Revisting thicker skins for laminar flow

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

stanislavz

Well-Known Member
HBA Supporter
Joined
Aug 21, 2016
Messages
397
Location
Lt
Found an older post on this subject, hence the subject line
Question:
Are laminar flow wings/fuselage always impossible with aluminum construction regardless of construction methods?
Does no construction method exist to solve this issue?
Prebent/molded thicker wing skins, aluminum over foam or other substructure, spar less, high AR wings with small distance between front and rear spars etc... does nothing solve this problem?
Understand the solution may not be worth the effort or weight, but still wondering if a solution exists.

Thanks in advance
I had asked similar question. And the answer is just google for "laminar airfoil waviness"

And the biggest issue is any local imperfection like rivet, or panel joint. Not some kind of flexible panel..

Structure - if no fuel in tank - foam cored wing with carbon fiber is the lightest option. But it will need some filling/sanding for glass like surface. Here are some calculations

Wing design for lightest possible wing
 

Norman

Well-Known Member
Joined
Nov 28, 2003
Messages
2,934
Location
Grand Junction, Colorado
The Questair Venture had thick, one piece stretch formed wing skins. No panel gaps.
The rivets can be filled with bondo. But I don't know if they bothered with a laminar airfoil. Top speed 300 mph.
Bondo is too brittle, it'll crack and ruin your paint job (wings flex a lot more than car bodies). Dry micro is somewhat tougher but a plasticizer wouldn't be a bad idea and maybe even a bit of flox. But in either case aluminum oxide will eventually creep under the filler and pop it loose if the surface isn't properly passivated. Steps, like the edges of flush rivets, are limited by the speed of the aircraft. At 100 mph it can be up to 0.012" (about 1/3 millimeter) but at 200 mph the allowable step height drops to about 0.006 inches. Puting a flat rivet in a curved surface creates steps and filing them down might freak out your A&P guy.
 

BBerson

Well-Known Member
HBA Supporter
Joined
Dec 16, 2007
Messages
12,826
Location
Port Townsend WA
I owned a professional aircraft paint shop. Bondo works fine on aluminum aircraft when thin, as usual.
 
Last edited:

lr27

Well-Known Member
Joined
Nov 3, 2007
Messages
3,729
Everyone is writing as if laminar flow is on or off. However, just about every wing will have a little laminar flow near the leading edge. The question is how far back it can be preserved.
 

Norman

Well-Known Member
Joined
Nov 28, 2003
Messages
2,934
Location
Grand Junction, Colorado
Everyone is writing as if laminar flow is on or off. However, just about every wing will have a little laminar flow near the leading edge. The question is how far back it can be preserved.
Fair enough. Here's an XFLR5 screen comparing a clean 10% thick older section (NACA 23010) compared to a more recent 15% thick laminar section (Riblett GA35A415), clean and dirtied up a bit. I've added some annotation for those readers who may not be familiar with this program. XFLR5 is a panel code like the Epplar code that Harry Riblett used to design his airfoil sections. These programs aren't reliable when there's extensive separation ie post stall but they're fairly accurate within the linear range. The lower left graph shows the chordwise point where the boundary layer transitions from laminar to turbulent. You can see that even this old NACA airfoil can have a lot of laminar flow at low AoA if it's clean enough but the transition point moves forward very fast with increasing AoA ie the transition ramp is very long. The riblet airfoil has a short transition ramp and the transition point stays relatively stationary up to near CLmax. This small transition zone is the defining characteristic of "laminar" airfoil cross sections. Anything that sticks up some fraction of the boundary layer thickness (rivet heads, squashed bugs, rain drops) will make waves (called Tollmien–Schlichting waves) in the BL that will cause early transition that fans out in a 14 degree triangular wake (called a turbulent wedge). When the BL flows over a wave in the skin that's above a certain length and height (Reynolds number dependent) that will also start a TS wave. Generally if you can feel a bump, wave, or flat spot (a flat spot counts as the trough between 2 waves) it's too big. clean-vs-dirty.png
 

dog

Well-Known Member
Joined
Dec 29, 2019
Messages
253
This topic and other considerations got me to ask the question"how big an extrusion can be made"
and of course forget right now ,but I think 26 inches,next question that I wasnt able to answer is
how thin the webs can be.At least one kit plane is doing the leading edge as an extrusion.
Sections dovetail,weld the joint,finish smooooth.
So laminar flow wing by the mile?
How much u wan?t
Must be a catch or things like ailerons and flaps would already be cut to length.
 

