Spinning an RV-10

[rvbuilder2002]

Originally Posted by jrs14855

I agree with most of what Vic says, however real world experience is a bit different.

Actually this real world experience is only useful when discussing these particular airplanes.

The effect on flight characteristics that can be induced by the moment of inertia, of mass mounted an extended distance from the C.G is well know. Part of that knowledge is the understanding that until testing is done to prove what effect a change will make on any particular aircraft with well known handling characteristics, any prediction is a guess at best.
This is why the FAA requires extensive testing of modifications to previously certificated aircraft such as tip tanks, etc.

From the description, I also don't think this incident was the result of a fully developed spin, but I agree with Vic (as I have already posted previously)... Flight testing a new aircraft is serious business. If it has modifications making it different from the original prototype, there should be an even higher level of caution used.


BTW, the original RV-10 prototype was spin tested extensively. Since it was a 100 % new design, it was done carefully and systematically, cautiously moving from the fwd CG limit, to the aft CG limit. The airplane was fitted with a spin recovery chute system and it was flown by a hired test pilot who wore a personal parachute.
Because of the successful completion of this testing, any RV-10 builder can likely expect no surprises regarding the stall and spin characteristics of their own airplane. But if they have done any significant modifications, nothing can be assumed. Only testing of their own aircraft can prove whether it has the same characteristics as the original.

[Mike S]

Polar momentum it the term I believe it is called.

Imagine a barbell with two 25# weights on it. The weights are slid to the center only 6" apart.

Pick up the barbell, in the middle where it balances with one hand, and twist it and then stop the twist.

Now, move the weights to the extreme ends of the bar, and repeat the twisting movement.

Same total weight, same balance point, but the location of the weight makes a lot of difference in how much effort it takes to stop the rotation.

[TSwezey]

I was going to add polar momentum to my aft cg comment earlier. Very important concept. It's why a single engine airplane can roll faster than a twin and why it takes more force to start and stop a twin from rolling.

[Ron Lee]

I do not understand the "polar momentum" issue. According to the pilot it was AT aft CG. It should behave adequately (known characteristics) at that CG regardless of whether it had an AC unit in the back or a large battery there.

[N355DW]

I think this might be a very important, and under-appreciated concept, it has been implicated in a couple of accidents where a Pitts with experienced pilots have spun in. It is popular in competition aerobatics (and with some airshow pilots) to get your cg as far aft as possible, often using dive weights strapped to the fuselage. As I understand it, and please correct me if I am wrong, an aircraft can be within cg limits, but out of polar moment limits. In this case it may be true that an incipient spin (less than two turns or so) can be recovered from, but a well-developed spin can be very difficult or even impossible to recover from. I do not totally understand the concept myself, especially on how to calculate it and what safe limits may be, though I get the general idea - which is one reason I shied away from the practice.

Extra 300L's are notoriuos for being nose heavy - it makes them terrible to teach spins in, crossover spins are nearly impossible to do, it recovers so quickly and easily. Good snap rolls can be difficult. I remember I did a cg calculation on my Extra that in order to have the cg up against the aft limit, I would have to put a 50 lb weight on the tail wheel! No thanks!

I do not recall the concept of polar moment ever being discussed in any of my formal training, other than it perhaps being mentioned as one reason you don't want to spin a twin because of the moment of the engines out on the wing. I think it may just be as important a concept to be aware of as cg, especially in the world of EAB's.

[pierre smith]

When the weight of an object is at either end, it has what is termed a "large" polar moment...think of a Porsche 911, or a Corvair with the engine hung in the rear...once they start going around, it's very difficult to stop...I owned two.

An Indy car or F-1 car, has the weight in the center...the engine and the driver, the reason they handle so well..a small polar moment of inertia, because the front and rear ends have little weight there...it's in the center.

An RV-10 has the battery behind the baggage area and the -10 in question also had an A/C unit back there, further aggravating the polar moment, as Vic pointed out.

Best,

[johnny stick]

OK here is my best shot 30 years after Berkeley Physics: CG has to do with the mass (weight) times distance, w*d, of an object from the datum. Polar moment is the mass (weight) times d times d, or d squared, w*d*d of the distance from the CG. So by adding weight to the tail to balance the plane, or bring the CG within range, this drastically increases the polar moment. 5 lbs at 100" is not the same polar moment as 10 lbs at 50". The first case is 50000 in**2*lbs, the later is 25000 in**2*lbs. So from a polar moment standpoint, the 10 lbs at 50 is better.

Polar moment is a big deal in spin recovery.

[1:1 Scale]

Originally Posted by rvbuilder2002

BTW, the original RV-10 prototype was spin tested extensively. Since it was a 100 % new design, it was done carefully and systematically, cautiously moving from the fwd CG limit, to the aft CG limit. The airplane was fitted with a spin recovery chute system and it was flown by a hired test pilot who wore a personal parachute.

So how much did weight did the spin recovery parachute and hardware add to the tail? I assume it was mounted pretty much under the vertical stabilizer/rudder.

I'm really enjoying this thread, btw. Lots of good info for someone shopping for an instructor and working on their ppl

[jrs14855]

I fully understand that my post was apples to oranges. however it was Vic who brought up the Art Scholl accident. I just read the accident report, one of the shortest reports I have ever seen. The important fact is that according to the report NOTHING was ever recovered. Given Arts experience with camera mounts on aircraft, I do not believe the accident had anything to do with "polar moment". Furthermore he had just recovered from an upright flat spin on the same flight. It is generally accepted that the inverted spin is a slightly quicker recovery in the Pitts because the rudder is operating in a "cleaner" airflow when inverted.
The accident cause states: "descent not corrected pilot in command", which of course is rather meaningless.

[Phil]

I can't believe what I'm reading. Who would have ever thought it was so easy to disprove the fundamentals of physics learned in a 101 level course.

I don't need wreckage to prove polar moment exist. (Which you could never prove with wreckage either)

And I don't need wreckage to tell me that the further away from center mass you move a mass of weight, the more pronounced it becomes.

This is simple stuff. I never realized how easy it would be to simply ignore the basics as being something insignificant.

Unbelievable.