craig saxon
Well-Known Member
The following post was on another airplane forum recently. Note, the names have been changed to x,y and z. I thought the warning I sent could be repeated here.
One thing I did that deviated from the standards was go with cabinet plywood for the spar facing and here is why. Back when xxxxxxx made his initial plane in poland as well as in the 30's when the chilton was made they used what materials they could aquire which in my opinion todays standars are much higher. That and the astronomical price of aircraft ply plus shipping, I believe my choice was good.
The plans I have are calling for 1/8 ply facing on the spar. I went with 3/16. The xxxxxx x-1 calls for 15mm which is over 1/2 thick. Either I am reading the plans wrong or there is a mistake in the plans. DWG #5 item 6 is the spar facing. Yet detail A-A, B-B and C-C does not support that for the spar is only 50mm wide. If anyone is willing to look at the x-2 plans, would love to know the spar construction details for that plan to compare with the x-1 and other homebuilts.
But one thing great about homebuilt aircraft is you can over build something if you feel the need to. Right or wrong you do have that option. The ply I am using is thicker than what is called for but in the grand scheme of things i dont think it will matter and I tapped out each piece after I cut them to ensure no voids or hollow spots in the laminations of the ply. After i am finished with the spars I plan on a layer of glass just to be on the safe side but in hind site I should of filled the areas with foam and glassed it in. 4 layers of glass on each side would of been much stronger than the ply.
Yyyyyy
I felt that I should comment on Ys plans and posted the following.......
Yyyyyy,
yes as builders of homebuilts we do have the options of deviating from the plans and using non approved materials. It's your ass in the air. The only trouble is that without knowing the tested strength of the materials we use, there is no way of doing a proper analysis of the design short of proof loading to the ultimate load, leaving you with an unusable structure. Even then, the use of non approved materials may change the natural frequency of the structure and may lead to destructive flutter. Some aircraft over the years have used marine ply successfully because the designer has known what he was doing and has done the sums. The use of cabinet ply is not just a matter of tapping it to search for voids. A great deal of this plywood has good laminations on the outside but uses punk wood for the inner plys. The glues used to bond the plys must also be considered suspect. The use of select Douglas Fir and other timbers that have known strengths is also possible, as long as you know how to grade the timber. It is not just a matter of grain slope, inclusions and number of rings per inch. Douglas Fir for example can exhibit compression shakes which cannot easily be seen with the naked eye. These are a result of impact with objects on the ground when the tree was felled. They seriously affect the strength of the timber and therefor any structure you build from it.
Saying that you intend to over build to make up for the deficiencies of the non approved materials will not work either. By strengthening part of a design in this way you may move the weakness to another part of the structure which may then fail at a lower loading, actually weakening the overall structure. By putting layers of glass over your wood spar, you have created a structure that the stiffer part thereof, either the plywood or the glass will take all of the load. the strength of one does not contribute to the other. Someone else mentioned that the foam in a sandwich structure was not under any load. This is dead wrong. The whole idea of a sandwich structure is that you will have tension on one face and compression on the other. This means that the foam must be able to take care of the shear loads. Sure, the foam core gives us a form to glass over but the structure will not carry load without it. Try making a sandwich structure from styrene foam and glass all the sides as well as the faces. fix one end to your bench.Test how much deflection you get with a weight on the other end of it. Now drill a small hole in one face and dissolve the foam on the inside with gas or thinners. Now look at what your deflection is with the same weight.
Yes as builders of homebuilts we do have the options of deviating from the plans and using non approved materials. Yes it's your ass in the air. The only problem is by so doing, if you fail and die, you will have bought the entire homebuilt community into disrepute and our legislators may decide that we all need to be protected from ourselves, and remove the freedoms that we currently enjoy.
Craig
Y replied.......
Craig, you bring up some valid points but for the most part I understand your concerns but dont agree with them all. The ply I am using is a hardwood ply and I questioned it before purchasing it including going to the manufactures web site for I too was concerned about the use of pulp for interior sections of the ply. The now canceled AC 43.13.1a had inspection criteria for ply and tapping is an approved method. ( it can still be accessed on the FAA web site but the FAA considers it as invalid now) Same with the use of douglas fir. It is a suitable alternate for spruce as well as has inspection details for what to look for when selecting your lumber which is why out of every 20 boards I looked at I would find one to two that I considered as usuable for my project. I am not going into this build carelessly or hapharzardly. Lots of time was spent thinking and planning what could be accomplished and how I would do it with a mind on safety because yes, it is my ass that will be in it and I plan on spending a lot of time flying the aircraft when it is built. Yet I am also trying to think along the lines of those early innovators who designed these aircraft that we are taking into the composite age. What did they have to work with, what was available to them, what was the purpose of such an item as a spar facing when some spars are solid with no facing. I do believe the facing does carry some load but in order for it to fail totally and ruin the weekend the whole spar will have also failed.
As for the foam we chose to use in our composite construction, guess we will have to keep an eye out for any signs of perferation of the glass to ensure what you described doesnt happen. It is a valid point of concern and I appreciate you bringing it to attention. Going by AC20-27G the FAA recommends approved materials in the construction of the aircraft, yet doesnt require we adhere to only FAA approved materials. Like Zzzzz I am taking an all wood design and making a composite aircraft so yeah, some major plan deviation have already occured and a lot of questions still need to be answered as the aircraft building progresses.
