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Old 03-15-2012, 05:29 PM   #1
Btro0515
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Hello again. Got alot of great info on my last question. Still waiting on plans to get here...but just out of interest...the +6 -3 g limit on the s1ss seems a bit small on the negative side. I know there have been many forums about this, but nothing too concrete in here. What i would like to know is how would one go about strengthening for harder outside acro? I am a first time builder, so if it is way too much engineering i could learn to love that limitation. But if there are some simple things to do to at least push a bit harder, id love to hear about it! Call me weird, but i enjoy outside a bit more than inside! I would be just as happy if the limit was 3 period, but if it can be done...why the hell not? (excluding utility limits, i want her to last a while!). Thanx for reading!

Brandon



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Old 03-15-2012, 06:51 PM   #2
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I'm going to take a shot at this and wait to stand corrected;
I believe the +6, -3 limits are a result of certification costs and not because the airplane is really that much different on the negative side. I do know for a fact that the S-1S airplanes are regularly flown to higher negative G loads with no apparent problems. My G-meter normally had +7, -5 on it after each flight. Since the meter only went to -5 before hitting the peg I'm not sure how high the loads were on the negative side. I was generally flying at less than gross weight while doing sequences but not 40% under.
The S-1T has different limits, something like +6, -4.6. I don't know why. That airplane sees about the same treatment as the other S-1 airplanes when flying in the higher competition categories.
I don't believe you need to make changes to the airframe to fly at -5g or even -6g. I did it for years without abusing the airplane.



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Old 03-15-2012, 06:58 PM   #3
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That sounds perfect! Thanks for the info, i know there are other posts, but nothin about reinforcements...that would probably be why that is. Thanx again sir!

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Old 03-15-2012, 08:50 PM   #4
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Brandon,


A few areas of reinforcement that has been incorporated over the years has been: Upper wing attach fittings, increase in thickness and flanges. They curled over time with higher -G loads. Also the finger straps added to the cabane struts, top and bottom. The increase in thickness of the control stick to .049. Also the increase in H tube thickness. All these changes were due to increased loads. Current plans reflect these changes.


Years ago when Sean Tucker was flying his Pitts airframe at Oskosh, Curtiss made the comment on the announcer stand, they he never designed the Pitts airframe to take such abuse. He finishedhis commentwith a smile


No monoplane will take the abuse that a Pitts airframe will endure. I don't want to tell you what the max gs I have pulled. Shame on me... I will just say the std G-meter does not go that high, only thespaceship ones do.


JSTreat

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Old 03-15-2012, 09:32 PM   #5
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The Aerosport Technical Committee performed a structural analysis of the S-1S as part of the approval process for aerobatics in Canada, back around 1970. I understand that the critical component for negative g turned to be the lower wing rear spar where it attaches to the fuselage. The load was claimed to be inward, and put a lot of stress on the wood / bolt junction.
Some time later a fix was promoted by Aerobatics Canada. The inboard four inches or so of the spar was to be encased in steel, replacing the straps. I think the idea was that the compression load would be taken by the steel boot, with the end of the spar carrying the load into the boot. I don't have any drawings; I'm just describing what I saw someone else install.
I didn't see any sign of damage to the inboard ends of the spars when I rebuilt my wings after 1000 hours, so I didn't make this change. (There were some minor cracks on the side I step on which resulting in replacing that spar, but the other side was fine.) Also, I couldn't visualize the inward load, so I didn't take this idea too seriously. I fly to the same +7 and -5 that cwilliamrose used. My guess is that even if that bolt hole is the wing part closest to its limit, it can tolerate at least -5 without problems. (I am a lot more careful about how I step on the wing.)
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Old 03-15-2012, 10:34 PM   #6
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Allan,

Do you recall at what G level the rear spar became an issue? A lot of us have seen worn holes in the attach fittings but I don't recall seeing anything that required repair on the rear spar itself. Apparently the airplane was approved as-is since there are a number of then flying akro in Canada.

