[Rick6a]

How do you determine early on during the fuselage build process if CG issues will dictate the need to install the battery somewhere in an aft location? Its not just the weight of the battery I'm concerned about but also the dramatically increased weight of the much longer battery cables required. With such a potentially lengthy cable run, is the PC-680 up to the task? At this point, I'm fairly sure the ELT, strobe power pack, and auto pilot pitch servo will be located aft. I do plan on an 0-360 and Hartzell up front, yet I anticipate a significantly lighter instrument panel than I installed in my 6A. I will also install Grove gear which will save some weight at its fuselage station. Again, how do I address potential CG issues very early on during the fuselage build process?

 

[William Slaughter]

You can't really, but ....

I spent quite a bit of time fretting over this same issue early on, but after reading all of the comments on this and other posts, I decided that the rear battery was pretty much always the way to go on 8's, and in fact everyone I've talked to with an 0-360/Hartzell/rear battery combination is happy with it. (Paul, comments?) I have a parallel valve 360, but with inverted oil system and will be using a composite prop. I, too, am using the Grove gear, but they're pretty close to the CG. As long as the battery cable is appropriately sized, I have no concerns about the PC-680 getting the job done - again lots of successful flying examples. I plan to address the battery cable weight issue by grounding the battery to local airframe, and using a CCA (Copper Clad Aluminum) positive cable from Perihelion Design. There may be somebody out there who's put the battery in the rear of an 8 and regretted it, but I've not heard of it.

 

[Ironflight]

I think William has the best point - I also haven't heard of anyone that put the battery in the back that said that they wished they hadn't. My guess is that as long as you have a C/S prop, getting that weight aft is a good idea. With my parallel valve O-360 and C/S Hartzell (aft mounted Odyssey), I am close to the forward end of the CG box when I am alone. The PC-680 is plenty of cranking power from back there, but frankly, I sometimes wonder if it does much to actually shift the CG aft - it isn't very heavy! But every little bit helps.

A thought on panel weights - I have come to the conclusion that Glass versus Steam probably doesn't make that much difference per se - the heaviest components in my panel seem to be the avionics (GNS 430, xponder, etc...), and they are usually the same in most panels.

Just random thoughts...

Paul

 

[Rick6a]

There may be somebody out there who's put the battery in the rear of an 8 and regretted it, but I've not heard of it.

The PC-680 is plenty of cranking power from back there, but frankly, I sometimes wonder if it does much to actually shift the CG aft - it isn't very heavy!

I received a rather interesting reply to my query from Van's. I posed the CG question almost identically to what I posted here. Although Van's concedes that it is impractical to anticipate what the CG will end up being, they do not recommend installing a PC-680 in the rear. In fact, it is their position that the ONLY (their emphasis) time a Concord battery should be installed in an aft location is if using an angle valve engine with a constant speed prop. A possible exception to this would be if installing a parallel engine/constant speed prop and intend to fly mostly solo aerobatics. They do acknowledge that others have placed a PC-680 in the back with no ill effects, but stressed they do not recommend it.

 

[Ironflight]

Interesting....

Did Van's give a reason why NOT to put a PC-680 in the back? I mean, their kit and plans show the aft battery config....and nowhere do they say why it shouldn't be equal to the forward battery.....

Just odd.

Paul

 

[Guy Prevost]

I'm bulding an 8a and struggling with the same questions. I've heard some evidence to suggest that the 8a's are not as nose heavy as the 8's since the main gear is aft of the CG and they don't have the gear boxes up front.

I put together a list of a bunch of rv8s with known good CG and handling properties to decide where I wanted my CG. All of this info came from Checkoway's W&B website.

I then created a spreadsheet to get a sense of what the location of the battery, strobe power supply, elt, etc. did to the CG. I would make this spreadsheet public, but it's a mess right now.

Here are my thoughts in general:

1. If I can avoid it, I don't want the battery in back. I have my reasons, but if I discuss them here, the thread will go off track.

