Wake propeller.

[tai626]

This is a wake propeller:
http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=AD0323899

If it is designed right, it "reduces" the fuselage drag by 90%!

Just wondering if anyone has modeled it.

[tai626]

This is insert from:

AERODYNAMIC DESIGN OF LOW-SPEED AIRCRAFT WITH A NASA FUSELAGE/WAKE-PROPELLER CONFIGURATION
FABIO R,GOLDSCHMIED * MONROEVILLE PA 15146
AIAA 86-2693

The concept of body wake regeneration for propulsion originated in 1865 with Froudel; his thinking was based on the overall momentum balance of the moving vehicle and he believed that the Rankin drag/thrust concept was an anachronism from the days when canal barges were towed by horses. When a force in one medium must be overcome by power input in another medium, or more generally when there is an impedance matching problem, the drag/thrust concept may have great merit. In steady motion through a single fluid, however, as with an aircraft, a LTA or a submarine, the drag/ thrust concept is misleading in its apparent simplicity and it invariably results in the adoption of reduced performance targets. For instance, when a fuselage is said to have a certain drag at a given speed, it is implied that the fuselage wake's momentum is condemned to useless dissipation, without possible recourse of any kind; it is implied that the drag can only be balanced by an equal propeller thrust, according to the Rankin concept. Still today, general aviation aircraft. are viewed essentially as powered gliders, with the thruster (propeller or jet unit) installed in a manner not conducive to efficient wake regeneration. Even fuselage-mounted pusher propellers are too large and are not tailored to the specific wake. The rational approach is to follow Froude's concept' and to expend power to prevent or to minimize the occurrence of the wake. Smith and Roberts,* Kuchemann and Weber,3 Edwards," 5 David- son,6 Goldschmied7 and many others have contributed t o the development of wake regeneration, with and without active boundary-layer control. For instance, Goldschmied7 8 has shown that, for an axisymmetric fuselage, the wake drag can be reduced by a factor of 10 with an efficient single-slot suction boundary-layer control aftbody design; the overall power was reduced by a factor of 2, for equal fuselage volume and speed.

[Airboatflyingshp]

Can you show diagrams of these please?

http://www.aircraftspruce.com/catalog/appages/vortelator_kit.php - might be of interest.

[nmasters]

Here's one. A ducted fan could provide the suction but I don't think it would give enough thrust for a plane with a fuselage this size.

--Norm

[tai626]

AIAA 86-2693 used to be free to download at www.cafefoundation.org

I did some drafts of wakes. By my understanding, the wake propeller "squeezes" the airship rear using the drag wake.

My goal is a long range electric model airplane. It should cruise at 55 mph efficiently.

I am just wondering if someone has modeled it before I invest more time on it.

Tai

[bwalt822]

nmasters, what you have posted there looks like a form of boundary layer suction, is that correct?

Is that what this paper is talking about? Are we talking a wake due to separation or just the wake from normal turbulent flow?

Once the wake has been created, the damage is irreversibly done. It may be possible to create some sort of propulsion that functions more efficiently in a wake but this would be highly impractical and wouldnt reduce drag. It would only increase the efficiency of the engines.

[tai626]

Here's one. A ducted fan could provide the suction but I don't think it would give enough thrust for a plane with a fuselage this size.

--Norm

That is exactly what I thought! My design is a in line bi motor airplane. The front motor is a normal big folding propeller to take off and the rear one is a pusher fan to power the airplane once it reaches cruise speed.

[bwalt822]

AIAA 86-2693 used to be free to download at www.cafe.org

I did some drafts of wakes. By my understanding, the wake propeller "squeezes" the airship rear using the drag wake.

My goal is a long range electric model airplane. It should cruise at 55 mph efficiently.

I am just wondering if someone has modeled it before I invest more time on it.

Tai

When you show the streamlines diverging like that behind the plane you are implying separation. Most planes do not have large separation areas which would make this sort of device kind of useless. This is essentially a form of boundary layer control.

This could allow for increased fuselage volume if you couldnt make the fuse longer instead but otherwise it has very limited functionality.

[tai626]

When you show the streamlines diverging like that behind the plane you are implying separation. Most planes do not have large separation areas which would make this sort of device kind of useless. This is essentially a form of boundary layer control.

This could allow for increased fuselage volume if you couldnt make the fuse longer instead but otherwise it has very limited functionality.

Thank you for your explanation on the streamlines. So this theory only works on fat and short fuselage...How about Skymaster 337?

[nmasters]

I did some drafts of wakes. By my understanding, the wake propeller "squeezes" the airship rear using the drag wake.

You've overestimated the size and position of the wake. Until you get past the trailing edge it's just the boundary layer and the flow outside of that is undisturbed. Right at the TE the disturbed region jumps to about the thickness of the body or a bit less. What you're trying to do is accelerate the BL back up to the free stream velocity before it gets dumped overboard where it mixes with the free stream and becomes the wake. The Cessna Skymaster does this to some extent, that's why its single engine performance is so different depending on which engine is out. If the aft engine is out performance is much worse than if the front engine is out. Basically they are using the aft propeller to clean up a very bad aft fuselage shape.
[/QUOTE]

The drawing I posted earlier includes a feature labeled "Ringleb cusp" but doesn't show details of it.
I posted a drawing of a Ringleb cusp in this thread (http://www.rcgroups.com/forums/attachment.php?attachmentid=2266751) . Good luck and keep us posted.

