. I'm currently living in an apartment, so that'll need to wait until I get a house with a garage. I more or less already did the '2D pilots on a side view' thing and concluded 65% looks tight, but anything above that shouldn't be too difficult. I currently have a student version of Autodesk Inventor that will last me about 7 months, so I need to figure out what I'm going to do when that runs out. I might have to take some classes at a local college so I can get another student version...:) I'd like to get my A&P anyway... While I agree that there probably won't be much of a material/construction cost difference between the different scale factors, the wetted area will increase for a larger aircraft, all else remaining constant, and that'll knock some off of my cruise speed. With a REALLY rough calculation of cruise speed, there's nearly 20mph difference between 65% and 75% scale. Also, above 70%, the Rotec is smaller than the cowl inlet, which will look stupid, I think. One solution to that may be to make this a T-28A instead of a T-28B/C, as the cowl on a "A" has a smaller opening because of the smaller engine that was originally used.

Chad-

Thanks! I hope it continues to progress:)

djschwartz-

We must think somewhat alike...I was using the RV-8A as a sanity check to my performance calculations. Airmaster Propeller makes an electric constant-speed prop of the right size, but I dunno if it'd work on a Rotec, as the Rotec doesn't seem to have anything to mount a brush block to. At the moment, it seems a Warp Drive ground-adjustable prop may be the best option, at least initially. Then I can pitch it for the takeoff performance I need that'll hopefully give me cruise performance I can live with. I had been meaning to talk to Rotec and a few prop manufacturers at Oshkosh, but I wound up not being able to go this year due to starting my job a week before. Next year I hope to get some answers for those things, if not sooner by emailing.

I am not crazy about the abrupt stall and hysteresis characteristics of the 23-series airfoils. I had a design professor in college (with lots of GA aircraft design experience) that cautioned us to not use them for that reason. Yes, washout will probably take care of most of the problems, but there are probably better airfoils to use. I've been reading up on the Riblett airfoils on here and a few other places...I think I may buy his book and see if I can find something that'll work for me.
 

djschwartz

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I am not crazy about the abrupt stall and hysteresis characteristics of the 23-series airfoils. I had a design professor in college (with lots of GA aircraft design experience) that cautioned us to not use them for that reason. Yes, washout will probably take care of most of the problems, but there are probably better airfoils to use. I've been reading up on the Riblett airfoils on here and a few other places...I think I may buy his book and see if I can find something that'll work for me.
My Stephens does have a very abrupt stall with no pre stall buffet. But its wing is tapered well more than 50%. Extreme taper tends to aggravate tip stall. That's great for an aerobatic ship where you want to do really fast snap rolls. not so good for a general purpose plane. Bonanzas use the 230 series airfoil. One reference says it is a 23016.5 at the root and 23012 at the tip. IMHO Bonanzas have a very docile stall, especially the straight tail variants (Debonair, F33, A36). The Taylorcraft also uses the 23012 and also is very docile. And as I said, the RVs use them and they are vey docile as well. The most you might need would be a bit of wash out or maybe a small stall strip near the root. I wouldn't reject the 230 series on that reason alone. That said, newer airfoils may work better if you can get the right one well matched to your performance requirements. Beware of ones that get talked about a lot but just don't seem to get used very often in real airplanes. There's a big difference still today between theory and practical results.
 

Tom Kay

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Hi Matt G;

Nice project. I wish you luck with it, especially since it would be rare or unique.

A couple thoughts, and please bounce these ideas off others, since I'm no expert.

You mentioned that crash worthiness is important, and I have heard that a steel tube fuselage gives a better margin of safety, generally, than an all-aluminum fuselage. You could do like the Titan-51, and use a comination of welded tube fuselage, with an aluminum skin for appearance.

While talking about the T-51, you might also consider the Harry Riblett airfoil that the T-51 uses. This is a 35-415, a 15% thickness ratio foil. It does taper toward the tip, but this is the root foil, I think.

