Flaps to aid climbing - truth or fiction

[Ed Cesnalis]

I agreed to test my CT and see if deploying flaps would aid the climb.  Fast Eddie is  challenging this technique and I tend to  use it to climb over terrain when my current climb path isn't going to make it.

 

I took off at 7,100' elevation 1,100lbTOW(approx) 61*F  and established a 500FPM climb to 12,000'.  

• The climb took 10 minutes  
• I elected to extend from the -6 setting to the 0 setting to maintain the 500fpm
• Max oil temp of 232 reached at 11,000'
• 232 maintained to 12,000'

Testing climb at 12,000 MSL

 

• 0* - trim for best climb rate = ~350FPM
• 15* - trim for best climb rate = ~550FPM
• -6* - trim for best climb rate =  ???  too close to stall, was near terrain, settled for ~100fpm @49kts IAS

My Conclusion:  My CT's climb is  ennahnced and extended to higher altitudes by extending flaps from either the reflex setting 1 or 2 notches or by extending from the 0 setting 1 notch.

 

Note:  I didn't even test full flap settings, no-one is arguing that they enhance climbing.

 

Photo:  Ritter Range and Thousand Island Lake from 12,000' over June Lake. 1/250 sec. f/11 24mm

[ralarcon]

I agreed to test my CT and see if deploying flaps would aid the climb.  Fast Eddie is  challenging this technique and I tend to  use it to climb over terrain when my current climb path isn't going to make it.

 

I took off at 7,100' elevation 1,100lbTOW(approx) 61*F  and established a 500FPM climb to 12,000.  

• The climb took 10 minutes  
• I elected to extend from the -6 setting to the 0 setting to maintain the 500fpm
• Max oil temp of 232 reached at 11,000'
• 232 maintained to 12,000'

Testing climb at 12,000 MSL

 

• 0* - trim for best climb rate = ~350FPM
• 15* - trim for best climb rate = ~550FPM
• -6* - trim for best climb rate =  ???  too close to stall, was near terrain, settled for ~100fpm @49kts IAS

My Conclusion:  My CT's climb is  ennahnced and extended to higher altitudes by extending flaps from either the reflex setting 1 or 2 nothces or by exting from the 0 setting 1 notch.

 

Note:  I didn't even test full flap settings, no-one is arguing that they enhance climbing.

 

Photo:  Ritter Range and Thousand Island Lake from 12,000' over June Lake. 1/250 sec. f/11 24mm

I agree with the above, and that has been my experience in the flat lands of Florida.

 

Cheers

[FlyingMonkey]

Thanks for taking the time do do this test, CT!

[Ed Cesnalis]

Thanks for taking the time do do this test, CT!

 

I still make it to work by 7:30AM

[FlyingMonkey]

I still make it to work by 7:30AM

 

[FastEddieB]

I'm  not ignoring this - thanks for the test - but I'll have to respond to it later.

[Roger Lee]

See Eddie, you told me you were old ???? so your books must be too. ????????

 

 

I get the same type of results as ED. I'm sure all the CT owners do and it's even more noticeable at higher altitudes.

[Doug G.]

My experience also.

[GravityKnight]

I recall resorting to -15 flaps high up in the mountains in the ctsw trying to climb... it was very clear that 0 flaps was much better than -6, and I felt like -15 was better yet.. this looks to confirm that. The only reason I wasn't convinced is that the ct's I rented had way too much prop pitch and it almost seemed if you could get your speed up level, with say 0 flaps, then climb with the prop rpm higher up and making more power, it would almost hold it there, and while trying to climb slow with -15 flaps you ended up at a really low rpm (4600 WOT iirc at high altitude). 

