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Weight and balance a pound saved is a pound gained

Vansrv7arob

Well Known Member
As RV7s are typically tail heavy, especially the arse draggers, FP and 0320's, has anyone calculated the effect/benefits of saving weight in the tail on cg. Obviously with the longest moment, adding or removing weight from this area would allow the greatest changes in WB with the smallest changes in Mass.

So what effect does a unit of weight in the tail cone equate to at the spinner, fuel tank CG, and baggage compartment?

Thinking of sealing and pressurizing my emp so I can fill it with helium or even better hydrogen to lighten it up, added benefit could act as a hydrogen reserve for my hydrogen powered alternative engine under development, don't tell RVG well I guess it is kinda a Lycoming.
 
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Rob,

You can simply calculate the difference. Add into your W&B spreadsheet a couple more rows with the appropriate arm for the spinner distance and tail distance, then "add" -1 kg (that's a minus one kg) to the tail line and +1 kg to the spinner line, and let the spreadsheet recalculate the W&B.

Hydrogen/helium in the tailcone might give you a few extra km if piped into the engine, but will cost you in weight. By the time you get the gas in there under pressure, you will have actually added weight rather than increased lift (think putting in a compressed gas cylinder).

cheers,
greg
 
Thanks guys,

I was taking the **** about the Hydrogen/Helium in the tail, to make it work you would need to use light weight composite high pressure tanks in the wings to keep the weight close to the CG.

however the JATO would make it easier to keep the grass down on short field operations.

My rough calculations say for a RV7a

1 pound saved in the tail =

approx. 2 pounds in the baggage compartment (not ignoring the 100 lb limit)

or

approx 20 pound at the spinner.

Happy to take on criticism if these numbers are completely out.

Food for thought.
 
A pound added IS a pound gained...

I added a 20lb crush plate to the prop (Catto 3-blade FP), and gained 18lbs in baggage capacity due to the improved CG. Way easier than trying to seal the tail for helium!
 
Thanks guys,

I was taking the **** about the Hydrogen/Helium in the tail, to make it work you would need to use light weight composite high pressure tanks in the wings to keep the weight close to the CG.

My rough calculations say for a RV7a

1 pound saved in the tail =

approx 20 pound at the spinner.

Happy to take on criticism if these numbers are completely out.

Food for thought.

And you're not alone, and i will probably join you with the catto as well, what I am trying to detirmine is with fore thought rather than after thought could you now possible own an aeroplane that has the same CG with a 21 pound lighter empty weight.

As I said I was just taking the **** about the helium/hydrogen.
 
...So what effect does a unit of weight in the tail cone equate to at the spinner, fuel tank CG, and baggage compartment...

It is a very straightforward mathematical ratio. The various arms are a multiplier (or divisor) for the opposing arms . One unit of weight located one unit of distance is equally offset by .250 units of weight at 4 units of distance.

The problem with this is that each airplane is different so the exact CG location will not be known until you weigh the aircraft. Until this is done you can just make an educated guess and take a stab at the CG location to get you in the ballpark.

In your example, you have to define the location of weight in the "tailcone" as a distance from the CG, then define the distance from the spinner to the CG. The resulting ratio in distance will be the same ratio in weight.
 
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As RV7s are typically tail heavy, especially the arse draggers, FP and 0320's, has anyone calculated the effect/benefits of saving weight in the tail on cg. Obviously with the longest moment, adding or removing weight from this area would allow the greatest changes in WB with the smallest changes in Mass.

So what effect does a unit of weight in the tail cone equate to at the spinner, fuel tank CG, and baggage compartment?

Thinking of sealing and pressurizing my emp so I can fill it with helium or even better hydrogen to lighten it up, added benefit could act as a hydrogen reserve for my hydrogen powered alternative engine under development, don't tell RVG well I guess it is kinda a Lycoming.

Rob,
Build the airplane and go fly it. The mental exercise of pressurizing the tail is time wasted. Hang an 0360 anyway you want and it will work. Press on, life it short.
 
Thank you for you imput Toobuilder.


David like I said twice already I was just taking the **** about the Helium, hydrogen in the tail (for a laugh) to see the response, i've now spent more time clarrifying that point than coming up with that gem of a hook.

You are somewhat correct David, yes if you hang an engine in a plane it will work, what I'm trying to work out is a simple way to make it work better. Do you find it ironic carrying unessary weight into the air with you, such as a 20 pound piece of steel attached to a very well refined and light prop such as a catto for example.