2. Moving the battery (PC-680 is the only battery I'm considering) from the firewall to the forward baggage compartment only moves the cg aft .1", so it's hardly worth the effort from a CG perspective.

3. If I put the battery on the firewall, I can mitigate the effect of this by putting the elt and strobe power supply in the aft section of the tail cone. This will require some effort to provide adequate access. The net effect on CG is that of putting the battery in the back seat.

I'll try to expand on this later when I have more time.

Guy

 

[Rick6a]

Did Van's give a reason why NOT to put a PC-680 in the back?...............

Paul,

They limited their response to addressing my question. In any event, I do feel comfortable going with my original instincts. I am going to place as many of the accessories I can.....aft. I feel IF there is a problem with CG, how hard would it be to bolt some lead under the empennage fairing? FACTOID: Even with the best design engineering brains in the business, CG was such an issue with the F-15 Eagle that all models carry 500 lbs. of lead ballast bolted just forward of the bulkhead that houses the nose radar!

 

[David-aviator]

Paul,
I am going to place as many of the accessories I can.....aft. I feel IF there is a problem with CG, how hard would it be to bolt some lead under the empennage fairing? FACTOID: Even with the best design engineering brains in the business, CG was such an issue with the F-15 Eagle that all models carry 500 lbs. of lead ballast bolted just forward of the bulkhead that houses the nose radar!

The 8, unlike the side by side versions, is more CG sensitive. For sure there is a limit to how big the back seat guy can be, no matter where the base CG is. There will be a point where forward bag compartment ballast will be necessary to accomodate chubby in the back seat.

That being said, I tried to calculate where the CG would be with the 7 and for the most part it was a waste of time. I used Van's base numbers as a starting point and tried to estimate what my stuff weighed but not knowing what their stuff weighed, it ends up being a big guestimate. If the machines were all built with exact same number of fasteners and equipment came out of the same box, it could be close but there still would be differences.

Certainly, the machine needs to be weighed and the numbers run before flight, but the proof of the pudding is when you fly it. My airplane was in the envelope but slightly forward. I did not like that in flight, so added 8# of lead at the HS. Now it flys better.

One guy I know, who weighs about 160 lbs, says if he puts his weight on the HS and the nose just comes off the ground, the CG is good.

 

[cdmiller]

My experience re battery location and CG (long)

I agree with the perspective put forward by William Slaughter above. I now have over 170 hours on my RV-8 with a rear-located PC-680 battery and I am happy with the result. My plane has a Superior IO-360 M1 parallel-valve engine with a Whirlwind 200 RV prop and a Christen Inverted Oil System. On the advice of Danny King, Randy Lervold, and others, I mounted my PC-680 battery aft of the rear baggage floor as shown in Van's plans. I mounted my ELT on the rear baggage shelf and my strobe power supply under the floor under the rear passenger's right leg. I used one (+) battery cable running forward and grounded my battery to one of the aluminum longerons near the battery. The PC-680 cranks like mad from the rear location, so that is not an issue. Furthermore, I have had no avionics interference issues, using the airframe as ground.

My engine/prop combination weighs about 58 lbs lighter than those with the Lycoming IO-360 and a Hartzell, and with the rear-mounted battery, my empty wt. CG is 78.21. I can carry a 200 lb pilot, a 240 lb pssgr, and 30 lbs of baggage on the rear baggage floor, 20 gal. of fuel, and still be forward of the rear CG limit. For those who want to play CG games, go to Dan Checkoway's CG database and look at CG calculations for N3TU, my plane. You can also compare empty CG's for various engine/prop combinations.

When I fly solo with full fuel, and no baggage or ballast, my plane is quite nose heavy (CG 80.2). I am able to 3-point it, but it is a challenge and stick forces are quite high. I usually fly solo with a 25 lb bag of tools strapped down on the rear baggage shelf. With this CG (81.4), I can trim the airplane for a pattern speed as low as 75 mph indicated, and flare to make a 3-point landing fairly easily. Furthermore, aerobatic elevator forces are more manageable, but still quite heavy.

