Just wondering on this aerodynamic theory.

How come in a combat airplane, with the elevator connected to the airfoil, and given a down control results in a downward maneuver. Whereas in a standard stunt airplane with a flap, which is connected to the airfoil, with a stabilizer and elevator with the elevator in the up position and the flap is in the down position the plane goes up... Why??  In the combat airplane the reaction is down and in the standard stunt airplane the reaction is just the opposite...up.

They both are giving lift but in opposite directions, but yet moving in the same direction. Puzzling...

Roger

Hey Roger:

When the elevator is in back and you command "up", it pushes the back of the airplane down.  So the whole plane starts going down, but it also starts pitching up, and some short time later the airplane goes up.  The control engineers in the group will start babbling about "unstable zeros" or "non-minimum phase systems" at this point -- ignore them.

When the elevator is attached to the wing and there's no other control surface back there, the same thing happens -- the instantaneous effect is for the plane to go down, but the cumulative effect is for the plane to go up.  On a combat wing the "down" part is very brief, so you don't notice.

On a stunt plane, when you command "up", the flaps go down and the elevators go up.  The wing is both lifting more, and trying to make the airplane pitch down.  The elevator is generating down-force, and trying to make the airplane pitch up.  On a well-trimmed airplane, this makes the airplane perform some pleasing balance of lifting more and rotating (and in Igor Burger's airplanes, there's a mechanism on the flaps that changes the flap:elevator ratio as a function of deflection, so you get less pitching at low deflections and more rotation at high deflections -- which makes a difference if you fly as well as the Big Boys).

As puzzling as it seems, I have overheard qualified commercial pilots answer this one incorrectly and it seems that we are alone with counter operating controls in the aeronautical world.

(Now that's sure to attract a rebuttal!)

As puzzling as it seems, I have overheard qualified commercial pilots answer this one incorrectly and it seems that we are alone with counter operating controls in the aeronautical world.

(Now that's sure to attract a rebuttal!)

  Commercial pilots, as a general rule, don't know how to design airplanes. And I am sure some of the exceptions - commercial pilots Ted Fancher, David Fitzgerald, Al Rabe, Todd Lee, and others probably understand how it works.

      People have been using flaps for maneuvering since before WW II. The P-38, for example, was rated to use flaps in the turns to improve the turning radius. Richard I Bong, top P-38 ace, used them to great effect to improve his combat chances.

    I am not going to bother describing how it works because it has been described clearly the modeling press since about 1948 or so (Glo-Devil) and Bob Palmer. Anyone who doesn't get it should go check out the radius of an outside loop performed in a Piper Cub. It's really that simple. Or hook up your controls backwards and see how it works.

     Brett

 Commercial pilots, as a general rule, don't know how to design airplanes. And I am sure some of the exceptions - commercial pilots Ted Fancher, David Fitzgerald, Al Rabe, Todd Lee, and others probably understand how it works.

      People have been using flaps for maneuvering since before WW II. The P-38, for example, was rated to use flaps in the turns to improve the turning radius. Richard I Bong, top P-38 ace, used them to great effect to improve his combat chances.

    I am not going to bother describing how it works because it has been described clearly the modeling press since about 1948 or so (Glo-Devil) and Bob Palmer. Anyone who doesn't get it should go check out the radius of an outside loop performed in a Piper Cub. It's really that simple. Or hook up your controls backwards and see how it works.

     Brett

Hi Brett,
             yes perhaps pilots don't know how to design airplanes but the confusion comes from the nomenclature of 'coupled.'
To most pilots coupled flaps means they are 'only' designed for banked turns, as in one drops and one lifts, not bidirectional landing flaps such as stunters use.

It took me about 15 minutes to explain to a good pilot friend of mine exactly why and what the use of being 'coupled' was for, and even then I am not sure he got it!

The one that wrinkles my brow is Canard flaps, up ...... down......... moving in opposition.............in unison ........ahh, my head hurts just thinking about it

Cheers.