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Old 03-16-2012, 01:27 AM   #7
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Sorry, Bill. This happened before I bought my Pitts. I didn't pay much attention to it; I just absorbed it from a Pitts owner who was overhauling his plane and installing the steel covers on the rear spar ends. Once I got my wings uncovered 10 years later I saw no indication of damage from inboard-directed force on the spars so I didn't go back to research it.
I thought I should throw this into the discussion since the Aerosport Technical Committee was quite highly regarded in Canada and likely got the stress analysis right, even if I can't see the way the force reaches the spar bolts. If they were right, then clearly -5 g is below the limit of damage to any part of the wings from routinely exceeding the factory's recommended limit of -3.
Maybe someone familiar with the S2 series could shed some light on this. Compared to a -B with a -3 limit, does the -C have beefed up inner rear spar ends for its certified limit of -5?

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Old 03-16-2012, 02:05 AM   #8
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6 and -3 are the limits for the S1S airframe for one simple reason. The FAA minimum certification requirement for aerobatic category is +6, -3 g limit load. The airplane was designed to these specs. What this means is that the structure can be subjected to unlimited # of cycles up to these g limits and not have permanent damage. The ultimate load factors of +9, -6 g is where the structure is designed to begin failure. In other words, the S1S was designed to be flown up to +6, -3 g but designed to not break until +9, -6 g. Loadings above 6 and -3 will cause measurable damage and loss of load capacity of the structure. Now, just how many times the thing can be flown beyond 6, -3 g is the question.

Since the airframe only had to have +6, -3g limit loads to be certified, that is how the structure was designed. The lower wing spars are in compression when in neg g flight. The lower wings were designed to be able to live at -3 g and fail at -6. If the certification requirements had been -14g, the structure would have been designed to meet that.

Consider this. These load factors are for a given weight of airplane. Not at whatever it comes out to be that is heavier and with a big and heavy pilot. Every ounce you add to "make it stronger" adds an once of weight to the airplane. This is where the acro gross weights of the design come into play. If you have a heavy airplane with lots of beef ups and a pilot over 165lb, chance are you are overloading the airplane when you fly it to +6, -3. Those that are flown at proper weights last a lifetime. those that are heavy and over beefed don't. Some people have the training and understanding of these structures and can make proper changes to increase performance in some areas while accounting for the affects to other factors in the airplane. Homebuilders generally do not have the training to sort these things out as is evident with the vast number of problems created by "I thought it would be better if's" that are so often done. If you have the engineering skills and experience, cool. Go for it. If not, don't mess up a good system. Build it light and straight and it will out perform the specs for it. So Brandon, don't think too hard trying to make it better. Concentrate instead on making it right and light.

FYI, the lower front spars in the Model 12 are larger than the upper front spars to allow for higher than FAA min neg g limits.

One more thing. Call Aviat and order a set of the factory S1S drawings they sell. These have a wealth of info that are not in the other plans sets available as they are the actual certified design drawings.Edited by: kjkimball

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Old 03-16-2012, 03:00 AM   #9
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Much appreciated everyone! Im no engineer and will be sticking with the plans (which got in today!!!!!!!!!). The steen ss wing plans seem impossible to build without their kit. I have no dimensions on ANYTHING!! The 1 and 4 upper ribs are designed compltly different than the rest! And not any info on how to build them. (i can see this befor truly even being able to comprehend the plans as of yet!!!!). Any suggestions, because i want to do the ribs myself, not order their kit! Im BROKE damnit!

