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  #21  
Old 05-28-2018, 05:39 AM
MConner MConner is offline
 
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Location: Snead Island, Florida
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Ok letís not get into a NASA research project here. The post is a query about unsecured shot bags in the baggage compartment of the family cruiser versus a 5# weight at the tie down ring. I have spent the past 15 years in Grummanís where spins are prohibited due to unsatisfactory recovery in testing.
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  #22  
Old 05-28-2018, 05:41 AM
Kyle Boatright Kyle Boatright is offline
 
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Quote:
Originally Posted by Larry DeCamp View Post
Without getting Dan H in the arena, is anyone qualified do an MoI comparison between acceptable CG with pax and baggage VS a stinger weight. Is the popcorn done yet ?
I'm not sure it is a question that can be answered by calculations. I think flight testing is how its done. The guys who do that for a living have spin recovery chutes on the airplane or at least a chute if they get into an unrecoverable spin.
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  #23  
Old 05-28-2018, 06:56 AM
brad walton brad walton is offline
 
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Many years ago (1980's) a sailplane pilot placed a lead weight behind his seat in a baggage area. A poor landing and the lead came forward and broke his neck killing the pilot. Unsecured or inadequately secured weight can become a missile in a sudden stop.
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  #24  
Old 05-28-2018, 07:54 AM
Mousse Mousse is offline
 
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Quote:
Originally Posted by MConner View Post
How and where will you secure your water ballast bag?

Mark
The bag is attached to cargo rings in the baggage compartment. Water can be loaded or disposed of anywhere as needed, whether at base or away from base.
Michel
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  #25  
Old 05-28-2018, 10:50 AM
RViter RViter is offline
 
Join Date: Jun 2009
Location: Mesa, AZ
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Wink Tom Mix highway ...

Quote:
Originally Posted by brad walton View Post
Many years ago (1980's) a sailplane pilot placed a lead weight behind his seat in a baggage area. A poor landing and the lead came forward and broke his neck killing the pilot. Unsecured or inadequately secured weight can become a missile in a sudden stop.
Some may vaguely remember Tom Mix the Hollywood cowboy movie star. There is a highway SE of Phoenix AZ that is the "Tom Mix Memorial Highway". Why? Because he was killed there. Story is it was an avoidable accident but that he was struck in the back of the head by a heavy object during the accident impact. "...Mix was smacked in the back of the head by one of the heavy aluminum suitcases he was carrying in the convertible’s backseat. The impact broke the actor’s neck and he died almost instantly."
As the kids say, google-it ...

And the other problem with 'unsecured' or inadequately secured objects is weight shift potential during any attitude change in flight. More than one accident attributed to the resulting CG shift.

MMOI? Try holding a 5 lb object against your chest and spin rapidly around (without falling), then tie it to a 10 ft rope and spin at the same speed. After it's well established, try stopping it rapidly. (And don't fall when that rope is wrapped around your ankles.)

I concur with the least prejudicial position and weight that achieves primary objective, moves CG aft for solo flight. Probably a rearseat 'dummy' in the seat harness. There will be passenger briefing to practice, and you'll have somebody to talk to (not with) on the trip.
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Last edited by RViter : 05-28-2018 at 10:50 AM. Reason: typo
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  #26  
Old 05-28-2018, 01:47 PM
Bill Boyd's Avatar
Bill Boyd Bill Boyd is offline
 
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Location: Landing field "12VA"
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Default Well, I learned something today

Researching the whole "moment of inertia" thing. I thought it odd that people smarter than me claimed there was a difference, rotational inertia wise, in a small weight in the tail and a larger weight (okay, "mass" is the correct term, but it's so snobbishly precise that only engineers use it ) in the baggage compartment that netted the same change in CG.

We all learned in ground school the effect of weight and station on CG is linear for each quantity.

Turns out, moment of inertia is a linear function for weight but a square function for arm. We can achieve a given change in CG with less corresponding increase in moment of inertia by using a heavy ballast a short distance from the original CG vs a lighter ballast out in the nose or tail.

The only net penalty in play is... gross weight. It's one savings at the cost of the other. We can achieve a CG station change with less gross weight penalty by adding 1 pound @ 100" in the desired direction, or 10 pounds @ 10" distance. There is a 9 pound gross weight penalty for the latter. But the moment of inertia I=MR^2. The one pound weight gives (1)*(100)^2 which I think is 10,000 inch squared pounds or something like that. The 10 pound weight calculates to (10)*(10)^2, or 1,000 what-evers. By cutting the gross ballast weight gain to 1/10th, we make the contribution to moment of inertia 10x higher. Trade-off's.

