Smith Miniplane Rigging Question

Discussion in 'Smith Mini Plane' started by Vintageav8r, Jan 1, 2015.

  1. Jan 1, 2015 #1

    Vintageav8r

    Vintageav8r

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    Some members of this forum may remember the posting of my recently purchased SMP. One astute member pointed out that the wing appeared to have anhedral. What couldn’t be seen from the picture was that the leading edge was washed out several degrees while the trailing edge was flat. After loosening all the wing tie rods and resetting the front spar and leading edge to 0 deg dihedral and 0 deg sweepback, the top wing now has what I believe to be excessive wash out. Here are the numbers; top wing angle of incidence is, Left: 1.8 and Right 1.7. I’m guessing it probably should be 1.5 or even 0 for the top wing. The bottom wing angle of incidence is 1.8(L) & 1.5(R). All angle measurements were made with plywood templates made to the profile (within about 1/32”) of the 4412 airfoil as depicted in NACA TR No. 824 and measurements taken with both a smart level and an angle cube (to one hundredth of a degree - you just gotta love the technology). The wing’s actual airfoil section only loosely matched the template, except at the wing walk which is spot on. So, the measurements are probably only accurate within a few tenths of a degree if that. Ok, If you have read this far, here’s the problem; the left wing is washed out 2 deg and the right wing 2.8 deg. The bottom wing is not a problem because as you know its washout is adjustable by the interplane struts and the dihedral is about where it probably should be (as a matter of opinion). I see no way to adjust the washout of the top wing short of cutting and welding. And I’m not opposed to doing that. But, before I do, I wanted to put the question before the panel of experts. So, whadda ya think. Is there a way, or do I cut and paste?


    Part of the reason I’m so reluctant to just start cutting is that this thing was built in 1967 and has flown almost 500 hours with several different owners. It’s hard to believe it’s been like this for all those years and hours. You’re going to ask how it flies. It does OK, cruses about 115 IAS at 2500’. The stall seems to me a little high at about 80 mph IAC, with little or no wing drop - very controllable. I’m hoping that I’ve missed something this panel would point out. Or say, quit whining and go flying.


    Sorry for the long description and thanks for reading. Any and all comments would be appreciated.

    Cheers,
    E
     
  2. Jan 1, 2015 #2

    Mong

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    Hmmm…I am probably the guy that told you that it looked like anhedral, and that stall speed does seem very high. An average Mong Biplane with 80 sq. ft. of wing stalls at 60 + / -…I understand that pitot tube errors are possible here...
    The Miniplanes are a tad heavier, but have about 98 sq. ft. of wing, so I am assuming that these wings are working against each other…..On a Stock Miniplane, the bottom wings have much more incidence than the top wing. I find that undesirable, and on our old Mini plane, we lowered the top wing rear spar to make the wings more or less match each other, and it flew way better…..Now, Yours is different than that, but I think you ought to fly it several times in different conditions , to see what the numbers are over and over again. I would do that before cutting struts, etc.
    You might just live with the washed out top wing, and do fine tune adjusting with the bottom wing rear spar 'n' strut adjustment…..Sometimes you have to say, 'If it ain't broke, don't fix it'…..Ed
     
  3. Jan 1, 2015 #3

    PittsDriver68

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    I will suggest that as noted above, first ask the question as to how accurate the airspeed installation is.

    Where is the static port for the system?

    Have you tried flying with a gps and comparing indicated vs GPS speed? That is, flying a box at a constant speed, say indicated 90 mph, and comparing the wind corrected GPS speed with the indicated airspeed?

    Your description of the stall behavior of the aircraft sounds like the airplane is telling you that the rigging is balanced laterally. A straight ahead power off stall with the ball centered is desired behavior.

    The next question is whether when you hold the stick fixed, the airplane flies straight with no tendency to roll. If so, the airplane is again telling you that the wings are rigged in balance. So if it stalls straight and flies straight, nothing to fix with the rigging.

