Flap area vs. flap deflection

[Tim Wescott]

I thought of this question reading Mike Griffon's "Imitation Stab" thread -- I didn't want to hijack that thread, so I'm asking here:

The thread has become a discussion about flap area -- small being nice if your plane isn't too heavy, with lots needed if you've built a porky plane.

My question is, what's the penalty that you pay for building a light plane with generous flap area and then just toning down the flap throw to match the plane weight.  Any?  If not, what?

Because -- flap throw is easy to change.  Changing the flap area without also changing the area and aspect ratio of the overall wing is (ehem) a bit harder.

(and no, you're not going to see my next plane with flap chord equal to 33% of the total wing chord -- I'm just curious).

[phil c]

I'd say bigger flaps with less travel probably generate less drag.  But if they are too big there's nothing wrong with trimming some off.  Just makes it hard to get that 20 pt finish back.

[Bill Little]

HI Tim,

After asking Ted a similar question a while ago, this is *my* understanding (and it makes sense).

Two of the same models built, one very light, one very heavy (for the design).  The "light" one will not need as much flap to help create more lift to maneuver.  The heavy one will need all the flaps it can get to create enough lift to maneuver.  SO I would say that "weight" is one of the important factors.  Of course, the actual airfoil will probably have something to do with this, also.  Plus the mode of power (is it enough for the total airframe?).

BIG Bear
RNMM/AMM

[Howard Rush]

Hinge moment increases roughly proportionally with deflection, but with the square of the flap chord.  Increasing hinge moment makes the airplane harder to fly accurately.  On the other hand, a larger flap chord may keep the airfoil operating in its linear range, which may make the airplane easier to fly accurately (see the article that Igor wrote for Stunt News). 

[Mark Scarborough]

Howard, when was that article published please?
thanks

[Howard Rush]

Bob Hunt is sitting on it until that Igor guy establishes some stunt credibility.  

[RandySmith]

Hinge moment increases roughly proportionally with deflection, but with the square of the flap chord.  Increasing hinge moment makes the airplane harder to fly accurately.  On the other hand, a larger flap chord may keep the airfoil operating in its linear range, which may make the airplane easier to fly accurately (see the article that Igor wrote for Stunt News). 

That's the way it works on my designs , I found the same thing after cutting down chord and area, then adding it back.

Randy

[Chuck_Smith]

If I remember correctly, and I need to check, I think the back-of-the-napkin estimate has always been the drag coefficinent changes by .0023 X (ratio of the flap chord to the wing total chord) X flap deflection in degrees.

What I'm not sure of is of it's .0023 or .00023. Makes a bit of a difference!

Since there is a direct relationship between the lift and drag coefficients (related to 1/Aspect Ratio)  it should be pretty easy to estimate using 2*Pi for Lift Coefficinet vs angle-of-attack and corrected for a finite wing, again a function of the Aspect Ratio. Square that product and divide by Pi*Aspect Ratio*.8 and you should be pretty close on the drag coefficent for the wing before the flap deflects.

Again, I'm trying to eat a yogurt with one hand, type with the other and remember all at once, so let me check my formulas before you pick up the Xacto.

But again notice that I really don't care about the airfoil - it's the ASPECT RATIO that's driving the bus.
 
We operate models at such rediculously low Reynolds numbers that there generally isn't sufficient energy in the flow to keep it attached, so it's pretty safe to say a separation bubble develops and most wind tunnel data would be irrelevant.
Chuck

[Howard Rush]

My limited stunt experience (and some ciphering) suggest that pitching moment is a lot bigger deal than drag.  My last dog had too much flap chord and too little stabilizer.  Even with magic doodads to reduce hinge moment, I had to fly it at a way aft CG to get it to turn a corner.

[phil c]

When I was young and foolish I built the ultimate profile carrrier plane(hah!).  Flaps were 30% of chord, the wing was 18% thick(including flaps).  The tail was over 25% of the wing.  With a good running ST35(I told you it was a long time ago), it could fly 13 mph in level flight, with the nose pointed down about 10 degrees.  Lots of flap pitching  moment there.

Anybody ever try a 6in. bellcrank and 1.5 in. horns to get the pushrod forces down so the flaps aren't driving the plane?

