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Boxy fuselage structures

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MalcolmW

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Jan 21, 2007
Messages
118
A question for Orion.

The other day you commented that your company had investigated an alternative to the boxy structures made from composite panels, but gave up on the ‘kerfing’ approach.

This raised a question in my mind about ‘boxy’ fuselages. Do they have a significant penalty in drag as compared to tubular fuselages? Or, do they have structural penalties?

I ask this question because I really don’t know enough aerodynamics to answer the question myself. I do note that there are quite a few aircraft that have ‘boxy’ fuselages, of which there are a couple which have high cruising speeds.

I also recognize that flat panel construction can be combined with sheet metal to produce more 'rounded' fuselage structures, but the basic question about the 'penalty' for boxy fuselages has me puzzled.
 

orion

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Western Washington
Depending on the details, yes, a boxy structure can exhibit additional drag as the air flows over and around the corners, potentially causing relatively draggy vortexes. One area of major concern is at the wing's root - the flow off the trailing edge next to the body is angled downward (downwash) past the bottom corner. The flow along the flat fuselage bottom wants to go straight back or actually slightly up (depending on tailcone shape). The interaction at that corner can be significant. The original BD-1 (predecessor to the Grumman Yankee) had the wing end bluntly at the fuselage, thus creating this condition.

When Grumman took possession of the design to produce it into the two and four place aircraft, they did quite a bit of optimizing before coming up with the eventual product line (reportedly the BD-1 was not really a good design - it just used the attractive structural concept which is what Grumman built on). One of the areas they investigated was that trailing edge interaction. The result was the wing root fairing (I think I remember that it was designed by LoPresti), which alone added well over ten knots to the cruise speed.

The structural penalties have only to do with panel stability issues. The trade-off of using the sandwich panel for the whole structure or only in the cabin area as Grumman did it, is weight. I am currently playing with an idea where the sandwich material is used for the entire fuselage, but then I like airplanes with more power so if I do have a bit of a drag rise, it will most likely not be too noticeable. I am using a pretty sizable root fairing.

The structure I'm looking to use combines the sandwich material with square aluminum tubing (hence my interest in bonding). The tubes take the primary axial and engine loads while the sandwich skin deals with the shear loads (this way the tubing does not have to be arranged in a truss configuration - just longerons and only a few uprights, no diagonals). Should be simple to build and assemble.
 
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mogren

Active Member
Joined
Jul 18, 2003
Messages
43
Air does not like corners, but the box section is easy to glue together:>
I am also, just now, drawing up the next two fuses. It is 2x2 alloy tube with the lower section all foam and glass. The tubes take the engine mount, wing mount, and the foam gets put in tension/twist.
This flying boat hull will be USA , UL legal.
The wings are tandem wing /canard style , pusher powered. This is based on my design of "Gretchen", a currently flying tandem wing Ul , that is steel tube fuse.
Mike Ogren , FL.
 

MalcolmW

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Joined
Jan 21, 2007
Messages
118
What is 'Elmer's Ultimate Glue?' What were the test substrates and what kind of surface preparation was used prior to bonding?

Orion's idea of using aluminum honeycomb panels in the structure, and carrying the shear loads should work. These panels have significant flexural stiffness, and except for highly stressed locations (depending upon skin thickness), should be able to carry most of the loads experienced in aircraft.

Now, I do agree that highly stressed areas should have a solid member (aluminum tubing sounds like a good idea) to tranfers concentrated stresses from one part of the structure to another (engine, wing, roll protection structure, and etc.).

I did a few quick calculations using a BD-4 as an example (using 0.02" skin & 0.50' core honeycomb, with .125" angles), and came up with a weight reduction. Not having a full set of plans prevented this calculation exercise from being full or rigorous.

I don't remember if the BD-4 has a root fairing. I do remember that it is a fairly fast (high wing) aircraft.
 

Mike Armstrong

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Joined
Aug 17, 2006
Messages
206
Location
near San Diego
Hi Orion, I've been meaning to bring this up. Maybe I'm just getting old but lately whenever you have more than a paragraph in your posts the letters are so small its now difficult ro read:depressed I realize your probably trying to save space and I really like reading your posts but your making it much harder to so, sorry. Anybody else finding it diffacult to read Orions posts?:emb:

Depending on the details, yes, a boxy structure can exhibit additional drag as the air flows over and around the corners, potentially causing relatively draggy vortexes. One area of major concern is at the wing's root - the flow off the trailing edge next to the body is angled downward (downwash) past the bottom corner. The flow along the flat fuselage bottom wants to go straight back or actually slightly up (depending on tailcone shape). The interaction at that corner can be significant. The original BD-1 (predecessor to the Grumman Yankee) had the wing end bluntly at the fuselage, thus creating this condition.

When Grumman took possession of the design to produce it into the two and four place aircraft, they did quite a bit of optimizing before coming up with the eventual product line (reportedly the BD-1 was not really a good design - it just used the attractive structural concept which is what Grumman built on). One of the areas they investigated was that trailing edge interaction. The result was the wing root fairing (I think I remember that it was designed by LoPresti), which alone added well over ten knots to the cruise speed.

The structural penalties have only to do with panel stability issues. The trade-off of using the sandwich panel for the whole structure or only in the cabin area as Grumman did it, is weight. I am currently playing with an idea where the sandwich material is used for the entire fuselage, but then I like airplanes with more power so if I do have a bit of a drag rise, it will most likely not be too noticeable. I am using a pretty sizable root fairing.

The structure I'm looking to use combines the sandwich material with square aluminum tubing (hence my interest in bonding). The tubes take the primary axial and engine loads while the sandwich skin deals with the shear loads (this way the tubing does not have to be arranged in a truss configuration - just longerons and only a few uprights, no diagonals). Should be simple to build and assemble.
 

orion

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Joined
Mar 2, 2003
Messages
5,800
Location
Western Washington
OK, I've been using Garamond - maybe I'll try Times New Roman. It's a bit bigger but still allows for a longer text without eating up a whole screen.
 

Topaz

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...Anybody else finding it diffacult to read Orions posts?:emb:
It's a little bit on the small side. I don't object to the font - I love Garamond (especially the Adobe cut of it, FWIW) - but perhaps a slightly larger size would be easier on my middle-aged eyes as well.
 

Mike Armstrong

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Joined
Aug 17, 2006
Messages
206
Location
near San Diego
It's a little bit on the small side. I don't object to the font - I love Garamond (especially the Adobe cut of it, FWIW) - but perhaps a slightly larger size would be easier on my middle-aged eyes as well.
For Topaz and the rest of us with middle aged eyes:ermm: , theres a couple of ways that I know of to better view the lettering on web pages. One is to 'decrease' your screen resolution and the other is a Microsoft program called ClearType. Its for LCD screens and its free.

http://www.microsoft.com/typography/WhatIsClearType.mspx

http://www.microsoft.com/typography/ClearType/tuner/Step1.aspx

Ofcoarse theres always those 'granny' glasses but so far, I flat refuse! (at least in public;) ).
 

Topaz

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Ok, I changed my original of the quoted post to Times New Roman and yes, that does look a bit more legible.
Yeah, that helps.

Readability is such a facinating thing, if you'll forgive a quick diversion off-topic. One of the problems with going to a small type size, all else being equal, is the larger number of letters and words per line. Makes for a more compact text, but those extra words-per-line actually hurt readability. The eye tends to wander up and down on a long line, making for eyestrain and more mental effort to keep focused on the line of interest.

There are actually charts that show the 'magic' length of line in number of characters (including spaces), as a function of type size.

And there's your typography trivia for today. :grin:
 
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