    I will note that if you are using a digital level, your measurement may be reporting more accuracy than you actually get out of the positioning of the rigging boards on your wings. You commented on the fit of those boards and if there is some slop between the boards and the wings you can easily have a + or - 1 degree of slop. Just because the tool reports to the tenth of a degree does not mean that your setup is actually that accurate.

    I would look hard at the accuracy of your airspeed system and declare success with your rigging work.

    Best of luck,

    Wes
     
    Last edited: Jan 1, 2015
  4. Jan 1, 2015 #4

    Larry Lyons

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    My experience with one of those cubes that measure degrees to a tenth of a degree are way to sensitive here. Even a move of a just a couple of inches could change my readings considerably. Mine has the lower wing incidence with the top wing 0 degrees. So I will see how that works out but measuring these old birds with this new stuff is a stress riser! Mine is set with zero wash out on the lower wing and maybe a 1 to 1.5 degree wash out on the top only because it takes that much just to get the front wires sort of taught. sorry can't comment on flying characteristics yet, no flights.

    L
     
    Last edited: Jan 1, 2015
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  5. Jan 1, 2015 #5

    cwilliamrose

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    I think it's best to pick up the airfoil in three places and not try to follow the entire shape. If you touch the rib over each spar and at the leading edge you can expect more consistent measurements. You could cut away your templates such that they only touch the ribs on those three spots.

    If you haven't seen the boards I designed based on the Livingston rigging boards you might want to take a look here. The pickup points are very close to the spars. In your case you could put them right over the spars since your airfoil is the same for all the wings.
     
    Last edited: Jan 1, 2015
  6. Jan 1, 2015 #6

    Vintageav8r

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    Thanks for taking the time to read my post and for your thoughtful responses. You’ve brought up some great points. I’ll try to address them as best I can. But first I want to say that the comment about loving the digital technology was my idea of a joke. It’s ludicrous to think it possible to measure a wing of this nature within .01 deg of accuracy.
    A Smith Miniplane if built as advertised will have 98.25 sq ft of wing area or 92.25 sq ft of lifting area. Ignoring Walter Diehl’s NACA Report # 458 and taking liberties with Munk’s biplane theory, this wing should quit lifting the 665# Miniplane at between 60 and 65 mph (according my quick and dirty calcs.) That’s quite a bit different than EAA’s advertised 50 mph. But roughly agrees with Mong’s estimate - which I am glad to know. So, 80 is a bit high. The Pitot tube is located on the left ‘N’ strut more or less according to plans along with the static port mounted just below it. Both are suspect as I have not put the manometer on either to check their accuracy along with the ASI. I did do the GPS thing. But it was inclusive because, frankly there’s not enough room to mount it and I couldn’t hold it with one hand, work the throttle with the other and the joy stick with the other. I’ll work on that. It does appear that the top and bottom wing have about the same angle of incidence. So, I was happy to read Mong’s comment about that working better. This thing is so well built and later modified that I’m thinking the people that did this were far wiser then me and they must have had a reason for the top wing washout.
    To answer Wes’s quested; with the stick free the airplane does a slight roll to the right and slightly nose down. So, you’re wondering, what’s the problem? The way the leading edge was forced down by the flying wires, I was concerned about compression failure in the main spar and who know what with the drag/anti-drag wires. Plus the stall seems a little high. Although the ASI is probably in error, the short test with the GPS seemed to confirm the high stall speed. Interestingly, this airplane has two stall modes. The one I’ve described thus far is a level stall from cruse attitude - like they teach in Cessna flight schools. If stalled with the nose up 35 deg or more, the stall becomes violent with the right wing dropping - but still manageable. Both these modes are with power at about 800 RPM. I haven’t tried a power on stall yet. That may be lots of fun.
    Larry thanks for the comment on the 0 deg washout on the bottom wings. I would have thought the bottom wings should carry the washout and not the top. You’re also right about the digital levels. I actually prefer my machinists’ vernier level. Except one gets spoiled to the instant readout of the digitals.
    Bill’s comment on the rigging boards is also good advice. I’d already started building one like Bill suggested. We are having an ice & snow event this week here in Texas so now is a good time to spend in the shop. I did do a search on not only this website but the IAC and others before asking for your time. My intent with the 4412 upper profile boards was 1)to see if the wing was built to a 4412 profile (it’s more like a 4411.5 but close). 2) To try to measure the angles. The trailing edge of the airfoil seem to be sloping up, much like Munk’s M6. So, my readings are off a little because of that.
    Thanks again for all your comments. I’ve learned a great deal by reading the posts in this forum. Sorry for the long response. But its cold outside. And in Texas we don't know how to cope with that. ;-)
     