Yyyyy
I tried...
Craig
One thing I did that deviated from the standards was go with cabinet plywood for the spar facing and here is why. Back when xxxxxxx made his initial plane in poland as well as in the 30's when the chilton was made they used what materials they could aquire which in my opinion todays standars are much higher. That and the astronomical price of aircraft ply plus shipping, I believe my choice was good.
The plans I have are calling for 1/8 ply facing on the spar. I went with 3/16. The xxxxxx x-1 calls for 15mm which is over 1/2 thick. Either I am reading the plans wrong or there is a mistake in the plans. DWG #5 item 6 is the spar facing. Yet detail A-A, B-B and C-C does not support that for the spar is only 50mm wide. If anyone is willing to look at the x-2 plans, would love to know the spar construction details for that plan to compare with the x-1 and other homebuilts.
But one thing great about homebuilt aircraft is you can over build something if you feel the need to. Right or wrong you do have that option. The ply I am using is thicker than what is called for but in the grand scheme of things i dont think it will matter and I tapped out each piece after I cut them to ensure no voids or hollow spots in the laminations of the ply. After i am finished with the spars I plan on a layer of glass just to be on the safe side but in hind site I should of filled the areas with foam and glassed it in. 4 layers of glass on each side would of been much stronger than the ply.
Yyyyyy
I felt that I should comment on Ys plans and posted the following.......
Yyyyyy,
yes as builders of homebuilts we do have the options of deviating from the plans and using non approved materials. It's your ass in the air. The only trouble is that without knowing the tested strength of the materials we use, there is no way of doing a proper analysis of the design short of proof loading to the ultimate load, leaving you with an unusable structure. Even then, the use of non approved materials may change the natural frequency of the structure and may lead to destructive flutter. Some aircraft over the years have used marine ply successfully because the designer has known what he was doing and has done the sums. The use of cabinet ply is not just a matter of tapping it to search for voids. A great deal of this plywood has good laminations on the outside but uses punk wood for the inner plys. The glues used to bond the plys must also be considered suspect. The use of select Douglas Fir and other timbers that have known strengths is also possible, as long as you know how to grade the timber. It is not just a matter of grain slope, inclusions and number of rings per inch. Douglas Fir for example can exhibit compression shakes which cannot easily be seen with the naked eye. These are a result of impact with objects on the ground when the tree was felled. They seriously affect the strength of the timber and therefor any structure you build from it.
Saying that you intend to over build to make up for the deficiencies of the non approved materials will not work either. By strengthening part of a design in this way you may move the weakness to another part of the structure which may then fail at a lower loading, actually weakening the overall structure. By putting layers of glass over your wood spar, you have created a structure that the stiffer part thereof, either the plywood or the glass will take all of the load. the strength of one does not contribute to the other. Someone else mentioned that the foam in a sandwich structure was not under any load. This is dead wrong. The whole idea of a sandwich structure is that you will have tension on one face and compression on the other. This means that the foam must be able to take care of the shear loads. Sure, the foam core gives us a form to glass over but the structure will not carry load without it. Try making a sandwich structure from styrene foam and glass all the sides as well as the faces. fix one end to your bench.Test how much deflection you get with a weight on the other end of it. Now drill a small hole in one face and dissolve the foam on the inside with gas or thinners. Now look at what your deflection is with the same weight.
Yes as builders of homebuilts we do have the options of deviating from the plans and using non approved materials. Yes it's your ass in the air. The only problem is by so doing, if you fail and die, you will have bought the entire homebuilt community into disrepute and our legislators may decide that we all need to be protected from ourselves, and remove the freedoms that we currently enjoy.
Craig
Y replied.......
Craig, you bring up some valid points but for the most part I understand your concerns but dont agree with them all. The ply I am using is a hardwood ply and I questioned it before purchasing it including going to the manufactures web site for I too was concerned about the use of pulp for interior sections of the ply. The now canceled AC 43.13.1a had inspection criteria for ply and tapping is an approved method. ( it can still be accessed on the FAA web site but the FAA considers it as invalid now) Same with the use of douglas fir. It is a suitable alternate for spruce as well as has inspection details for what to look for when selecting your lumber which is why out of every 20 boards I looked at I would find one to two that I considered as usuable for my project. I am not going into this build carelessly or hapharzardly. Lots of time was spent thinking and planning what could be accomplished and how I would do it with a mind on safety because yes, it is my ass that will be in it and I plan on spending a lot of time flying the aircraft when it is built. Yet I am also trying to think along the lines of those early innovators who designed these aircraft that we are taking into the composite age. What did they have to work with, what was available to them, what was the purpose of such an item as a spar facing when some spars are solid with no facing. I do believe the facing does carry some load but in order for it to fail totally and ruin the weekend the whole spar will have also failed.
As for the foam we chose to use in our composite construction, guess we will have to keep an eye out for any signs of perferation of the glass to ensure what you described doesnt happen. It is a valid point of concern and I appreciate you bringing it to attention. Going by AC20-27G the FAA recommends approved materials in the construction of the aircraft, yet doesnt require we adhere to only FAA approved materials. Like Zzzzz I am taking an all wood design and making a composite aircraft so yeah, some major plan deviation have already occured and a lot of questions still need to be answered as the aircraft building progresses.
Yyyyy
I tried...
Craig