I would not want my 8 to be any more nose-heavy that it already is. I think that the rear-located battery and ELT is a decent compromise between having the ability to 3-point the plane, trim it for pattern speed, and still carry a decent-sized passenger and some baggage. When flying solo, those of you with angle-valve engines, Hartzells, and front-located batteries are likely to have to carry 50+ lbs of ballast in the rear to be able to trim for pattern speed and 3-point the airplane.

Dan Miller
Battle Ground, WA
RV-8 N3TU. IO-360 M1, WW 200RV, 170+ hours since 7/16/06

 

[grantcarruthers]

I wondered awhile back about putting a lead ballast on a track in the tail so one could alter the cg with something like a manual trim wheel to move the ballast forward and back during preflight or inflight for that matter.

I'm an efficiency nut and always try to get as aft cg as I can so the tail doesn't have to fly "down" so hard and drag the cruise speed with it (at least for x-country and without adding weigt overall). I put everything I can as far back as I can.

A track system that runs from the tail cone to as far forward as space allows would allow a pilot to tune the cg and flight characteristics for the given mission and load (heavy vs light / x-country vs acro / solo vs passenger).

I have no idea how much an aft cg adds to cruise speed in a 172 like I unfortunately must use until the -8 is done. I doubt it's much. It's just the principal of the thing that bugs me knowing the tail is nothing but drag in cruise.

 

[McFly]

Go to Dan's site

He has a pretty good sample of all of the various RV's with the weights and CGs.

Look for RV's equiped like you plan to equip yours (prop, engine, ect) for some ballpark numbers and you can plan accordingly. The builder is also listed, which may be helpful for further questions. Good luck

 

[TShort]

How much does a PC-680 weigh?
How much does the long run of positive cable to the front of the plane weigh?

I believe all those that have gone before and are happy with the aft battery; however, I still like the idea of having it in the front better. With a composite prop and a parallel valve engine, I'm wondering if putting it up front and then (if necessary) adding ballast like Rick6A said ... I wonder if the ballast would be comparable to the weight of the additional cable needed to run to the front of the plane?? I don't want to add any extra weight anywhere, but if it is either cable or ballast and the mass is the same then ... any thoughts?

T.

 

[westwinds]

Rick, It is difficult to predict the empty CG during build. You have been given good advice to check Dans website. You should look at Jack Fromms weight and balance in that site as it is an 0-360 with Hartzell prop, Grove gear and battery on firewall. That is the same combination I plan to use at this time myself. Jack told me that he flies solo with a case of oil in the back and that the plane really likes to be wheel landed at that CG. I have no doubt that flying solo with a forward CG will hurt cruise performance also as the tail will have to create more drag with the forward CG. I fly a piper Comanche 250 (tricycle gear) that is actually slightly forward of the forward CG limit when i fly solo unless i keep a bag of tools and some oil in the back. even with the ballast it is nose heavy and impossible to keep the nosewheel off the runway when for more than a few seconds on landing. I know the RV8 will respond similarly with respect to being a taildragger and be more difficult to get the tail down a lower airspeeds for a flare in a three point attitude and will also lose some efficiency in cruise. It should be much easier to get the tail up early in the takeoff run tho. Since i do not have a flying RV8 yet i do not know how the braking characteristics will be with a fwd CG.
I plan to put my battery on the firewall to keep the leads short and to keep cranking performance up. I am tired of the Comanche 250s poor electrical performance with the battery in the back even with a healthy new copper cable coming fwd. Granted my battery is a flooded lead acid with poorer performance than the PC680. Because i fly at higher altitudes here in Colorado i plan to put a large oxygen bottle in the back of the baggage compartment (will have to come up with a good mount that can take the pull test) as my partial ballast since i also need the oxy. Then flying solo i will adjust with a case of oil or a tool bag if necc. I feel that will give me the most flexibility to go on a long trip with my wife and full tanks and baggage and still be within CG and gross weight limits. It will also make it possible to take up a heavier friend with partial tanks and stay within the CG and weight limits for aerobatics. Your choice is yours to make, but it can be done both ways.
Battery in back if you wish to not carry ballast solo or to minimize it, and battery in front for slightly better cranking performance if you do not mind using ballast to get a better solo Cg range or flying and seeing if the fwd CG is ok with you solo. Good luck with your project. Tony