  7. Jan 1, 2015 #7

    Mong

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    I may have mentioned this before, in another thread, long ago…..The 'stock' Smith Miniplane will have a secondary stall, of sorts, at least that's what I remember….Heres why….With the top wing at zero incidence, and bottom wings at 2.5 to 3.0 positive incidence, being that the top wing is ahead of the bottom, when approaching a stall, the ailerons got sluggish, and the bottom, or 'back' wing, stalls first, which shifts all the CP to the forward, top wing, but just an instant before that, the nose will pitch up in the buried stall….The CP is fwd. of the CG, so it pitches up. This does not feel good, or natural….
    All of the Miniplanes that had the top wing lowered, and the incidence changed to closely match that of the bottom wing, stalled more naturally, and were generally faster airplanes, all things considered.
    Several built in the Davenport, Iowa area in the late sixties had this mod, and they were real performers….We had a few in the northeast Ohio area also.
    The Mong Sports have a half degree more in the top/fwd wing than the bottom…They stall predictably, and fall thru at around 60…...Ed
     
  8. Jan 1, 2015 #8

    Vintageav8r

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    That's very interesting about the gap. I've a collection of pictures of Miniplanes and have noticed the difference in gap. Mine has a larger gap than most. Your comments have helped solve a riddle I've had for a long time. I was thinking that the larger gap the less interplane interference and therefore the less drag. But, you're saying in practice this is just the opposite. How interesting. So far I haven't experienced the pitch up just before both wings stall. That does sound like fun though. Until I get the airplane completely 'fixed' and I'm sure of its structural integrity, I'm not going to do anything that I might regret. If you know what I mean.

    BTY: I think I knew Mr. Mong. If it is the same person, he was/is an unassuming stocky fellow. The man I knew was living in the Dallas/Fort Worth area during the mid to late 1970's. Time has made the details vague today, but I remember him as being a fountain of information and in fact my go to guy when I was trying to understand aerodynamics. (I’m still trying!)

    Thanks again for your post, you’ve been a great help. The history of the design is as interesting to me as the philosophy of the design. I’ve come to know that Frank Smith built (or at least owned) a Stits Playboy before building his biplane. I have a Playboy and many of the same design features are apparent in the Smith. However, the Smith is a better design, I think.

    Cheers
    E
    P.S. Is it OK to admit that I have a monoplane?????
     
  9. Jan 1, 2015 #9

    Mong

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    Heck yes, it's cool to have a monoplane! (I am Restoring a Sonerai 1 right now, simultaneously with a Mong Sport)
    I knew Ralph Mong from the late '80s on. He just passed away the week of thanksgiving, he was 89. I wouldn't have called him stocky. He lived in Hurst, TX, near DFW airport. He was about 5' 1" tall. I bought the design rights from him in the early '90s.
    Yes, Frank Smith did build a Single seat Playboy. The fuselage truss on the Miniplane is similar, but shorter. The Playboy I flew was loads of fun with the 125 GPU for power. A 65 h.p. Playboy does not perform nearly as well as the GPU one. Lou Stolp also built a Playboy before he designed the Starduster 1…
    As long as there is a minimum of one chord length between top of bottom wing, and bottom of top wing, I see no airflow interference problem…..Ed
     
  10. Jan 1, 2015 #10

    EAABipe40FF

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    I wouldn't call 80 mph stall a "bit" high. If real it's TOO high and I suspect someday you will get "bit" by it. I hope not but that's nuts IMO.

    I'm going to stop saying "Best of luck", I'll leave it to Wes. I don't believe in luck but I do 2nd Wes's best wishes. Whatever, the landing speed suggested by a 80 mph stall is pushing it whatever it's called.

