The question came up about aluminum spars, and Earl Brown asked me if I would not mind chiming in, so I will do so -
Yes, I am using the aluminum extruded spars. However, I purchased mine from D&E Aircraft (https://www.de-aircraft.com/index.html
) out of Lake Worth FL. I wrote a piece for the Newsletter under the previous administration, but somehow it never got printed, so I will attach it below. Some of the suppositions and figures came from about 2 years ago, so some of my numbers may have changed a little bit. But the gist of the article is the same. I will repeat it in its entirety below -
A Discussion On The Use Of Aluminum Spars On The Pietenpol Air Camper
In my profile that I wrote on the Pietvair Forum recently, I mentioned that I planned on using aluminum extruded I-beam wing spars on my aircraft. That started a series of emails between Bob Dewenter and myself in which I explained the why and how of my choice. Below is a discussion taken from those emails and expanded a bit. Hopefully it will create further discussion on the forum. Let’s be clear at this point - I am not necessarily advocating that anyone else use aluminum wing spars. I am simply explaining the logic behind my choice -
Why Am I Using Aluminum Wing Spars?
1. They are a proven design on certified aircraft. There are 2 sources of aluminum I-beam wing spars that I know of. There may be more, but these are the two that I am familiar with. They are D&E Aircraft of Florida (http://www.de-aircraft.com/
) located in Lake Worth FL, and Carlson Aircraft (http://carlsonaircraft.com/
), located in East Palestine, OH. Many builders are familiar with both companies as sources for aluminum lift struts. In looking at their websites, you can see that these manufacturers are the source for aircraft such as the Legend Cub, Dakota Cubs, and Backcountry Cubs (formerly Turbine Cubs of Wyoming), the J3 and the PA15 Vagabond (experimental variants, anyway). In researching on the internet, information indicates that around 1946 Piper switched from wood spars to aluminum spars. William or someone else on the forum may be able to confirm this or expand the discussion on that point, in any case.
2. Cost of Aluminum Spars vs Wood Materials. Let’s look at an apples to apples cost comparison. Or at least as “Apples-to-Apples” as we can get.
D&E Aircraft advertises aluminum spars that fit my Riblett 613.5 airfoils in the necessary dimensions and at the following prices-
Spar Blank - 4.625" X 204" - $120.00 each / Subtotal $240.00
Spar Blank - 5.683" X 204" - $130.00 each / Subtotal $260.00
(D&E Aircraft only sells 17 foot spars)
Carlson Aircraft advertises the similar sizes in the necessary lengths (the spars in the three-piece plans are 13’ 2.5 “, or 158.5”. So a minimum 14 foot, or 168” length of spruce is required for each spar) at the prices below-
Spar Blank - 4.500" X 168" - $103.00 each / Subtotal $206.00
Spar Blank - 5.683" X 170" - $119.00 each / Subtotal $238.00
(Note - these prices do not include shipping and handling, which can double the cost in some cases. I was fortunate that I was able to have my spars delivered to this year’s Sun-n-Fun and then delivered to me in Atlanta by a friend at no cost. Thanks Skip and Cinda Gadd!)
Compare the cost of solid 1” spars (to be routed) in similar necessary sizes from Aircraft Spruce-
1” X 6.00” X 168” - $199.50 each - Subtotal $399.00 1” X 5.50” X 168” - $168.00 each - Subtotal $336.00 Total $735.00
Compare the cost of solid 3/4” spars in similar sizes from Aircraft Spruce-
3/4” X 6.00” X 168” - $175.00 each - Subtotal $350.00
3/4” X 5.50” X 168” - $161.00 each - Subtotal $322.00
What about built up I-Beam spars? On another Pietenpol forum there is floating around a design for a built up spar. It consists of a plywood shear web from 1 sheet of 1/2” X 48” X 96” and 1/4” X 1.00” X 224 Feet of spruce capstrip assembled with T88 epoxy
1 sheet of 1/2” X 48” X 96” - $310.00 each - Subtotal $310.00
1/4” X 1.00” X 224 Feet (ACS only sells in full foot sections. the 13’ 2.50” flanges require purchasing 16 each 14 foot sections) -
$0.90 per linear foot - Subtotal $201.60 T88 Epoxy - $38.70 for the quart kit - Subtotal $ 38.70
(What needs to be pointed out with this design is that at some point along the length of each spar you will have to scarf a joint in to make the full 13’ 2.5” length. Not impossible, but it does add another layer of difficulty to this method. Just a point to think about.)
The overall point to be made is that the price of the aluminum extruded I-beams is slightly cheaper than similarly sized spruce. Your materials cost may be less depending on your ability to find acceptable wood locally and at a cheaper price.
