What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Spinning an RV-10

Status
Not open for further replies.
jrs14855 said:
I agree with most of what Vic says, however real world experience is a bit different.
Click to expand...

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.
 
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.
 
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.
 
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.
 
Polar moment

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.
 
Last edited:
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,
 
polar moment

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.
 
rvbuilder2002 said:
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.
Click to expand...
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 :)
 
art scholl

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.
 
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.
 
Last edited:
I have never seen anything until this thread that suggests that some mythical "polar moment" impacts how a plane recovers from an unusual attitude. NEVER.

Aft CG...yes. Now if it is factual that there is a real aerodynamic difference in how you get to a specific CG, then we all need to change our POH and training needs to be changed as well.

Voodoo physics doesn't cut it.

While we are at it...perhaps angular momentum is the term you should be using.
 
polar moment exists?

Polar moment does exist and I think affects spin recovery. For single engine small planes the polar moment differences are small since the engine and passengers are right next to each other and the tail feathers are so far back. But for the big birds, the airliners, the polar moment due to the fuselage as compared to the wings makes spin recovery a little different. I remember reading a Flying article a few decades ago talking about how the spin characteristics of an airliner is drastically different than a small plane. Previously someone has mentioned about a twin verses a single. As experimenters, we get into all sorts of trouble that the certified engineers haven't clued us in on.
 
Momentum, not moment---if there is a difference

Ron Lee said:
I have never seen anything until this thread that suggests that some mythical "polar moment" impacts how a plane recovers from an unusual attitude. NEVER.
Click to expand...

I mentioned it six years ago, http://www.vansairforce.com/community/showpost.php?p=69218&postcount=27

There are other posts, do a search.


Ron Lee said:
While we are at it...perhaps angular momentum is the term you should be using.
Click to expand...

You may be right, I am not an engineer, just going on prior input, but it is the term I was taught many years ago.

The aft C/G contributes to the likelihood for a spin, the polar ----or angular----(take your pick :D) momentum contributes to the energy developed in the spin, and the corresponding effort needed to recover.
 
Last edited:
Ron Lee said:
I have never seen anything until this thread that suggests that some mythical "polar moment" impacts how a plane recovers from an unusual attitude. NEVER.

Aft CG...yes. Now if it is factual that there is a real aerodynamic difference in how you get to a specific CG, then we all need to change our POH and training needs to be changed as well.

Voodoo physics doesn't cut it.

While we are at it...perhaps angular momentum is the term you should be using.
Click to expand...

its more about the KINEMATICs than AERODYNAMICs. Aerodynamically any additional weight added to the aircraft not in contact with airflow will not impact the aerodynamics: the same forces will be generated by the control surfaces for given flight conditions. However kinematically speaking with a larger longitudinal polar moment it will take more force to arrest a developed pitch rate. Since the aerodynamic forces have remained the same the commanded pitch rate will be less with large longitudinal polar moments - less pitch rate may in some circumstances make it more difficult to recover from a spin, or at least will take a bit longer. An added consideration is that the polar moment also affects the vertical axis (yaw) thereby requiring greater control power (rudder) to arrest the developed yaw rates - certainly this will reduce spin recovery margins.
 
Ron Lee said:
I have never seen anything until this thread that suggests that some mythical "polar moment" impacts how a plane recovers from an unusual attitude. NEVER.

Aft CG...yes. Now if it is factual that there is a real aerodynamic difference in how you get to a specific CG, then we all need to change our POH and training needs to be changed as well.

Voodoo physics doesn't cut it.

While we are at it...perhaps angular momentum is the term you should be using.
Click to expand...

Polar moment (or moment of inertia) is an attribute of the object even when at rest. The angular momentum only appears when the object gets spinning. To get the thing to stop spinning, you've got to overcome the momentum, which is due to the moment of inertia.

In any case, we don't need to put quotes around these terms as if we're talking about sasquatch or something.

P.S. Van himself mentioned the issue of polar moment when writing on his Facebook page about the extremely fancy RV-10 that was featured in Sport Aviation.
 
Last edited:
RV8R999 said:
its more about the KINEMATICs than AERODYNAMICs. Aerodynamically any additional weight added to the aircraft not in contact with airflow will not impact the aerodynamics: the same forces will be generated by the control surfaces for given flight conditions. However kinematically speaking with a larger longitudinal polar moment it will take more force to arrest a developed pitch rate. Since the aerodynamic forces have remained the same the commanded pitch rate will be less with large longitudinal polar moments - less pitch rate may in some circumstances make it more difficult to recover from a spin, or at least will take a bit longer. An added consideration is that the polar moment also affects the vertical axis (yaw) thereby requiring greater control power (rudder) to arrest the developed yaw rates - certainly this will reduce spin recovery margins.
Click to expand...

Very well put. It has been 26 years since I have had an Aero class.

