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M.T.O.W. modifications

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Posted

Here's a question regarding a M.T.O.W. modification for the Club Fat peanut gallery...

The official gross of my old Avid Mk-IV with Heavy Hauler wing (144"L x .083" wall T6061 spars, 7/8" x .035" wall 4130 lift struts, aluminum drag tubes, ribs 12" o.c. and 1/4" x 16" ply spar stiffeners) was either 1150 or 1200lbs, depending on the lunar phase and who you talked to. I believe 1200 because it had all the latest Mk-IV mods as the last factory fuse produced. By comparison, the official gross of the Avid+ with the exact same wing is 1250lb. supposedly due to improvements such as stronger fuselage and larger dia. rear carry thru tube, etc. The only other difference I found is the 2" longer ply spar stiffeners supplied with the Avid+ kit.

When building my wings, I decided to replace the 1/4" x 18" ply wing spar stiffeners (part W-24, inserted into the spars and secured under the lift strut brackets) in favor of a longer, stronger spar stiffener design. I followed a tip in an old newsletter and made up a new set of 24" long wooden 'I-Beam' stiffeners similar to that used in the Avid Magnum. My goal in doing so is to safely increase M.T.O.W. on the Fat Bastard by an additional 70lbs, to the Light Sport Aircraft max of 1320lbs. And no, I didn't just pick the 70lbs outta my arse. Here's my rationale, such as it is...

The early Avid Magnum design at 1500lbs. gross, used identical 0.83" spars and ribs, but added 1" x .049 wall 4130 lift struts, steel drag tubes (vs. aluminum) and 1/4" x 24" long ply stiffeners. Shortly afterward, Magnum gwt was increased to 1650lbs. with the addition of 1/8" x 24" aluminum stiffeners similar in shape/size as the original plywood type, however, two strips of 1/8" x 1/2" aluminum were riveted along the top edge of the aluminum stiffener to form a kind of 'T-Beam'. All other wing components being equal, the 2nd stiffener was good for a 150lb. (10%) gwt. increase. Even later, gwt was raised to 1750lbs. with the addition of a 36" extruded aluminum 'I-Beam' style stiffener.

Obviously, adding beefed-up Magnum style spar stiffeners will strengthen the Avid+ wing. But how much?? Since 70lbs. represents a 5.6% increase in gwt for the Avid+ (much less than the 10% factory increase on the Magnum for essentially the same stiffener modification), I don't think raising the gwt by 70lbs. after the mod is excessive or unrealistic. I will, however, do calculations on the wing loading and consult the G forces per bank angle charts. To be conservative, I may find I need to keep bank angles below 75*-80* degrees, probably not a huge problem with floats hanging under. At any rate, a 1320lbs. gwt would increase the useful load for pilot, passenger and gear from 450lbs. to 520lbs. (or from 300lbs. to 370lbs. with full fuel, and from 375lbs. to 445lbs. with half fuel) on the amphib floats. Not bad.

So while I hate to add weight unless it's safety related, this seemed like a good trade off. At 4.2ozs. for the original 18" W-24 ply vs. 17.5ozs. each for my new 24" 'I-Beam' stiffeners, the weight penalty is 13.3oz. each x (4) stiffeners = 53.2ozs., or about 3-1/3lbs. total. Weight penalty aside, these suckers proved to be such a royal P.I.T.A. to get into the spars I gave up and simply added a 12" length of hardwood shoe mold to both sides top and bottom of the supplied 18" W-24 ply stiffeners. So basically I ended up with new wooden spar stiffeners 2" longer than the Mk-IV and modified to the 'I-Beam' shape. The new 18" 'I-Beam' stiffeners I ended up using weigh out at 10.3ozs. each, or a total weight penalty of ~1-1/2lbs. total.

Now I'm no aerospace engineer but I figure this mod has got to add --some-- strength to the wing structure. At least I can back up my rationale with the Magnum factory specs to help justify my 70lbs increase to M.T.O.W. Or, am I'm not thinking straight and it was all a big ass waste of time? Tell me what you really think.  :-

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Posted

Please understand I am not presenting the following questions to be argumentative, but, (the question being) what makes a weight "official"?

At what point  during the registration process or when acquiring a experimental airworthiness certificate is there anything "official" about the maximum weight of an experimental amateur-built aircraft other then having weight and balance information that shows the aircraft is within it's CG envelope for each flight.

I understand that the intention is to make the aircraft stronger and safer, but would it be considered illegal, e.g., a FAA violation, to fly a "Avid Mk-IV with Heavy Hauler wing" up to 1320# MTOW?

I am aware of the structural design limits the designer/factory implies for max weight, but when and where does it become a legally binding limit?

