BLC wing small sport plane feasibility.

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gschuld

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I’ve read a good bit about BLC (suction specifically) on everything from jets to gliders. Much of it was theoretical, CFD analysis, wind tunnel studies, university thesis papers, the extensive work of Boerman and others. There were a number of powered plane real world tests, much of which focused on jets and some on gliders. But I’ve seen very little if any real world testing on small highly efficient powered aircraft.

I imagine there is good reason for this:

1- most small powered aircraft lack the quality (accuracy and smoothness) laminar flow wings.
2- most small powered aircraft would almost certainly need to have all new wings made to account for the suction surfaces and layout, ducting, etc.
3- a consistent issue with boundary layer suction on wings(or other surfaces) that is all but impossible to solve is contamination of the wing surface orifices regardless of the specific layout. This makes it a hard sell for regular aircraft use(a constant maintenance headache). It seems more naturally geared toward a technology demonstrator use airplane. That greatly reduces the interest pool.
4- the air removed from wing must be re accelerated to flight speed to not incur a significant drag that combats the drag reduction of the BLC in the first place.
5- the air removed must be powered in some way. The more significant the force required, the more impractical and unsustainable this often becomes.
6- powered planes(tractor planes specifically) has the propeller disturbing the flow behind its wake, coming out at a somewhat shallow angle from its arc. But for the most part, the entire fuselage, the tail surfaces, much or all of the landing gear, and the area near the wing root will all be affected by this disturbed air. This leaves the majority of the wings only operating in “clean” air. I presume the area behind the prop wash will be at least somewhat less effected by BLC methods.
7- the added weight and complexity of the system over a non suction wing NLF wing.
8- there are many areas of an airplane that are far easier and cost effective to reduce drag. Cooling drag, fairings, air leakage drag, aerodynamic shaping, propeller design, minimizing exterior of airframe bits(antennas, hinges, anything). So anyone considering BLC on a small powered aircraft must as mush as possible optimize all the other parts of the low drag equation before even considering BLC.

I’m sure there are other reasons…but collectively it certainly makes sense that experimentation with BLC on small powered planes hasn’t taken off. Experimentation in gliders has been stunted by the fact that the rules for glider competition require no external power source. So even though Loek Boerman says the performance increase for gliders can be substancial, implementations that are against the competition rules hampers development.

So for the sake of science, lets consider a 620lb empty weight, high compression 0-200 with 125HP at 3300rpm, 20.5’ win220mph at sea level at 3300rpm side by side low wing tractor taildragger fixed gear aircraft. Not Paulo Iscold/Klaus Savier level specs or performance, but a realistic starting point.

There is a lot of energy in the FWF. Using the engine exhaust speed and volume to create a vacuum source per side is not a compilated concept. Optimizing output and determining true vacuum and volume numbers would take some work.

But, without question, a constant source of vacuum that is naturally accelerated well beyond the flight speed at minimal weight added is doable. Yes, vacuum will vary on rpm, with peak at WOT. But we are talking about cruise rpm being 60-80% WOT.

The question becomes, is the exhaust driven vacuum source sufficient to “power” meaningful BLC suction on the wings? I’ve read reports from massive to minuscule the vacuum required for efficient BLC from Jets to gliders.


The link above referenced suction on a glider tail. Testing at 40 m/s (about 90mph) suggested that 1mm holes at 10mm spacing across the span at that speed was quite effective. And one row at 70% chord made a worthwhile improvement in laminar flow with the airfoil used. Improvements increased up to 4 rows of holes but diminishing noticeably by then.

The suction required to make this work was quite small, with only a tiny exhaust extractor in the low pressure area on the tail sufficient to pull enough vacuum. The tail airfoil reported a 20% drag reduction but the extractor added enough drag to counter it completely.

Let’s take a fairly modern wing airfoil, the AS5045 (pic below). I’m not certain, but I’d guess 55-60% laminar up top and maybe 70% on the bottom(pulling numbers out of thin air)

If a new pair of wings (from the fuselage out) were made with BLC in mind, using as an example the 1mm holes at 10mm spacing of the linked study, I’m wondering how many rows of suction holes per wing I could power per exhaust driven extractor to be useful at 200mph.

FWIW, average chord(tapered wing) is .85m or 33”, 90m/s velocity or 200mph, for an average Reynolds number of 5,400,000.

George
 

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gschuld

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FWIW, as an example, If I had 3 rows of 1mm diameter suction holes at 10mm spacing on top, and 2 rows of 1mm at 10mm spacing on bottom of each wing at 8’ along the span, I’d have the equivalent of 1.5 sq inches(9.7 sq cm) a 1.38”(3.50cm) diameter hole.

I’m guessing that is not a huge volume to evacuate at low speed with an extractor powered by a high speed 1.5” exhaust pipe.
 
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gschuld

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Regarding Synergy, I have, it’s been a few (bunch of) years though. Back to when John started building his project.

