Yellow Arc Question

[RanchPilot]

Anyone know what safety factor they used when establishing the yellow arc for the PA-12 and/or PA-18? I heard a rumor that it was something really high--like 30% or more. I have also read other rumors that (i) there has never been a proven in-flight structural failure on a Cub wing; and (ii) the Vne on the Cub was actually a function of what the windshield would stand, rather than the wing. I'm not vouching for any of these statements--just curious if anyone can confirm or debunk them.


I have no desire to fly in the yellow arc in rough air, whatever the answer to these questions may be. Rather, my question is motivated by that sinking feeling I get whenever I'm cruising along in the yellow and I hit that first big bump...

[Grant]

I get that feeling too.

[Jeffrey Flinn]

Ranch Pilot I remember a thread on here somewhere titled "Can a wing be too stiff" that may answer your question but I'm not sure. Check it out.

[sierra bravo]

Yellow arc? Put some big tires and Cub gear under that thing and you'll be in my realm, 25mph shy of the yellow arc in WFO cruise. I'd have to dive to see yellow arc speeds in my -12, literally.

[RanchPilot]

I'd have to dive to see yellow arc speeds in my -12, literally.

I know what you're saying, SB, and I assume I will eventually end up there too. Before I do, though, I wanted to experiment a little. With my current configuration (no flaps, stock -12 gear, 8.00 tires, 74/52 prop) I was able to true out at 111 mph at 2400 rpm.

I have a 74/56 prop sitting in my hangar that would, presumably, put me further into the yellow for cruise. Like I said, probably not where I will ultimately end up, but fun to experiment nevertheless. Part of the purpose of my question was to determine whether pushing my cruise up into the yellow would be largely a waste of time, as I don't plan to spend much time there anyway if the air isn't glassy smooth.

[aviationinfo]

Darin, the other interesting thing to consider is that the yellow arc isn't linear in terms of how much turbulence is acceptable. A certain amount (we don't know how much or what the fudge factor is) is ok until you hit the magic number, which is 138. Then it's just...too much. it's a CAR 3 airplane and I think you can get a look at historical FARs on the FAA website. Hopefully the certification standards are on there.

Really looking fwd to your results. Of course you need to take the racing tires off first!

Andrew

[RanchPilot]

Yellow arc? Put some big tires and Cub gear under that thing and you'll be in my realm, 25mph shy of the yellow arc in WFO cruise.

BTW, I love the acronym, SB. That's a new one to me, and I'm definitely going to add it to my plane's performance specs in my POH:

Vx
Vy
Vne
Vwfo

I may even make up a new sticker to put on the north end of my throttle range!

[aktango58]

and those tests were done with brand new metal holding them wings together

Now that it is almost 50 years old, and has how many flex cycles...


Bonanza pilots I think did a bunch of test flying when they were first out. Did not work out well for them or their aircraft.

Just my thoughts on stress

[RanchPilot]

Darin, the other interesting thing to consider is that the yellow arc isn't linear in terms of how much turbulence is acceptable.

Andrew, that non-linear stuff makes me nervous. I have enough trouble comprehending numbers higher than 10 (unless I have my shoes and socks off), much less exponential ones. Add in AK's comments about stress cycles, and I'm certain I don't have any need to be the test pilot trying to determine the accuracy of that 138 number.

That said, I'm still curious to try and understand what fudge factor they used when they officially proclaimed that yellow was yellow.

[mike mcs repair]

... I have also read other rumors that (i) there has never been a proven in-flight structural failure on a Cub wing; .....

I changed rear spars on both wings of one cub that bent them doing loops.... that was back in mid 90's

[RanchPilot]

I changed rear spars on both wings of one cub that bent them doing loops.... that was back in mid 90's

(he wasn't a Bonanza pilot, was he...?)

[tophand]

Darin, the other interesting thing to consider is that the yellow arc isn't linear in terms of how much turbulence is acceptable. A certain amount (we don't know how much or what the fudge factor is) is ok until you hit the magic number, which is 138. Then it's just...too much. it's a CAR 3 airplane and I think you can get a look at historical FARs on the FAA website. Hopefully the certification standards are on there.

Really looking fwd to your results. Of course you need to take the racing tires off first!

Andrew

That there's funny. I'm still laughing.

..........Mike