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RV-8 Weight n Balance question

Bones McCoy

Active Member
I am finishing my 8, my weight is 1084lbs. I have a 200hp with CS prop. My W&B puts my Datum at 73.67. That makes me pretty nose heavy. I ran the numbers solo and my W&B stays so far fwd that it looks really ugly once I burn the fuel out. I ran numbers with 45lbs in aft cargo floor, the numbers look better but....
My tail weighed in at 29lbs. Any thoughts on weight mods to the tail?
Thanks
Bones
 
common problem with the angle valved set up.

Yep this is seen a lot with the angle valved engine and heavy prop set up.
The first thing is that the battery or batteries need to go in the back. You can ballast by putting some lead in the tail. I would only add as much as it take to get you 3/4 to an inch and a 1/4 behind the forward most CG though as you want to keep the weight down as much as you can. Always remember the most aft CG moves forward and you gross weight goes down if you want to do aerobatics later on. Other than that try to sift as much of your weight aft of the fuel tank moment that you can. I am glad you are seeing this as it is something that has gotten a few in bad situations before. If you can get the CG as close to the front of the box as you can with the 8 you are doing the right thing as other than the front baggage most every thing you will load for a trip makes the CG move back not forward. Hope this help. Yours, R.E.A. III #80888
 
W&b

I have the same set up and a prop spacer ( James Cowl )
My battery is behind the panel , 4.5 gallon smoke tank (and ballast ) where the plans call for the battery and made a new tail wheel that weighs 16 oz more than stock . Mines 1094 with 42 on the tail .
29 sounds pretty light , have you verified the weights ?

I can email you pics of the tailwheel .
 
If you do not have your battery mounted aft of the baggage compartment, move it there. If it is already mounted there, add a second battery in the same spot. If you are going to add weight you may as well get some benefit from it. A second battery, even if you don't optimize the power distribution design, will add electrical reserve.

Carl
 
W&B

I wouldn't move the battery , especially considering the weight is almost nothing now days , consider adding weight under the vertical fin on the rear deck and making a heavy tail.
 
Also try to remove some weight from the front.
Install a lighter starter and alternator, that's as close to the front as you can get (except the prop) Get a light weight flywheel (Ouch $!)
Install a light weight sump, like a Superior or Sky Dynamics.
When you move the battery to the back, you get unwanted additional weight, like more than 3 pounds of battery cable!
I went the opposite direction when I removed my powerful but weighty Mazda Turbo Rotary and installed a Parallel valve conical mount IO-360 with Catto prop and light starter & sump.
I move the battery forward. The removed cable alone was 3.2 pounds!
My son Peter had a rear mount alternator on his Pitts, so he machined the alternator pulley off the flywheel to save weight. He also switched to an AeroVoltz LiFPo4 battery. It all counts!
G Zero has a good point, add a little weight very far aft.
 
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I wouldn't move the battery , especially considering the weight is almost nothing now days , consider adding weight under the vertical fin on the rear deck and making a heavy tail.

OK, I really don't know what you mean by "weight is almost nothing". But if you are considering adding weight under the vertical fin read this:
http://www.vansairforce.com/community/showthread.php?t=133740

Disregard the negative comments. The mod works!
 
Is it painted yet? Can make a significant difference as most of the paint is behind the c/g...

If it is, a little weight added to the tail = a whole bunch taken off the nose if it's already equipped as you want it...
 
If you care, a PC925 on its side fits the stock rear battery tray. And it will blow up a lot of air mattresses.
 
Great words

I did mount the battery aft, but being light weight it didn't do much. I talked to Danny King at Oshkosh and he suggested the big Concord battery for weight. I will consider the double battery option and the the smoke tank. I looked up smoking airplanes smoke tanks and they weigh in at 45lbs.
The tail weight increase, I'll work on that mod today.
Thanks again folks for the great words.
Bones
 
Don't know what others would do, but in lieu of adding a heavier battery in the back to help your problem, I'd (and I do) put together a tool bag and keep it in the rear baggage area. That way the extra pounds have some utility, not just adding weight.
 
I love Odyssey batteries. I followed Dan's advice recently and removed my 5 year old PC680 and put in a 925, layed on its side and with terminals facing backwards. I sure wouldn't go with the Concorde battery for a number of reasons.

I'm not sure that batteries "lack utility" as Veetail88 suggests. My tool bag almost never gets used, thankfully, but my battery get used every flight. ;-)
 
One of my RV8s had a similar problem, but it was great with a passenger in the back
I just made up a heavy tool kit in a soft bag and strapped it into the baggage compartment so it wouldn't float around
If I needed to for baggage or a heavy passenger, I just took it out.
Plus you have the added benefit of a complete tool kit if you need it
Worked for me.
 
If you care, a PC925 on its side fits the stock rear battery tray. And it will blow up a lot of air mattresses.

My plan is to put one of those back there since it's sealed and good for acro. The Gill I have back there is a fine battery but it can leak a little.

As you say the battery has to be rotated 90 degrees so that the terminals point backwards in order to fit into the tray. But to anchor it in the tray, different angle brackets have to be installed on the sides of the battery - the footprint of the sides of the 925 is not the same side to side as the bottom of the Gill.
 
I'm not sure that batteries "lack utility" as Veetail88 suggests.

The lack of utility comment was in regard to simply installing a heavier battery than the PC680 or some lighter battery. I'm quite aware that a battery has utility......
 
The lack of utility comment was in regard to simply installing a heavier battery than the PC680 or some lighter battery. I'm quite aware that a battery has utility......

I was pretty sure you did, thus the ;-) at the end of my post

. I really should learn how to make :);) these things show up as they are more effective at showing humor, etc.
 
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