Vortex Generators

[RC Storick]

I got a great idea why don't you build a website and turn it into the best in the world and I will come there and bash you?

Bashing you? No that wasn't bashing you, but disrespecting opinions and idea's ranks low on my list of qualities I admire..

Sure thing - Ive got a better idea, why dont you build a take apart plane travel 1/2 way across the world - Qualify for Top 20 Finals and bash me there ?

Exact quote from a NASA engineer they are the wrong shape and in the wrong place to do much good at low speeds. As far as coming there not a chance. You said everything I do is a Joke. You wouldn't call that bashing?

[Steve Thomas]

Going back to Dave's experiences, I'm wondering if it wasn't simply a case of too much angle on them?  If you think of them as like a tiny wing, then an appropriate angle of attack will give you low pressure on one side, high pressure on the other, and a resulting vortex round the tip.  But too much AoA , they stall, and the system breaks down - you don't get the pressure differential you're trying to create, and just end with an aerodynamic mess and a bunch of drag.  So stalled VGs wouldn't give any benefit to the boundary layer (and would bugger it right up instead), and would certainly hurt the glide by virtue of the additional drag.  Any chance you could try them again with less angle, Dave?

[PJ Rowland]

Exact quote from a NASA engineer they are the wrong shape and in the wrong place to do much good at low speeds.

It doesn't really matter who said they are wrong ...  - They work : FACT and Countless Open Nats #1 trophies are sitting in the rooms of guys who agree enough to sprinkle them on their setups.

On another point of order - I don't see many people using Fibrous refractory composite insulation on their stunt planes - perhaps ask him if he thinks that's of any use for us.


" You said everything I do is a Joke. You wouldn't call that bashing? "

If we are exact quoting.. that's not what I said..

" Sure, I knew it was a joke - most of your stuff is. "

Most isn't everything.

It was not a personal attack or a bashing - it was simply a double entendre , said with intent ,reflective of the initial pretext statement of being "a Joke" - perhaps my sense of humor runs a different axis.

[Howard Rush]

Exact quote from a NASA engineer they are the wrong shape and in the wrong place to do much good at low speeds.

Does he (or she) speak from having flown stunt with VGs, from having experimented with stunt airfoils at our Reynolds number in a wind tunnel, or from an analysis of the boundary layer physics?  Let's see the data.  

[Howard Rush]

...Countless Open Nats #1 trophies are sitting in the rooms of guys who agree enough to sprinkle them on their setups.

Come on, PJ, you should be able to count that high. It's only 20 or so.

[Howard Rush]

Exact quote from a NASA engineer they are the wrong shape and in the wrong place to do much good at low speeds.

Oo.  Come to think of if, maybe that's me you're quoting.  I did spend time at NASA Langley.  I don't think I said they're the wrong shape, but I did figure that PJ's were in the wrong place until I tried them there.  I don't think I would have said "low speeds": I would have related it to Reynolds number, but I don't know how their effect would vary with Reynolds number except for maybe how big to make them.  I do agree that the configurations I've tried experimentally don't do much good.  I think that for my airplane the VG arrangement I have now does a little good.

[Dave_Trible]

Going back to Dave's experiences, I'm wondering if it wasn't simply a case of too much angle on them?  If you think of them as like a tiny wing, then an appropriate angle of attack will give you low pressure on one side, high pressure on the other, and a resulting vortex round the tip.  But too much AoA , they stall, and the system breaks down - you don't get the pressure differential you're trying to create, and just end with an aerodynamic mess and a bunch of drag.  So stalled VGs wouldn't give any benefit to the boundary layer (and would bugger it right up instead), and would certainly hurt the glide by virtue of the additional drag.  Any chance you could try them again with less angle, Dave?

Steve I sure might at some time down the road a little.  I'm not going to fix that airplane-it's borderline too heavy as is and my flying season is rapidly winding down here due to weather .  I'm trying some new fuel tanks in my best airplanes now while I can. I really have too many pots stirring already to do much now.  Trying new things is good and I will.  If my theory is right about the VGs it's still a good thing used in the right way. 

