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BLC wing small sport plane feasibility.

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gschuld

Well-Known Member
Joined
May 9, 2007
Messages
438
Location
Toms River, New Jersey
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.

https://repository.nwu.ac.za/bitstream/handle/10394/38061/25105280 A Smith.pdf?sequence=1
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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