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Old 03-16-2012, 04:38 AM   #10
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Thanks for the explanation. I'm hoping you will expand on this:

Quote:
Originally Posted by kjkimball

The lower wing spars are in compression when in neg g flight.
Well of course the spars in the lower wing are in compression during negative g. The landing wires resist the lift load by turning it into a compression load on the spars. It seems to me that most of the compression load should be on the front spar because the landing wires are aligned with it. How much compression load can the drag - antidrag wire system transfer to the rear spar? Can the wood of the lower rear spar where it attaches to the fuselage really be the most critically stressed part of the wings?
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Old 03-16-2012, 07:38 AM   #11
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Quote:
Originally Posted by Btro0515
Much appreciated everyone! Im no engineer and will be sticking with the plans (which got in today!!!!!!!!!). The steen ss wing plans seem impossible to build without their kit. I have no dimensions on ANYTHING!! The 1 and 4 upper ribs are designed compltly different than the rest! And not any info on how to build them. (i can see this befor truly even being able to comprehend the plans as of yet!!!!). Any suggestions, because i want to do the ribs myself, not order their kit! Im BROKE damnit!
See my suggestion above to get the Aviat S1S plans. The most info available for the S1S structure.
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Old 03-16-2012, 07:45 AM   #12
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Quote:
Originally Posted by allanf
Thanks for the explanation.Â* I'm hoping you will expand on this:
Quote:
Originally Posted by kjkimball
The lower wing spars are in compression when in neg g flight.
Well of course the spars in the lower wing are in compression during negative g.Â* The landing wires resist the lift load by turning it into a compression load on the spars.Â* It seems to me that most of the compression load should be on the front spar because the landing wires are aligned with it.Â* How much compression load can the drag - antidrag wire system transfer to the rear spar?Â* Can the wood of the lower rear spar where it attaches to the fuselage really be the most critically stressed part of the wings?
Allan,

I didn't want to counter the info offered on the wing mod. The front spar does have the greatest load in it. I had not heard of the mods you referred to before this thread. Prior to that, I knew only what Curtis Pitts had told be which was the lower rear spars see the greatest load on the ground when a 200+lb pilot steps on the wing. S1 rear spars are very small compared to front spars. Same in a Model 12. Front lower spar 1", rear, 11/16" thick.
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Old 03-16-2012, 11:02 AM   #13
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Not to question what you said Kevin, just taking a historical perspective;

The S-1S was designed in the late 60's for pilots flying higher category competition and airshows. The pilots flying them in the Unlimited category were surely flying beyond -3g during their flights and designing the airplane to that number when the mission clearly required more would seem to be foolish. At the 1970 world contest there were at least four S-1S airplanes entered by the US team. This was about 2 years before the certification of the airplane. One of those airplanes is very familiar to me (wax on, wax off) because I worked for the owner during the early 70's. On the rare occasions when the G-meter was not zeroed after the flight I saw what the pilot was pulling and pushing. An outside snap will get you 3.5g without trying hard but this pilot was trying hard to do crisp snaps and it showed.

So I go back to my original comment that the +6, -3 G limit was the FAA minimum for the aerobatic category and likely not the actual limit of the structure. I'm sure Curtis knew they were already flying the C's above the minimum negative G limit of -3. To certify the airplane beyond the minimum would surely have added cost and time to the process. I'd love to know the actual limits and I'd be very surprised if the answer was -3g given the history of the airplane and its intended mission.

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Old 03-16-2012, 08:20 PM   #14
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Kevin,

It appears to me that you've made a math error here:

Quote:
Originally Posted by kjkimball
6 and -3 are the limits for the S1S airframe for one simple reason. The FAA minimum certification requirement for aerobatic category is +6, -3 g limit load. The airplane was designed to these specs. What this means is that the structure can be subjected to unlimited # of cycles up to these g limits and not have permanent damage. The ultimate load factors of +9, -6 g is where the structure is designed to begin failure. In other words, the S1S was designed to be flown up to +6, -3 g but designed to not break until +9, -6 g. Loadings above 6 and -3 will cause measurable damage and loss of load capacity of the structure. Now, just how many times the thing can be flown beyond 6, -3 g is the question.


Your ratio of the positive ultimate load to limit load is 1.5, which is what I understand to be correct. However, your ratio of negative ultimate load to limit load is 2. If we use 1.5, we get a negative ultimate g load of -4.5, beyond which structure should start to break.