Again, ask others if they agree, but you can always call Harry Riblett and ask his opinion on the airfoil. He's not hard to find in the USA.

Good luck. Tom.
 

Matt G.

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Still using my awful 3-views and section cuts, I've come up with a fuselage I can live with for now. Here's what it looks like at the moment, sans air intake and the canopy-to-fuselage fillet. How bad does it look? I'm still not happy with the cowling; it currently looks like a cross between the "A" and "B/C" model cowlings, but is better than my first half-dozen attempts...



Tom-

Thanks for the input. From reading some other threads on here, it seems that structures designed with any typical aircraft construction method can be made either crashworthy or not crashworthy. I'm not quite sure how I'd analyze the 'aluminum over steel tube structure' idea, as that is not typically done. I'd be more comfortable analyzing a more traditional structure. My current 'design for crashworthiness' ideas include a firewall that slants aft at the bottom as not to dig into the ground in the event of a gear up landing off-airport, seats with a 'crush zone' that absorbs crash impact energy, 4-point harnesses to keep the occupants from hitting the instrument panel, and an attempt to design the cockpit such that crash damage to the fuselage is less likely to trap the occupants' limbs, preventing them from exiting in a crash. I'm certainly open to other suggestions. Thanks for the tip with the T-51 airfoil; Ribblett's book is on the list of books I need to acquire in the near future.
 

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RJW

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Looks beautiful. But it looks like a lot of compound curves. Is this true? I can’t really tell from your model. If the plane is to be made at home from aluminum you might want to compromise some on appearance and include as much flat-wrapped surface as possible. Compound surfaces are extremely difficult to make at home. What's funny is, though easy to make in the garage, flat wrapped surfaces are difficult to make/see on a computer! :suprised: Just my two cents.

Nice work,
Rob
 

Matt G.

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Well, I think the aft fuselage is going to be pretty close to flat wrap...the loft rails are straight lines in that area. I suspect if I really want to do this justice I'm going to have to learn how to use an english wheel. I certainly don't want to cheat with the cowl and make it out of fiberglass...:)

This model may change a bit once I am able to look at/photograph an actual T-28 again. I took a bunch of pictures of one earlier this year, but at the time I didn't know what I was going to need detail photos of. I would suspect that for ease of production, they'd have made as much of the original aircraft as possible flat-wrap, but I can't say for sure.

I am tempted to try to make a small scale model (well, not real small, 12-16" or so) of the fuselage out of aluminum roof flashing and cut-up beverage cans once I have at least the preliminary structural sizing/layout to try to figure out what will be difficult to manufacture. That'll also help me figure out how I'm going to jig the fuselage to assemble it once I get to the real thing.
 

Will Aldridge

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Looking at the stuff snaildrake posted it looks like there are only 2 areas other than the cowling where you would need any kind of compound curvature (if you were bound and determined to do it that is). The first is just beneath the windscreen and the second is just behind the canopy. It looks like the designers spent some time making the shape as simple as possible for ease of manufacture. The tail cone looks like it is straight taper all the way from the aft tip of the canopy to the rudder and looking at the top view the fuselage sides are parallel in the cockpit area. In other words from the windscreen all the way to the rudder you only have 2 flat wrap cones with 2 small compuund curve transitions.

I am a paper model designer (see attached photo of paper model I designed which is all flat wrapped) and so I automatically look for ways to simplify and flatten things and the T-28 looks really simple. I am fairly good with Rhino (got it because it could unroll stuff for my model design) and can give you some pointers if you want.
 

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Jay Kempf

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Still using my awful 3-views and section cuts, I've come up with a fuselage I can live with for now. Here's what it looks like at the moment, sans air intake and the canopy-to-fuselage fillet. How bad does it look? I'm still not happy with the cowling; it currently looks like a cross between the "A" and "B/C" model cowlings, but is better than my first half-dozen attempts...