Most of us focus on gross empty weight as we add items to our build, myself included. Few of us probably know how relevant a few extra inch-pound-inches of inertia really are. Until we need a little extra yaw or pitch damping, we probably won't care. Where's the red line we will wish we hadn't crossed? Probably not answerable or relevant for 99% of our flying lifetimes.

Carry on
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Last edited by Bill Boyd : 05-28-2018 at 01:50 PM.
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  #27  
Old 05-28-2018, 04:31 PM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
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Default Thanks Bill !

Bills explanation is the answer I was looking for. Plan B is duly implemented.A little extra weight behind the pilot (RV3 or RV4) sounds like a smart trade-off 😊
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  #28  
Old 05-31-2018, 09:43 AM
MConner MConner is offline
 
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Location: Snead Island, Florida
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Quote:
Originally Posted by Bill Boyd View Post
Researching the whole "moment of inertia" thing. I thought it odd that people smarter than me claimed there was a difference, rotational inertia wise, in a small weight in the tail and a larger weight (okay, "mass" is the correct term, but it's so snobbishly precise that only engineers use it ) in the baggage compartment that netted the same change in CG.

We all learned in ground school the effect of weight and station on CG is linear for each quantity.

Turns out, moment of inertia is a linear function for weight but a square function for arm. We can achieve a given change in CG with less corresponding increase in moment of inertia by using a heavy ballast a short distance from the original CG vs a lighter ballast out in the nose or tail.

The only net penalty in play is... gross weight. It's one savings at the cost of the other. We can achieve a CG station change with less gross weight penalty by adding 1 pound @ 100" in the desired direction, or 10 pounds @ 10" distance. There is a 9 pound gross weight penalty for the latter. But the moment of inertia I=MR^2. The one pound weight gives (1)*(100)^2 which I think is 10,000 inch squared pounds or something like that. The 10 pound weight calculates to (10)*(10)^2, or 1,000 what-evers. By cutting the gross ballast weight gain to 1/10th, we make the contribution to moment of inertia 10x higher. Trade-off's.

Most of us focus on gross empty weight as we add items to our build, myself included. Few of us probably know how relevant a few extra inch-pound-inches of inertia really are. Until we need a little extra yaw or pitch damping, we probably won't care. Where's the red line we will wish we hadn't crossed? Probably not answerable or relevant for 99% of our flying lifetimes.

Carry on
Bill,
Thank you for your considered response. I have finally measured moments and calculated the amount of weight necessary to affect the change I seek. Although it is less at the very tail, the distance from datum is not as large as I first thought due to the datum being well in front of the aircraft.

The amount of weight at the tail to get the aircraft in forward CG with two 200# folks and 10 gallons of fuel is almost 15# at the tie down ring. That is far more than I imagined, and way beyond what I would ask the structure to support without major reinforcement not to mention the MOI issues you described.

I have calculate that a 25# weight secured at the base of the aft bulkhead (183" aft of datum) in the baggage area is minimum. A 50# weight would take the plane to 1" behind the forward limit. I am working on ideas for securing that weight, the standard we used in the cargo indusrty was 9 G's. If shot bags were used, a designed weak point would spill the shot in a crash without making the bag a projectile reducing the engineering of the restraint required to attach the bag(s) to the airframe.

Mark
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  #29  
Old 05-31-2018, 11:22 AM
rv7charlie rv7charlie is online now
 
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Data point: When I first got into flying, I saw a Vari Eze that had shot bags in the nose for ballast. The plane was likely less than 10 years old at the time (this was late '80s/early '90s). The bag(s) had rotted, and the shot was scattered along the floor of the plane. A bit of yanking & banking could have moved almost all the shot to the rear of the fuselage.
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  #30  
Old 05-31-2018, 11:29 AM
Bill Boyd's Avatar
Bill Boyd Bill Boyd is offline
 
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Location: Landing field "12VA"
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Default Mark,

makes you want to consider a single alternator, dual lead-acid AGM battery in the back electrical system, doesn't it.

That said, I still think there's more overall utility to letting the -10 be nose heavy solo and requiring ballast (water bottles or tool kit) than to having the CG further aft and never being able to carry full baggage and rear seat pax with min. fuel. Won't really know what I like best until it's too late to undo some design changes along the way that affect these numbers.
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