(Again, these prices do not reflect shipping and handling costs)
3. Weight. Here is a comparison of the weight of each type of spar. The aluminum weights come from the Carlson Aircraft website as shown below. It is assumed that the D&E spars are of a similar weight. The weights of the wood come from the current online Aircraft Spruce Catalog.
-Aluminum spars weigh between 0.93 and 1.00 oz per linear inch. Therefore, using the 1 oz. weight times the total length of the four spars (4 x 158.5”) is 634 inches. 634 ounces equals 39 pounds, 10 ounces. You will be using 30 each 5/16” X 1” X 5.5” plywood slats and 30 each 5/16” X 1” X 4.5” slats for attaching the ribs to the spars (more on that later). A 5/16” X 24” X 48” plywood sheet weighs 30 pounds, or 1.33 ounces per cubic inch. The slats would weigh approximately 33 ounces or 2 pounds 1 ounce, for a total weight of 41 pounds, 11 ounces. The weight of any T88 epoxy used to attach the slats to the aluminum spars is not included.
-Spruce weighs 27 pounds per cubic foot. so a 1” X 6” X 158.5” solid spruce spar weighs 14 pounds, 14 ounces, while a 1” X 5.5” X 158.5” spar weighs 13 pounds, 9 ounces. 2 of each spar has a total weight (before any routing is done) of 56 pounds, 14 ounces.
-If you decide to use the 3/4” solid, unrouted spars, each 3/4” X 6” X 158.5 “ spar weighs 11 pounds, 2 ounces, and each 3/4” X 5.5” X 158.5” spars weighs 10 pounds, 4 ounces. the four spars together weigh 42 pounds, 12 ounces.
-If you build up your I-beam spars, a sheet of Aircraft grade plywood (7 ply) in the size noted above weighs 50 pounds. If you add 631.5 cubic inches of spruce capstrips for the flanges (1/4” X 1” X 2526 inches [158.5” X 4 flanges per spar X 4 spars]), the flanges weigh 9 pounds, 14 ounces. The built up I-beam, therefore, weighs roughly 59 pounds, 14 ounces. That does not include the weight of the T-88 used in the assembly.
So what does all the math tell me? It tells me that aluminum spars are slightly lighter than 3/4 spars and even lighter than 1 inch or built-up I-beams. At most the difference is roughly 18 pounds. If you are looking for lighter, then the aluminum is the lightest of the bunch. A little bit lighter compared to some, and a lot lighter compared to other methods.
4. Time. Ask any builder how long it took him to build up his I-beam spar, or rout his one inch thick solid spar. I don’t have that answer, because everyone is a little different. However, I can answer this question - From the time the aluminum spars arrive at your shop/hangar, how long before you can use them to assemble your wing? The answer is almost immediately. They come to you ready to cut to length and begin assembly of your wing. I know that it is not exactly true in that you have to cut 60 slats for attaching the ribs to the spar, but it certainly is quicker than I-beam assembly or routing of solid spars.
(I will interject here that this was written before I had begun work on the spars so the response was a bit naive. There is more work than just sliding your ribs onto a couple aluminum spars. You have to add a plywood spacer at every point that a rib attaches to the spars, as well as wood spacers for compression struts and the internal support pieces as shown on the plans. There is work to be done to prep the spars for use, but having said that, for reason number 5 listed below, I would still make the same choice to use them.)
5. The “Hand Factor” is removed. This is more a criticism of me than anything else, so that is why I put it last on my list of reasons. If you are a master builder, woodworker, etc. and are confident of your skills, then just pass on by this point. This is, in my opinion, a risk management assessment, not really an advantage of one type of spar over another. I am comfortable with the quality of my work. If I were not, then I would never get in any airplane that I had built. Although being comfortable with the quality of my work, it does not change the fact that I am human and humans make mistakes. Did I rout my solid spars correctly, or did I just make expensive sawdust? Did I use enough T88 in assembling my I-beam spar? Was it mixed properly? Was the temperature in my hangar right when I assembled? Did I use enough clamps? The right clamps? Too much/too little pressure? Did I square up the spar prior to assembly? I know that T88 is incredibly forgiving, and I know that in the 1930's high school dropouts in the heart of the Great Depression assembled wooden airplanes in factories. I get all of that. I just feel more comfortable that a factory extruded aluminum spar is less likely to have human flaws affecting its construction. At least MY human flaws. YMMV.
So, how do I plan to assemble my wings using extruded I-beam aluminum spars? Did I pique your interest? Read my next thread. It should be up late tonight or early tomorrow. Stay tuned.
I hope that the info above at least answers a few questions. I am not the engineer that Oscar is, so I can't do the analysis that he can. I am using them based on discussions with William Wynne as well as the fact that this type of spar is used on hundreds of J3 Cubs and similar aircraft, all similar in size and weight as the Pietenpol.