Tom-

Thanks for the input. From reading some other threads on here, it seems that structures designed with any typical aircraft construction method can be made either crashworthy or not crashworthy. I'm not quite sure how I'd analyze the 'aluminum over steel tube structure' idea, as that is not typically done. I'd be more comfortable analyzing a more traditional structure. My current 'design for crashworthiness' ideas include a firewall that slants aft at the bottom as not to dig into the ground in the event of a gear up landing off-airport, seats with a 'crush zone' that absorbs crash impact energy, 4-point harnesses to keep the occupants from hitting the instrument panel, and an attempt to design the cockpit such that crash damage to the fuselage is less likely to trap the occupants' limbs, preventing them from exiting in a crash. I'm certainly open to other suggestions. Thanks for the tip with the T-51 airfoil; Ribblett's book is on the list of books I need to acquire in the near future.
Looks like my starting attempts at modelling and saving on commands like using symmetry and letting the loft command do what it wants instead of what you want it to do. The only way to incorporate flat wrapped surfaces with compound surfaces is to define the key intersections and then use straight lofts, sweeps, or any of the other standard modelling techniques to form the flat wrapped sections first. Then after that is done you loft from those parts making the new surface lofts tangent to the first parts. Hard to work backwards from the lofts to the flat wraps. What's nice about this particular modeler is that once you define the surfaces and if they are behaving that you can then either enclose solids by adding a fixed thickness or by shelling or build sheet metal parts for each individual facet thereby giving you flat wrap patterns. Takes a bit of trial and error of combinations of features but once you have it down you get rid of all the symmetry errors and the sharp corners where you want nice smooth surfaces traversing the transitions between panels.

As far as crashworthiness, the more distance you put between the occupants and the eventual impact with something firm the more opportunity to add structures that absorb a portion of the hit. Long crush zones can be lighter than short ones. Aluminum structures deform nicely and predictably on impact. That large round nose will do well if the seat is well up off the bottom of the structure on another sub floor with lots of bulkheads and subbulkheads as well as shear webs around the occupant.

I still think the T-28 is one of the archetypes for a normal, conventional aircraft. Very pretty in it's own way. Nice and beefy while being graceful.
 

Matt G.

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Will and Jay-

Thanks for the input. I'm very interested in learning your methods for getting more flat-wrapped areas. Part of the problem is I have very little to work from as far as fuselage cross-sections; I have the cowl just behind the inlet, the firewall, between the pilots, and about halfway down the aft fuselage. This is probably one of those things where it's easier to do when you have a clean-sheet design and you aren't trying to make it look like something that already exists. Oh well, I like a challenge :)

Oh, and Will, that Corsair is beautiful!
 

Matt G.

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Well, the scale factor debacle continues...

I now have enough definition in the fuselage to assemble it with the engine and two representative 6'3" pilots (try not to laugh too hard...I know they don't look very good) with limb dimensions from one of the Roskam design books. At first glance it appears there is plenty of room at 70% scale, but the pilots' shoulders are right at the edge of the canopy/fuselage intersection. The fillet in that area that I haven't added yet will help, but of course the thickness of structure in that area will take away any gain there. Increasing the scale factor to 75% will only increase the width of the canopy by 1.5", and the cowl inlet is bigger than the engine at 75%. So I've either got to massage the positions of the pilots so their shoulders are below the canopy, which would probably reduce their vision quite a bit, or try to re-shape the canopy slightly to give the pilots enough room without making it look stupid. That's going to be a challenge, for sure. And yes, I'm aware the front pilot's feet are currently more or less up against the firewall; I need to get them back far enough where there is room for the rudder pedals. Due to the width of the fuselage, I'm thinking I can get the rear pilot's feet outside the front pilot's seat so he can be a bit further forward, a la SGS 2-33. I'd certainly want dual controls, but I wonder if the rear seat pilot needs an instrument panel, as well? I'm not sure there will be a lot of room...
Fuse12.18.11_1.jpg
Fuse12.18.11_2.jpg
 

Will Aldridge

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Okay a slightly closer look at the 3view shows that there are 3 flat wrapped sections not 2. As long as you have the 4 cross sections indicated by arrows you can create a simple yet good looking model in the computer from which you can get any other cross sections you want. Forget about stuff like the canopy and fairings for the moment. That stuff all gets built later onto the basic structure. Granted I did simplify things a bit but start simple and add complexity later.
 

