[Noah]

Question-

Are you completing a weight and balance prior to finishing building, and prior to weighing the plane?

I guess what I'm asking is this: When you are building, you need to make decisions about where to locate certain equipment so as not to adversely affect the CG. But the problem is, what is your starting point? Does Vans or anybody else have an empty weight and CG of a stock 7A with no instruments, engine, or prop (essentially airframe only), so that you can take it from there and add what you want for options?

I've heard people say that the 7A is tail heavy, and putting a heavy prop on it helps in this regard. But this is qualitative. As far as I can tell, you really don't know how much of an effect you've had on things until you get the plane complete (or at least on the gear so that you can establish the weight and CG). It seems to me, this is too late to "discover" what you have, and be able to change things if necessary. I'd really rather start with a basic "stock" airframe W&B and then add all of my equipment from there. When I'm done, hopefully the spreadsheet calculations would be reasonably close to the as-weighed values.

I'm familiar with Checkoway's database of W&B for completed planes, which is good, but you still have to back out what everybody in there has for equipment and you still don't know where things are located (ELT and strobe power supply, for examples). This is more of a "top down" number, and so has somewhat limited utility as compared to a "bottoms up" W&B.

So does anybody start a weight and balance spreadsheet early, during the build, as a tool to help make informed decisions about equipment location and ensuring your empty weight isn't ridiculous??

 

[N941WR]

Noah,

I've been on the board for a number of years and do not recall anyone weighing an in progress project. That doesn't mean it hasn't been done, just that I have never heard of anyone doing it. This might be because most people weight them on the wheels and while under construction, the wheels usually are not on until very late in the build.

That said, I made every effort to build a light aircraft and to place things like the strobe power pack and ELT under the baggage compartment, as far forward as I could.

Remember, the more you can center the mass in the middle of the airframe, the better the aircraft will handle.

Other than that, I selected the lightest components I could and figured that if it was extremely light but out of CG, weight could easily be added back.

 

[zkvii]

Hi Noah,

Great question and yes it is a 'if I knew that then I would have done it differently'.

For your specific problem - 7A tipup with 'heavy' engine that is pretty close to us. During the build we tried to move things 'forward' where possible, we have ended up with a pretty good CofG and I can't load it out the front or back in any flying configuration.

For us, the addition of about 5 kgs to the luggage area makes a HUGE difference to the nose weight during landing - with out it and one up full fuel I need nearly 50kts to get the nose off. With it, nose lifts around 30 kts.

For us, strobe pack is under baggage floor, ELT on rear turtle deck. IO-360 (180hp) with Hartzell BA CS prop. The big thing for us was second battery and the ECUs on the firewall - this probably dragged the CofG forward by .25 (roughed out estimate). Paint added about 3.5 kgs to the total weight and moved the CofG aft by ~0.15". The finished basic weight (including fluids) is 538 kgs (1186lbs), CofG 78.57" aft of datum.

Any further specific questions PM me,

HTH,

Carl

 

[L.Adamson]

That said, I made every effort to build a light aircraft and to place things like the strobe power pack and ELT under the baggage compartment, as far forward as I could.

On the other hand, in my 6A, I placed the strobe power pack and ELT almost back to the horizontal stab, to offset the weight of my Hartzell C/S prop.

And I'm glad I did, as the plane is nose heavy enough, that it requires at least a light weight pilot to make the CG range.

L.Adamson

 

[SteelMike]

Hmm... this is a really good question and I don't know if it's been fully answered yet. I'd be interested to know if there's more to say about it other than to relate various anecdotes (not to take away from what has already been contributed).

 

[pierre smith]

Precedents.......

....are easily followed and the results are fairly well known for most all the RV models. If you're going with a known heavy CS prop, then it'll need a lightweight Skytek starter or similar. Ditto for a 200HP engine. If you pile on lotsa paint, it'll move the CG rearward, since most of the surface area is behind the CG.

The most common method of acheiving a desirable CG is obviously moving the battery, a well known fact and can be decided on whenever you make your engine/prop combination choice.

Regards,

 

[Noah]

....Precedents are easily followed and the results are fairly well known...

I mean no disrespect by this comment Pierre, but well known to whom? Where are these precedents documented?

Again, these are qualitative and anecdotal comments. What about a heavy (200 hp) engine AND prop? Where will that put my CG?

For the thousands of hours and tens of thousands of dollars one puts into building one of these, this seems to me to be one area where we are "rolling the dice" and "hoping for the best" as builders rather than applying simple engineering principles... arithmetic, really.

I for one do not want to get my plane finished only to find that my CG is too close to one end of the range, resulting in some loading limitation I can't live with. And as a result having to perform surgery to correct it, like moving the battery.

Come on guys, there has got to be a better way!

 

[JHines]

I too would love to know the empty weight and CG of each of Van's airframes, completely built with cowl and engine mount, but nothing else FWF and no instruments, interior, electrical, fuel system, etc. That would be a great starting point for planning.