P.S. I do know that applying too much flap opens up the maneuvers due to excess pitching.

Meaning, coupled to the ailerons?

Yes, well put.

The one that wrinkles my brow is Canard flaps, up ...... down......... moving in opposition.............in unison ........ahh, my head hurts just thinking about it

       Once you start looking at that one, you will very quickly find out why canards are NOT a good idea. The answer is that you want the flap on the wing to move to camber the wing, just like on a regular airplane. And then the canard needs to get very much larger, because the CG has to be very far ahead of the wing, so that pitches it down very strongly. So you have to get far more lift out of the canard than you would a conventional tail. So you might be inclined to make the canard larger. But then you have to move the CG further forward to make it stable with a larger canard.

    It turns out that once you get the canard about 4x the area of the wing, you hit about the optimum arrangement, and, you start needing to move the flaps on the wing in the conventional direction.  So, canards work great as long as you make the canard large enough, QED.

     Brett

It comes down to things we call moment coefficients and stability derivatives.

Some aircraft will pitch up when the flaps go down - a Cessna 150 is a good example. Some pitch down, like a Mooney 201 or my HP10.

It's all a complicated dance between the wing's downwash on the tail and how it alters the flow field around the aircraft.

I find that flaps on a CL ship make corners smoother, but not necessarily tighter.

Other's experience may be different.

In the full-scale word we use flaps to steepen the descent. They lower the L/D and and so we don't need to be at 500 AGL 6 miles out.

Notice that very, very few unlimited performance full-scale aerobatic aircraft use flaps.

Makes you wonder, eh?

Great question!

It's interesting.  Several times I've read quotes from German Me-109 pilots that routinely dropped the flaps to the first stop, 10 deg, when starting combat maneuvering.  ME-109 had a small wing which used leading edge slat, a forgiving airfoil, and flaps.  According to the link below a fellow who owns both a Mustang and an ME-109(Harold Kinsvater) the 109 has much better handling 109 is far superior to either the Mustang or Spitfire for air combat.  It's main flaws were that it was a bit too small and couldn't carry enough fuel when the needed extra range.  And the landing gear apparently killed or wounded more pilots than flying combat.  It was a xxxch to take off and land and ended up in the hands of thousands of barely trained pilots. Once in the air the overall(7-1) tells a lot.

I often wonder how WW-II would have turned out if the Germans had actually strategically planned their options instead of going by Hitler's whims.

Forgot the website:  http://www.scientistsandfriends.com/aircraft.html

During one of the VSC meets, I was practicing with my 1952 Nobler. Seems I was still using 50's "technology" and the washer that was soldered to the elevator push rod vibrated loose at the flap horn. I noticed the Nobler was tending to flying a tad lower each lap, so I gave a little up. BLAM, into the asphalt so hard the Fox .35 ended up in the adjoining circle and almost hit the pilot in the center. SO, flaps without a countering elevator become defacto elevators.  FWIW.

I've heard about 10 different versions of this story.  A scant few save the plane.
Phil

Isn't all of this just the relationship between the centre of pressure of the model vs the centre of pressure of each lifting surface?

This works very well, as the tail moment on the sailplanes is very long (although the tail volume is very small, relative to our C/L stunt planes).

Are you keeping in mind that tail volume is tail moment arm times tail area?  And that the tail volume coefficient is something yet again?  I'm not sure that sailplane tail volume coefficient is all that teeny, when all is said and done.

The pitching moment on the wing due to flaps is (roughly -- everything is "roughly" when you're talking aerodynamics) proportional to the percentage of wing chord taken up by flaps times the overall wing chord.  On a high-aspect ratio sailplane, that pitching moment is going to be pretty small compared to an equivalent-area stunter with a 5:1 aspect ratio.  Add in the long tail moment arm, and I'd expect that the overall effect of flaps is to pitch the plane up when the flaps are lowered, and visa-versa.

