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Long Steel Fuselage
Posted: Tue Jun 26, 2018 2:10 pm
I'm planning out my pietenpol build and was wondering if anyone has tried to build a long version of the steel fuselage. I drew up the space frame in cad and increased the length of each bay by the same amount that you would going from short version wood layout to long version layout. Is there some reason you wouldn't want to do this with the steel frame? The other big question I've been trying to answer is how the wing location adjustment works for keeping the CG within limits. Does the frame get modified or the just the attachment clips or does attachment location on the wing change change?
Re: Long Steel Fuselage
Posted: Sun Jul 01, 2018 12:02 pm
The cabane fitting locations on the fuselage and the wing do not change.
The fittings are designed to allow the wing to move fore and aft to get the CG in the right position.
Re: Long Steel Fuselage
Posted: Sun Jul 01, 2018 2:04 pm
I've been working on a long steel tube fuselage project for a few years, at least the drawing and weight and balance part of it. I'm not a big fan of sweeping the wing back, so I've been using a spreadsheet to check the balance as I go. I fly Ken Perkins Piet, short wood with a Ford model A, and we had a balance problem at first. We moved the engine to cure it. There is a thread about this on this site. For my project I'm using a heavier engine, so that helps with us modern, heavier, people. I could look at my drawings for the stations I came up with.
Re: Long Steel Fuselage
Posted: Mon Jul 02, 2018 2:39 pm
I am doing this from memory, so I hope no one shoots me if I don't get this 100% correct. But here goes...
A few years ago there was a group of builders in Georgia that built longer and wider steel tube Pietenpols together and they were commonly referred to as the "Big Piets". A couple of them had issues as I recall with gear collapsing and failure of the lower part of the fuselage. Their recommendation to all steel tube builders was to add a strap across the bottom to reinforce the gear and lower fuselage members. I asked William Wynne about the need for the strap and he surmised that the builders had merely stretched the dimensions but had not gone up in size in terms of tube size and/or tube wall thickness. Once you lengthen the tubes, you create totally different load requirements for which the tubing may be insufficient. I suspect that William is correct. He is a pretty smart dude about such things.
The moral of that story is this - the tube size and wall thickness in the plans is for that plan only. Not yours. I am not saying that you can't lengthen or widen the plans, but I would recommend you speak to an engineer about loads, or at a minimum rethink tube sizes and wall thickness of the tubes that you use. Remember that even Bernard Pietenpol had an engineer look at his steel tube plans and the engineer pronounced the plans to be plenty strong - for those plans. Not yours.
Re: Long Steel Fuselage
Posted: Sat Jul 07, 2018 11:46 am
I agree with Terry on this subject.
When I first started my tube Piet project I had to learn basic stress analysis, statics, so that I would feel good about what I was changing. I ran an analysis on the tube Piet that comes with the Pietenpol family drawings. Using standard category loads (FAR 23) I found an area on the lower longerons, the station just behind the pilots position, that is marginal in my opinion. That is the area on the wooden Piet that has the long doublers running along the lower longeron, fore and aft.
The engine I wanted to use is heavier than the Ford and I wanted to use the long fuselage. My fuselage is 3" deeper at the aft cabane attach station, tapering to the firewall and the tail post by the same percentage. I wanted the head room (that I don't have in Ken Perkins Piet) without raising the wing, also I wanted to be a little more inside the airplane, for wind etc.. I also stretched the longeron between the cabane stations to an even 30", to make the classic 15%, 65% spar stations possible, I'm using the GA612 airfoil.
I started my analysis using the tube sizes called for on the original drawing, stretched to the long wood length. The only change I made was I didn't use the 1" tube shown on the drawing. I found that the deepening of the fuselage 3" lowered overall longeron loads quite a bit. There was still a weak area on the lower longerons, just behind the pilots station, where the tubes drop to 5/8". I tried different spacings on the tail cone stations, but all I did was move the problem around. I also tried adding stations, that helped, but added weight. After messing with it for several weeks, the easy solution came to mind, one that would work and not add as much weight as the extra stations (isn't that how it always works?). All I had to do was increase the 3/4" tube run on the lower longeron one more station, the next aft of the pilots position.
