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lr172

Well Known Member
I am considering building a cargo pod for my RV-6A. I have plenty of Gross Weight margin, but can't put a lot of weight behind the seats. I got the idea of building a cargo pod like the one available for the 10. I know it would slow me down a bit, but I have a few mission profiles that involve more baggage and struggling to see find a way for the 6A to work without having to move up to a 10:)

My question is where to account for the weight in my design W&B calculations. I would have four attach points on the pod. Consider the following for my question. Assume my pod was four feet long and had attach points one foot in from each end. Let's say I loaded 150 pounds in it with the weight evenly distributed. That would put 75 pounds on each pair of attach points. For W&B purposes, do I apply that 75 pounds at the datum for the attach point or do I have to balance the weight across the span of the pod/plane?

What if the attach points were at the extreme forward end and 1.5' in from the aft end? Do I still put all of the weight at the attach point.

I am struggling to understand how an attached appendage is factored in the W&B calcs.

I am hoping someone with more experience or training can help guide me on this.

Thanks,

Larry
 
In a CG calculation, it really doesn't matter where the pod attaches. What matters is that it is attached and where the CG of the pod is relative to the datum point on the airplane.

So, if you had a 150 lb pod which had a CG right in the middle of the 4' pod, and the middle of the pod was 70 inches from the datum point, you'd consider it as 150 lbs at a 70 inch arm.

Does that help?

One thing I would say is that a pod and attach hardware will be relatively heavy. Could you (just as quickly, and far more easily from a fabrication standpoint) bolt on one of Saber's big crush plates or add nose weight in some other expedient way so you could use the baggage compartment instead of going to the trouble of a pod?
 
Think of it as the same thing as locating something simpler like a normal baggage compartment inside the airplane. The center of the baggage compartment is a certain distance from the datum and you assume the baggage cg is at the center of the baggage compartment. It's one location and one weight.

If your baggage pod is symmetric then it should be easy to figure out where the center of gravity is. If it is not symmetric it will be a bit tougher.

The load on the attach points is only of interest when calculating the stress on those points.

For baggage, figure out what the max weight of the baggage will be and assume the baggage is of uniform density and assume the cg of the baggage is the same as the cg of the baggage pod.

If you have a sketch of the pod and where it will be located on your plane send it to me with your W&B and I'll either check your work or develop the updated W&B. [email protected]

I have been a mechanical engineer for 33 years and cg calculations are common in my job.
 
It's tough to get an engineering degree by asking questions on this forum. I highly recommend that you find a friend or hire someone who will help.

This is not just a W&B problem, but also structural and aerodynamic. Fastener loads are critical and must be considered for high G loads (including imperfect landings) and asymmetrical cargo loading. Hardpoints on the fuselage must be structurally analyzed as well.

It's fun to make modifications of our aircraft, but this one does need some professional involvement. Good luck.
 
Kyle Boatright said:
In a CG calculation, it really doesn't matter where the pod attaches. What matters is that it is attached and where the CG of the pod is relative to the datum point on the airplane.

So, if you had a 150 lb pod which had a CG right in the middle of the 4' pod, and the middle of the pod was 70 inches from the datum point, you'd consider it as 150 lbs at a 70 inch arm.

Does that help?

One thing I would say is that a pod and attach hardware will be relatively heavy. Could you (just as quickly, and far more easily from a fabrication standpoint) bolt on one of Saber's big crush plates or add nose weight in some other expedient way so you could use the baggage compartment instead of going to the trouble of a pod?
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This does help; Thanks! I have a 19lb crush plate. It helped quite a bit and gave me close to a 70lb potential in my baggage compartment. My old Remy starter bit the dust and I put in a Skytech. I have another old starter and may some day put it on to get back another 9lbs up there, but I really like the high RPM starter:)
 
Bubblehead said:
Think of it as the same thing as locating something simpler like a normal baggage compartment inside the airplane. The center of the baggage compartment is a certain distance from the datum and you assume the baggage cg is at the center of the baggage compartment. It's one location and one weight.

If your baggage pod is symmetric then it should be easy to figure out where the center of gravity is. If it is not symmetric it will be a bit tougher.

The load on the attach points is only of interest when calculating the stress on those points.

For baggage, figure out what the max weight of the baggage will be and assume the baggage is of uniform density and assume the cg of the baggage is the same as the cg of the baggage pod.

If you have a sketch of the pod and where it will be located on your plane send it to me with your W&B and I'll either check your work or develop the updated W&B. [email protected]

I have been a mechanical engineer for 33 years and cg calculations are common in my job.
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Thanks for the offer to help, I really appreciate it! I am still at the feasibility stage, but will take you up on your offer if I decide to proceed.
 
As already hinted at, a baggage pod would likely weigh much more than the amount of weight you would have to add to the nose to get a full 100 lbs of utility for your baggage compartment. That would mean you would lose more useful load while gaining a better C.G. position.
That, coupled with the labor to design and build a pod, along with at least some level of performance reduction, makes the decision an easy one (in my mind anyway).
 
But...

Is it primarily a weight capacity issue, or a volume capacity issue? If it's volume, fixing cg won't help. If it's a weight issue, 19 lbs of ballast?? Ugh.
 
rv7charlie said:
Is it primarily a weight capacity issue, or a volume capacity issue? If it's volume, fixing cg won't help. If it's a weight issue, 19 lbs of ballast?? Ugh.
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The desire for the cargo pod was both a volume issue and a weight issue. I want to be able to carry more stuff for longer trips. The crush plate was purely a weight issue. The 6A is naturally a bit tail heavy with the 320 and composite FP prop. The 19 lbs added a full inch forward on my W&B and let's me carry 60+ pounds in the baggage area. A side benefit was a smoother engine. Even with the ballast I am only 1045 lbs total weight. It is worth being able to actually use the existing baggage area.