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
7,373
Location
North Carolina
An extrusion of the required aspect ratio won't be accurate enough. It will distort as it cools. It would have a horrific setup cost, too.
 

stanislavz

Well-Known Member
HBA Supporter
Joined
Aug 21, 2016
Messages
397
Location
Lt
This topic and other considerations got me to ask the question"how big an extrusion can be made"
and of course forget right now ,but I think 26 inches,next question that I wasnt able to answer is
how thin the webs can be.At least one kit plane is doing the leading edge as an extrusion.
Sections dovetail,weld the joint,finish smooooth.
So laminar flow wing by the mile?
How much u wan?t
Must be a catch or things like ailerons and flaps would already be cut to length.
Where was an link in HB to article with glider design paper built from extrusion, and one tandem wing airplane build from wing extrusion.

upload_2020-2-4_12-45-25.png

here it is : https://books.google.lt/books?id=MYJWAAAAMAAJ&pg=PA419&lpg=PA419&dq=glider+designed+from+extrusion&source=bl&ots=mVMSlA3R13&sig=ACfU3U1i923ySvZj8srpeq16P-TbvUyzzw&hl=en&sa=X&ved=2ahUKEwjW-ICG6bfnAhUNxqYKHUPMBjAQ6AEwC3oECAYQAQ#v=onepage&q=glider designed from extrusion&f=false

It shall be a no-no option for lightest ones, with low a/r, but quite ok, for faster airplanes/sailplanes. And if you go for al-li alloys - it shall cover even more.
 

dog

Well-Known Member
Joined
Dec 29, 2019
Messages
253
ok,then how does piaggio get the wing on the three
lifting surface pusher to work?
all objections and other considerations aside,just look at the size of the wing compared to the fuselage and its clear that they have got an unusualy efficient wing,and fueselage combo.
want laminar flow,give laminar flow now,want
 

stanislavz

Well-Known Member
HBA Supporter
Joined
Aug 21, 2016
Messages
397
Location
Lt
It as all depends on sum of totall. Even Wittman Tailwind, have a really tiny wings, buts have performance compared to best composite aircrafts with laminar foils. Due to nice flow over fuselage.
 

lr27

Well-Known Member
Joined
Nov 3, 2007
Messages
3,729
Seems to me that really stable epoxy might reduce the maintenance. Ditto for that extruded wing. Still, an extruded wing will have a lot of excess,weight.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
4,565
Location
US
Anything that sticks up some fraction of the boundary layer thickness (rivet heads, squashed bugs, rain drops) will make waves (called Tollmien–Schlichting waves) in the BL that will cause early transition that fans out in a 14 degree triangular wake (called a turbulent wedge). When the BL flows over a wave in the skin that's above a certain length and height (Reynolds number dependent) that will also start a TS wave. Generally if you can feel a bump, wave, or flat spot (a flat spot counts as the trough between 2 waves) it's too big.
Norman, thanks.
So, this addresses localized discontinuities (bumps, dips, etc). Is there similar guidance/rules of thumb regarding how accurately the entire profile needs to match the "as drawn" airfoil if the wing is to have the same laminar flow properties as the "as drawn" airfoil? To give an extreme example: the skin of a tightly-stretched Monokote wing might be very smooth in the chordwise direction (along the air's path), but the airfoil shape between the ribs will not be the same as the rib airfoil.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
6,314
Location
Saline Michigan
You guys have been addressing the static shape of the foil and how that can trip laminar flow to turbulent pretty well. The other issue in our little airplanes is that with moving air outside the foil and stationary air inside, these airfoils become somewhat inflated in flight. You can watch this happen when up in any sheet metal winged airplane - the skins bulge between lines of rivets. The size of the bulge is quite large compared to the max waviness allowable to maintain laminar flow.