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Jay Kempf

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Your human model has sharp corners which isn't representative of a real human. And you can also angle the seat bottom to make the torso more like 90 degrees with the same seat back slant. That is standard and will fix your shoulder clearance issue. Make sure you look at the range of motion required to move the controls like extending your toes to work the rudder pedals the brakes. That means you start from the firewall back surface and then put in the pedal, brake, foot and work your way back to the seat. I position one foot on my human model at full pedal forward and one at full pedal back, one hand at full forward like messing with a dial on the panel and one at the rear stick position. Helps in the long run to visualize and do clearances. Once you have the loft around the humans you can shell with different thicknesses just to check clearances. Use the shell command and offset certain surfaces like the canopy at 1/4" and the rest at 1" and it should approximate the need for materials, doublers, longerons, etc for initial layout. Then later when you get to detailing you can just delete the shell and add another one.

With electronics you can put a rudimentary panel in the rear seat with everything except the redundant mechanical gauges.
 

Matt G.

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Will-

Unfortunately, I don't have cross sections at all 4 of those locations. Here is what I do have:

Naval Fighters 05 - N.A. T-28 Trojan_Page_34.jpg

I also am not sure I trust the 3-view that was posted on the previous page, as it doesn't quite match up to my other ones (particularly in the aft end of the canopy), which do look very much like the actual aircraft I have photographed. I do think you may be onto something though; the actual aircraft appears to have a major joint in the fuselage where you've sectioned it just forward of the 'NAVY' lettering, and another one slightly ahead of where you drew the one forward of that. I'll have to play with that further and eventually make a new model, but all I'm after at the moment is choosing a scale factor so I can proceed with verifying that this project is going to be both possible and feasible from a stability and control aspect before I go much further.

Jay-

Thanks for the suggestions! I had forgotten about the 95th percentile human model that I'd used in my preliminary design class in college. I swapped it in, adjusted the positions of the pilots as per your suggestions, and there no longer appear to be any clearance issues. I've also found that Inventor's thickness/offset/shell command still sucks. I tried to thicken the OML and it crashed Inventor. I remember having similar results with all but the most simple shapes in my previous experience with this program.


Here's what it looks like now:

Fuse12.19.11.jpg
I think a cockpit mockup is going to be necessary before the detail design phase so I can make sure everything works out. Having something look right in CAD with the positioning of the pilots and having it work out in real life may be two different things.
 

Jay Kempf

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Sorry, didn't realize you were using Inventor. I may be doing a bout with it soon. I use Solidworks.

Crashing an Autodesk product is something I am familiar with. I got sick of it so I don't use it anymore.

What I would do is start measuring or tracing over your 2D that you want to anchor to. Normally that means splining anchoring curves to the wider lines in the bitmap. Can be done. Put the three views on three intersecting planes and start laying in curves. Then build ellipse, straight lines and radiuses to approximate the bulkheads wherever you think it is faithful to the 2D. Then loft that and see where you are. Trends of geometry should come out of that effort. Then you can tweak. I bet you will find that there is more complex geometry that was done with hard tooling than flatwrapping. That was the way they did things back then. If you know a place to go see one of these beasts you can actually measure or take pictures from different angles to help you dial in the loft. Then it is up to you to fight with Inventor enough to get a real structure if that is your goal. I am sure it can be done but Autodesk's kernels have never liked a lot of organic geometry.

Nice vest on the models. Nice touch.
 
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