 

[KirkGrovesRV8]

Just a diffrent view

Hi Fellas,
I guess I am the odd fellow out.. on the advice of a local RV-8 builder and pilot who I respect immensely, I put my PC-680 on the firewall with a 0-360 and a BA Hartzell prop. My empty CG is 77.9 and when full of fuel and solo it is nose heavy, but in my very limited experience I like it that way. The plane lands and handles very well. And quite frankly I like the fact that I can load a fairly large passenger in the plane and be within limits. Now I will say I have flown with 200+lb passengers and the plane does trim out much, much better in the landing phase. But I like having the option of putting weight back there if wanted as opposed to telling someone "sorry no can do" not that I mind telling someone no I just wanted to have more options with loading the plane.
P.S My empty weight is 1096

 

[rv8180]

I'm bulding an 8a and struggling with the same questions. I've heard some evidence to suggest that the 8a's are not as nose heavy as the 8's since the main gear is aft of the CG and they don't have the gear box...........

Guy

Guy,

I'm very interested in what you've found out and what your impressions are.
I'm about to hang my BPA AEIO-390 now. There will be a Christian inverted oil system up there too. The prop will be an 18 lb Catto.
Currently I have the strobe power module and the ELT mounted back by the push rod bell crank (behind aft baggage compartment).

I've already made both the firewall and aft battery mounts, so can go either way. Right now, it's firewall mounted. I'd like to leave it there but don't want a nose heavy RV8!

The AEIO-390 weighs 308 lbs, dry. The inverted oil system weighs about 8 lbs. If I've got my figures anywhere near correct,
I think that would make me around 334 lbs total, as compared to a 180 hp, metal fixed pitch prop (say 40 lbs), no inverted oil, at about 325 lbs.
That same engine, with a Hartzell would be about 345 lbs, so it seems I will fall somewhere in between (180 fixed = 325 lbs, 180 CS=345 lbs, me=334 lbs).
So I'm struggling with where to put the battery and would be very interested in your opinion

I'm leaning toward trying it on the firewall, but sure would hate to have to move it after it's flying.

A good flying, well balanced plane is more important than carrying elephants in the back seat.

I'll greatly appreciate anyone's input on this.
Thanks!

 

[Ironflight]

I'll greatly appreciate anyone's input on this.
Thanks!

In addition to all of the comments in this thread, this topic has been discussed numerous times in the past, so if you do a little search, you can find a lot more inputs from folks. After reading all of the opinions though, I doubt that you're going to find any new and different breakthroughs, but you might be able to tally a few more votes. In reading the many posts on the subject over the years, one might very well come to the conclusion that it doesn't make any difference - people are generally satisfied with where they put their battery, either forward or aft.

Perhaps the question that would generate some new information is..."Is there anyone who regrets where they put their battery (either forward or aft) - and why?"

Paul

 

[rv8180]

In addition to all of the comments in this thread, this topic has been discussed numerous times in the past, so if you do a little search......

Paul

Thanks! Paul!
I've done alot of research on this. The WB data base (Dan's, I think it was) was helpful too.
There seems to be conflicting information that keeps me from making my mind up.
Maybe this comes from what you said....they're happy with where "their" battery is.

It would be nice to have more info in the WB data base, like battery location and large or small battery and comments about anything else that might affect the WB (like having an inverted oil system, a loaded panel, etc) and tendency towards nose heaviness, etc.
I don't know if that could be easily done, though.

By the way....I love your plane. Hope you're really enjoying it!