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Posted

lol, shoulda said 'Avid's official recommended gwt' from pre-LSA terminology time. I'm just talking structural safety limitations, not operating or certification limitations, for my Experimental, not LSA, registered. I followed the other list -uhem- conversations and your posts here, took a bit to sink in but you convinced me (and schooled a few others in the process too I thought, ha!) citing the FAR facts. Anyhow, if I fly under Sport Pilot rules with 1320lb M.T.O.W., I just figured the 1320lbs would make for a handy max gwt target too. Real question is whether the new stiffeners gain me 70lbs additional structural safety to justify that gwt.  ;)

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Posted

Doug,

A couple of comments/questions.  When I was trying to find out what the gross weight on my C model would be with floats added, I was told by Avid that the gross is 1250 lbs, or 100 lbs more than with wheels.  Apparently the fuselage is strengthened by the float struts to a degree that 100 lbs additional weight could be supported.  Have you asked if the Avid + benefits with the addition of floats?  If it does, you should be able to safely go to 1350 lbs with floats.  The addition of the spar reinforcement should be a plus, but not necessarily the thing that will allow you to fly at 1320 lbs.

Have you talked to Brett at Airdale?  When he and I were talking a while back he mentioned a conversation he had with John Larson.  The weight at which the original Avid wing failed was somewhere in the 9 G range.  If you haven't talked with him, it might be worth your time.  Good luck.

Jack 

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Posted

Jack - interesting you say that, I remember reading the following post on the Yahoo group with more than a little skepticism. I guess I never really bought into the old argument that floats fly themselves.

It is widely accepted that when adding floats to an aircraft rule of thumb says "increase in gross weight = to weight of the Floats added" within reason--Amphibs not quite... Assuming Avid Straight floats weigh about 55lbs EACH an increase in gross weight of 90lbs is how the Factory advised Safely the increase to original gross weight. Just to cover their --s. The assumption is based on the fact that the floats, with their aerodynamic shape etc. when flying, add Lift to the airframe thus reducing the wing loading---i.e. the average float generates their own equivalent weight in lift in cruise... So if an Avid is placarded with a gross weight with floats then it is ONLY when floats are fitted... Remove the floats and the Gross weight should revert back to original Avid number... Which on an Avid C model, without any airframe modifications, is a MAXIMUM of 1050lbs. (Hauler Wing). Less for STOL wing, Less for Aerobatic Speedwings and even Less with the ordinary Speedwing. BUT being "experimental" the Builder is classed as the designer and can literally stipulate the gross weight he wants REGARDLESS of what the kit manufacturer recommends... a way of covering Their --s when flying "overgross" for insurance purposes... Hope this makes sense... Checkout a J3 Gross weight and then Gross weight with Certified floats--- Don't try justifying something to yourself that isn't there! Hope this understandable. Steve at Airdale

That said, I can see where float rigging might provide a small measure of added structural strength although 10% seems really big to me. There was a mod for early KF and Avids where a strengthening brace was added under the fuse between the lower lift strut brackets to help take wing stress off the lower longerons. I suppose float rigging might do something similar, although cross brace flying wires wouldn't be nearly as effective as a solid brace. In any event, even if float rigging alone provided some benefit, since the Avid+ has a more substantial full width bridge truss between the brackets than a C model, I'm skeptical it would be as large??

I've read many times there was a fairly high safety factor in the Avid wings. The picture of the factory static load test with all the sand bags on the 911lb gwt Mk-IV STOL wing shows +5.7/-2.25 G at gross vs. the factory advertised +3.8/-1.5 G. That translates into ~50% safety factor. I know this isn't the correct way to look at things, however, if you have a 50% safety factor and you're adding 70lbs, or 5-6%, it's no where's near falling under the heading of totally stupid. Even so, some time ago Steve emailed me a reply in part "The Avid+ was designed around 1250lbs gross and to allow this to be increased to 1400lbs (the Airdale Flyer gwt) would entail much more work than just beefing up the wing." He always pounded the table that wing construction and lift strut components were the primary limiting factors for MTOW on the various models, but not the ONLY factors that come into play. Here's a post that details better with the Avid+ info highlighted:

There are also several other areas in the HH wing that run out of "stress related" safety factor margin over 1250lbs. These are: The Heim Adjustable rod end, the rod end attach Bolts, the rod end welded screwed thread, the root attach bolt and bushings, the wing attach bolts, the lift strut attach brackets and the carry through tubes. As well as the wing stiffener. Yes, on the Airdale an I beam Alu extrusion was used..same as the Magnum of 1700lbs Gross but the Normal Magnum style spar stiffener, 1/8" Aluminum plywood style, ran out around 1500lbs. The Alum I beam extruded Spar stiffener is no longer available. It was designed to fit .083" Wall Spars. Airdale had some stock over from when Avid went out of business in 2001 but I think they are all gone! or not enough to do whole wing! However, an Aluminum spar stiffener IDENTICAL to original Magnum, (similar shape to Plywood MkIV HH but longer,) works well to over 1400lbs. Also at Higher gross weights, compressive / Drag loads are GREATER so, Airdale wings as per Magnum, ALL Steel Drag tubes needed, with the plywood gussets installed. The Airdale wing also used a Magnum Style Lift Strut Rod end which was stressed in excess of 1700lbs. Regards, Steve