All I'm saying is can it be done a diffent way, that makes more sence, yes life is short especially at the pointy end, I have 50 more years flying ahead of me based on the precedence you are setting and over that period there will be some very significant changes in Aviation cost of fuel power plant choices etc. I would like to fly my plane for as long as I can post build so I'm looking at future proofing the design in simple ways that don't add build time but may reduce them.

One thing is for certain however any extra weight you take with you in the air costs you in many many more ways than just $.

Any way tonight I spent 10mins working out I can easily wipe 1.65 pounds of my tail weight which based on my rough calcs equates to 33pound at the nose.

With a little more effort I found i can wipe a further 12 pounds of my tail roughly 240 pounds at the nose, I'm just trying to decide apart from a nice light prop what else might I like to lighten in the front end producing further weight savings or do I want to just carry more useful weight in my baggage area or fuel rather that an otherwise very well manufactured lump of steel.

I maybe missing some thing here but apart from correcting CG what other advantages does carrying a 20pound fly wheel into the air give you apart from helping return the aircraft to earth faster?
 
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.......... With a little more effort I found i can wipe a further 12 pounds of my tail roughly 240 pounds at the nose ......

I maybe missing some thing here but apart from correcting CG what other advantages does carrying a 20pound fly wheel into the air give you apart from helping return the aircraft to earth faster?

Curious how you would eliminate 12 lbs of weight in the tail without compromising strength?

And a (minor) advantage of adding a steel ring to a lightweight prop is that it adds rotational mass, which smoothes out the idle, can make starting easier and smoother, and in some cases allows the idle RPM to be set lower, which can allow a shorter landing distance.

BTW, the Landoll steel ring weighs 11-12 lbs, not 20. There may be a 20 lb ring out there.

PS: The phrase "Taking the p_ss" is not a well-known slang term "upover" (opposite of "downunder"). If I remember, it sorta means "Pulling your leg".
 
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Thanks Dave,
Great site, reading through your older posts, it appears we may have a similar build philosophy, to you have a web site or build log? I would be interested in finding out more about your approach to the build.

Will you be using one of these batteries?
 
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saving weight on the tail...

The simplest is probably to polish the fuselage and tail feathers instead of painting them. Might save a pound or two and cost a bit of time.

Curious what potential weight savings would be gained by making carbon-fiber fairings for the tail? I suspect not enough lighter to be worth the cost, but I don't know composites well enough to judge...

Is electric elevator trim lighter than manual elevator trim?
 
....Is electric elevator trim lighter than manual elevator trim?
Yes, and the weight difference is dramatic. The steel cable assembly for the manual trim system all by itself weighs approximately 2.5 POUNDS. I don't know what the mount, knob, and associated hardware add to the total installed weight of a manual trim system. Anyone? By comparison, a complete electric trim system including servo, wiring, LED indicator etc. weighs in at little more than 7-8 ounces.
 
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...With a little more effort I found i can wipe a further 12 pounds of my tail roughly 240 pounds at the nose...

With a ratio of 20, that 12 pounds sounds like it's in the empenage... (240/12 = 20) i.e. nose (arm) X 20 = tail (arm) which means structure...

...If so, where did you find an "extra" 12 pounds of structure in the empenage?
 
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Rob,

I don't have an online blog of my build. I am planning on putting what I have up though. It's on my to do list. I'm focusing on simple and light for my build.
 
I love the idea of building a light plane that has all the available baggage capacity. A light tail would help this greatly. Not sure how this could be done but here are a few ideas to discuss or shoot down. I'm not sure of the structure or validity of these, so I am just throwing them out there.

- no paint on the tail feathers
- electric trim
- more lightning holes in the ribs
- some how remove the need for the lead counterbalance
- internal rudder stop
- carbon fiber elevator and rudder
- a nose wheel or a very light tailwheel

A RV-7 under 1000lbs with a 100lb baggage capacity would be great. Even better if you could some how work in a constant speed prop too.
 
Yes, and the weight difference is dramatic. The steel cable assembly for the manual trim system all by itself weighs approximately 2.5 POUNDS. I don't know what the mount, knob, and associated hardware add to the total installed weight of a manual trim system. Anyone? By comparison, a complete electric trim system including servo, wiring, LED indicator etc. weighs in at little more than 7-8 ounces.
True... However! Adding the trim servo to the elevator (the stock location) requires adding lead to the counterbalance to offset the extra weight and balance the surface again. Instead, mount the servo in the tail cone and run a short cable assembly out to the elevator.
 
