Christen Eagle

[Doug Rodrigues]

I was fortunate to fly with a pilot several years ago that had a Christen Eagle and he really knew how to fly it. I am curious about two things, First off, he said he heated up the oil before starting the engine. Is that typical protocol for this aircraft and why would he do that (I assume it would be to extend the life of the engine by reducing wear?). Second, what kind of G-forces can the plane take? We did one maneuver that had a +4 immediately followed by a - 3.5 and I almost lost my lunch. I had previously flown in a Pitts but the Christen Eagle was far more exciting. I assume a good portion of that was due to the pilots' experience. Best, DiverDoug

 

[texasclouds]

What was the outside temperature when he preheated the oil? Cold winters many people will preheat oil with an electric heating element affixed to the oil pan.

Wiki says +9/-6 G

welcome to POA. I’d advise you run while you still can. You be hooked on aviation soon, followed by broke as a joke.

 

[Doug Rodrigues]

What was the outside temperature when he preheated the oil? Cold winters many people will preheat oil with an electric heating element affixed to the oil pan.

Wiki says +9/-6 G

welcome to POA. I’d advise you run while you still can. You be hooked ok aviation soon, followed by broke as a joke.

texasclouds, thanks for the feedback. I recall the outside temperature was about 60 degrees F

 

[RoscoeT]

The Eagle is rated for +6/-3G operational load. +9G is ultimate load where things may start to bend. Glad you're interested in aviation and acro. The Eagle has virtually the same overall performance as the 4-cylinder Pitts S-2A, but was designed for improved ergos. It has less performance than the single seat Pitts' and the 6-cylinder Pitts S-2B/C, but at a certain level it's a bit of a Cub vs. Champ sort of thing...Pitts being the Cub and Eagle being the Champ.

 

[Spinka]

The Eagle IIs I've flown were all limited to +7/-5 G.
I prefer the feel of the Pitts for competition style aerobatics, but the Eagle sure is more comfortable and slightly faster than the S-2A too If you're going somewhere.

 

[Plano Pilot]

I remember watching the Eagles Aerobatic Team years ago and being very impressed. A video of them I found on YouTube.

 

[Dana]

The Eagle is rated for +6/-3G operational load. +9G is ultimate load where things may start to bend.

+6/-3 is the minimum limit load for certification in acrobatic category, but most aerobatic planes are far stronger. Parts are allowed to bend at the limit load, but can't break until they reach the ultimate load of 1.5 x the limit load.

+4 isn't that much, but -3.5 is certainly enough to be uncomfortable.

 

[RoscoeT]

+6/-3 is the minimum limit load for certification in acrobatic category, but most aerobatic planes are far stronger. Parts are allowed to bend at the limit load, but can't break until they reach the ultimate load of 1.5 x the limit load.

No, for example an aerobatic airplane with a metal wing spar rated for standard +6/-3G is not allowed to have the wing spar permanently bend at 6G. Ultimate load (+9G) does not necessarily mean that's where things actually break, it's the territory where permanent deformation (damage) may begin to occur. But things may also actually break at the ultimate load. Depends on the airplane.

 

[Dana]

No, for example an aerobatic airplane with a metal wing spar rated for standard +6/-3G is not allowed to have the wing spar permanently bend at 6G. Ultimate load (+9G) does not necessarily mean that's where things actually break, it's the territory where permanent deformation (damage) may begin to occur. But things may also actually break at the ultimate load. Depends on the airplane.

I should have worded it better. The ultimate load is where things can break (because the material's ultimate tensile strength is reached or something buckles), and must be at least 150% of the required limit load. That 150% is the safety factor. But most materials' elastic limit is less than the tensile strength, so there may be minor permanent deformation (it bends and stays bent without actually breaking) somewhere between the limit and ultimate loads. However, many aircraft structures are limited by elastic buckling, which must not happen up to the ultimate load.

FAR 23 says the structure must support the limit loads without "detrimental permanent deformation".

 

[brcase]

I should have worded it better. The ultimate load is where things can break (because the material's ultimate tensile strength is reached or something buckles), and must be at least 150% of the required limit load. That 150% is the safety factor. But most materials' elastic limit is less than the tensile strength, so there may be minor permanent deformation (it bends and stays bent without actually breaking) somewhere between the limit and ultimate loads. However, many aircraft structures are limited by elastic buckling, which must not happen up to the ultimate load.

FAR 23 says the structure must support the limit loads without "detrimental permanent deformation".

It appears FAR 23 rules were written around the use of Aluminum which generally has an ultimate strength about 150% of it's yield Strength. Meaning in very general terms it will yield at 6G but not break until 9g's in acrobatically stressed aircraft.
Paper clip example bend it a bit and it will spring back to it original shape, bend it more and it will stay bent, this means you exceeded its yield Strength.
This also ignores Fatigue effects, way oversimplified version is when you bend the paperclip back and forth until it breaks. Mostly, but not entirely if you don't exceed the yield load on it then it won't Fatigue much, but can which Is why you end if with life limits on some airframes (Tomahawk Wings).

It gets even more complicated when you start designing with composites that don't act the same way aluminum does and has other potential issues as well.

Brian
CFIIG/ASEL
Former Engineering Assistant/Draftsman for Papa51 (Thunder Mustang)
There many more people qualified to discuss this than I (Real Certified Engineers) but they didn't here (yet)

 

[Country Flier]

texasclouds, thanks for the feedback. I recall the outside temperature was about 60 degrees F

Although a high performance, modified engine may have different requirements, 60 degrees is way above any engine manufacture recommendations for preheating that I've ever heard of (usually they recommend preheating below, say, 30 degrees or so)...perhaps an abundance of caution, or just a "can't hurt" attitude.

 

[Capt. Geoffrey Thorpe]

This also ignores Fatigue effects, way oversimplified version is when you bend the paperclip back and forth until it breaks.

From the no nit to small to pick department:
That's work hardening - you are causing plastic deformation. But it is a common erroneously used example of fatigue...
Fatigue involves only elastic deformation - loads below the yield strength - but it's real hard to give a simple example because it typically requires tens of thousands to millions of cycles...
But you did say "oversimplified", so STFU Geoff.

And, yes, the rules appear to be more or less written around aluminum.

 

[brcase]

From the no nit to small to pick department:
That's work hardening - you are causing plastic deformation. But it is a common erroneously used example of fatigue...
Fatigue involves only elastic deformation - loads below the yield strength - but it's real hard to give a simple example because it typically requires tens of thousands to millions of cycles...
But you did say "oversimplified", so STFU Geoff.

And, yes, the rules appear to be more or less written around aluminum.

Good point, Please nit pick, I can learn new stuff or understand it better to. Or least get my terminology better.
Thanks for the clarification.

Brian

 

[Ed Haywood]

No need to over-analyze. To be rated for 6g, a structure must be designed for at least 9g. Margin for error, a buffer for variations in materials and construction.