I'll rebut that statement about being alone with counter operating controls.

More than happy to be corrected!

Its good to learn something  new.

I get it, "reverse snap flap"  is a different thing from spoilers. 

I get to set up my new DLG this weekend, but my radio has less than the recommended number of mixes and no 3-position switches.

Phil

Doesn't the flap system on our stunt planes act to increase lift when deflected in either direction by changing the airfoil section? The speed of the wind(?) going over and under the surface of the wing the airframe is forced into more turmoil providing the increase in the direction of the deflection?  In that way they then aid in the ability of the stabilizer to provide direction for the airframe.

I am not an aerodynamics engineer (nor do I play one on TV), but that is the explanation I have always been most comfortable with.

Fly the models we do, things like Bernoulli and Reynolds numbers play a small enough part as to be negligible.  So, Ackem's razor applies.  The simplest explanation is usually the correct one.   

Yep.  Here's a picture of what flaps do on an Impact airfoil.  The x axis is angle of attack, the y axis is lift coefficient, a measure of the force on the airplane perpendicular to the airstream.  The green line, flaps deflected to 30 degrees, gets to a higher value than the black line, flaps centered.  Also, behold that for a given angle of attack, deflecting the flaps increases the lift.  Angle of attack in all cases is measured relative to the centerline of the airfoil with flaps undeflected.  




Alas, the internet and popular press are flush with simple aeronautical explanations that are hogwash.  

Yes Howard, but now show us the pitching moment and drag as function of flap deflection. That's the rub. Yes, the flap deflection will move the Lift(AoA) curve to the left, but the slope will remain the same (2*Pi) and the pitching moment coefficient will increase as the flap deflects, as will the drag coefficient .  So you increased the lift, but you increased the nose-down pitching moment. This means the horizontal stab/elevator needs to push down harder so the net gain for the entire airplane in lift isn't what it appears.

Also, being picky the curves you post appear to be for 2-D flow and don't account for the reduction in slope due to wing geometry, but yes, they show the shift to the left nicely.  The AR will also determine the AoA for stall, and that's why airplanes have a lower AR for the tail than the wing. You want the tail to still be working when the wing stalls. So that means the efficiency of the tail is low and so it will add more drag to counter the moment increase of the flaps...

And agreed, there is lot of "wisdom" about aerodynamics and stability and control. Most are taken completely out of context. The classic 2D flow picture of a wing never happens except in a wind tunnel. Once you build an actual airplane the flow becomes 3D, and if you're subsonic what the airplane does to the air surrounding it influences the air in front of it.

If you ever want to really bang your head, go to the R/C forums and read what people post about dihedral!!!!

And does that increase in lift along with forward momentum influence the fact that we 'slush' our way through corners, Chuck?  

And is that why we kept moving toward longer tail moments to make the tail more efficient while not overloading the wing stall?

Layman's terms please!  

I wouldn't say we "slush" through them. YMMV, but for me a well designed and built CPLA plane will "fly" the corners and that's what makes it a great ship.

Anyway, it's always more complicated for an airplane than an airfoil. As I've discussed before, the wing will have a downwash behind it. The more lift, the higher the downwash. The downwash will change the effective angle of attack for the horizontal tail. This means that we get some "free" up elevator when the flap goes down. So simply by being there, the tail changes the way the ship responds.

Here comes the really fun part: Let's build a 70's era combat ship. Let's put the elevator 6" behind the trailing edge. Elevator goes up, nose goes up. Now let's put it 3" behind the trailing edge.  Elevator goes up, nose goes up. Now, let's attach it to the trailing edge. Elevator goes up, nose goes up.

Nothing magical happens pitch-moment-wise when it gets attached to the wing's trailing edge except that it becomes less efficient as the moment arm decreases.

