# Ideal Weight distribution

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#### NeedHelp12

##### Member
Hi,

Let's suppose I know the wing load and total weight of a plane

Is there a way to calculate or find out the ideal weights of the individual parts of the plane like fuselage and wing?
Or with other words: What is the ideal weight distribution?

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
Can't sleep tonight and the Don Q 151 is starting to help so................

The ideal weight is zero - but we can never come close.
As for distribution it really doesn't matter much until you start doing structural calculations and spin testing. The more weight in the wing the more the inertia reduces the bending moment compared to a plane of the same mass with a light wing, and a heavy body. However distributing the weight along the wing tends to up the moment of inertia on that axis (tip tanks as an extreme example) and this can result in poor spin manners.

It's all a compromise and the interplay of the variables can get complex. But that is what makes it fun for some of us. The learning never stops.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
+1 on Hot Wings' comments.

For what purpose? WEIGHT IS THE ENEMY of all performance measures in airplanes, so ideally we have as little airframe and powerplant weight as will haul our mission people and bags and fuel. This point on weight is very important...

Distribution of weight, how it is spread along the span (BL) and spread along the fuselage (FS) does impact handling. All of our control inputs are designed to rotate the ship. Ailerons give us roll with mass moment of inertia along the span involved there, while Rudder and Elevator give us yaw and pitch with mass moment of inertia along the fuselage in involved there.

If you were building an unlimited class aerobat, you want instantaneous response to all inputs. To do this, the ratio of moments to inertia should be as high as you can get it. Usually this means large control surfaces and driving weight toward the CG as much as possible. Engine, fuel tank, pilot as close together as you can, taper the wings, optimize wing and aft fuselage design to minimize weights towards the tips and tail... You then have an airplane where a control input makes fast response to inputs, and it will also tend to come out of spin/snap rolls more easily than if the weight were more spread out.

If instead you are building a fast tandem cross country ship with baggage capacity, you have already decided to spread the humans along the fuselage, some bags will go aft, you will probably have to put some bags between people and the engine, and fuel will go in the wings. All add inertia along all three axes, slowing down response to controls. This can still make very satisfying airplanes - RV8 and the two seat Extras as examples.

Want to make it feel like an airliner? You fly instruments a lot, like the airplane to not change attitude much in bumps, etc. You might actually like spreading the weight out fore-and-aft and out towards the tips. Understand that making the airplane stiffer in the axes makes it less pleasant to fly - it requires more anticipating for roll inputs to both start and stop roll, and the roll rates are lower. In general, it makes for a less pleasant airplane to maneuver. Folks with tip tanks can tell you about how much more enjoyable the airplane is to fly when they are empty. Well, unless the tuna tanks are the mains, then they are only empty when you are out of fuel... Oh, and all that inertia means it will be slow to enter spins and snap rolls, but it will feel like it takes an age to stop those maneuvers with opposite rudder, and will probably drive more tail area and control surface area than for the same mission achieved with less inertia in the axes.

If you want a better ride in turbulence, what you really want is higher wing loading.

So, it depends on your mission, your preferences, etc.

Billski

#### TFF

##### Well-Known Member
Ideal weight distribution is all of it at the perfect CG point. X, Y, and Z. Impossible.

You have to look at what you are trying to design and what you want to use. Your wing sticks out this far and another one sticks out the other side. What’s it made out of? What type of construction is it? How strong does it need to be? How efficient does it need to be? Answer those questions and like ones for every part and you will get that answer.

If you want to reverse it, it’s like reversing an equation. put in what you know and figure out what you don’t.

#### Aerowerx

##### Well-Known Member
The ideal weight is zero ....
Really?

I thought you would need some weight. Wing loading and all that. Otherwise your plane would be tossed around by the slightest breeze, like a tissue in a tornado.

#### Riggerrob

##### Well-Known Member
Dan Raymer's "Simplified Aircraft Design for Homebuilders" is a good start because includes estimates of wing weight, versus fuselage weight, etc.
Dan provides a rough guide for early configuration estimates.
Later in the design process, you will have to weigh individual components to confirm that they weigh close to your early guesstimates.

#### BJC

##### Well-Known Member
HBA Supporter
At least one pilot (could have been Harold) commented that the Krier Kraft one-off aerobatic biplane was too light. I suppose that one could make the counter argument; that the wings are too big.

Too light or not, that is one airplane that I would love to fly.

BJC

#### wsimpso1

##### Super Moderator
Staff member
Log Member
At least one pilot (could have been Harold) commented that the Krier Kraft one-off aerobatic biplane was too light. I suppose that one could make the counter argument; that the wings are too big.
I concur with the counter argument.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Really?

