Spinning an RV-10

[TSwezey]

Remember Phil most people don't have engineering degrees and few have aero degrees.

[Ron Lee]

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.

[johnny stick]

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.

[Mike S]

Originally Posted by Ron Lee

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.

I mentioned it six years ago, http://www.vansairforce.com/communit...8&postcount=27

There are other posts, do a search.

Originally Posted by Ron Lee

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

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 ) momentum contributes to the energy developed in the spin, and the corresponding effort needed to recover.

[RV8R999]

Originally Posted by Ron Lee

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.

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.

[szicree]

Originally Posted by Ron Lee

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 (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.

[TSwezey]

Originally Posted by RV8R999

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.

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]

Originally Posted by TSwezey

Mike it has been six years since we discussed that! Holy Cow! Time flys.

Yep, lot has happened in that time.

[TSwezey]

Originally Posted by szicree

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

Thanks for the laugh.

[N355DW]

Originally Posted by Mike S

The aft C/G contributes to the likelihood for a spin, the polar ----or angular----(take your pick ) momentum contributes to the energy developed in the spin, and the corresponding effort needed to recover.

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.