If that is correct, it reinforces cwilliamrose's opinion that the plane was designed for well more than -3g. I know of a few S-1Ss that were flown routinely to -5 for 500+ hours and and nothing was found damaged in the wings that could be attributed to negative g overstress. (It's difficult to fly Advanced with a limit of -3, and impossible to fly Unlimited. I don't know of anyone who was able to fly the higher categories without routinely reaching and sometimes exceeding the ultimate load of -4.5.) If it really was designed for -3 as the limit load, then it was overdesigned!

(Or maybe the ratio of ultimate to limit load is supposed to be different for positive and negative g.)

Allan

Edited by: allanf
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Old 03-16-2012, 09:01 PM   #15
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Ultimate limit load is point where structure start to chance form permanently.....but not catastrophic failure point. Catastrophic failure point is always beyond ultimate limit load.
Designers must design (at least these days) aircraft certain point, where structures start to chance permanently, but not break catastrophic, and this is point where ultimate limit load is given. Aircraft structures cant break at ultimate limit load, or that ultimate limit load must determine lower value.
This is basic rule for certified aircraft design....however at homebuild business it might be different story....

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Old 03-17-2012, 06:14 AM   #16
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The encasing lower wings attach fitting reminds me of what is depicted on the Ultimate 10-200 plans (sorry not at home so no pics available)...
Surely the same than on the Ultimate wings for Pitts S1, certified by Transport Canada for +8,5/-? according to an ad from Ultimate found here : http://moleski.net/ULTBIPE/ulthist/dy6.jpgEdited by: Xavier

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Old 03-17-2012, 11:37 AM   #17
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Bill,

I did not write that the S1 structure would fail at -3g. As I posted on the 15th, the limit load is an FAA certification minimum that is met and is a published number that applies to the S1S structure as designed. The ultimate load is the designed failure point and even at that, Curtis' designs had some safety factor.

Keep in mind as designed includes designed weights. Gross for an S1S to T is 1150lb. So, if 200lb pilot, 20lb chute, 20gal gas, handheld gps, some other typical cockpit stuff, you can have an empty weight airplane of 740-750lb and fly to +6, and -3 g limit loads and conform to the design intent of the airplane. If the airplane is heavier, less gas, lighter pilot and still adds up to 1150lb, OK. But, so many of these airplanes are 800-850 even 900lb empty and still get a 200+ pilot and full gas that flying at the limit loads exceeds the design intent work load on the structure.

The Limit load values are the point where beyond this load, there is some part of the structure that exceeds the plastic limit (engineering term not material type). Example for you guys. A straight wire type antenna on a car. It can vibrate, shake, be pulled pretty far over and rebound. But, take it too far and it will not return to the original shape and position. Grab it with two hands and bend its sharply, it remains bent. As long as it returns to its original shape and location, it was inside the plastic limit of the material it is made from. If it doesn't return, it exceeded the limit. This is the point of the LIMIT LOADS in aircraft structures. The antenna didn't break, but it isn't the same and like any material, subsequent loads at that slightly yielded point require less force to exceed the remaining material strengths. The more you bend it the easier it is to bend right up until ultimate failure.

Allan,

It is up to the designer to pick values that he feels are needed for the structure and how it will be used. On these types of airplanes, it is pretty easy to exceed the -3 limit. This is why there is more safety room there. Look, the margin between the limit loads and the ultimate loads is not some extra space where you can routinely fly the airplanes. If you know it was designed to fail at -6 and you like to fly at -4.5 all the time, chances are you will inadvertently exceed -6 sooner or later by a mistake, bad air, whatever. Plus every time you fly it past the -3 limit load, you have changed the structure and it no longer is as tough as it first was.

This operating over the limit load failure take time to show up. Look at the S2B longeron AD as an example. Planes were begin flown over acro gross and over limit loads. First time...Ohh nothing broke so it must be OK. So, many times later, it feels funny or one airplane I knew about actually had slots pulled in the fuselage sheet metal in the cockpit and cabane area as the skins were being stretched under g loads since the longerons were broken.