True... However! Adding the trim servo to the elevator (the stock location) requires adding lead to the counterbalance to offset the extra weight......
Not my experience Rob. With the servo installed per plans in the elevator in both my -6A and -8, I did not add any lead to the counterbalance. There was simply no need to.
 
Really? Every time i've read about putting the electric trim in someone has pointed out that the elevator would need to be re-balanced. So servo plus access panel hardware equals the weight of the portion of the manual cable that runs through the elevator? That doesn't seem intuitively right. Did you convert your -6A and -8 from manual to electric, or just build them that way from the beginning?
 
I am not aware of a note in the drawings or builder manual that discusses the possibility of adding mass to the lead counterbalance. You would think that if precise balance was an important concern when installing the popular electric trim option, it would be noted somewhere in the plans. If the information is noted, I missed it and perhaps you can direct me to the relevant text.

In my case, the electric trim system on both RV's were built per plans. The only thing I did different on the -8 was to add a nutplate through an existing tooling hole so elevator balance could be perfectly achieved pre paint, post paint etc. with the simple addition or subtraction of an area washer or two. I would have done that regardless of what trim system was used because it is such an easy mod to do.

ka5ylf.jpg


I did not do that on the -6A because at the time, I was not aware of the nutplate trick. In the end, the nutplate wasn't needed anyway and probably not needed on the -8 either.
 
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As RV7s are typically tail heavy, especially the arse draggers, FP and 0320's, has anyone calculated the effect/benefits of saving weight in the tail on cg. Obviously with the longest moment, adding or removing weight from this area would allow the greatest changes in WB with the smallest changes in Mass.

So what effect does a unit of weight in the tail cone equate to at the spinner, fuel tank CG, and baggage compartment?

Thinking of sealing and pressurizing my emp so I can fill it with helium or even better hydrogen to lighten it up, added benefit could act as a hydrogen reserve for my hydrogen powered alternative engine under development, don't tell RVG well I guess it is kinda a Lycoming.
So only a decade on I will shortly close this loop on w&b. It’s a shame the factory had little time for the solution and better (simple) offering to solve this common customer conundrum. Very sad that Barb Billman has left the building she might have been the smartest person in the building for a time… Dick works from home right….. :)
 
So only a decade on I will shortly close this loop on w&b. It’s a shame the factory had little time for the solution and better (simple) offering to solve this common customer conundrum. Very sad that Barb Billman has left the building she might have been the smartest person in the building for a time… Dick works from home right….. :)

To save weight on the tail, for $500 you can save 1lb by using the Skydesigns titanium tail spring and another 1lb by using Flybys lightweight tailwheel tire.
 
Math Stinks

By the way, the CG math breaks down when you talk about adding helium or hydrogen to the tail. It may weigh within CG limits on the scales, but when flying, the CG will be farther back. This is the one case where CG someone weighs is not the same as aeronautical CG. All the helium does is make it like the tail has some lift, but the mass of the helium has been added, so this actually moves the CG farther Back. JMHO
 
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Mine Wants to be Tailheavy, Too

Our RV-6 has an O-320, featherweight Catto prop and a lightweight starter. CG likes to run toward the aft limit. I installed a Flyboys lightweight tailwheel and used the kinda hat-shaped nuts on the hinge bolts with minimal washers, just to shave a bit of weight. Looked into that **** hydrogen and helium stuff, but concluded noweightium would be better. It’s backordered everywhere I inquired about it. I know one thing… no need to spend all that $ buying an EarthEx battery that will just make things worse.
 
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At risk of starting a primer war, I only primed mating surfaces in the tail feathers. Rest of the plane is getting full primer.

Was done as more of a learning/experimental exercise, but there are weight benefits too.
 
Nobody has mentioned that you could tie a bunch of helium balloons to the tail. That way you could get extra lift without adding weight.
 
Nobody has mentioned that you could tie a bunch of helium balloons to the tail. That way you could get extra lift without adding weight.

Put a helium balloon inside the tail!

The previous remarks about pressurized heavy containers and adding weight do not make any sense. Try to get a balloon into the air if all the helium is inside a pressurized container. Lighter than air, folks!

Or pipe hot air from the engine through a balloon in the tail ;)

Of course I wonder if the volume in the tail is big enough to make any difference.

Finn
 
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