Mike it has been six years since we discussed that! Holy Cow! Time flys.
 
Mike S said:
The aft C/G contributes to the likelihood for a spin, the polar ----or angular----(take your pick :D) momentum contributes to the energy developed in the spin, and the corresponding effort needed to recover.
Click to expand...


Well put! Once the spin develops sufficient momentum (energy), the effort needed to recover may now beyond the effectiveness of the flight controls.

It's not an issue in normal flight, or even "unusual attitudes".

I agree it's an important issue that builders of EAB's should be aware of, especially aerobatic EAB's. I'm glad it was brought up.
 
spins

A few facts about certified aircraft: spin testing the Cirrus was apparently waived because of the chute.
There is NO spin testing requirement for US certified light twin engine aircraft.
AC23.8A Twin engine airplanes can not display an "undue tendency" to spin from an unaccelerated, wings level stall with the critical engine failed.

Cessna took the Skycatcher far beyond the testing requrements fo LSA , and as a result lost two aircraft before getting things sorted out.

My main point in this is that someone posted about spin testing airliners. I don't believe that any modern US airliner has been spin tested. I do know of alleged cases where the Lockheed Electra(4 engine turboprop) and the Boeing 707 were inadvertently spun.
 
I do not recall every prior post but if this test was at the aft CG limit, was it loaded that way or is the basic CG such that it could be loaded to a point AFT of the aft CG limit with people in the back seat and/or lots of luggage?
 
Ron Lee said:
I have never seen anything until this thread that suggests that some mythical "polar moment" impacts how a plane recovers from an unusual attitude. NEVER.

Aft CG...yes. Now if it is factual that there is a real aerodynamic difference in how you get to a specific CG, then we all need to change our POH and training needs to be changed as well.

Voodoo physics doesn't cut it.

While we are at it...perhaps angular momentum is the term you should be using.
Click to expand...

Ron,

The Laymans term for polar moment is the dumb-bell effect.. It is real, not voodoo science and concentrating weight at the ends of an airplane has a dramatic effect on spin recovery... Tip tanks being an extreme example of dumb-bell effect. if you have fuel in the tip tanks of an airplane on the C.G the amount of fuel would have no effect on CG, but a drastic effect on spin behavior and recovery...

There is so much more to aviation than the rote level learning we acquired as private, commercial, or ATP, students... The total volume of what is unknown in the universe so far exceeds the volume of what is known, and aviation is no exception. Much of the stuff that us left brainers hold to be gospel fact, is actually the best guess of some guy who came up with a simple way to make us left brain pilot learners understand a concept.... The real truth of it is that much of our understanding of aerodynamics is still a hypothesis... However, polar moment, or dumb-bell effect is not a hypothesis, it is well understood and well documented in the literature... But, not in the private, commercial, or ATP, PTS.

Once we realize how much we don't know and understand, we can open our mind to the reality of how much we have to learn about the art of aviation.... Yes there is science in it, but at the end of the day, like medicine, it is an art. and those who do it well, are in a practice.... Those who believe they have it all figured out are missing most of it....

Doug Rozendaal
Practitioner..
 
Last edited:
Phil said:
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.
Click to expand...

You obviously didn't see the torque wrench thread....
 
I can vouch for polar moment, its introduced in any Dynamic Stability and Control class and is a player in equations of motion. It can be considered about any of the three axis not just the longitudinal. It came up in the tanker as someone claimed to have recovered at 180 deg of bank. I suggested if they really knew their stuff they would've continued the roll and recovered faster considering the time and effort to dissipate and reverse the angular momentum they had created.
 
Polar moment

I remember my first polar moment. New years day 93. Pulled into KPNA as a full-time resident, stepped out of my 72 Bago into -27 degrees. I blinked and my contact lenses froze and fell on the ground.
 
aerhed said:
I remember my first polar moment. New years day 93. Pulled into KPNA as a full-time resident, stepped out of my 72 Bago into -27 degrees. I blinked and my contact lenses froze and fell on the ground.
Click to expand...
I don't care who you are: THAT THERE IS FUNNY!!! :D
 
RVZoomie said:
I can vouch for polar moment, its introduced in any Dynamic Stability and Control class and is a player in equations of motion. It can be considered about any of the three axis not just the longitudinal. It came up in the tanker as someone claimed to have recovered at 180 deg of bank. I suggested if they really knew their stuff they would've continued the roll and recovered faster considering the time and effort to dissipate and reverse the angular momentum they had created.
Click to expand...

Is that like somebody doing a 360 to turn around and go in the opposite direction?
 
B25Flyer said:
Ron,

The Laymans term for polar moment is the dumb-bell effect.. It is real, not voodoo science and concentrating weight at the ends of an airplane has a dramatic effect on spin recovery... Tip tanks being an extreme example of dumb-bell effect. if you have fuel in the tip tanks of an airplane on the C.G the amount of fuel would have no effect on CG, but a drastic effect on spin behavior and recovery...