    I 2nd Bill, rigging board over spars.

    Jack
     
    Last edited: Jan 1, 2015
  11. Jan 2, 2015 #11

    TFF1

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    Assuming the airspeed is 100% right, the CG may be too forward. A local guy built a LSA fun plane. A heavy engine forward CG had the stall up in the 50s; added weight to tail, in the 30s. New short engine mount soon. As for fixing the build errors vs rigging as best as you can, do what will make you happy in the long run.
     
  12. Jan 2, 2015 #12

    Vintageav8r

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    Jack, you made me laugh. Yea, I agree 95 over the fence and a stall at 80 is "nuts". That's why I'm looking at the rigging, Pitot and Static Source. My experience doesn’t include miniature airplanes. But, I’m learning. Bill is right. I’ve just finished building a new set of rigging boards closer to his design with three points of contact, two above the spars. Saturday the WX is supposed to warm up a little and that will be a good time to try them out.


    TFF1, The airplane came with a recent weight and balance. I’ve got a set of scales, but haven’t taken the time to weigh it yet. I can say it flies with very little up elevator. I appreciate the comment and will put a higher priority on a new weight and balance.


    Mong, I wish I had known that Ralph was living in Hurst. He probably wouldn’t have remembered me, but I would have called him anyway. I thought he had moved away. we were in EAA Chapter 34 together. Thanks for clarifying the one cord length gap. That syncs with what I thought I knew. My Miniplane is setup with 36.5” gap and 36" cord. So that fits as well. I bought my SA3A when I didn’t have much to do. It had been wrecked and deregistered. I’ve repaired the wing (except for cover. It’s covered with Polyfiber. I’ve got gallons of Nitrate and Butyrate, but no Polyfiber. Go figure.) Most of the fuselage forward of the seat was destroyed or damaged. (The pilot walked away with minor bruises.) So far I’ve cut away all the damage and designed a new forward section that a human can actually sit in. I was working on the stress analysis of the new forward section when the Smith came along. And now I’ve got several weeks of revenue projects to do that I can’t pass up. I’m rambling so I’ll quit.

    Happy New Year!! One and All,

    E
     
  13. Jan 2, 2015 #13

    EAABipe40FF

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    Yea, I've got a monoplane too........ People think I'm strange sometimes or have a twitch in my neck. Flying the mono I forget that I don't have duck/weave/bank and stretch my neck to check for traffic that might be behind the top wing.... My EAA biplanes nose was above the horizon in cruise, I had to fly sideways to see where I was going.....

    X my heart

    Jack

    GEDC3458 (800x600).jpg
     
    Last edited: Jan 2, 2015
  14. Dec 18, 2017 #14

    Tramp

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    In doing some research for my book (which includes a chapter on the Miniplane's origins and, obviously, Frank Smith), I independently learned he'd built one of the first Playboys from plans. I came across this brochure that has two pictures of his example in flight with, I assume, Frank Smith at the controls:
    http://zoomaviation.com/playboy2/stits_sa3a_single_seat/drawings/poster.pdf
    In zooming in on the top photo, you can just barely make out (guess really) that the numbers are 4 and 7. In poking around in the depths of the internet, I found this image of N47K more or less unused at Bohunk's Airstrip near Lancaster, California. The build year is 1954 (Prototype was 1952 and the EAA Chapter that Stits and Smith were both members of began meeting in 1953) AND the serial number is FS-1...so this is quite likely Frank Smith's Playboy.
    As of 2015, the registration was valid but it doesn't look like its moved very far in a while.
    Anyway, thought I'd share!

    N47K.jpg
     
  15. Dec 18, 2017 #15

    Lotahp1

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    So wild that the history of the Vans line comes directly from the Playboy. The Spezio and Playboy are very similar also. I’ll have to ask Tony if it was any influence or if Ike was his only influence.
     