[Ted Fancher]

That's the way it works on my designs , I found the same thing after cutting down chord and area, then adding it back.

Randy

Randy,

and it is exactly what Wild Bill told us decades ago in Control Line Aerodynamics Made Painless.  He was not only a big sweet man, but also a very knowledgable one!

p.s  this (hinge moment) is also exactly why none of my designs have had "low aspect ratio" flaps.  The fact that I also build quite heavy compared to many top fliers yet remained competitive with small flapped high wing loading ships lends credence to my contention that flaps are more often too large than too small...and that lowest achieveable wing loadings will automatically move one up the competitive ladder!

Ted

[RandySmith]

Randy,

and it is exactly what Wild Bill told us decades ago in Control Line Aerodynamics Made Painless.  He was not only a big sweet man, but also a very knowledgable one!

p.s  this (hinge moment) is also exactly why none of my designs have had "low aspect ratio" flaps.  The fact that I also build quite heavy compared to many top fliers yet remained competitive with small flapped high wing loading ships lends credence to my contention that flaps are more often too large than too small...and that lowest achieveable wing loadings will automatically move one up the competitive ladder!

Ted

Hi  Ted

I agree  with that, the only times, generally I saw people having to add more chord to flaps, was back in the late 70s , early 80s when we went to the 60 ships, or the extra large ST 46 ships, many dropped in the corners, and adding about 1/4 inch extra chord along the length of the flaps, really cleaned up the corners and stopped the sinking/stalling, Of course I also remember a couple of pilots using more than a 1 to 1 ratio and moving the flaps just a bit more than the elevator. Then a few other tried  running the planes more nose heavy in order to use more deflection of the flaps and generate more lift that way... I thought building just a little light was a better way to go   

It seems strange to remember I had 700 Sq IN ships in 1973 with 45 and 46s, and 750 and 725 sq in ships as the First large SVs in 1975 and 76. these had 46s and 50s. Now the ships are 670 sq in with  65s and 75s LOL

Randy

[Ted Fancher]

Hi  Ted

I agree  with that, the only times, generally I saw people having to add more chord to flaps, was back in the late 70s , early 80s when we went to the 60 ships, or the extra large ST 46 ships, many dropped in the corners, and adding about 1/4 inch extra chord along the length of the flaps, really cleaned up the corners and stopped the sinking/stalling, Of course I also remember a couple of pilots using more than a 1 to 1 ratio and moving the flaps just a bit more than the elevator. Then a few other tried  running the planes more nose heavy in order to use more deflection of the flaps and generate more lift that way... I thought building just a little light was a better way to go   

It seems strange to remember I had 700 Sq IN ships in 1973 with 45 and 46s, and 750 and 725 sq in ships as the First large SVs in 1975 and 76. these had 46s and 50s. Now the ships are 670 sq in with  65s and 75s LOL

Randy

Yup, Randy.  Lift goes up as the square of airspeed (or down as the square when there's inadequate ponies to haul uphill).  Sure, I wish I could build lighter but have pretty much surrendered to my well documented historical inability to do so.  I think the added ponies and their ability to keep us from slowing, combined with the desirability of reducing wing area to perform better in strong winds (reduced "stunt kite" effect) has had a salutory impact (pardon the pun) on stunt's hard and fast Golden Rules.  My sole purpose in reprising my comments about competitive performance by "gravity challenged" stunt ships has been to encourage stunt fliers not to give up hope if they fail to build a zero ounce stunter.  Thanks to you, by the way, for your efforts in the "ponies" category!

Ted

[Ted Fancher]

My limited stunt experience (and some ciphering) suggest that pitching moment is a lot bigger deal than drag.  My last dog had too much flap chord and too little stabilizer.  Even with magic doodads to reduce hinge moment, I had to fly it at a way aft CG to get it to turn a corner.

Howard,

Gotta admit,  I loved reading this. 

Ted

[Tim Wescott]

Sure, I wish I could build lighter but have pretty much surrendered to my well documented historical inability to do so.

(snip)

Ted

Whoa!  Epiphany!  Does this mean that if anyone makes derogatory comments about the weight of the Fancherized Twister that I'm building that I can say "no, it's really Fancherized"?

(Sorry.  Couldn't resist.  I'll get back to work now).