Odd questions

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

stanislavz

Well-Known Member
HBA Supporter
Joined
Aug 21, 2016
Messages
496
Location
Lt
Now, i am actively reading forum from back to now, seeing some invaluable info / trying to gain more knowledge. 99 % of mine already asked question was answered before on HBA. But some question - was not. Or not to level, which i would like.

1. Laminar/turbulent airfoils. Fabric wing should be used with turbulent airfoil, laminar should be used only with rigid skin wings and not used when you have to many insects/ rain / debris etc. And laminar airfoil have to be build with high geometric accuracy.

But - can it be explained in number ? ie - to achieve laminar flow, nose section should be build with 0.1% geometric accuracy, and surface finished to high gloss ? Deflections due to wing load less than 0.03% ? And - where all have to be very accure (ie nose), and where all should be just ok (ie bottom)

2. Rutan / Long ez etc wing connecting using "bolts"

upload_2020-1-6_19-9-19.png

I can understand "metal thinking" in some composite builds, using metal like connector to connect wing.

Most factory builds are using "central box / glider box" to connect wings. Which looks to be lighter and stronger (Closed shape/vs open shape). Ct wing mount is below :

upload_2020-1-6_19-19-57.png

And the question is - do Rutan way was chosen for easier life of one off builders ?

3. Long ez have fuel in tanks placed in central wing station. But wings are build as foam cored using PS foam..

Which according to forum knowledge - have to be spaced as far as possible. Is it ok solution ?

Thanks !
 

ScaleBirdsPaul

Active Member
Joined
Jul 6, 2015
Messages
38
Location
Connecticut
Ive been looking into laminar foils more and I’ve had the same question as #1. Jack Lambie’s composite book says that a geometric flaw of only a few .001s of an inch is enough to trip the laminar flow, but that surface roughness is not as much of a factor as shape. It is important to have a consistently increasing thickness up until the maximum thickness point. To me this means imperfections such as divots around a countersunk rivet would trip the flow, but profile deviations that do not have local reversals would be acceptable.

I haven’t spent a lot of time researching, so I’m eager to se what the far more knowledgeable posters have to say.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,013
Location
Memphis, TN
One thing to take into account is the design is 40 years old. If designed today, it would be a completely different design. People might be using CNC cut foam and fancy fabrics today; it was designed for someone to use a electric carving knife and a 2x4 with sandpaper taped to it to shape everything. At least that’s how the one was build next door to a high school teacher of mine.

Very successful, but you do have to look at it in 1975 eyes. It’s an alien spaceship then.
 

Victor Bravo

Well-Known Member
Joined
Jul 30, 2014
Messages
7,414
Location
KWHP, Los Angeles CA, USA
(somewhat on-topc, somewhat slightly off)

Well I don't have any advanced scientific or theoretical knowledge, but I have extensive and painful knowledge about 6 foot long sanding blocks with 600-800 grit black "wetordry" 3M sandpaper, used to maintain 200 square feet worth of real laminar flow airfoils on the contest sailplanes. If we were doing this today I'd jokingly call it "being waterboarded". Back then (1983-87 in my time), Rick Wagner (Rick's Glass Tech, Glass Tech Unlimited) called it simply "the torture board".

I'm pretty sure BoKu knows all about this piece of sadistic equipment, with even greater pain and suffering than I. (There's a guy named Freddy Jiran (who taught Rutan what composites were) and another guy named Alan Bikle (the most iconic and famous sailplane race tuner of the fiberglass glider era) who know more about it than anyone... they'd give us all an earful :) )

The point is that a real long-run laminar airfoil takes a lot of work to build, or a lot of work to build molds for, and then it takes a horrific amount of time to re-do later if you really have a case for maximizing natural laminar flow.

Nowdays they have vacuum bagging and heat-curing cycles and probably more stable resins... so it may be possible to build a laminar wing and have it stay within the .001 or .002 that some of the airfoils need.

But if you travel back in time to the ancient age of solid foam core and wet layup, it's a whole lotta work to build and keep a "real" hi-performance laminar wing. The truth is that a decent semi-laminar wing is probably good enough for almost any HBA for normal use. Remember the Long-EZ and Quickie and many other airplanes were pretty fast and efficient using reasonably smooth surfaces on semi-laminar wings.