--Norm

[nmasters]

Hay!!! Slow down, I'm typing with one finger on my left hand

[tai626]

I will. Thank you!

[nmasters]

I will. Thank you!
Naw, brain storm all you want. I was just surprised to see that 4 messages came in while I was typing

[tai626]

I will keep you posted! Thank you!

[Tulo]

It's boundary layer control at work. Goldschmied was a smart guy. Here's design that used it successfully (though not commercially):

http://www.avinc.com/uas/adc/broomstick/

[tai626]

Wow. That's a silly airplane!

Waaaay too advance to me.

I am trying something easier, inspire on Rutan's Voyage: an in line bi motor, capable of 2 hours flight at 55 mph average. 2 models come up - Cessna Skymaster 337 and Rutan 74 (both .40 size). I hope I can model this boundary layer control and get some extra miles :-)
Thank you all for comment.

[dhaverlah]

I didn't read all of this thread (yet). But, from what I remember in fluid dynamics, the boundary layer on a 747 is in the realm of 12-24 inches at the extreme aft end. I'll read this thread and probably kick myself later for this post, but this is my initial input for discussion...BTW the kids (& me) need to go to bed now.
DH

[nmasters]

from what I remember in fluid dynamics, the boundary layer on a 747 is in the realm of 12-24 inches at the extreme aft end.
I've heard that the BL is ~7+ at the start of the tail cone so that doesn't sound unreasonable to me. Given the forum that he has posted in I had just assumed that tai626 was contemplating something a bit smaller. :D

[nmasters]

nmasters, what you have posted there looks like a form of boundary layer suction, is that correct?
It is boundary layer suction combined with a thick pressure recovery fuselage. Notice that the fuselage aft of the suction slot is concave. When the BL is sucked off the potential flow swings around the shoulder and plasters itself against that surface and you get a favorable pressure gradient all the way to the jet outlet, thus practically no wake (or at least that's how I understand it's supposed to work). I visualize the flow as like an aerospike engine. Some of Fabio Goldschmied's papers are hosted on the CAFE foundation's web site (http://cafefoundation.org/v2/pav_tech_lib.php) . I think thias one is titled "Shaping of Axisymmetric Bodies for Minimum Drag In Incompressible Flow". I've seen discussions in other fora suggesting that this may have to be attached to the wing by struts because the wake of the wing is too disrupted. If that's correct then even struts may be a problem if they're not faired in properly. This apparently can work with submarines and torpedoes (http://www.solarnavigator.net/torpedoes.htm) but I haven't seen it on an airplane

--Norm

[markdrela]

Once the wake has been created, the damage is irreversibly done. It may be possible to create some sort of propulsion that functions more efficiently in a wake but this would be highly impractical and wouldnt reduce drag. It would only increase the efficiency of the engines.
Not quite. The real goal is to reduce overall viscous dissipation of power, which ultimately must come from the fuel or batteries. Dissipation occurs in the surface boundary layers, in the wakes, and also on prop or compressor blades.
For a well-designed airfoil or fuselage the dissipation fractions are roughly 85% on the surface and 15% in the wake. A wake-ingesting propeller or jet engine can in principle eliminate the wake dissipation, but not the surface dissipation. So a "perfect" wake-ingesting propulsor can in principle reduce power by about 15%, but no more.

However, the nonuniform wake might increase dissipation of the prop or engine itself (more blade profile drag), which will eat into the 15% benefit. On the other hand, a wake-ingesting engine needs to send out a slower propulsive propwash or wake, which will increase the 15% figure.

If there are boundary-layer ingestion systems onboard, the dissipation inside the flow passages chews up power just like in the outside flow. So it doesn't make sense to suck off the boundary layer unless it reduces dissipation elsewhere, via delay of transition or separation for example.

[tai626]

I've heard that the BL is ~7+ at the start of the tail cone so that doesn't sound unreasonable to me. Given the forum that he has posted in I had just assumed that tai626 was contemplating something a bit smaller. :D

Much, much smaller :-)

If the real fuselage speed is 140 mph, and I want 55 mph on my model, according to Froude's number (V/(scale)^1/2), my model should be around 1/6 to 1/7 scale.

[tai626]

CAFE Foundation and NASA is organizing a contest. The prize is over 1.5 million $$$ to the airplane that can achieve the efficiency of 200 statute mile-passanger per gallon of fuel, with minimum cruise speed of 100 mph, if I remeber correctly. In 2011, we will see some really efficient designs, maybe a wake propeller and Goldschmeid's fuselage.

[enduro_man996]

CAFE Foundation and NASA is organizing a contest. The prize is over 1.5 million $$$ to the airplane that can achieve the efficiency of 200 statute mile-passanger per gallon of fuel, with minimum cruise speed of 100 mph, if I remeber correctly. In 2011, we will see some really efficient designs, maybe a wake propeller and Goldschmeid's fuselage.

but one can not beat a magical broom stick... :rolleyes:



Sorry, I could not resist after checking out the awe inspiring broomstick design. :o

[bwalt822]

CAFE Foundation and NASA is organizing a contest. The prize is over 1.5 million $$$ to the airplane that can achieve the efficiency of 200 statute mile-passanger per gallon of fuel, with minimum cruise speed of 100 mph, if I remeber correctly. In 2011, we will see some really efficient designs, maybe a wake propeller and Goldschmeid's fuselage.

Shouldn't steve fosset's around the world plane have done this?