I thought you would need some weight. Wing loading and all that. Otherwise your plane would be tossed around by the slightest breeze, like a tissue in a tornado.
It still has to carry whatever the mission people/ bags/ fuel is, and performance keeps getting better the lighter the total package is. You just keep reducing the foil dimensions as the gross weight drops maintaining wing loading. So, ideal is zero empty airplane mass. We all know it is unobtainable...

Billski

#### Lendo

##### Well-Known Member
Need Help 12,
Don't we all need help at some time or other!

What I ended up doing was cutting the fuselage drawing up in slices for Surface area calculations, found the circumference between slices (as an average) and added them all up, then I calculated the area between those slices and applied the weight of materials to them, similarly the wing skins Spar and ribs. I then measured the internal Composite pieces at added them up. I measured the internal control mechanisms, found their weight as well as Instruments etc.

So it can be done manually, but it does take a little time, no short cuts unless your an experienced designer and builder, even then it depends on size, design and materials.

I know this is not want you want to hear, but there's no shortcuts - hope it helps.
George

#### NeedHelp12

##### Member
Sorry for the late response.

So is it then possible that an airplane can fly if it has a heavy fuselage and relatively light wings? What would be the differene to an airplane that has the same mass but a light fuselage compared to relatively heavy wings?

I assume that in both cases the wings have the same size and thus have the same wing loading.

#### BJC

##### Well-Known Member
HBA Supporter
For simply being able to fly, the airplane needs to lift the total wright plus trim force.

If all the weight were in the fuselage (impossible, but illustrative), the wings would need to be stronger in bending than they would with some of the weight in the wings.

BJC

#### NeedHelp12

##### Member
Ok, thanks.
I now have a rather silly question but please dont make fun of me

Would a real aircraft (no scale model) be able to fly if its wings were made out of paper?

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Sorry for the late response.

So is it then possible that an airplane can fly if it has a heavy fuselage and relatively light wings? What would be the differene to an airplane that has the same mass but a light fuselage compared to relatively heavy wings?

I assume that in both cases the wings have the same size and thus have the same wing loading.
It looks like you are playing the "what-if" game without doing your own thinking. I say to you what difference do you think it makes if you vary where the weight is in the airplane, as long as the same weight is lifted by the same planview and airfoil shape? Get a little analytical and then we can discuss it.

Billski

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Ok, thanks.
I now have a rather silly question but please dont make fun of me

Would a real aircraft (no scale model) be able to fly if its wings were made out of paper?
Harrumph. What is the difference between wood and paper? Both are cellulose fibers held together by resins. We have wooden airplanes. "Composite" structures are fibers held together by resins too. Put on your own thinking cap and you will be able too answer your own questions.

Billski

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
I now have a rather silly question
It may sound silly to some of us but the age difference between you and this group as a whole means that many of us have decades of experience you don't. Also the culture was quite different when we grew up and a much higher percentage of us fixed our own bicycles, built tree houses and even filled plastic bags with acetylene/O2.

The point being the intuitive 'feel' for all things related to physics has been diminished over the years among the general population. One way to get this 'feel' is to actually go out, build things and make a few mistakes. A college level class in physics for non-majors, that has a lab associated, is a close second.

You might also add a little more to your profile so we can better tailor our responses.

#### dog

##### Well-Known Member
the problem with "paper", or any wood in general as an engneered product is,lignin, which holds the
fibers together, and is very,very difficult to remove without destroying the fibers themselves.
If lignin could be removed, leaving behind very long celulose fibres,this then would form the basis for a true super material,and it would also be suitable for wrapping things, scribbling on etc.

#### davidjgall

##### Active Member
Ok, thanks.
I now have a rather silly question but please dont make fun of me

Would a real aircraft (no scale model) be able to fly if its wings were made out of paper?
It's been done. TPG is Taylor-Paper-Glass wherein Kraft paper is infused with resin. The primary structure of the Holcomb "Perigee" was constructed entirely from TPG with glass reinforcements. Quite a successful airplane, and I wish I could find one of the few sets of plans that were sold....

#### davidjgall

##### Active Member
the problem with "paper", or any wood in general as an engneered product is,lignin, which holds the
fibers together, and is very,very difficult to remove without destroying the fibers themselves.
If lignin could be removed, leaving behind very long celulose fibres,this then would form the basis for a true super material,and it would also be suitable for wrapping things, scribbling on etc.
Research "BioMid Fiber" and be amazed. It is as strong as glass fiber, significantly lighter, 100% crystalline cellulose, 11 micron continuous filament fiber, available as roving or woven into cloth, etc....