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Personally, if I were in the market to change engines from, say an Aero Vee to a Corvair, I wouldn't hesitate in the least. Its not a big design change and if the worry is that just because one hangs more pounds on the airplane, the firewall is going to depart for places unknown; I think that's completely unfounded.
With care and reason its not something that would bother me in the least. Super Cubs mostly started out with 115 HP. That went to 150 and 180. Obviously, reasoned minds looked at the structure, did some calculations, added what was needed and wallah! No engines falling off or planes falling out of the sky.
What does bother me is this continuous ongoing string of unknown engine failures. Without and answer from the NTSB, or other persons that may or may not have information.
Corvair installs have proven to be pretty sturdy installs and holding up well. That's completely a different feeling than I had about 5 yrs ago.
I had no confidence in the Corvair as a conversion. I know feel I was wrong on that front.
The Corvair is now a very understood platform and unlike some engines, isn't strained to the nuts to do what it does.

Larry
Waiex121YX, Camit 3300, Dynon Skyview

For the record, I like the corvair. Not too keen on some of the opening statements, like flying way past Vne, super high empty weights, double FWF weights, and so on. I wouldn't want my kid to do any of that. That is one reason it is important to have some balanced discussion. Somebody new may read that and think it is therefore ok to have 400 lbs FWF or to fly 250 mph.

This debate may be over looking the intended mission one is building their plane for. If intending for a lot of yank and bank aerobatics, while flying over terrain that provides ample emergency landing opportunities, structural and aerodynamic considerations may be of more importance than engine reliability.
and using one of the Sonex supported engines that is within the weight limits seems advisable. However, if doing more "normal" flying, but over inhospitable terrain, best engine may be that one the builder has the most confidence in. Most would like the best of both, but may lean one way or the other depending which mission they feel is the more critical. There is not likely one simple answer to this, but the more information one has available when making a choice, the better.

David A. Sonex TD, CAMit 3300. (Hilltop airport, surrounded by deep valleys and forests.)

Bryan,

Not trying to be snippy or split hairs here. I just think that Dale was pointing out the sturdiness of the design. Not implying that he had done all the things on the list.
I'm not an engineer of any type. I went to the design school of hard knocks. Some folks can look at a mechanical project and decide a proper course of action and be very successful at their endeavor. Others can get the best advice and all the best materials and still turn it into a mess.
Countless homebuilders have done amazing builds that others openly scoffed at and condemned roundly.
I would encourage builders to keep an open mind because not everything being discussed here is going way out on a limb.
Experimental design should be responsibly encouraged. We're getting down to a level where experimentation is frowned upon for no reason other than, "the outcome is not guaranteed". That in my mind is a cookie cutter world. Manufacturers of kits have a vested interest in no one going off the pathway. For them its all about image and liability. I understand that. They just ask, when your done, don't call it a Sonex, or whatever kit you started off with. Its a Whizbang 5000, or whatever.
I applaud those that think outside the box and build better mouse traps.

Larry

Just so we're clear: I'm not at all against experimentation. (The 3D-printed Aerovee intake should make that clear! :-)

But there are facts and there are judgement calls. These are facts:

• Exceeding the design limits does eat into the safety margin.
• Absence of previous failures is not evidence that there will be no such failures.

If you disagree with any of those two statements, you're deluding yourself.

Now, these are judgement calls:

• What remaining safety margin am I comfortable with?
• How many instances of existing modifications, that have not failed, do I need to be comfortable with it?

Everyone's answers to those will be different, and they will determine how that individual proceeds. But in order to make those judgement calls, you gotta have some data to work with.

If people have actually done their own calculations to conclude that they're comfortable with the extra weight, please post it for everyone's benefit.

lutorm wrote:Just so we're clear: I'm not at all against experimentation. (The 3D-printed Aerovee intake should make that clear! :-)

But there are facts and there are judgement calls. These are facts:

• Exceeding the design limits does eat into the safety margin.
• Absence of previous failures is not evidence that there will be no such failures.

If you disagree with any of those two statements, you're deluding yourself.

Now, these are judgement calls:

• What remaining safety margin am I comfortable with?
• How many instances of existing modifications, that have not failed, do I need to be comfortable with it?

Everyone's answers to those will be different, and they will determine how that individual proceeds. But in order to make those judgement calls, you gotta have some data to work with.

If people have actually done their own calculations to conclude that they're comfortable with the extra weight, please post it for everyone's benefit.

I agree with all of this but I question that if one decides not to allow aerobatics in their plane, is exceeding the FWF limit by lets say 20lbs still "eating into the safety margin"?
Here's how I see this (I'm not an aeronautical engineer so if I'm looking at this the wrong way, please correct me):
In the Aerobatic Category the plane can be taken up to 6G's so with an FWF weight of 200lbs, that's 1200lbs hanging from the nose during those instances.
In the Utility Category the plane can be taken up to 4G's, so with and FWF weight of 220lbs, that's 880lbs hanging from the nose.

So are we really eating into the safety margin if we don't allow aerobatics?
It might be as you say "a judgement call".

Who says FWF weight was the limiting factor? When they test for ultimate load they turn the plane upside down and load the wing to 9x aerobatic gross. What does that have to do with hanging an extra 20 lbs on the nose? Did they put 9x200lbs on the engine mount and the structure failed at that point? Maybe a 200 lb artificial limit was placed to ensure the market did not have to be shared with the other auto conversion? Maybe not. I bet Pete Buck would know.

rizzz wrote:So are we really eating into the safety margin if we don't allow aerobatics?
It might be as you say "a judgement call".

Sure you are. Then you're getting some back by imposing more strict operating limitations. :-)

Direct C51 wrote:Maybe a 200 lb artificial limit was placed to ensure the market did not have to be shared with the other auto conversion? Maybe not.

I only know that if *I* was the designer, and I had decided on a 200lb FWF max weight, I would design the rest of the plane to meet that limit and nothing more, rather than make the rest of the structure artificially heavier and stronger for no purpose.

Strength isn't free, designs are always tradeoffs.

lutorm wrote:

rizzz wrote:So are we really eating into the safety margin if we don't allow aerobatics?
It might be as you say "a judgement call".

Sure you are. Then you're getting some back by imposing more strict operating limitations. :-)

That's probably the correct way of viewing this.