I won't go into all the details of the tube fuselage, but I will elaborate on the way I did all this. I did the analysis using the classic "statics" analysis in triangles. All drawing was done on a cad program and the loads, allowable and actual were done on a spreadsheet I set up. I did all loads to utility category loads, +4.4, -2.2 g's, plus the usual 50%. The tubing is 4130, all .035" wall thickness. Since I'm not an engineer, just a poor old aircraft mechanic, I bought a FEA program and did the whole thing again. It verified my finding. There really is no reason to test to utility loads, other than I'm not an engineer, but this is after all Experimental Aviation.
Long story short. If you are changing the design, check it out.
Re: Long Steel Fuselage
Posted: Sun Jul 08, 2018 5:33 pm
Not to get off into the weeds too far on this, I thought I'd use a representative example to illustrate this subject. Scott Liefeld, who flies the heck out of his airplane and posts regularly about his flights and experiences, just flew an out-and-back to a very hot desert lakebed in company with Ed Knouse, who now owns Gary Boothe's NX308MB. As usual, Scott posted pictures, and as usual, I zoomed in to look at details. That brought a few "ah-hah!" moments, especially after I followed that up with snooping around at various pictures of Scott's airplane on the Westcoastpiet site. If not for the fact that Scott has decades of experience flying and maintaining his airplane, plus the fact that he's technically very savvy AND very willing to share his experiences and knowledge without a scowl on his face or a condescending tone in his voice, I wouldn't have picked on his airplane as an example.
Scott's airplane has a steel tube fuselage, and without going into the whole discussion of Pietenpol Air Camper vs. Grega GN-1 Aircamper, looking at the photos of Scott's airplane will provide the steel tube builder with a LOT of very good information and ideas. Scott's airplane has the elevator walking beam bellcranks outboard of the empennage fabric with the elevator cables running exposed out to the elevator bellcranks like the late Dick Navratil's beautiful "Woody" Air Camper except with the walking beam pivot shaft at the bottom of the fuselage. The geometry created by the lowered position made it necessary for Scott to run the upper elevator cables through the fabric of the horizontal stabilizer. We run cables out the sides of the fuselage, so running them through the stabilizer fabric is perfectly acceptable. Scott's airplane also has the bottom end of the control stick protruding out the belly of the aircraft so that the aileron cables can be routed outside the cockpit floor and fabric and up the wing struts, guided by pulleys.
The main landing gear rear attach point at the lower longeron is not as far aft as it is on the Pietenpol, a distinctive of the Grega that simplifies the fabrication of the cluster bracket at the rear lift strut attach point.
The tailwheel is connected to the bottom of the rudder by a bellcrank with chains and springs and thus eliminates the use of separate tailwheel steering cables. Constructing the rudder of welded steel tube readily accommodates the twisting force of steering the tailwheel bellcrank at a distance down the tail from the rudder bellcrank, where using this setup on a wooden Piet imposes twisting forces on the wooden rudder spar and structure that they are not as readily able to deal with as steel tube.
The cabane struts on a Piet are designed and constructed so as to permit pivoting the wing backward to adjust the CG. On a Grega, as on Scott's airplane, the mounting of the cabanes does not permit this wing shift. Running the weight & balance periodically during construction and calculating the CG as you go makes this perfectly acceptable, as the landing gear and engine mount can be fabricated to place the empty CG where it needs to be without wing shifting. An engine swap, adding accessories, or making other significant changes to the weight might make the wing shift a useful feature though.
Lastly, from the photos I looked at, Scott's airplane doesn't use the undercambered Pietenpol airfoil. As we know from Scott's frequent postings about flights here and there and everywhere, and his many hours of operating the airplane, it flies just fine with whatever airfoil it is using. Perhaps he'll chime in on this (or anything else I've commented on, or stated incorrectly). Meanwhile, steel tube fuselage builders would do well to look at photos and read narratives about who-did-what-why-and-how. Attach points, gussets, tabs, swivels, all sorts of other details- readily "harvestable" on sites like Westcoastpiet where Scott's airplane, Jake Schultz's airplane, Terry Hand's, the UK Piets, and William Wynne's website. Speaking of which, going to this webpage of William's will keep you busy reading and looking for a good while, all by itself: https://flycorvair.net/?s=steel+tube