Next, let's split it in half and move the halves to the wingtips and make them move in opposite directions" One goes down and one goes up. The one that goes down  increases lift and the one that goes up decreases lift. The airplane rolls.

But the one that goes down increases drag and the one that goes up decreases it. This wants to drag the nose of the plane to the side so we need a rudder input.

What the above is *hopefully* demonstrating is that the changes in lift, drag  and moment due to a surface deflection are related, but separate forces.

Making any sense?

Modern RC sailplanes use this same flaps with elevator mixing, adding camber to up elevator, and reflex to down elevator, they call this "snapflap".
We don't have the advantage on them of a propeller pulling the plane though. And still, this is a more efficient way to change pitch, because you want a higher cambered wing profile when you have a higher angle of attack than when you are asking for closer to a zero or nearly zero lift from the wing. Or so I am told.

R,
Chris

So that means the efficiency of the tail is low and so it will add more drag to counter the moment increase of the flaps...

I assume that's a joke for my benefit.  I have smilies turned off.  If not, see replies 29 and 35 here: http://stunthanger.com/smf/stunt-design/horizontal-tail-aspect-ratio/

The AR will also determine the AoA for stall, and that's why airplanes have a lower AR for the tail than the wing. You want the tail to still be working when the wing stalls.

Hmm.   Interesting.  Thinking about how this would apply to stunt, I would guess that the continued effectiveness of the low-aspect-ratio tail while the wing lift is petering out at high angle of attack might give an effect similar to that of Igor's variable-ratio linkage.  It might be time to take some measurements with the pole-on-the-Miata wind tunnel.

Hmm.   Interesting.  Thinking about how this would apply to stunt, I would guess that the continued effectiveness of the low-aspect-ratio tail while the wing lift is petering out at high angle of attack might give an effect similar to that of Igor's variable-ratio linkage.  It might be time to take some measurements with the pole-on-the-Miata wind tunnel.

Howard, it's all been done already. Most of the polars for NACA  four-digit airfoils show a formula for converting the 2D for test section to 3D. Just some simple algebra.


BTW Howard - You deserve some kudos for any aero-knowledge I may possess. When I was in my late teens flying Sneekers I could beat anything until a Nemesis went against me.  I read the magazine article about a hundred times and decided I needed to go to college to get a better understanding of what was going on. I thought " These college guys know stuff I don't." I was a crappy student and on a really bad trajectory until that happened.

So, I owe you.
 
Thank You.

Chuck

No one? No one sees this as a bit incongruous?

Ok, carry on.

Not at all, If the flap goes up and we reduce the lift so we reduce the induced drag, i.e, the relative angle of attack decreases. The wing's geometric angle of attack is fixed to the angle it makes with the flight path, but the relative angle of attack ( the angle at which the wind actually hits the wing) is dependent on it's geometry, loading, and a few other things. It has to do with the tornadoes you can see coming off the wingtips of a large airplane, and why we have a very real thing called "ground effect".  The ground interrupts the wingtip vortices so the aircraft has a reduction in induced drag, and so it "floats" down the runway.

An airplane that is turning uses lift to turn and therefore ALWAYS increases drag in a turn. Lift makes drag.  More efficient designs make less drag than poor designs.  But they all increase drag while creating turning force.

The difference in combat and stunt is that in combat we want to turn tight and go fast.
In stunt we want to turn tight and don't mind going slow to turn tighter.

Thanks for the answer Chuck, I see what you are getting at but must admit that its a very transient state.

If you are describing a roll here with simple rudder correction only then I would expect the craft to drop or slip towards gravity, you would need up elevator to correct that.

If the craft starts to roll or place its span away from horizontal then surely the geometric angle of attack is not fixed to its flight path due to excessive slippage?

Thanks.

Well, I'm not going to get into flying lessons but if you look up "adverse yaw" on the interwebs thingy you'll find what the rudder is for, and learn about how to make a coordinated turn without slipping or skidding.