Re: Dimples On Model Aircraft Could Greatly Extend Range

People with light aircraft often notice a black plastic tape applied to the prop's leading edges as minor erosion protection. As far as I can recall, it's not an STC item.

Brian W

See this:

STC yes; 337 no.

Ouch! If they want $74 for a tape, you bet it's STCed even as a minor mod (sorry for the cynicism - thanks for the URL)

Brian W

Oh it's Bret. IGNORE his stupidity.

You need laminar flow wings for best airliner etc efficiency.

Graham

Or in Airbus's case not using rivets at all as far as possible ! I think Boeing are catching up on that one too btw.

Graham

And just about most things else too.

Graham

Even the smoothest skins will still have flow separations leading to turbulent flow.

While the problem can be addressed, it takes a bit more than dimples to stick the boundary layer down tight over the entire wing.

Consider the X-21A program. It worked exceptionally well, but unfortunately proved to be unmaintainable.

X-21A

First Flight: April 18, 1963 Mission: Full sized test bed for testing "Laminar Flow Control" (also referred to as boundary layer control) theory Major Accomplishments: Proved that while Laminar flow control was possible, it was not feasible with existing technology. Power Source: General Electric J79-GE-13, 9,400 lb thrust max. Wing Span: 93' 6" Length: 75' 3' Weight (Loaded): 83,000 lbs Maximum Achieved Speed: 560 mph Maximum Achieved Altitude: 42,500 ft

Additional Information: Only two X-21's were built, and were actually highly modified Douglas WB-66D's. The X-21 was flown to test the "Laminar Flow Control" theory. The basic concept is that the exterior surface of the aircraft can be designed to create a slight suction during flight. Slots are incorporated in the aircraft's surface to produce the suction. Though the concept works, environmental considerations including rain, dirt, dust and other particulates required excessive maintenance on the aircraft.

The last known location of both X-21's was Edwards AFB, where they had been gutted of most instrumentation and left out of doors to deteriorate.

These are not mysteries and have been studied and initially implemented as riblets.

Forgive me, I have no idea what SM board is.

Not quite. As far as riblets go, it is my understanding that the height and spacing of the riblets is specified according to the boundary thickness such as to prevent the growth of turbulent bursts which causes an exchange of low momentum fluid near the surface with higher momentum fluid from above. This momentum exchange being a loss/drag mechanism. The other point of importance is the orientation of the riblets along streamlines.

The fundamental studies were done at NASA Langley Research Center in the late 70s/early 80s. The first open use of riblets was on the boat Dennis Connor (sp?) used to win back the America's Cup, being applied via a special 3M tape. The wind tunnel test articles from that study reside in the basement of the building next door to the one the branch I work resides. I my be wrong but It's possible the same the facility used in the study was also used for the work leading to the Speedo LZR Racer suit.

That's a lot different that your original point that it was not well understood. Also after the original riblet research was performed similarities to shark scales/skin were observed.

German researchers concurrently came up with the same conclusions from studying shark skin, they've continued with considerable efforts in 'printing' them into UV cured paints and other techniques. In any case the riblet effect has been known for over 50 years.

Riblets aren't the only way; hair and feathers have a similar effect.

"With Robert Brodkey, professor of chemical engineering, Koeltzsch has now turned his attention away from sharks, to penguins and seals. He hopes to determine whether hair makes these aquatic mammals more hydrodynamic. Initial studies by other scientists have shown that natural and artificial fibers can reduce drag by amounts that vary from 1.5 to 50 percent.

Continued research could show whether hair would improve the design of boat hulls and even airplanes, Koeltzsch said.

"Wouldn=92t it be something if, in the future, airplanes had hairy surfaces?" he asked. "

None of this is new. When I was into waterskiing way back in the '80s the best slalom skis had varied textures along the bottom. Toward the rear the surface was just a little rough, like 1000 grit sandpaper, which reduced drag on the water. Nearer the front, the surface was much smoother so that the skier, just by flattening the ski on the water, could slow down quickly. I've noticed that when we paint an airplane with the really shiny smooth urethane paints, it seems to lose a little cruise speed.

Dan

| None of this is new. When I was into waterskiing way back in | the '80s the best slalom skis had varied textures along the bottom. | Toward the rear the surface was just a little rough, like 1000 grit | sandpaper, which reduced drag on the water. Nearer the front, the | surface was much smoother so that the skier, just by flattening the | ski on the water, could slow down quickly. | I've noticed that when we paint an airplane with the really | shiny smooth urethane paints, it seems to lose a little cruise speed.

Depending upon the size of the aircraft paint can add a considerable weight burden. However, when the paint is really smooth it can drag along with it an increasingly thicker layer of turbulent air building toward the aft end of the aircraft. Dragging this thick turbulent boundary layer causes increased drag which seems counter-intuitive to smoothness. This is why various small surface patterns (seems to me somebody should try fractals) often decrease drag - they decrease the extent of the turbulent boundary layer thus the drag caused by it.

-- Gregory Hall

Dear Gregor Hall:

The external tank of the space shuttle used to be painted. They stopped for exaclty this reason.

... [increase] ...

... better to say that it allows for earlier boundary layer separation, essentially increasing the cross sectional area of the shape.

It sure does.

NASA had an aircraft where they sucked air into the upper surface of the wing, to try and create a "laminar" boundary layer. I think it took more power than they got extra-lift / reduced-drag.

David A. Smith

Probably reduced lift as well. Seems to me sucking air into the upper surface of the wing would degrade the low pressure caused by the Bernoulli effect because it would tend to slow down the sped-up air traveling across the top surface of the wing.

-- Gregory Hall

"N:dlzc D:aol T:com (dlzc)" wrote

I was reading about that one, just a few days ago.

It said one of the largest problems was keeping the vent holes open, from bugs and stuff, and rain changing the laminar flow.

What theory do you mean? It seems it's well understood how to design the riblets to minimize drag. If you mean the theory of the underlying fluid dynamics then I wouldn't hold your breath. We still don't have adequate turbulence models to begin with.

Not sure by what you mean by this. The requirements to design the riblets are known. What more do you want?

Fluids as it effects most every situation we deal with, including this one, deal with the macroscopic properties and not the microscopic or molecular properties of the media. There is no quantum relationship between the fluid and the surface in continuum flow which we are addressing here. That only becomes important in rarefied gas dynamics.

During WWII they decided to repaint the Spitfire with a flat matt finish paint so it would be harder to see at night. It worked, they were harder to see at night. They lost 20mph because of the increased drag of the matt finish paint. If you noticed a loss in cruise speed with a slick paint job, I would suspect your data collection procedures.

Paint can add considerable weight to the aircraft. What in the world makes you think a smooth surface causes the thickness of the boundary layer to increase? Have you been mislead by the installation of Vortex Generators to stir up the boundary layer?

Highflyer Highflight Aviation Services Pinckneyville Airport ( PJY )