Hi Had not noticed it before but I've been assembling/disassembling the tail section a few times in the course of getting it set up right and painted, and I finally saw that the horizontal stabilizer is not strut braced to the lower fuselage. There are struts from the vertical stabilizer to the horizontal stabilizer but nothing from the horizontal stabilizer to the bottom of the fuselage. The only horizontal stabilizer connection to the fuselage is the four bolt arrangement, all of which are in a fairly narrow mid-range section of the stabilizer. Apparently this meets required engineering criteria, but it does make me curious as to how many people have run some sort of strut or cable from the bottom of the horizontal stabilizer to the fuselage to firm up this area. Or in fact is this something that should not be done? So I guess I am asking if I should add such pieces or rest easy that they are not needed. Thank you. Kim Brown New Hampshire
Kim, Remember that in normal flight loads the stab is generating a downward force, holding the nose up. At first glance a lot of things on these little airplanes look suspect, but after hundreds of successful airplanes, the design has stood the test of time, so to speak.
Kim, you raised a good question. I too had a lurking unease regarding this aspect of the design. This is how I tried to rationalize it. At the front of the stabilizer where it is bolted to the fuse, a very sturdy RS-9 cross-piece sits there, meaning there is no way this sturdy joint can separate from the fuselage. Same goes with the rear attachment, where the wood backing on the fuselage and stabilizer is plenty strong. So if a failure on the stabilizer has to occur, if at all, it has to be outside of these 2 strong joints. The lone strut in the design will prevent the rear spar of the stabilizer from folding upwards/downwards in a wind-gust's loading. But I still find a slight weakness on the front /leading edge of the stabilizer. (And in my build I have a light tail, where the leading edge is weaker than the conventional stabilizer's leading edge.) The leading edge can still flex when gust loads exceed unexpectedly.
Also if you compare the stabilizer to the main wings, the wings have 2 struts holding the leading and trailing edges secure, where as the stabilizer has only one strut holding the trailing edge secure. Agreed, the nature of loading is vastly dissimilar. Probably the designer feels the strength of the leading edge of the stabilizer is adequate, and that it can go without a strut on the leading edge. Another reason the designer may have chosen the strut to sit on the trailing edge could be that the trailing edge should absolutely not flex and should always remain in a straight line or else the elevator hinges would get jammed during flexing of the trailing edge.
As I said, this is just the way I tried to analyze and re-assure myself and you could certainly pick holes in my reasoning. If I were to make any mods, I will try to make good the weakness on the leading edge of the stabilizer, and not do any changes to the bottom of the stabilizer. I was thinking of adding a small jury strut from midway on the existing strut, going forward and meeting the leading edge, shown in green line on attached picture. Either I will leave the design as is, or do the above mod. I still have not decided which way I will eventually go.
This is a very proven design. Be very wary of modifications. Beefing up one area can lead to a stress magnification in another area that can actually weaken the structure. My 2 cents B-
Brian is right. Often "adding strength" actually creates a stress concentration somewhere else and weakens the overall structure. For more on this I recommend the section on stress concentration in Roark's "Formulas for Stress and Strain". In some cases actually removing material, in very particular ways, spreads stress out more and makes an overall structure stronger. The bottom line being it is really risky to make changes, unless you are willing to do a lot of reading, calculation, and testing to verify what you are doing.
I agree with Brian and Tom, the Minimax is one of the few Ultralight designs that has a full Structural analysis, personally I would never change anything structural. Having said that I love messing with the non structural parts.
Look at the stabilizer spar at the root. It is triple thick where the stress is highest. The designer did reinforce the spar after analysis. Also note the spar doubler that tapers, another clue. All the bending is resisted by this spar so the leading edge gets no struts and neither are struts on the underside of the stabilizer required. Reinforcing the spar is simpler, lighter and cleaner.
The leading edge on my stab is built with 1.5 mm plywood as base board ( a kind of spar web ) with a 0.8 mm ply cover leading edge, shaped to a D. Two RS 1 sticks reinforce the 1.5 mm ply lengthwise along either edge, much like spar caps in the wing's main spar build.. This is not a particularly strong set up. While assembling the tail unit with the strut last week, I noticed the leading edge of the stab moving as much as 3/4 inch up and down ( total of 1.5 inches transversely ), at the center when pushed gently, with the corners blocked up. This is what caused me to take notice and I felt it was kind of too flimsy a build. It is quite likely wind gusts can do far worse than the gentle force that I exerted on the stab.
The leading edge on the stabilizer of all other maxes, has 4 pieces of RS-17 glues together. That is a very robust leading edge, and I would not think of doing anything to that stabilizer's strut arrangement.
I am not sure that Team is continuing to recommend the light tail, but if they have discontinued it, there must be good reason. Also more than a thousand Maxes have been built, but the ones with light tail may not possibly touch 3 figures. Does someone have the numbers on this ?
I am attaching a figure to show the light built tail's spar section. The build is somewhat similar to the aileron except that the aileron has 3 mm ( 1 / 8 ) ply as spar web, where as the stabilizer has 1.5 mm ply as spar web. ( Vertical stab is shown in picture, but build is same for horizontal stab too ).