One big question I'd like to seen an answer to... which brings this into relevance with ScaleBirdsPaul's interest in laminar wings... is how much do the molded airfoils for the more recent Lancair and Glasair models move around and lose contour? Are they using methods and materials that maintain their shape within a couple of thousandths? Or are the fast Lancair guys all spending the winter sanding and sanding and filling and sanding to "profile" their wings?
 

lr27

Well-Known Member
Joined
Nov 3, 2007
Messages
3,822
The accuracy required, I think, depends on the scale you're measuring at. I'm sure you could have a wing that's within .010" everywhere, but which is sufficiently wavy to trip the flow early. On the other hand, I'm sure that you could have a wing that's .1" too thick and still would allow a reasonable stretch of laminar flow, as long as it's smoothly curving everywhere. Or, at least, that's the impression I get from Strojnik's Laminar AIrcraft Technologies. There's a reasonably detailed section in that book on how to get your wing smooth enough, but I found it hard to boil it down to something short and pithy.

I'm sure the difficulty of maintaining laminar surfaces depends a lot on what's under them. I know someone who used to vacuum bag the top of his foam cored rc sailplane wings and then put the slot for the spar in later. The slot wouldn't reach the top surface. This made the top significantly smoother.

I once spoke with the builder of a Windrose motorglider. I can't remember if he said he spent 1,000 or 2,000 hours sanding and filling the wing. OTOH, I've read articles pitching ways to do it that are supposed to reduce the finishing to a few days. Might have been in an EAA magazine, but I'm not sure, nor am I sure that they're right.

My impression is that you don't need a glossy surface, just a smooth one. From what I've heard, something like 300 or 400 grit paper is enough. That probably depends on the size of the wing and the speed it will fly at. As information, it's probably worth what you paid to read it.
 

stanislavz

Well-Known Member
HBA Supporter
Joined
Aug 21, 2016
Messages
496
Location
Lt
Loud thinking. 0.01" accuracy, at 45" chord will not survive any sunlight / or other kind of reasons for thermal expansion - ie just flying on sunny day, top skin is getting warm, bottom - still coll.

But - you can hot wire with 0.01" accuracy OK, full block, if foam block is fixed to rigid base. Add some skin with 0.02" - should give you an 0.015", if skin can have an tolerance of +-0.005" of geometric accuracy without any finish ?
 

Victor Bravo

Well-Known Member
Joined
Jul 30, 2014
Messages
7,414
Location
KWHP, Los Angeles CA, USA
My impression is that you don't need a glossy surface, just a smooth one. From what I've heard, something like 300 or 400 grit paper is enough. That probably depends on the size of the wing and the speed it will fly at. As information, it's probably worth what you paid to read it.
Experiments were made with all types of surface, from polished gloss where you could easily see yourself all the way to 240 grit surface with the sanding marks all at 45 degrees to the wing chord line.

Ross Briegleb once did a custom experimental finish (on my friend Trip Mellinger's AS-W20) where the leading edge back to .2 chord was mirror polished white lacquer, and from .2C to the rear spar was 400 grit with the sanding marks perfetly aligned straight chordwise (simulating the NASA "shark skin" research... shark skins had lengthwise micro-grooves that were studied to have lower fluid dynamic surface drag than smooth). Behind the rear spar, the control surfaces were polished. A lot of hours went into that, Trip didn't do any better in contests than he did before, and the sanded surface kept him very busy cleaning the fingerprints out of the texture!
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
6,778
Location
Saline Michigan
Now, i am actively reading forum from back to now, seeing some invaluable info / trying to gain more knowledge. 99 % of mine already asked question was answered before on HBA. But some question - was not. Or not to level, which i would like.
You my friend are AMBITIOUS!

1. Laminar/turbulent airfoils. Fabric wing should be used with turbulent airfoil, laminar should be used only with rigid skin wings and not used when you have to many insects/ rain / debris etc. And laminar airfoil have to be build with high geometric accuracy.