Dave

[RC Storick]

If you guys wish to use them DO IT. I will not be taking a aeronautical intercourse and a rolling piece of pastry anytime soon. But one piece of advice get off my back.

[Paul Walker]

If you guys wish to use them DO IT. I will not be taking a aeronautical intercourse and a rolling piece of pastry anytime soon. But one piece of advice get off my back.

Yeah, but I knew that!

OK, take your ball and go home so no one else can play!

[RC Storick]

I have not shut this thread down have I? No I am just telling everyone I have had it. Kind of like when the dog growls at you. I had had more patience than Leonard would have had.

[Ted Fancher]

Hmmmm.  Late to the debate.  I've gotta try to keep up better.

I grew up working at the Renton, Washington airport where my father had an FBO/Cessna dealership.  I was privileged to see a number of historic airplanes as that was where Boeing built and flew the maiden flights of a  number of iconic airliners.  I watched the maiden flights of the original 707 and 727, both very memorable events.  Of interest with respect to this thread is the following.

You pretty much all know the 727 had three engines.  One on each side of the aft fuselage and a third exhausting at the tail end of the fuselage.  The outboards had direct airflow into them in the conventional manner.  The middle engine, however, had the intake of the engine itself somewhere just aft of the last row of coach passengers.  Boeing had to figure out a way to get large amounts of air into a cleverly contrived "S" duct that had it's intake on the top of the fuselage just forward of the vertical stabilizer.  The intake was connected to the front of the engine by ducting in the shape of an "S" through which huge masses of air had to flow smoothly to sate the hungry middle engine.  A convoluted but clever solution, right!

On the maiden voyage the crew spooled up all three engines at the south end of the one mile long runway heading toward the far end that abutted Lake Washington.  Once the engines were up to thrust they released the brakes and the first 727 flight was about to begin.  It raced down the runway, got to Vr (rotation speed), the nose was raised to the appropriate angle for lift off...and a big BOOM! was heard fractionally after flames shot from the exhaust of the middle engine AND out the intake...shooting halfway up toward the nose of the airplane.  Too late to reject the takeoff they lowered the nose slightly in anticipation of losing the engine but the (what turned to be compressor stage stalls) ceased as the body angle was reduced.

Evaluation post flight determined that at required rotation angles the flow of air through "S" duct separated and the engine made it's displeasure very obvious.

The solution?  You guessed it.  Vortex generators were located strategically within the "S" duct (most likely at the "inner radius" of each of the "S" curves but I'm not certain of the exact location) to maintain laminar flow (that might not be the right technical term...but the bottom line was to prevent reduction of flow through the duct) back to the engine which, henceforth on hundreds and hundreds of 727s and 727 variants, operated without the rotation stalls thanks to the VGs.

A curious side note.  Compressor stalls much earlier in the takeoff roll did become a known problem on the 727 and a technique was developed to prevent them.  It would occur just at the start of the takeoff rolls as the engines were accelerating on runways experiencing strong cross winds.  The crosswise flow at the intake once again interrupted laminar flow and the engine would bark it's displeasure.  As the airplane accelerated and the relative airflow was more direct the stalls would cease and normal operation returned.  The technique for such conditions consisted of slightly delaying takeoff thrust on the center engine until forward motion was established and the thrust lever was then more gradually positioned to take off thrust.

I've no idea whether any thought was given to redistribute additional VGs within the duct to deal with this situation.  It might have been that there were simply to many variables given varying angles and amounts of cross wind conditions to predict anything like an ideal "one size fits all" location.

Ted

[Mike Keville]

Thanks for that one, Ted.  All through the 1980s I was a frequent "back of the bus" flyer on literally dozens (perhaps hundreds) of 727s (100s and 200s) in connection with business travel for Hughes Aircraft Co.