 

[rv72004]

I have always liked the 8 but built a 7 due to the side by side set up for my kids.
I have this burning desire for a 8 which wont go away. However if I read between the lines, the 8 could be 'different' due to c of g .

My question is ; if you fly mostly solo at between 1/4 and 3/4 fuel levels , is the 7 a nicer flying plane than the 8 or are they similar ?

Anyone who has piloted both could maybe give their point of view. EJ

 

[KirkGrovesRV8]

YMMV

Take this for what its worth
I my very, very humble opinion I do believe the 8 is a easier plane to handle than the 6, was for me at least. I flew for around 60hrs before I flew with the plane loaded to gross and a aft CG(I used horse feed bags ) and honestly It was fine, I am not sure were the perception of the 8 being "different" comes from? It does handle different but not touchy or bad in anyway.
So this little note probably does not help much but for me the 8 is easier to fly and land than the 6 was. Either way I don't think you can go wrong with a RV

 

[grantcarruthers]

Regardless of W/B issue, how about environmental concerns for the battery.

In the fuse it's protected from some of the extreme temps inside the cowl.

Inside the cowl can go from 0 C to 150+ C in winter flight (just guessing on the numbers but you know what I mean).

Plus the vibration on the firewall has to be far worse than in the fuse.

And my understanding of batteries is they prefer to avoid both temp extremes and vibration??


So how much longer is an expensive battery going to last in the fuse vs the harsh environment on the firewall????????????????????????

 

[Rick6a]

Regardless of W/B issue, how about environmental concerns for the battery.In the fuse it's protected from some of the extreme temps inside the cowl.Inside the cowl can go from 0 C to 150+ C in winter flight (just guessing on the numbers but you know what I mean).Plus the vibration on the firewall has to be far worse than in the fuse.And my understanding of batteries is they prefer to avoid both temp extremes and vibration??So how much longer is an expensive battery going to last in the fuse vs the harsh environment on the firewall????????????????????????

Well, at risk of comparing apples and oranges here, my 150 and many other larger Cessna's I'm acquainted with come equipped with a firewall supported...if not necessarily mounted.....battery box. In my experience, I could reasonably expect 4 or more years of service before replacing that expensive and approved battery.

To touch upon another point that was considered in another reply. I tend to agree that by the time you install "X" amount of total weight (not to mention build complexity and serviceability) to mount a battery at some station in the aft fuselage, how much LESS weight in ballast would otherwise be required if the ballast were bolted much further aft..... such as under the empennage fairing?

 

[Stephen Lindberg]

how much LESS weight in ballast would otherwise be required if the ballast were bolted much further aft..... such as under the empennage fairing?

Adding a ballast weight in the tail will increase the fuselage moment of inertia in a spin, which could adversely affect spin recovery even though the overall aircraft cg is in the proper range. The only way to know for sure would be a spin testing program. Approach with caution and good luck. (Ref: "Anatomy of a Spin", John Lowery, Airguide Publications, 1981)

 

[dbuds2]

Searching for RV-8 Weight BAlance Spreadsheet

I surfed on to someones site with an Excel Spreadsheet the other day that was excellent at doing loading scenarios for the Rv8. I can't find it again. Anyone know that wonderful person's site or a link?

 

[tomagin]

If anyone is looking for a "contrarian" I am putting the PC-680 up front and as others have mentioned putting the ELT pretty far back along with the strobe power supply and the TruTrak servo is back there too. As with many others it is a point without clear direction and only some indications about things like carrying more weight aft, solo aerobatics, etc. A friend of mind has the PC-680 up front with similar equipment and no issues around solo flight. Mine is a parallel valve IO-360 though with the C/S prop. If I had the parallel valve engine I'd put the battery in the back. Guess I'll only know the score when I weigh it! Typically I am carry a load of "standard baggage" in the back of my other plane and expect it to be the same ---- tool kit, oil, tiedowns, etc that will add to the compensating effect of the ELT and power supply. Anyway, I just want to make the point that with the parallel valve engine plus C/C prop there are lot's of happy campers with the PC-680 up front.