Based on all this, I'm comfortable with 1320lb gwt and MTOW with my new stiffeners but will certainly ask Brett for his input. It was clear from the earlier discussions on the other forum much of this info is not very well understood by the Avid community. I found many Steve's comments extremely helpful and the safety aspect so important they are worth re-posting on this forum for future reference.

thanks

__________________________

Misc. helpful Avid spec info...

Some Lift Struts 4130 details

Avid A, B,C, & Bandit--STOL Versions NOT Hauler used 3/4" .035 wall 4130---gross weight around 1000lbs.

Avid B,C Speedwings, NOT Aerobatic, used 3/4" .035 wall 4130

Avid B,C, D MkIV Hauler used 7/8" .035 wall 4130 Gross weight around 1200lbs

Avid Magnum used 1" x .049 wall 4130. Gross weight 1500-1750lbs depending on who's selling it and where to? Longer lift struts than Flyer

Airdale uses 1" .049 wall 4130. Gross weight of 1400lbs with a Magnum style adjustable rod end. Same length as Flyer.

Avid Amphib /Catalina used 7/8" .035 wall 4130 with 2 Jury Struts, (AD compliance, as they failed in -ve at 1150lbs gross!) Longest lift struts.

These details could be used as a quick check to see what gross weight capability on a used airplane / kit etc...there are many mixed and matched variations out there.

Regards,

Steve ukav8r at Airdale.

The Spar Stiffener/G factor

Avid used two thickness of Spars--.065" and .083"-- at one time a few Commercial grade spars 2.5" x .090" were shipped --long story but very few out there. The standard Spar stiffener on ALL the Avid Flyer and Catalina was a piece of 1/4" plywood about 16" long that was slid inside the spars spaced around the lift strut attach bracket. This was good for 1150lbs on the MkIV---in fact when the stress analysis of the wing was done, with the .083" wall spars, this plywood stiffener was never included in the calculations and the wing found to comply. On the Magnum, only .083" wall tubes were shipped. The very early Magnums used a longer Plywood stiffener but shortly afterwards changed to a piece of 1/8" aluminum about 2ft long similar in shape/size as the Flyer plywood type---however two strips of 1/8" x1/2" aluminum were riveted along the Top edge of the aluminum stiffener to form a kind of T--both types of stiffener were held in place by 6 of the lift strut attach bracket rivets---these passed through the bracket and spar and crimped inside the stiffener---just to stop it rolling around inside the spar. When Avid wanted to INCREASE the Gross weight of the Magnum from 1500lbs to 1750lbs (to compete with the Glastar---yep that's right) they offered a Gross weight increase option. This consisted of two x 6ft lengths of extruded Aluminum in the form of an I beam--the lengths were cut into 3ft and 1 piece slipped inside each spar across the Strut attach bracket in place of the Aluminum built up Stiffener---the Magnum from that day forth was advertised as a 1750lb Gross airplane---- We (Airdale) use this I beam, extrusion in our wing with the .083" spars as part of our wing improvement to take the gross weight up to 1400lbs... Skystar, in their Kitfoxes, used two spar thicknesses---original was .055" but was extruded along its length with the centre web--Kitfox 1,2,3 etc... (heavy + expensive as die wore out very quickly). As .083" is not a standard wall tubing Skystar on the IV's, V's, VI's and VII's use a .065" wall spar and a similar I beam extrusion to the ones we use--BUT you cannot put a .083" I beam into a .065" spar and vice versa they won't fit---bigger/smaller O/D on the I beam. Both I beam stiffeners--Kitfox or Airdale are rivetted along their length into the spar. Now here's the rub----If you wish to buy the spar stiffener or .083" wall spars. 1.) They are non-- industry standard--so you have to have the Die made 2.) A supplier will only make / extrude the stiffener/spars in minimum quantity ---eg 500lbs. Similar with spars--- As each stiffener/spar weighs 10lbs approx---then you have to order/expend $ for a minimum of around 50 Extrusions / spars----cost??? 2 extrusions per airplane = 25 airplanes worth----$$$ to be sat on inventory till needed!!! Hence high selling price! Hope this clears things up--- Steve at Airdale..