Now that you've mentioned it, I seem to recall that on takeoff quite often the center engine didn't fully kick-in until near V1 or Vr (though perhaps that's my faulty memory).

At any rate, my thanks to Boeing engineers and the "G/GUF" (guys/gals up front) for the many hours of safe flying.  The ol' 727 was quite a machine.

As to compressor stalls, the only time I experienced that was aboard a DC-9-32 out of Melbourne, FL just prior to cruising alt.  Still climbing, when suddenly there was this alarming "Bang!" (I was seated near the rear).  Certainly does get one's attention!  There was no communication from the flight deck.  "Just another day at the office?"

[Steve Thomas]

I watched the maiden flights of the original 707 and 727, both very memorable events.

Now that really is something.

Our F-111s had a bunch of VGs in the inlet ducts, for the same reason.  I'm aware of at least one incident involving one of our ground crew and a young lady, where the VGs would've caused a certain amount of discomfort and inconvenience... Probably outside the bounds of this discussion, though!

[Douglas Ames]

<snip>
The solution?  You guessed it.  Vortex generators were located strategically within the "S" duct (most likely at the "inner radius" of each of the "S" curves but I'm not certain of the exact location) to maintain laminar flow (that might not be the right technical term...but the bottom line was to prevent reduction of flow through the duct) back to the engine which, henceforth on hundreds and hundreds of 727s and 727 variants, operated without the rotation stalls thanks to the VGs.<snip>

Ted

Ted, et all,
I have over 19 years overhauling all versions of the Boeing 727 with two different Airlines. What Eastern called the "S duct", American called the "Worm duct", same-same.
Yes, they had VG's installed along the inner radius of each bend. I have scars to prove it! The early -100 series had an oval intake that was changed to a conventional round intake when the stretch (-200 series) versions came out. Also there are VG's installed forward of the split rudder hinge line because the intake for #2 is wider than the verticle fin.
I remember Eastern got rid of their "Shortys" in the early 80's (I was at MIA). A couple showed up as FedEx Freighters at our overhaul line at American (TUL) in the late 90's. It was like seeing an old friend. We also overhauled converted Pan Am and Braniff 727's (FedEx).

[Douglas Ames]

 If you sealed the hinge lines on a 777 (I have no idea how you would do that, one hell of a roll of tape!!) it would do the same thing. Keep air from traveling through the gap.  VGs create a vortex. Doesn't really matter what they are on.  That's what they do.

Boeing 777's use flexible silicone gap seals reinforced with a mesh. They're sealed.
So are the ailerons and rudder. Flaps are sealed in the UP position.
Agree with your point. But on full scale they are used for boundry layer control and to control stall progression at high AoA's.
On some aircraft you'll notice them installed fwd. of the airlerons only.

[Chris Wilson]

And the new Enya venturi inserts have vortex tabs in them also for yet another entirely related reason.

[Ted Fancher]

Boeing 777's use flexible silicone gap seals reinforced with a mesh. They're sealed.
So are the ailerons and rudder. Flaps are sealed in the UP position.
Agree with your point. But on full scale they are used for boundry layer control and to control stall progression at high AoA's.
On some aircraft you'll notice them installed fwd. of the airlerons only.

Doug,

My memory may be incorrect on the following but IIRC early Boeing jets utilized "balance panels" between some movable and fixed surfaces (stabs and elevators maybe?)...sort of triple hinged affairs that utilized the pressure differential between the high and low pressure surfaces to reduce control input forces necessary to deflect the control surface.  As an added bonus they served as gap seals...although I suspect that it was more likely the desire to seal the gap provided the impetus to design them so as to do the "balance" part of their roles.  I sort of remember that early Boeings used control tabs for reducing input forces...looked like trim tabs but were driven by the pilots wheel inputs to aerodynamically drive the much larger ailerons in the desired direction.

Does any of that sound at all familiar?