Let's say you can only afford 0.006 waviness, and you built to 0.004 ( that is tough to achieve in metal construction), and the base skin bulges 0.020" in the 2" near an underlying stiffener or other reinforcement - you would need to make the skin at minimum 10 times stiffer to just get to the max waviness. A minimum of 215% thickness of the original skin. If it was 0.025, it would have to go to 0.054, which does not exist, so 0.060. And that won't do you any good unless you can achieve fiberglass level waviness in build of this sheet metal airplane.

Next, just because you have stiffer metal skins does not mean you can afford to spread out the ribs to make up for it, skin stresses go with thickness squared and panel size squared. This going to give you a weight gain...

In little airplanes, the route to extensive laminar flow at reasonable weight is composites with cored skins, then profiling work.

Billski
 
Last edited:

dog

Well-Known Member
Joined
Dec 29, 2019
Messages
253
The other issue in our little airplanes is that with moving air outside the foil and stationary air inside, these airfoils become somewhat inflated in flight. You can watch this happen

just because you have stiffer metal skins does not mean you can afford to spread out the ribs to make up for it, skin stresses go with thickness squared and panel size squared. This going to give you a weight gain...

In little airplanes, the route to extensive laminar flow at reasonable weight is composites with cored skins, then profiling work.

Billski


Do you have a number for the pressure difference
between the inside and outside of a wing?
Just curious ,it must be a fraction of one psi, and vary with construction and airifoil.
.5 psi=72 lbs ft which might be a design consideration in its own right.
 

Norman

Well-Known Member
Joined
Nov 28, 2003
Messages
2,934
Location
Grand Junction, Colorado
Do you have a number for the pressure difference
between the inside and outside of a wing?
Just curious ,it must be a fraction of one psi, and vary with construction and airifoil.
.5 psi=72 lbs ft which might be a design consideration in its own right.
It depends on where the wing leaks. If the wing is hermetically sealed the internal pressure will be equal to where it was built but that probably never happens. Control linkages always open up into the fuselage and that'll usually be your biggest wing leak. Next biggest leaks would probably be where the surface control push rods leave the wing. Fuselage internal pressure might be pretty close to free stream pressure but probably won't be. Since the skin panels are arches and arches withstand compression a lot better than they withstand tension it might not be a bad idea to make sure that all your major leaks are in low pressure areas. With fabric wings it's a different matter. Fabric can't help but bulge or sag when there's a pressure differential and laminar flow can not persist past the edge of the D-tube if the skin sags however a bulge would be slightly less bad. Generally spar print-through is the end of laminar flow regardless of materials or stiffness.
 
Last edited:

lr27

Well-Known Member
Joined
Nov 3, 2007
Messages
3,729
There are ways around print through, though I suspect anything like that would increase the weight of the spar. Wondering about a metal-foam-metal sandwich, though I suppose the skins would need some kind of special treatment for an adhesive to stick. Might be relatively lower maintenance.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
6,314
Location
Saline Michigan
Sounds like a consensus.

The pressures outside go with local outside velocity squared. Even pressures we might consider really small, each panel defined by ribs and spars and the like are hundreds of square inches. It does not take much to deform a thin aluminum skin enough to destroy laminar flow. Yeah, you could attempt to vent the wing insides to a low pressure place. I am still suspicious that the metal skins will be too flexible to maintain laminar flow....

Billski
 
Last edited:

stanislavz

Well-Known Member
HBA Supporter
Joined
Aug 21, 2016
Messages
397
Location
Lt
Sounds like a consensus.

The pressures outside go with local outside velocity squared. Even pressures we might consider really small, each panel defined by ribs and spars and the like are hundreds of square inches. It does not take much to deform a thin aluminum skin enough to destroy laminar flow. Yeah, you could attempt to vent the wing insides to a low pressure place. I am still suspicious that the metal skins will be too flexible to maintain laminar flow....

Billski
Blanik L13, have laminarflow and all metal wing..

https://en.wikipedia.org/wiki/LET_L-13_Blaník
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
10,505
Location
97FL, Florida, USA
2
Group Builder
Top