  16. Dec 18, 2017 #16

    Vintageav8r

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    While not a biplane, the lowly SA3A is one of the most unappreciated homebuilts ever. It deserves a degree of fame it's never received. From an academic view point, it isn't much compared with designs of today. But it's influence is still being felt in today's designs. One could argue Ray Stits' 1st set of plans, the 'Play Boy', begat the SA100 which begat the SA300 which begat the SA750. It begat the DSA-1 And who knows where that would have gone had Frank Smith lived. (There was after all a 2 place Miniplane [sort of] by another designer.) And of course the SA3A morphed into the RV-1 which begat the RV-3, 4, 6, 8,....etc. And somewhere in that mix at Flybob airport was Ed Maquart building his biplanes, although apparently not of direct lineage, they had to have been influenced by the SA3A to some degree.I'm sure I've left out some designs.. Maybe Dave Baxter, the historian and knower of all things Starduster, will fill in the blanks.

    Warm blue skies to all,
    E
     
  17. Dec 18, 2017 #17

    Knight Twister

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    There were quite a few Playboys built and in their day they were the hot rod. I have an original set of SA3A plans with a copy of that brochure. I also have a brochure of the Flut R Bug. The whole kit was 1200 bucks. Ray Stits did a lot for the homebuilt airplane movement, he was quite the designer an innovator.
     
  18. Dec 18, 2017 #18

    Dave Baxter

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    Well not sure I can fill in all the blanks, as it would take quite some time to do so. The Play Boy certainly was an airplane that influenced many of the early builders, and designers. But also Les longs designs the Longster, and Wimpy later named the little Gee Bee George Bogardus flew back to Washington in the late 40s that stared the home building thing, that begot the Flybaby.

    https://airandspace.si.edu/collection-objects/story-little-gee-bee

    For me as a young lad about 11 or 12 I use to ride my bicycle out to Flabob and pay some axe murderer a couple of buck to give me an airplane ride. The first Stits airplane I remember seeing at Flabob was the Flut-R-Bug, the Play Boy, the Sky Coupe and the many variations of them as well as the Play Mate all made an impression on me.

    Don Stits, Rays youngest son and I did were friends and spent quite some time together. As a new PP I was able to fly the SA-9A (Which by the way was certified), about 13 hrs. After Lou Stolp got upside down in building hangers at Redlands airport and about all but one got laid off, I worked for Ray welding Nose gears and other items for the Play Mate before going to work at Rohr Aircraft.

    Ed Marquart worked for Ray prior to starting his own business, and had worked for the Piacecki Helicopter Corp prior to that. But I am sure like all designers Lou Stolp and Frank smith were influenced by all that was going on in aviation at the time and Curtis Pitts was also a player before and during this time.AKA Betty Skelton and the Little Stinker these airplanes influenced a lot of builder as well.

    It is kind of interesting and ironic that one of the reasons Ray quit selling plans was because no one would build them to the drawings! Ray was quite a vocal and opinionated guy, not like us? Also yrs later Dick VanGrunsven bemoaned the same when builders did the same with his designs. But as you know the RV-1 was a modified cantilever winged Stits Play Boy.

    Lou Stolp once made the comment that he never designed the Starduster Too he said all of the parts were there for the taking, all he did was put them together in the right package. But then most all airplanes today owe their or at least some of their heritage to the many airplanes that proceeded them.

    I remember getting a ride in Art Scholls Ranger powered Chipmunk N13A and Morgan (Bud) Schracks Starduster Too N5461 my first experience in aerobatics, with incredible pilots like them.

    Looking back with today's prospective I did not realize at the time Flabob Airport was was quite a unique place, I thought all airports were the same? There were certainly others, Santa Paula was also a hot bed of home built and antique aircraft construction and re building, as were other place around the country, I am sure others could like me add to their humble beginnings in aviation, as well as the people and airports that made and impression on them, and so many that are no longer around! Dave

    Stitts Sky Coupe SA-9A 1.jpg

    Stits Sky CoupeLog Book entries.jpg

    Stits Sky Coupe SA-9A 2.jpg

    Playmate1-360x145.jpg

    N13A  Chipmunk Art Scholl 1.jpg

    N5461 at Flabob with Morgan Schrack.jpg
     
    Last edited: Dec 18, 2017
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