He was at the time being a bit protective of the details behind his theories. But for sure John was thinking along similar lines regarding exhaust driven suction, along with a host of more ….controversial concepts. (Please no Synergy related thread drift 🙏)

I was reluctant to bring up the example as anything Synergy related tends to get toxic quickly for a number of reasons. I have concentrated heavily on researching primarily conventional proven methods of drag reduction, performance improvements (outside BLC stuff). Though undoubtedly BLC isn’t unproven as a concept. But it’s practical real world use application in small tractor ICE powered aircraft is a bit of a mystery.

George
 

ShindenKai

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The most kick-ass aircraft EVER made with BLC, on ALL control surfaces too (used a 5th turbine embedded in fuselage..)!! Probably the largest STOL aircraft ever created as well. The Shinmeiwa PS-1 and later US-2

PX-S STOL Flying Boat.mpg - YouTube

Fun facts: Shinmeiwa/Shinmaywa is formerly Kawanishi... aka made the H6K Mavis and H8K Emily flying-boats during WW2 and the same guy that designed both of those aircraft also designed this beast.

P.S.- Yamaha + Shinmaywa have apparently joined forces to build a small GA type of aircraft (kinda old news, actually)

Yamaha Teams With Shinmaywa To Provide Engine for New-Generation Light Aircraft (news18.com)

-Not tryin' to thread jack-
 
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Victor Bravo

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Slight OT, but I had a sailplane that attempted to use BLC. It was blowing, not sucking, which is of course a different thing, I know. Mine was the AS-W20BL sailplane, with the super whiz-bang Horstmann & Quast airfoil, which used a thousand hypodermic needles embedded in the lower wing surface at 80-85% chord. A plenum pressurized with a separate ram air pitot provided the pressure and airflow.

This airfoil worked great in the wind tunnel, tested and blessed by no less than the very famous aerodynamics researcher Loek Boermanns at Delft University.

Out on the competition circuit, in the real world, it wasn't any better than the previous model of the same glider (which I had sold to buy the new advanced version).
 

gschuld

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The PX-S is cool, thanks for the link.

On the glider, that’s fascinating. Shame on the disappointing result in the air. Hypodermic needles….that’s ingenious in and of itself.

Talk about tiny jets of air….and that air had to be pressurized from somewhere, enough to push out all 1000 tiny holes at great enough velocity to make any meaningful effect on re invigorating the turbulent flow at 85%.

I’d be happy to hear more details on this for the sake of BLC science.

George
 
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Victor Bravo

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All you actually need to know is that the guys who had these sailplanes (IIRC I think the Glaser-Dirks DG-300 also had them) spent a lot of time with little pieces of wire cleaning out these millions of blow holes after every time we waxed the wings, or sanded them, or flew in a dusty environment.

The tiny tubes were bonded into the wing skins at a certain angle too, such that they swirled little micro-vortex air jets. I have no idea how there could be enough ram pressure and mass flow into a 3/8 inch pitot probe to accomplish whatever HQ or Boermanns needed. I know they would not have built the gliders with this airfoil if it didn't work in the wind tunnel, because it added extra parts and a lot of work to create the plenum and the micro-jets.

(way OT, sorry, disregard if irrelevant)
The B model glider was introduced in 1984, and the little blow holes were cutting edge aero at that time. There were some other significant differences between the A and the B besides the airfoil; the B wing was a lot heavier and less flexible, because they wanted it to carry a lot more water than the A model. The B was specifically built for a strong weather National or World championship in Hobbs NM, they were flying at 11 or 11.5 pounds a square foot, something like 45-50 gallons of water in the wing bladders. We even modified my B model with a 2 gal. ballast tank in the tail to keep the GC at 105-110%.

But the price for being able to carry this much ballast was high, the wing panels of the B each weighed 125 pounds or better, they put in carbon tow I think, and it lost that magic flexi-diving board-rubber band wing that we loved so much. You could see people desperately fleeing away from your line of sight when you opened the trailer and started looking for a helper to put the glider together. It was less of a problem when you needed to put the glider back in the trailer later, because the people who watched you make a "contest finish" (blasting across the finish line at 140 MPH in low ground effect with long graceful trails of water vapor flowing behind you like contrails) were just hypnotized and wanted to come see the glider :)

The A model flew the best out of all of them hands down. The B was faster and heavier but lacked the "sportscar" handling. I only have seven or eight flights in the C model (loaned to me for the '87 Nationals at Barstow after 25 of our gliders burned up in a hangar fire). The C model had the new airfoil and the blow holes, but it had the lighter more flexible wing structure so it handled almost as good as the A.

The B and C models also had 40 degrees of "landing flap" instead of the A model's 55 degrees. So when you had to go into a small field, parking lot, backyard, etc. you had to have a smidge m ore room available. The reason for the reduction in landing flap wasn't structural or aerodynamic, BTW, it was because at 55 degrees the flaps would try to peel off the wiper seals at the flap/wing junction! We were always having to re-glue the Mylar back on when we put wiper seals on the A models!
 

gschuld

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The maintenance needed in unclogging all 1000 holes every time the wings are waxed is an almost comical example of the real world limitations of the BLC issue.