There is so much more to aviation than the rote level learning we acquired as private, commercial, or ATP, students... The total volume of what is unknown in the universe so far exceeds the volume of what is known, and aviation is no exception. Much of the stuff that us left brainers hold to be gospel fact, is actually the best guess of some guy who came up with a simple way to make us left brain pilot learners understand a concept.... The real truth of it is that much of our understanding of aerodynamics is still a hypothesis... However, polar moment, or dumb-bell effect is not a hypothesis, it is well understood and well documented in the literature... But, not in the private, commercial, or ATP, PTS.

Once we realize how much we don't know and understand, we can open our mind to the reality of how much we have to learn about the art of aviation.... Yes there is science in it, but at the end of the day, like medicine, it is an art. and those who do it well, are in a practice.... Those who believe they have it all figured out are missing most of it....

Doug Rozendaal
Practitioner..
Click to expand...

Excellent post, Doug. I couldn't agree with you more.

I've been at this longer than I care to admit and still feel like a student in some respects. I guess that's what keeps it interesting.
 
180 deg bank= inverted

TSwezey said:
Is that like somebody doing a 360 to turn around and go in the opposite direction?
Click to expand...

It was someone who apparently was caught in wake turbulence vortices and claimed it caused him to roll all the way over (inverted) b/c the wake was so strong (he was out of position). I argue to continue the roll to recover if the aircraft is already inverted rather than fight the angular momentum and wake vortex as well.
 
The key to surviving a botched roll close to the ground is getting the nose above the horizon. That means stopping what you are doing and pushing the stick or rudder, whatever it takes, (and what it takes is a hard control input that is very uncomfortable,) to put some air under the nose. In a non-inverted airplane, this means the engine quits... but there is no advantage to having the engine running when you hit the ground upside down....
 
Nope

RVZoomie said:
It was someone who apparently was caught in wake turbulence vortices and claimed it caused him to roll all the way over (inverted) b/c the wake was so strong (he was out of position). I argue to continue the roll to recover if the aircraft is already inverted rather than fight the angular momentum and wake vortex as well.
Click to expand...

I'm with Doug R on this one! If the airplane was rolled inverted, the nose would already be pointed further downhill than before the half roll. Continuing the roll will probably result in a near 45 deg impact!

Best,
 
There seems to have been a bit of thread drift here.

Looking at the original incident, there seems little doubt that it was not a spin but a spiral dive. The recovery from a high speed, nose low departure is to close the throttle, un-load, roll to the nearest horizon and pull to level.

Having said that, I agree that in some specialist aerobatic aircraft, you may be able to push your way out of it but the situations where that would help are minimal - if you can roll, then the postiive g available is typically twice the negative g.

I refer back to my original comments some pages ago. Whilst I commend the honesty in reporting this incident, if you don't know the difference between a spin and a spiral dive, then you didn't ought to be flight testing an aeroplane.

Edit: The roll should be unloaded
 
Last edited:
pierre smith said:
I'm with Doug R on this one! If the airplane was rolled inverted, the nose would already be pointed further downhill than before the half roll. Continuing the roll will probably result in a near 45 deg impact!

Best,
Click to expand...

I agree, but just to be clear, the best path would both push the nose up and continue the roll at the same time, but I assume that's what you and Doug both mean. I can't honestly say for sure I would automatically continue the roll, my first instinct would probably be to try to stop it. I guess it would depend on how fast I was rolled inverted. If I was rolled to the left, my guess is I would be more likely to continue the roll, as that is the way most competition pilots prefer to roll. I do I know I would push the nose up without thinking about it, I have done so many competition style rolls (as opposed to barrel rolls) it's automatic. The great thing about having aerobatic experience is, especially at low altitudes you don't want to have to think to much about the proper use of the controls, you need to just do it!:)

Sorry about the thread drift!
 
you don't want to have to think to much about the proper use of the controls, you need to just do it!
Click to expand...
Yah, it should just be point and shoot. If you have to actually think about the necessary recovery steps that could be involved with a maneuver, then you should not go near said maneuver solo until its automatic. That's what keeps a pothole from becoming a crater.
 
Because of a very specific, personal request by another forum member, I've agreed to post an update on my test progress.

Since my last post, I have had a discussion with Van's, completed another inspection of the plane, plus had an inspection of the airplane by two other sets of eyes (an IA and a mechanic together) just to be sure. No signs of any damage was found, and so I got back on the horse and went flying. I have completed all of my formal test program objectives, starting right where I left off in the stall and cg testing, and am now within 1 hour of completing my 40 hours of Phase 1 testing. No issues have been uncovered with the plane.

I'm not sure I will continue on with this forum so please do not reply to this post.
 
Status
Not open for further replies.
Back
Top