Weight & Balance Spreadsheet

An acquaintance is building a W&B spreadsheet that will allow for addition/subtraction of components like avionics and that has other handy features. He's asking if anyone has the math equations behind the "Weight on Nosewheel vs. Gross Weight & CG" graph on p. 14-10 of the -7/7A build manual (attached). No success in querying Van's.

Two-Beam Formula

You have a weight (Gross Weight of Aircraft) suspended on 2 beams (the mains and nose wheel).

1. A is the nose wheel weight, and B is the weight on the mains, GW is gross weight.
2. a (little a) is the distance from where the gross weight is centered (aka the CG calculated using normal methods) to position A.
3. b (little b) is the distance from CG to position B.

Then the weight on A is the ratio b/(a+b) of the weight of GW. And the weight on B is the ratio a/(a+b) of the weight of GW.

So using their sample from the graph on 14-10, and distances from the datum on 14-8, I get this:

Let a = (CG - Nosewheel location) = (80.48 - 39.11) = 41.37
Let b = (Mains location - CG) = (93.96 - 80.48) = 13.48

A = GW * (b / (a+b)) = 1463 * ((13.48) / (41.37 + 13.48)) = 359.54

Which is pretty close to their graph of 362# on the nose wheel (and they fudge by saying CG was estimated).

If you do another sample, and choose places on the graph where lines intersect to try and be more accurate, I get:

GW = 1350
CG = 78.7
A (aka weight on NW) = 1350 * (15.26/54.85) .... = 375.58# on NW, corresponding exactly to the limit on the chart at that loading/GW.

The theory is correct, but I might have mistyped something in this post, so check my work

Nose wheel weight calculation

Hi Stephen,

Thank you for your post above. My RV-7A is nearing completion so I have been looking at the W&B process and requirements before weighing my aircraft. My interest was caught by the graph on Page 14-10 in the Construction Manual and I was wondering how I could put formula into my W&B Excel spreadsheet to define the nose wheel load for different configurations. Thankfully I was intrigued by post.

Subsequently, I have input the Van's figures from page 14-8 into my empty weight portion of my spreadsheet and then, in the remainder for different configurations, I input different combinations of fuel quantity, pilot weights, no passenger or differing passenger weights, plus no baggage or maximum baggage.

Next to each combination I input your formulas that read directly from that spreadsheet configuration to obtain the nose wheel weight. Then I used the graph from Page 14-10 to calculate the nose wheel load using the same data. The figures from both procedures were almost identical (i.e. within 2-3 lb) keeping in mind that the graph can be open to interpretation.

The interesting thing I have gained from this exercise is that the full fuel, light pilot, no passenger or baggage configuration is the most limiting of them all.

This picture is of this configuration using my weight for the pilot.

https://flic.kr/p/2ncNrMC

The next picture shows the empty weight is increased by 20 lb by increasing the nose wheel weight but the other figures remaining the same. You will see from the calculation that the actual weight on the nose wheel is slightly above the maximum limit.

https://flic.kr/p/2ncHic2

In this last picture I have decreased the pilot weight (i.e. a light male or a female) by 10 lb but kept everything the same from the second picture. It indicates the nose wheel weight is now truly over the limit.

https://flic.kr/p/2ncHisT

This was a scenario I had not contemplated before now and I thought others may be interested in my findings which is why I am posting this information.

griffg said:

In this last picture I have decreased the pilot weight (i.e. a light male or a female) by 10 lb but kept everything the same from the second picture. It indicates the nose wheel weight is now truly over the limit.

https://flic.kr/p/2ncHisT

This was a scenario I had not contemplated before now and I thought others may be interested in my findings which is why I am posting this information.

Click to expand...

This is actually quite common in stock plans-built (e.g., nothing heavy in the back, like air conditioners, etc.) RV-10s. Even more concerning, for CG's close to the limits (either fore or aft), the CG moves further towards or past the limit as you burn gas. So it's important to check CG at the estimated landing configuration. I just carry 20 lbs of water in a plastic jug in the aft part of the baggage area for flying solo. If I have people or baggage in the back, I pour the water out. I've attached a copy of my Excel wt and bal spreadsheet, it may give you some ideas. You start by filling in the yellow line near the top, with people and baggage weights, plus fuel gallons. The program will then calculate gross weight, and CG locations with front seats full forward, bags full forward (within their areas); and again with front seats full aft, bags full aft. If this is okay then you don't have to enter any moment arms. If it is not okay, then you enter the actual front seat and baggage locations in the yellow line that's 1/3 of the way down. Again, the program will warn you if this is not within limits. Finally, a little further down, the program will calculate the zero fuel CG, and again warn if this is out of limits. The plot shows RV10 allowed limits; CG at max forward and max aft locations (horizontal red line); and CG calculated with moment arms (if entered), the intersection of the blue dotted lines). You'll also see, to the right, where Y or N can be selected for modifications, like rear seats and/or seat backs removed, E size oxygen tank on-board, etc.