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Removable ballast

MConner

Well Known Member
I spend most of the time flying with one or two folks and little baggage. The CG is always just barely at the forward limit.

I know some folks toss in unsecured shot bags in the baggage compartment but that could seriously injure someone if there was a sudden deceleration. I considered a much smaller weight that mounts as far back as possible but was easily removable when the mission called for four seats and bags.

I am wondering if the tail tie down bolt?s nutplate and structure would be strong enough for around 5# of conformal weight.

Mark
 
Yes many sailplanes have a water tank in the tail for CG control. Not a bad idea but still needs to be secured and at the tail.
 
I just bought a Swiss Army water bag for this purpose. Its holds up to 20 litres and is made of strong rubber with two handles that can be easily fastened. Available from Army surplus stores for $18.

Michel
 
Similar profile !

I fly mostly solo so plan to make a tail wheel spring weight to put my RV at most rearwardACRO limit for solo flying. It will come off with one bolt for dual flight....FWIW
 
I just bought a Swiss Army water bag for this purpose. Its holds up to 20 litres and is made of strong rubber with two handles that can be easily fastened. Available from Army surplus stores for $18.

Michel

How and where will you secure your water ballast bag?

Mark
 
Be careful. That could drastically change the aircraft's spin and recovery characteristics.

Kyle, caution noted. I am not planning to load the plane outside the normal CG limit but right now it is at or ahead of the forward limit with no baggage/ballast.
 
Some time ago I proposed a slug of lead in an aluminum tube spanning the fuse sides under the stab to address the RV-8 solo CG issues. Have not implemented it yet, but the cautions for spin recovery are valid until proven otherwise. Even a little weight that far aft will alter the MMOI of the vehicle.
 
Kyle, caution noted. I am not planning to load the plane outside the normal CG limit but right now it is at or ahead of the forward limit with no baggage/ballast.

The issue is the moment of inertia. Think of a two pound stick that's a couple of feet long, but most of the weight is located towards the center. Give that stick a spin and it'll spin rapidly, but slow down relatively quickly too. Take that same stick and put the weight out towards the ends and it'll be harder to get spinning, but once it begins spinning, it'll want to keep spinning - think about a majorette's baton.

Putting a weight on the stinger will make your airplane more like a baton - with more weight out towards the ends. Slower to generate a yaw rate, but once you have a yaw rate (say in a spin), it'll be more difficult to slow/stop that yaw rate.
 
But if it takes 50 lbs in the baggage area, it might only take 2 lbs on the tailspring. Moment of inertia should be the same, right?
 
But if it takes 50 lbs in the baggage area, it might only take 2 lbs on the tailspring. Moment of inertia should be the same, right?

I agree, the CG is what it is. I do not do acro and and the chance of me getting a fully developed spin is almost zero.

Mark
 
The issue is the moment of inertia. Think of a two pound stick that's a couple of feet long, but most of the weight is located towards the center. Give that stick a spin and it'll spin rapidly, but slow down relatively quickly too. Take that same stick and put the weight out towards the ends and it'll be harder to get spinning, but once it begins spinning, it'll want to keep spinning - think about a majorette's baton.

Putting a weight on the stinger will make your airplane more like a baton - with more weight out towards the ends. Slower to generate a yaw rate, but once you have a yaw rate (say in a spin), it'll be more difficult to slow/stop that yaw rate.

But if it takes 50 lbs in the baggage area, it might only take 2 lbs on the tailspring. Moment of inertia should be the same, right?

No, that?s not the way it works.

I'm getting my popcorn. This might get interesting fast :D
 
I agree, the CG is what it is. I do not do acro and and the chance of me getting a fully developed spin is almost zero.

Mark

For your -10, the spin issue is probably a minor consideration. For folks who are trying to move the CG aft for aerobatics, the moment of inertia is a real consideration because aerobatics increase the odds of entering an accidental spin.
 
Kyle, your point duly noted !

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 ?
 
My solution

Over two years ago I installed a ballast container in the tail of my RV-8 to improve aerobatic performance when flying solo. The concept received many critical comments when I posted it on VAF but I have found that the addition of 20 pounds to the tail of the aircraft has greatly eased the stick forces required in aerobatic maneuvering, especially in inverted flight, and I have no problems with spin recovery or other control issues. The thread can be viewed HERE.

http://www.vansairforce.com/community/showthread.php?t=133740
 
Ok let?s 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 Grumman?s where spins are prohibited due to unsatisfactory recovery in testing.
 
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.
 
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.
 
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
 
Tom Mix highway ...

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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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 :D) 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 :p
 
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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 😊
 
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 :D) 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 :p

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
 
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.
 
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.
 
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.

Not like you are going to be doing acro in the -10 anyway.:rolleyes:
 
Didn't mention acro - just using the full GW capacity of the airframe with typical max loading arrangement (4 souls on board, full baggage, and low tanks near completion of flight) without exceeding aft CG. Reportedly hard to do without making 10's more nose-heavy empty than they typically emerge from the shop.
 
-10

I'm finishing my ten built in the next 6mo or so and I'm thinking about my CG. I have a G900X which is a little on the heavy side and my 3 blade weighs about 5 lbs more than the standard hartzell. I did put my adsb receiver in the tailcone next to battery and elt under the tailcone fairing. I'm using the odyssey 925 and still anticipating having to use ballast for solo. The scales will tell the tale but hoping I'm on the right thought.
 
Just to level set. If you build the RV-10 with the stock engine, Hartzell prop, and the battery in the per plans spot the CG should be just fine for solo to four people and baggage. At any rate it was on mine.

I had two PC-625 batteries mounted in the normal spot, ELT mounted in the same area, no AC or other such stuff. Oxygen bottle mounted on top of the tunnel cap between the rear seats.

The only caveat being with four people and baggage I put the biggest passenger in the co-pilot seat.

Happy to provide the W&B data if you PM me your email address.
Carl
 
Didn't mention acro - just using the full GW capacity of the airframe with typical max loading arrangement (4 souls on board, full baggage, and low tanks near completion of flight) without exceeding aft CG. Reportedly hard to do without making 10's more nose-heavy empty than they typically emerge from the shop.

Bill,
I can load 4ea 200# pax, 100# bags and 35 gallons of fuel and am in CG. Even with 10 gallons of fuel left, I am just at the aft CG.

Mark
 
Mark-

that's good to hear. I had come across posts years ago suggesting this might not always be the case.

2 questions for you, if I may:

what equipment do you have installed in your 10?

do you require ballast to fly solo or two in front? how much?

-Bill
 
Bill,
O-540 with Hartsel blended prop. G3x (3 screen) SL-40 comm, 327 transponder, minimal interior.

Two folks and full Fuel is 25# at the aft baggage bulkhead.

Mark
 
As Mark implied, you always need to check the cg on a -10 at the expected landing configuration. If you start close to a limit, the cg moves closer (or past) the limit as you burn gas.
 
So... 5 gallons of water in a collapsible jug from the camping store. Easy to add/subtract at any FBO.
 
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