XP-82 Update for Oct. 14, 2017

[BK]

Engine runs yesterday: https://www.facebook.com/Xp-82-Twin-Mus ... 783063916/

[Zac Yates]

*faints* Awesome.

[menards]

So the XP-82 the props are counter rotating the top of propeller spins towards the center of the plane. On the P-38, the top of the props spin outward towards the wing tips. I find it more interesting that both planes had to reverse the rotation of the engines during initial development. Why is that?

[maxum96]

Awesome, but that happened a year ago. Look at the date on the Facebook page with the video.

[BK]

Awesome, but that happened a year ago. Look at the date on the Facebook page with the video.

Scroll down to see the video from Saturday, with both engines running and the plane mostly complete.

[maxum96]

Awesome, but that happened a year ago. Look at the date on the Facebook page with the video.

Scroll down to see the video from Saturday, with both engines running and the plane mostly complete.

Oops! I missed that one. Sorry. Thanks for setting me straight.

[airnutz]

So the XP-82 the props are counter rotating the top of propeller spins towards the center of the plane. On the P-38, the top of the props spin outward towards the wing tips. I find it more interesting that both planes had to reverse the rotation of the engines during initial development. Why is that?

Initially the XP-82 props rotated as the P-38 did which was figured to optimize single-engine aircraft control in an engine-out situation. When it came to the test flight of the XP-82, she wouldn't take off. They traced the problem due to the center wing/fuselage configuration and a prop rotation aerodynamics conflict, lift in the center wing section was negated. Their solution was to switch to engines with each other and the -82 flew. That how I recall the explanation...I'm sure someone in the wings may have a better way to put it.

[Dan K]

So the XP-82 the props are counter rotating the top of propeller spins towards the center of the plane. On the P-38, the top of the props spin outward towards the wing tips. I find it more interesting that both planes had to reverse the rotation of the engines during initial development. Why is that?

Initially the XP-82 props rotated as the P-38 did which was figured to optimize single-engine aircraft control in an engine-out situation. When it came to the test flight of the XP-82, she wouldn't take off. They traced the problem due to the center wing/fuselage configuration, and a prop rotation aerodynamics conflict, lift in the center wing section was negated. Their solution was to switch to engines with each other and the -82 flew. That how I recall the explanation...I'm sure someone in the wings may have a better way to put it.

I believe your answer is pretty close, except for the first sentence: Outward rotation of twin props (as with production P-38's) is not optimal for single-engine operation. In fact, it makes it more challenging. Single-engine operation with inward rotation (such as with production F-82's) would provide torque that would lift the "dead" side of the aircraft. During operations, F-82 drivers would occasionally report that they were initially unaware that an engine had even failed.

[airnutz]

Ahhhh! Thaanks Dan! Rusty memory, never a dull moment...

[C VEICH]

Simple physics makes it obvious that inward rotation engines are the optimum. Which is why they tried it that way on the P-38 first and only changed it when other aerodynamic factors demanded. So my question is, knowing this, what made North American think that outward rotation was the way to go on the P-82 initially?

[DH82EH]

I have commented on this before and they discussion just kinda dropped dead.
I suspect that a major contributing difference is the fact that on the P-38 the thrust line is below the mean chord of the wing.
On the '82 the thrust line is above.
Now we just need someone smarter than me to do the math based on my theory

Thoughts anyone?

Andy

Oh yeah. Totally freakin' cool that she is this close to flying. Such amazing craftsmanship.
Thanks to all involved for having the wherewithal to resurrect this amazing bit of aviation awesomeness!!

[airnutz]

So my question is, knowing this, what made North American think that outward rotation was the way to go on the P-82 initially?

Wasn't there some thought in the original concept with this design that the outward rotation would come together to actually enhance lift on the center wing section? I need to hit the library, but time is short lately...

[bdk]

Wasn't there some thought in the original concept with this design that the outward rotation would come together to actually enhance lift on the center wing section? I need to hit the library, but time is short lately...

I just can't imagine that the force of the airstream rotation has much affect on the wing that close to the prop, and the faster you go the less of an effect the rotation would have on the airframe. The tail is far away from the prop and has a huge moment arm by comparison and the effect is arguably not large there either. P-Factor seems to have a much larger affect by comparison.

[airnutz]

Wasn't there some thought in the original concept with this design that the outward rotation would come together to actually enhance lift on the center wing section? I need to hit the library, but time is short lately...

I just can't imagine that the force of the airstream rotation has much affect on the wing that close to the prop, and the faster you go the less of an effect the rotation would have on the airframe. The tail is far away from the prop and has a huge moment arm by comparison and the effect is arguably not large there either. P-Factor seems to have a much larger affect by comparison.

Dunno where I drug that up from Brandon...some vague memory tying to trigger someone else's vague memory. The old guy who I used to query about such minutiae passed a year and a half ago. Like I said earlier, I'll try to do some reading...