The Heavy Hauler was the name attached to several Avid models. A Heavy Hauler B or C model was 1050lbs. A Heavy Hauler MkIV was originally 1150lbs and then, to compete with the Kitfox 1200, "re-marketed" as 1200lbs. When Airdale sold upgrades from an Avid to an Airdale or a "FatAvid" they clearly stated that the gross weight of the upgraded model was as per the existing wing construction. The Airdale airframe, Landing Gear, Tailwheel, was designed for 1400lbs gross weight. The Avid+ or "FatAvid" Frame etc. was designed for a gross weight of 1250lbs. Fit a STOL wing from A model and gross weight would be 850lbs, Wing from Avid B/C model 1050lbs, Avid MkIV HH 1150-1200lbs. STOL wing from Avid MkIV / Bandit 911lbs-1000lbs. Gross weight increase is not just about the wing. There are many inter-related other things to take into consideration. Landing Gear, Bungees, Tailwheel, Tailspring, Attach bushings, attach bolts, Wheels, Brakes, Axles, Carry through tubes. Even to the extent of Bigger Rudder, larger elevator, trim tab requirement, heavier duty fabric etc... BUT as the builder of an experimental becomes the "designer" ANY gross weight can be stipulated prior to flight testing, provided it could be "shown" to have been demonstrated. This is the reason I said one has to be careful around gross weights and Wings.....the wing is only 1 small part of the equation. You can see similar changes around gross weight and airframe / wings etc with the Kitfox models.... and I hate to admit it But.... they were a bit more scientific about it. Regards, Steve

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Posted

Lotsa good info in that post, thanks for putting it up!

I have a C HH with most MK IV mods.  When I rebuilt the wreck I beefed up the fuse by plating it on each side from the firewall to behind the strut attach point with 4130 .030 ? plate.  I also added 4130 gussets to the fuse truss where the bungies wrap around.  My struts were pretty rusty so I cut them off and used 1" .059" tubing.  It started off as 1085 gross (as stated on the paperwork from Avid on the sales ticket) so I dont even think twice about putting 1200 in it.  I dont fly heavy unless winds are pretty calm so turbulence is not an issue. 

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Posted

Leni - owner / builder of the first Avid Flyer Alaskan Tank model, ha! I cut 1/4" marine ply gussets for the seat truss and triangles below the door sill back to the rear gear bracket on my Mk-IV, partly because I only own a stick welder and partly because I was concerned about heat hardening / softening the tubes. I think the Mk-IV seat truss used 5/8" x .035" wall on the top tube and 1/2" x .035 for the bottom and bridging. The top, bottom and bridge tubes on the Fat Avid seat truss are all 5/8" x .058" wall and there is also more vertical bridging between top and bottom truss. So, much stronger and no beefing necessary even with the wide gear legs.

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Posted

Doug,

Thanks for posting all that info in one place.  Very valuable, for us and particularly for someone new to the Avid.

Jack

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Posted

PPS...Wouldn't it be great if someone could convince Steve, now that he's "retired,"  to put his knowledge of the Avid into a book, so we wouldn't have to depend on old messages and memories of conversations to understand the limitations of our birds.

Steve are you listening?

Jack 

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Posted

hows the fat bastage projects coming along?  been awhile since we had an update... dont give us any crap about life or work slowing you down, this should be the #1 priority :lol:

:beerchug:

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Posted

Hope to have wings set to fuse this weekend, need another set of hands....

Took last weekend off for a little spring splash r&r, but not before I got the fabric plates and lift strut brackets done, designed and cut tubing for the new amphib bull wheel support bracket, lined up an A&P oxy welder to do a bunch of welding, and scrounged materials to make a metal brake so I can start bending up a console. Was going to make this one, then EAA Experimenter online showed up in the email with an article on this one. Decisions, decisions!

One question on the lift strut bracket install... how the h*|| do you seat that one rivet under the tab on the rear brackets?

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Posted

Doug,

There used to be exploding rivets, I wonder if they would work?  I think they were set by placing them in the hole and then heating them with a soldering iron or some other heat source.  I have to laugh thinking about some experimenter making his own.....  Really though, there were some rivets designed this way.  No fooling!

Jack

Leni,  Moving very slowly on my Avid+,  throttle arms and paint coming, when the engine hook-ups are completed, focus on surface prep for priming and painting.  Going to have to learn to fly again, been so long rust is very thick.

Jack

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Posted

On the rivit under the strut bracket... cut a short piece of steel tubing, just long enough to clear the bracket but short enough for the rivit gun to catch the rivit nail.

it is also ok to bend the nail a little. You can also use a 1/4" drive socket for the tubing.

AK.. Don't wory about the rust, I didn't fly for 21 years but a few landings and S turns with Leni and back in the groove. Well, it wasn't fun on the first landing but

Leni saved it.

Bob

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Posted

Explosive Rivets......

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Posted

Good tip Bob, thanks. Although explosive rivets do sound kinda fun...  :beerchug:

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