But - can it be explained in number ? ie - to achieve laminar flow, nose section should be build with 0.1% geometric accuracy, and surface finished to high gloss ? Deflections due to wing load less than 0.03% ? And - where all have to be very accure (ie nose), and where all should be just ok (ie bottom)
The wing profile needs to be a pretty close replication of the design, but the big thing that trips flow early is excess chordwise waviness. Search on "laminar airfoil waviness" and all sorts of articles come up. The methods used for profile sanding do a great job of leaving surfaces without significant chordwise waviness...

2. Rutan / Long ez etc wing connecting using "bolts"

I can understand "metal thinking" in some composite builds, using metal like connector to connect wing.

Most factory builds are using "central box / glider box" to connect wings. Which looks to be lighter and stronger (Closed shape/vs open shape). Ct wing mount is below :

And the question is - do Rutan way was chosen for easier life of one off builders ?

3. Long ez have fuel in tanks placed in central wing station. But wings are build as foam cored using PS foam..

Which according to forum knowledge - have to be spaced as far as possible. Is it ok solution ?
2 and 3 are related issues. The Long Eze and derivatives have polystyrene foam cored outer wing panels, canard, winglets, and control surfaces. These foams are sensitive to fuel, but since they are completely covered in glass and epoxy and bolt on with zero fuel internal to them, they are fine. The strakes and header tank and fuselage are built with PCV and/or polyurathane foam cores, which are unbothered by fuels. The fuel containing strakes straddle the CG, so fuel state has only very minor impact on CG. Separation between the two ensures no fuel damage to styrene foam cores.

The glider type of wing attachment had to be known to Burt Rutan, but several things seem to mitigate against that scheme for the Long Eze and derivatives.

First is that the fuel containing strakes can not share volume with styrene foam inside a fiberglass envelope. The styrene cored pieces must be separate pieces of the airplane, but the fuel tanks can share volume with the fuselage. So attaching the strakes to the fuselage makes all kinds of sense.

Second is that longitudenal packaging is everything in making a rear engined airplane's CG go in the right place. The engine is pulled as far forward as you can, the back seater is on the CG so the presence or absence makes little difference, and then the front seater is way forward and must be within certain bounds. When you do this, do you really want two beefy spars stuffed into that tight space? Burt decided to run a center section spar with the outer panels overlapping but only through the strakes.

This has some other advantages. Maximum spar lengths that the builder has to produce are shorter, which should make for easier construction. If you pinned the two wings together at the fuselage walls, the mounting loads that are shear in the spars will be much bigger than if they mount further out and are further apart than inside that narrow fuselage. Lower pin loads allow less web thicknesses on all of the spars and smaller bolts connecting them. In general, a three piece wing appears to be easier to build and with lighter hardpoints to put things together.

My airplane has a three piece wing too. At the time, I just could not fathom making everything straight enough on my first build over that whole length, but it would have saved me 20 pounds or more to do so. If I were starting knowing what I do now about making all this stuff, my wings would be two piece, overlapping through my 47" wide fuselage, like a sailplane.

I hope this helps with your understanding.

Billski
 
Last edited:

stanislavz

Well-Known Member
HBA Supporter
Joined
Aug 21, 2016
Messages
496
Location
Lt
You my friend are AMBITIOUS!
No, just trying to save other time, and "summarize" HBA knowledge/wisdom. + fast reading.

In general, a three piece wing appears to be easier to build and with lighter hardpoints to put things together.
And Pazmany did that on metal wing PL1 - and built an one piece wing afterward :)

Can it be said - Rutan style wing join is easier to build and easier to align ? But will weight more ?
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
6,778
Location
Saline Michigan
I summarized the distinctions between two piece and three piece wings above. A one piece will save some more weight, but it gets much more difficult to build and have it straight when done. You need a full span table and somehow you must build and tram in two full sets of wing fixtures that are mirror images of each other.

Making the wing in two or thee pieces lets you build on much shorter tables and symmetry can be assured by building one side then knocking down and setting the same fixtures back up in mirror image. For my three piece wing, max table needed was 12 feet. Same airplane with a two piece wing is 16-1/2 feet, one piece wing is 29 feet. I started in a 2-1/2 car garage that was 31 feet on the diagonal, so the one piece was completely out of the question, and the two piece was daunting. Three piece worked out for my space, and it allowed for lots of fitting to make the outer panels and center section all symmetric. Water levels are your friends at each stage.