Larry
 
lr172 said:
The desire for the cargo pod was both a volume issue and a weight issue. I want to be able to carry more stuff for longer trips. The crush plate was purely a weight issue. The 6A is naturally a bit tail heavy with the 320 and composite FP prop. The 19 lbs added a full inch forward on my W&B and let's me carry 60+ pounds in the baggage area. A side benefit was a smoother engine. Even with the ballast I am only 1045 lbs total weight. It is worth being able to actually use the existing baggage area.

Larry
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A lot of it comes down to how you manage the weight. On heavy trips with baggage, I put some of the heavier bags just in front of the main spar - basically under my (and my passenger's knees). Moving 30 lbs from the baggage compartment to in front of the spar has a very beneficial impact on an aft CG.
 
I've done something similar in 2 different -4's I've owned. For OSH/SNF camping, I I'd stand the dome tent (basically a big sausage) & air mattress on the front cockpit floor, extending up and behind the instrument panel.

Someone mentioned the wingtips as lockers. I like the idea, & I'm considering doing that. I've seen doors in the top of the tips, and I've also seen them set up with piano hinge with pins that can be easily pulled. Engineers (I'm not one) say that loads outboard on the wings can actually help with gross weight issues in flight (with negatives of landing loads and spin recovery issues).

FWIW, at least one company made lockers ('bombs') that bolted to the wing tiedowns on RV's. It used to be a common practice on EZ type pushers. Same upsides and downsides as the tip locker idea.

Either option should help keep the load closer to the CG, I would think.

Charlie
 
Wing lockers

rv7charlie said:
Someone mentioned the wingtips as lockers. I like the idea, & I'm considering doing that. I've seen doors in the top of the tips, and I've also seen them set up with piano hinge with pins that can be easily pulled. Engineers (I'm not one) say that loads outboard on the wings can actually help with gross weight issues in flight (with negatives of landing loads and spin recovery issues).
Click to expand...

skwztk.jpg
 
lr172 said:
The desire for the cargo pod was both a volume issue and a weight issue. I want to be able to carry more stuff for longer trips.
Click to expand...

Clearly you own the wrong airplane for your mission. Instead of trying to stuff 10 lbs into a 5 lb sack, evaluate your need THEN choose your tool.
 
Option

If you had a -7, then you could use the -7a gear attach points for your hard points. Then you could get it close to CG ,,,,,, but loading it would be a bear.

Cj
 
Have been thinking of adding the heavy saber crush plate to the front of my RV9 due to almost always a rear limit CG with low fuel tanks and baggage. Also have a O 320 with fixed pitch composite prop. What would be the typical weight that would be added at the prop crush plate with that saber plate? Guessing I would also have to replace prop bolts.
 
DennisRhodes said:
Have been thinking of adding the heavy saber crush plate to the front of my RV9 due to almost always a rear limit CG with low fuel tanks and baggage. Also have a O 320 with fixed pitch composite prop. What would be the typical weight that would be added at the prop crush plate with that saber plate? Guessing I would also have to replace prop bolts.
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Sorry, I don't know the answer to your question but it has always struck me as odd that we contemplate adding more weight to our airplanes after we get them flying, while during building, we obsess about keeping weight down. :confused:
 
RVbySDI said:
Sorry, I don't know the answer to your question but it has always struck me as odd that we contemplate adding more weight to our airplanes after we get them flying, while during building, we obsess about keeping weight down. :confused:
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Proves that choosing a light weight prop because it will reduce empty weight is somewhat a losing proposition.
Anyone doing so needs to do everything they can to put all systems weight as far fwd as possible but even that wont be enough. They should just plan on using a heavy crush plate & extension.

As for how much weight to add after the fact? It is a simple weight and balance problem by determining the arm position where you will add the weight (just like for the baggage compartment) and calculate with different weight values until you get the amount of fwd shift of the empty C.G. that you want.
 
rvbuilder2002 said:
Proves that choosing a light weight prop because it will reduce empty weight is somewhat a losing proposition.
Anyone doing so needs to do everything they can to put all systems weight as far fwd as possible but even that wont be enough. They should just plan on using a heavy crush plate & extension.
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Actually, I would have to disagree with this statement considering what I have with my 9A. I have a 12 lb 3 blade Catto prop with a 8.5 lb Sabre spacer. My CG is exactly dead center of the 9A range. I did not specifically install a light weight prop for the purpose of reducing empty weight as much as I did it in order to keep as much weight off of the nose gear. Since my plane lives on a grass strip I did not want any unnecessary weight on that nose gear when landing/taking off on grass. My nose wheel weight came out to 252 lbs partly due to the light weight prop. In my case, doing so did not cause any undue problems with CG. When it comes to CG issues with my plane there are no real world scenarios where I can move the CG fore or aft of the max limits.
 
Did you spec the 8 lb weight or is that just what the crush plate weighs. ? Do you think there might be some option for weight based on selection of thickness of various plates? Guess I need to look closer and find out how much it would take to shift CG fwd a bit.
 
DennisRhodes said:
Did you spec the 8 lb weight or is that just what the crush plate weighs. ? Do you think there might be some option for weight based on selection of thickness of various plates? Guess I need to look closer and find out how much it would take to shift CG fwd a bit.
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I am assuming the question was posed to me. If so, it was 8.5 lbs and yes, I actually weighed the spacer myself. I weighed every component separately before installation so I would know what I was dealing with if ever I had to alter anything.
 
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