It has been shown in many designs that repeated operation beyond the limit loads results in accelerated deterioration of the structure. Some are OK with this and willing to rebuild every year or two. Some want the airplanes to last 2 lifetimes and fly the airplanes like they own them instead of a rental.

Anyone interested in continuing this conversation with me can stop by my booth at SNF in a few days. Far easier to talk about it than write.

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Old 03-17-2012, 05:21 PM   #18
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I talked to Curtis once about load testing the S2A at his field in Homestead for certification. The jig he used was hanging from the ceiling and they loaded the airplane with sand bags to positive G. I asked him if they had to flip the airplane and do it -G. He said ,No , they did it mathematically. I asked why the airplane was only stress for -3 g's "Was it because of the geometry of the flying/landing wires? He answered the airplane was the same strength + or - . He didn't want to tell me how much sand they put on the airplane. He did tell me the airplane only defected so much and returned to it's original shape / position.

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Old 03-19-2012, 11:46 AM   #19
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Quote:
Originally Posted by kjkimball
Bill,



I did not write that the S1 structure would fail at -3g......
I didn't take it that way Kevin, sorry for the confusion. I was trying to say that an airplane could be built to +/- 12g and be certified for +6g, -3g. The certified limits may be conservative if all the manufacturer wants is an Aerobatic category cert. Based on actual use, the S-1S seems to be good for at least -5g since flying at this level does no short term damage and seems to hold up well long term too. And, as I stated previously, the mission for the airplane would require more than -3g, certified or not.

I was more concerned about the +7g I was flying at than the -5g. The airplane was light -- built by Curtis for Bill Thomas (the last one he built in Homestead, I think) and rebuilt at the factory with a fairly light finish. No electrical system, no radios, nothing extra except a canopy and belly Plexiglas. It didn't even have a big rudder. I was under max gross except for X-country flights.

Maybe someone can comment on this;

The original g limits for Aerobatic category certification were +5, -2. That was changed and I believe the S-2A may have been the first airplane to apply for a TC in the Aerobatic category after the change. The Decathlon was being done about that same time so maybe it was first?? Airplanes like the Citabria were certified under the old +5, -2 limits. At the time this was being done it was a "new" company certifying an airplane and the rules were new for the feds.

Edited by: cwilliamrose
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Old 05-23-2012, 12:31 PM   #20
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to answer to allanf, the factor of safety is the same for positive and negative g and usually il 1.5, except for some particular cases. some definitions from far 23 below if you want more details.

I think Curtis has been wise not to answer about the maximum loads, because if he would have said e.g. 10 g, some people would have pulled more thinking that there was still margin on that, but remember that the structure is not supposed to be used above limit loads, ultimate load are just there to get you home safely, after that the aircraft must be carefully checked.
but the question remains: how many g for S1 and S2? would be useful to know the mod's in the -2a, b and c to increase the negative g limit and also on special pitts like Sean Tucker's one

Alberto

§ 23.303 Factor of safety.
Unless otherwise provided, a factor of
safety of 1.5 must be used.

§ 23.305 Strength and deformation.
(a) The structure must be able to
support limit loads without detrimental,
permanent deformation. At
any load up to limit loads, the deformation
may not interfere with safe operation.
(b) The structure must be able to
support ultimate loads without failure
for at least three seconds, except local
failures or structural instabilities between
limit and ultimate load are acceptable
only if the structure can sustain
the required ultimate load for at
least three seconds.


§ 23.301 Loads.
(a) Strength requirements are specified
in terms of limit loads (the maximum
loads to be expected in service)
and ultimate loads (limit loads multiplied
by prescribed factors of safety).
Unless otherwise provided, prescribed
loads are limit loads.

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Old 06-01-2012, 03:33 PM   #21
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Interesting discussion. I always wondered why the s1-11 lower wing attach points were reinforced as much as they were. Here are some pics of the s1-11 wing attach points.

http://mars58superstinker.wordpress.com/2010/01/10/lower-wing-attach-angles-in-place-on-the-right-wing/

This discussion seems to have shed some light on that...

Good stuff!!



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