I don't have any experience with the light tail so I can't really address your concerns. What you describe does sound like insufficient stiffness however I would note that air loads don't act like the point load you applied to the leading edge, small comfort I know. Rather than a brace strut you might add a second ply web to the other side of the RS1's. The "jury" strut you propose, I think, would bend the other strut which is the opposite of what a true jury strut does. If you really want a second strut run it up to where the other strut attaches to the fin.
I don't have any experience with the light tail so I can't really address your concerns. What you describe does sound like insufficient stiffness however I would note that air loads don't act like the point load you applied to the leading edge, small comfort I know. Rather than a brace strut you might add a second ply web to the other side of the RS1's. The "jury" strut you propose, I think, would bend the other strut which is the opposite of what a true jury strut does. If you really want a second strut run it up to where the other strut attaches to the fin.
Great advise, Bob. I will see if I can glue in an extra web of 1.5 mm or 1.0 mm birch ply. That might mean an extra 1/2 lb, but the added strength is something I would like to have. Thanks
Because it was harder to set up, draggier, more expensive and ultimately unecessary. As a non structural improvement to it, a couple of balsa fairings on the struts knock 85% of the drag off - about 35 square inches, which, if you visualise it as a 7x5 plate in the airstream makes you want to do it.
Amen on the struts fairing. Several times people have suggested that people couldn't possibly get the performance figures they are reporting but they aren't taking into account the enormous gains in performance that careful fairings and other "clean up" can produce. Much more useful and cheaper than larger engines.
I would be wary of adding a jury strut leading from the tail strut to the stab's leading edge. Although most of the time the stab is carrying a downward force, you can apply an upward force if you push the nose down. In the end, any twisting of the stab could load the jury strut and cripple (as in buckle) the main tail strut. I believe that the heavily tapered design of the horizontal stab is meant to provide the structure with adequate torsional stiffness/strength without having to support the stab's leading edge. Not having looked at the stress report in a lot of detail, I believe that the tail struts are there to support the vertical stabilizer more so than to provide bending strength to the horizontal stab.
Without careful consideration I would not modify the structure.
Having reviewed the Team stress report and Hiscocks's "Design of Light Aircraft", I believe the horizontal tail spar takes all the bending load under the design condition. The maximum load occurs at the design dive speed, 110mph, and 4.4g load factor and is 220lbs. This is combined with a 76lb side load from rudder deflection transmitted via the struts. This makes the maximum bending moment about 117ft-lbs at the stabilizer root attachment. Under the design load condition, the chord-wise lift distribution of the stabilizer/elevator is a maximum at or slightly ahead of the elevator hinge line. When the elevator is not deflected, like in level flight where gust loads might be imposed, the chord-wise lift distribution is more centered about the 1/4 chord point. With a mean aerodynamic chord of 34", the centroid of the lift distribution is about 11" ahead if the spar, therefore the spar still takes 2/3's of the load and gust loads are much less than the design case.
Keith I installed flying wires on my 103 because it looked better and did make the tail stiffer. I did install them on bottom running from the existing tailwheel spring mounting bolt and on the top replacing the struts and on the top replacing the struts. Might have added a little drag but on this high drag a/c I did not notice.
Keith I installed flying wires on my 103 because it looked better and did make the tail stiffer. I did install them on bottom running from the existing tailwheel spring mounting bolt and on the top replacing the struts and on the top replacing the struts. Might have added a little drag but on this high drag a/c I did not notice.
Monte
Thanks, Monte. I saw the picture of your Max with wire braces , in an earlier thread.
From earlier discussions on this forum it looks like wire bracing was at one time recommended for the light tail. Not sure when this was discontinued. (My paper plans of 1997 are for the light tail, but it says to use only Al tube struts. )
In fact Team had handed out plan diagrams for the tail bracing with wires. There is one plan diagram posted by Larry.
Links to some earlier threads on same subject are below:
From earlier discussions on this forum it looks like wire bracing was at one time recommended for the light tail. Not sure when this was discontinued. (My paper plans of 1997 are for the light tail, but it says to use only Al tube struts. )
There are three versions of tails now , the first had all hollow built-up leading edges and spars and and only one center bolt at the rear attachment, this one must use the four wire bracing.
The second version has hollow leading edges but reinforced spars , Also the rib lay out is different and the tube attachments are at different location than the cable attachments. the third version has both leading edge and spars solid , those later two have four bolts total at the fuselage sides attachments and are designed for the two tube bracing.
Non of the current downloadable plans contains the original cable braced light tail drawings.
There are three versions of tails now , the first had all hollow built-up leading edges and spars and only one center bolt at the rear attachment, this one must use the four wire bracing.
Just to ease your mind I built a skypup which is wood and foam cantilevered construction with no wires or struts. The horizontal stabilizer bolted to the fuselage with two bolts and two more bolts went through the vertical stabilizer spar. I was always surprised how much the vertical stabilizer would flex if you pushed on it. I put over 100 hours on that bird before selling it.
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I put over 100 hours on that bird before selling it.