Billski
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,013
Location
Memphis, TN
Answering the question, “Where did you come from?” Rutan may have been a rogue designer for homebuilts, but he had to have a starting place. In the US, you learn how to design airliners. Different is only different from a reference point.

Rutan concentrated on the composite for anyone. Someone else would have done it, but for history, you are only studying it because of him. Why come up with different wing mounts when a proven workable solution can be copied with not enough penalty to matter? Funny airplane shape out of plastic and sand is enough to tackle. Some constants have to be in play just to finish.

If he had come from competition European sailplanes, his frame of reference would be completely different and the EZ planes would not have been designed. Total different mindsets.

You already know more about it than he did at the time; you have an unfair advantage.

Fuel placement is always going to be a compromise. The EZs are all around planes. Most people concentrate on speed and range. It also has agility in its bag. Put the fuel in the outer panels and you will loose some agility and you add sloshing problems. Fuel in the strakes is an interesting compromise. Some lifting and low polar of inertia. Not fully supported by wing and not the lowest inertia placement, but you get OK and low GC change. They are separate from the main structure and yet part of the main structure.

With any design, what is brainchild and what is necessity to make work? It’s kind of like a Beatles song. You can enjoy it on the surface of a nice song, you can enjoy it for different cords played from normal and other unorthodox sounds, you can enjoy the production, you can enjoy the history. Pretty hard to believe it was meant to be played over a scratchy AM radio in mono. Also pretty hard to change it.
 

stanislavz

Well-Known Member
HBA Supporter
Joined
Aug 21, 2016
Messages
496
Location
Lt
Answering the question, “Where did you come from?” Rutan may have been a rogue designer for homebuilts, but he had to have a starting place. In the US, you learn how to design airliners. Different is only different from a reference point.
I had come from childhood :) Which teach me to think differently. And i still try think like that and - find, why others in one chosen way, but not another.

EZ, for me personally, look like "sum optimisation" - an example, you have profile which looks good on paper - high cl, low drag at you cruise cl etc.. but high pitching moment which in total plane Cd ( sum with tail cd for required pitching moment/ minus Cl from tail too etc ) - give your worse Cd
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,013
Location
Memphis, TN
Can your numbers fit into that shape? I think Rutan had general shape ideas but let the numbers push how it looks. The Vari Viggen does not look like the EZ even if it’s a canard. An EZ will force certain numbers. New numbers may force a different shape. Questioning is good. Not following the answers is bad if trying to constrict the outcome by some arbitrary fixed situation.
 

stanislavz

Well-Known Member
HBA Supporter
Joined
Aug 21, 2016
Messages
496
Location
Lt
Can your numbers fit into that shape? I
Easy build. With sacrifice on performance side. But not safety. And whole optimisation.

But not yet. Still have something to practice on (an tail composite section with fin/stabilator)
 

proppastie

Well-Known Member
Log Member
Joined
Feb 19, 2012
Messages
4,558
Location
NJ
I read somewhere a "laminar airfoils are better even when laminar flow is not maintained" I think it was Roncz.....
Rutan was a model builder as a young man, and the structural design of the EZ was patterned like building models.
Design is a compromise....there is "no one right answer" except..... is it strong enough? and how well does it work?
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,013
Location
Memphis, TN
For best numbers, slide rule is needed. No scientific calculators. My dad had a desk top adding machine that was about as big as a CRT computer monitor. If you put in 335x12, it would add 335 twelve times mechanically. The little printer and gears churning away.
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
11,224
Location
97FL, Florida, USA
FWIW, the highly vaunted “laminar flow wing” on the P-51, uses an airfoil that, in a wing tunnel, with a well made, smooth, section, achieves significant laminar flow. The actual wing on the P-51 did not achieve the hoped for benefit due to the as-built profile.


BJC
 

stanislavz

Well-Known Member
HBA Supporter
Joined
Aug 21, 2016
Messages
496
Location
Lt
My dad had a desk top adding machine that was about as big as a CRT computer monitor. If you put in 335x12, it would add 335 twelve times mechanically.
LOL I have it too now. Kids a playing with it now.

This one :
 
Top