Slotted flaps are, of course, unsealed when extended in order to do their job properly redirecting airflow over the multiple surfaces that are deployed.

Ted

[Howard Rush]

...although I suspect that it was more likely the desire to seal the gap provided the impetus to design them so as to do the "balance" part of their roles. 

No, the balance was needed because you didn't eat your Wheaties.

[Douglas Ames]

Doug,

My memory may be incorrect on the following but IIRC early Boeing jets utilized "balance panels" between some movable and fixed surfaces (stabs and elevators maybe?)...sort of triple hinged affairs that utilized the pressure differential between the high and low pressure surfaces to reduce control input forces necessary to deflect the control surface.  As an added bonus they served as gap seals...although I suspect that it was more likely the desire to seal the gap provided the impetus to design them so as to do the "balance" part of their roles.  I sort of remember that early Boeings used control tabs for reducing input forces...looked like trim tabs but were driven by the pilots wheel inputs to aerodynamically drive the much larger ailerons in the desired direction.

Does any of that sound at all familiar?

Slotted flaps are, of course, unsealed when extended in order to do their job properly redirecting airflow over the multiple surfaces that are deployed.

Ted

Not sure on the 707's, but yes on the 727's. The balance panels had a habit of collecting snow and ice. I remember bird nests being a problem on our USAF KC-135's, so I'm thinkin' they had them also.

[frank williams]

There is no reason to wonder if the vg’s work or not.  Chris’ video of the wing with tufts going through maneuvers both with and without vg’s shows that the vg’s have a real measurable effect.  The video goes pretty fast even at the slow motion rate and you have to look close.  The maneuver to look at is the entry into the triangle.  There is a definite stalled area of the wing with no vg’s and it is just barely disturbed when the vg’s are in place.

The stalled area is the middle section of the rear third of the wing and flap.  Apparently close to the fuselage the prop wash is enough extra velocity to prevent stall.  The very tip area of the wing is influenced by the tip vortex and is also less affected.  But, the center rearward portion of the wing clearly has significant flow separation on the entrance to the triangle ……. for this plane anyway.

This was a fairly light weight plane with a nice thick airfoil, probably not tending to stall very much.  I think that the average plane with a bit thinner airfoil and a bit “porkier” may be seeing this partial stall on more of the harder square corners.  This “slight stall” is probably what pilots refer to as “inconsistent pullouts”.  The fix of the vg’s is to remove the inconsistencies.  A picture is worth a thousand words.

[Randy Cuberly]

Hi Frank,
That is a very interesting observation, and obviously there is separation present without the VG's.
I did notice one thing of intrest however (at least to me).

The tip of the Stab and elevator are also visible in both photos and in the one with no VG's there is still significant up elevator deflection, wheras in the photo with the VG's the elevator appears to have returned to approximately neutral.  I would think that this means that the turn was completed at the point of no elevator deflection, but not yet completed in the photo without the VG's, since the elevator is still deflected.

It might be possible that the stall on the wing occurred because the turn was more abrupt and driven harder by more elevator deflection.

Also, it may be just a trick of the angle of the photo, but there doesn't appear to be much flap deflection in the one that still has elevator deflection.

Is it possible to capture both photos with the same approximate elevator deflection.

[frank williams]

The tip of the Stab and elevator are also visible in both photos and in the one with no VG's there is still significant up elevator deflection, wheras in the photo with the VG's the elevator appears to have returned to approximately neutral.  I would think that this means that the turn was completed at the point of no elevator deflection, but not yet completed in the photo without the VG's, since the elevator is still deflected.

Hi Randy
I think its the other way around ..... the deflected elevator is with no vg's ..... the neutral elevator is with vg's .....
actually the video WITH vg's doesn't have any real disturbance ... so I just selected one of the frames
I've included one with the vg's and the elevator matching the frame without vg's.

[Howard Rush]

Frank, how did putting them just ahead of the flaps work?