I’d be curious to know the ID of those particular needles FWIW.

It would help greatly for me to get a feel for real world wing drag reduction potential with suction over a decent laminar flow wing airfoil itself.

The wings generally account for 25-30% of total drag on a really clean aircraft. So by reducing the wing drag by, perhaps, 30% will have a not earth shattering but a noticeable reduction of total airframe drag in a 225+ mph aircraft.

Top modern glider airfoils now have reported laminar flow around 90-95% lower surface and 70-75% upper surface. Frankly, that doesn’t leave much room for improvement, never mind the complexity of systems and competition rules restrictions. Granted the laminar flow wasn’t nearly as extensive 20-30 years ago.

Tractor ICE sport plane 15% airfoils as far as I know, do not have near the real world laminar flow amounts compared to top gliders. Perhaps 55-60% on top, not much better below unless we are talking about pure race plane airfoils with significant shortfalls outside their confined perimeters.

Judging viability IMHO needs to start with:

What is the potential performance gain.
A- how much drag reduction is feasible in the wings
B- how much cost in weight/complexity
C- how much suction can be provided by the engine.

If there isn’t enough incentive from A, nothing else really matters beyond a mental exercise.

George
 

WonderousMountain

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Place the Vacuum Turbine? In Wing Tip Pods,
with Independant systems & engine pto split
to each side. Using a hollow section Carbon
Aft spar would cover nicely the construction
Symmetry expected. Just close off the end,
with hardware joints. Another option would
be a 'working' strut tube with vacuum drawn.
 

Vigilant1

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Just to make sure you've seen it, here's an old (1947) NASA document on the impact of BLC on small aircraft (4-5 seat) TO and landing performance Boundary Layer Control
Takeaways:
1) BLC has more relative value at higher aspect ratios
2) BLC may reduce stall speed by about 25%
That last one means a lot less energy on landing and might be more significant in many applications than any prospective reduction in cruise drag.

The discussion so far is a bit focused on drag reduction, but BLC can have other utility.
 
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gschuld

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Vigilant1,

Yes I’ve read that one. And for sure BLC can benefit low speed lift (stall) greatly as the suction helps keep the flow attached at a surprisingly high angle of attack.

I was just trying to keep focus on drag at cruise speed to WOT.

thanks for the thoughts,

George
 

Victor Bravo

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If BLC could be accomplished in the real world, with a net reduction in drag (more drag reduction than extra power required to pump air), they would have it installed on every glider in the world.

If it created enough reduction in drag even to pay for an engine-mounted air pump... it would be installed on every airliner in the last 60 years, with drooling, cigar-smoking bankers and VC's holding loaded guns to the heads of every Boeing and Airbus engineer at their drafting boards.

If the Spanish Fly pills and pheromone sprays offered in the back of men's magazines for the last 50 years worked as promised, I would have had an entirely different experience in high school.

Some things sound better than they actually work :(
 
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Slight OT, but I had a sailplane that attempted to use BLC. It was blowing, not sucking, which is of course a different thing, I know. Mine was the AS-W20BL sailplane, with the super whiz-bang Horstmann & Quast airfoil, which used a thousand hypodermic needles embedded in the lower wing surface at 80-85% chord.
Yeah, the BLC didn't help the -20 much, but the B model had the best landing gear of all the ASW-20 series. You could land out with confidence, though you could still knock the tail off if you snagged a wingtip while the tail was still up...
 
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Jim Bede drilled a bunch of wee holes in his BD-2 (pusher with a ducted fan) wings to try to prolong laminar flow. As far as I know, only one was ever built, and she was donated to the EAA in the early 70's and stored in Oshkosh. I'm not sure where she is now, nor what Bede learned from his experiment.
 

Victor Bravo

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Yeah, the BLC didn't help the -20 much, but the B model had the best landing gear of all the ASW-20 series. You could land out with confidence, though you could still knock the tail off if you snagged a wingtip while the tail was still up...

Come to think of it, you're probably right. The 1157 pound B model gear was probably heavier duty than the 1000 pound A or C model's gear. I never tested it thankfully, always managed to dump all the water before I landed out.

Klaus Holighaus landed his 1200 pound Ventus with all the water still in it every day during the nationals at Uvalde. I asked him why he did that, and he said "far more convenient"! I guess if you wrote the manual you weren't required to read the manual :)

I did manage to pull the main gear out of my Mini-Nimbus on a dirt road though...
 

gschuld

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Gliders ran into rules issues. They are not allowed any external power source, among other restrictions.

I agree that BLC is not practical. But I will contend that the concept is capable of an improvement in performance(though small) in the right circumstance, albeit with a totally unacceptable clogging/maintenance issue beyond a demonstration type situation. And it would likely have to be designed and built by someone far more knowledgeable on the subject that I am. And on an airplane layout that would most benefit from the BLC.

George
 
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