Validating Aircraft Design/Modifications

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I have been reading about building various aircraft, and some of the modifications that are done to them.

I am leaning toward an all-metal build, but some research has resulted in some interesting thoughts. The 3 things that really made me think were:
1)A CX-4 where the wing ribs were redesigned to be CNC cut but used bent tab fasteners rather than the dimpled rim design.
2)Reading about the KR2 design and safety record. Basically (as I understand), almost no one built a stock KR2, and that makes some unsafe, but those modifications resulted in the KR2S, which has a much better safety record.
3)A Trent Palmer video where he added a leading-edge cuff to his wing and, after testing, found that it increased his stall speed rather than decreasing it (as he intended).

While these things seem only marginally related, for me they underscored how modifications can make an existing design better fit your mission, but can also have unintended consequences. I am looking to understand how to avoid the latter.
 

Voidhawk9

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Trent's cuffs were temporarily affixed and appeared to disrupt the smooth shapoe of the airfoil over the upper surface, likely leading to premature separation that may not take place if they were permanently faired into place. Airfoil performance is very sensitive to stuff like that, especially near the leading edge.
 

TFF

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To avoid is build a vetted design with no modifications. If you want to experiment, you realize you really are experimenting. If you know how to do what you want, it probably will work. If you are just scabbing an idea you like on something, it could be a major health risk. I think many times people just like the shape or the performance numbers if true. The best way to pick a plane is to understand why it was designed. The reason why it exists. Why Wittman, Pitts,VanG, Rutan did what they did speaks why. When and where they were designed has bearings too. All these things have shortcomings to some; to the designer it may have been a positive.
 
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I understand the risks of making changes, that's why I am asking this question. Honestly, when I build, I plan to stick to whatever design I choose.

But my curiosity asks things like "What would happen if I made the flaps on a Sonerai 2L into flaperons?" and I would like to be able to answer that without becoming a test pilot.
 

Voidhawk9

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For some things, a good X-plane simulator model will get you a decent 'ballpark' answer.
 

TFF

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It’s not intended to be a smart answer, but to know if you are not a fluent engineer in aircraft design, you can’t know, unless guess and try looking for results. It’s why the guys here want to talk engineering. Because the engineers are serious, they never give” do it, it’s ok” answers. They want to run numbers. The idea might be fine and easy but if you want to know before it leaves the ground, you have to run numbers. If you have lots of practical aircraft experience, probably your guess would be close to right. There are no magic programs. Even if you do it all right, it’s all about execution. You are going to be a test pilot, it’s just the degree of scare.
 

pictsidhe

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I understand the risks of making changes, that's why I am asking this question. Honestly, when I build, I plan to stick to whatever design I choose.

But my curiosity asks things like "What would happen if I made the flaps on a Sonerai 2L into flaperons?" and I would like to be able to answer that without becoming a test pilot.
It will affect the aerodynamics, it will affect the stresses on the structure. It will affect stick forces. It could make it a lead sticked, spin happy or likely to snap a wing off. Or something else. Possibly a combination. Or, it could be great. If you don't know how to check potential modifications for adverse scenarios, it's best not to modify it. As TFF says, the engineers won't say yay or nay to an idea as they determine things by numbers and/or testing. Modifying things to change construction method is somewhat easier, you don't need to look at aero affects, assuming any stiffness change doesn't change the aero.
 

jedi

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But the designer can not do everything. There will be changes to any experimental (EAB) airplane that should or could be made and there may be valid reason for the designer / builder not making those changes as in he already has a working aircraft.

It is called evolution and persistent designs go through that. Consider the Boeing 737 as a current interesting example.
 

pictsidhe

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True, but making changes when you don't have the aptitude to assess their affects is somewhat risky in aviation. For instance, many people are unaware that painting a composite aircraft black instead of boring white can result in bad things happening.
 

Urquiola

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Perhaps a wise way to test modifcations in known designs could be changing the airplane into a remote controlled version, in Extremadura, Spain, they have a full-size, remote controlled version of Cri-Cri, built by an aeromodeller's club member, they never attempted building it under experimental/ homebuilt aviation rules, this would have allowed testing it as pilot.
Radio Control could miss the 'feeling' flying on board may have, but the advantage in military drones: losing pilots is extremely expensive, specially for pilots themselves. Agur. Salut +
 

Alan_VA

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The best way to ‘test’ a modification is to ask the designer. He will have all of the background studies and experience that enable a person to make a knowledgeable decision. In the case of the CX4, call Dave Thatcher or write an email to Glenn Bradley. Regarding the CX4 wing rib question - I’m about to start building a CX5, and I asked that same question after seeing the same (or similar) video. Dave said that either way would be acceptable. If you ever saw his shop, you’d understand why “CNC” is a phrase that will never be spoken there. Old school, all the way.

Alan
 

pictsidhe

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Burt Rutan left the plans market as he was fed up with defending himself from the grieving relatives of builders who had made fatal alterations to his designs...
 

Marc Zeitlin

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Burt Rutan left the plans market as he was fed up with defending himself from the grieving relatives of builders who had made fatal alterations to his designs...
While Burt certainly wasn't thrilled with being sued for builder's ridiculous mistakes, he stopped selling plans for liability reasons when he started Scaled Composites - he didn't want any liability to flow to the new company. Minor nit.
 

Dennis DeFrange

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Doesn't hurt to study modifications to particular aircraft and to their design , as well as the results of the outcome . The Pitts Special is a great example . Not a one of them are even close to the original design but everything that has evolved has rendered them one of the most outstanding airplanes in the air today . Watch what others do and have done . Ya don't have to hold a degree in anything if ya just use your head and don't do stupid stuff . If ya think ya can just design an airplane and don't know diddly squat about airplanes , you're gonna kill yourself or someone else . Common Sense go's along way but back it up .
 

BJC

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Curtis Pitts not only knew airplanes from his business and from working on them, he also had several engineers looking over his shoulder and providing input.


BJC
 

Hephaestus

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If you go back and look. Some of the work like you've mentioned on the kr2 - was an iterative process. Small mods that worked and were incorporated into larger as they refined and tested and refined again. The stretches started small 4-6" now are 2'-4'. 2" to width became 6-8" and the dragonfly canopy since it's prettier and cheaper. Krnet is still very active to this day, and there's still a core there who will pay money to have a design idea validated by a professional (like the new airfoil see http://www.krnet.org/as504x/ )
 
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