[Randy Cuberly]

The tip of the Stab and elevator are also visible in both photos and in the one with no VG's there is still significant up elevator deflection, wheras in the photo with the VG's the elevator appears to have returned to approximately neutral.  I would think that this means that the turn was completed at the point of no elevator deflection, but not yet completed in the photo without the VG's, since the elevator is still deflected.

Hi Randy
I think its the other way around ..... the deflected elevator is with no vg's ..... the neutral elevator is with vg's .....
actually the video WITH vg's doesn't have any real disturbance ... so I just selected one of the frames
I've included one with the vg's and the elevator matching the frame without vg's.

Uhhhh......NO....The deflected elevator in your photos is with VGs...

My conjecture, and I admit it's strictly conjecture, is that the indication might be that the triangle corner with no VG's could have been stalled by pulling harder and faster on the corner to make it tighter and thereby induce the stall.

However I do understand your point that it is simply a frame out of a video and the frames did not match in time.  They do seem to match closely in relative angles, so I came to the conclusion that the corner may have been softer on the one with VG's.  

Also there appears to be a significant disturbance in the covering aft of the high point in that same area.  I'm not at all sure if that could influence what is showing.  It appears to be more "dimpled" in the one with VG's.  It seems to be torn and taped, and the tape is deflecting more in the one with VG's

[Derek Barry]

The tip of the Stab and elevator are also visible in both photos and in the one with no VG's there is still significant up elevator deflection, wheras in the photo with the VG's the elevator appears to have returned to approximately neutral.  I would think that this means that the turn was completed at the point of no elevator deflection, but not yet completed in the photo without the VG's, since the elevator is still deflected.

Hi Randy
I think its the other way around ..... the deflected elevator is with no vg's ..... the neutral elevator is with vg's .....
actually the video WITH vg's doesn't have any real disturbance ... so I just selected one of the frames
I've included one with the vg's and the elevator matching the frame without vg's.

The pictures are impressive but I think it could be something as simple as Chris' inconsistencies from one flight to another, where the amount of control input is concerned. OR it might be directly related to the VGs, this one would be hard to prove. It is obvious in the pictures that the VGs are cleaning up the air over the wings. I love the taste of crow.

Derek

[frank williams]

Randy
Old people shouldn't post after 9pm .... sorry 'bout that .... but the point is that ... WITH VG's I couldn't find a frame with obvious separation.

Howard
I flew one plane with vg's right before the flaps, trying to get the vortices to help hold the flow on the flaps ..... once again it was a subtle effect ... nothing jumped out.
I can tell you though that a control surface deflection greater than 15 degrees, the flow over the flap or elevator is pretty much separated.  The little turning vanes I had on some planes back through the years did help...... not so much on the flaps as they did on the tail surfaces.
Probably one of the best solutions recently is Igor's "blended airfoil/flap" intersection .  I should note also that Al Rabe did the same thing with his airfoil shapes some years back.   

Derek
I think that these little "mini stalls" probably occur more often than we are aware.  A little extra handle in the corner might cause these "inconsistencies" that make the plane be just a little off of where we expected it to be.  Maybe also wind conditions might effect the flow just enough to change the angle of attack just enough to push the wing into a separated flow.
From Chris' video, I do think that we see that the area of concern is the mid wing area and that vg placement in that range of the span would probably be  the most benificial.

[Chris_Rud]

All,

I put this test on hold because my daughter was about to be born. This winter I intend to build a profile ship that I can mount a camera on different locations. The current plane doesn't allow very many camera positions. I would invite anyone to "request an experiment" and I will do my best to test it. I would love to know what people want to know NOW so I can finish the design of the test ship with the experiments in mind.

Thanks,
Chris

[Chris Wilson]

Probably one of the best solutions recently is Igor's "blended airfoil/flap" intersection .  I should note also that Al Rabe did the same thing with his airfoil shapes some years back.   

Didn't Claus Maikis back in the 80's feature something like that with his 'knuckle jointed' flap hinges also?