 

I need to play with flaps at high altitude in my rans.. I jerk in full flaps on take off and I can pop off the ground at 40mph indicated sometimes less- and its ready to fly, I can get it off at a lower speed but it needs a high AoA and is real mushy of course.. I have done some vx climb testing with 1 notch, 2 notches vs no flaps. My results led me to usually pull full flaps, then leave it in ground effect for a couple seconds until I reach 60mph and have the flaps out, then go into a steep 60mph climb (usually 1000-1200fpm leaving at 6650ft elevation). I have the speed wing, only 116sq ft, so at high altitude I might also benefit from 1 notch, maybe 2 of flaps (flaps are not real large or 'powerful' on this wing..... as in, the stall speed continues to get lower on each notch of flaps, where as a ctsw 40 degrees doesn't produce a lower stall speed - or more lift, just more drag etc).

 

Good test

[FastEddieB]

I find CharlieTango's results quite interesting.

 

Knowing what I do about aerodynamics, they would seem to point to one thing:

 

His CT, and perhaps all CT's, have less drag at 10° flaps than they do clean.

 

Which I suppose is possible, depending on the airfoil shape, the shape and movement of the flap, and some interplay between the two.

 

Since a plane climbs due to excess thrust, and in each of CT's examples the thrust is the same, the only conclusion is that somehow there's less overall drag at 10° flaps - unusual but apparently not impossible.

 

To show how unusual it is, I cannot think of a POH that shows best rate of climb, or service ceiling, as being achieved at partial flaps. You'd think that a manufacturer would want to publish the best numbers possible, so if a higher rate or a higher ceiling was achievable via flaps, would they not be incentivized to publish that data?

 

Again, because I have not seen it does not mean it does not exist. Can anyone recall a Best Rate Of Climb chart that specified flaps for any plane? Or one that called for flaps to achieve the highest service ceiling? Seems unlikely, but since Ed's CT seems to show better performance with flaps, I have to admit it's possible.

 

I'm going away this weekend, but soon I plan to see how my Sky Arrow does under simulated high-altitude conditions. I'll let you know what I find.

[Roger Lee]

Hi Eddie,

 

There's 1800+ more just like ED's. That's a pretty good sample size. 

[FastEddieB]

Then CT's are draggier "clean" than with 10° flaps.

 

At least, that's the only thing I can think of to explain a higher climb rate with 15° flaps. If there's another explanation, I'd love to be edified - no pun intended!

[Roger Lee]

Clean is for flying faster and flaps for slower and or climb. They do increase drag, but not to the point of being detrimental to climb in small  flap settings.. If you did the same test at 30-40 flaps then you would be correct. It's all a balance.

[FastEddieB]

As a general rule, flaps can only hurt climb rate, though they may help climb angle.

 

If flaps do, in fact, increase climb rate in a CT, the CT is an odd duck, for whatever reason.

[FlyingMonkey]

The airplane is reconfigured, with an entirely different shape wing.  That can't have *anything* to do with it?

 

I guess the point of the other thread is that wings don't have anything to do with how an airplane climbs, only power does...?

[FastEddieB]

The airplane is reconfigured, with an entirely different shape wing.  That can't have *anything* to do with it?

 

 

Sure it can. It's just unusual for flap deflection to decrease drag. That seems to be the case for the CT.

 

In general, more lift directly means more induced drag. Somehow the CT must be decreasing parasiitic drag to more than compensate for that increase in induced drag.

 

Weird, but must be true if Ed's numbers are accurate.

[Ed Cesnalis]

 http://www.flyingmag.com/can-flap-deflection-help-you-climb

[Anticept]

The general rule is called a "general rule" for a reason. It doesn't apply to all cases!

 

CTs tend to level out quite a bit with flaps, so I'm betting there might be some cross section component affecting drag.

 

Anyways, we were't told what the airspeed is. Newer CT AOIs have a higher Vy and best glide speed than the older ones. I was told it's because it was approximated on old ones, and they got much better research equipment and redid the tests a few years ago and found the numbers were too low.

[FastEddieB]

Thanks. I had read this recently in support of my assertion.

 

This explains a lot:

 

"It's difficult to generalize about the drag that flaps produce. Increased camber usually reduces drag at higher lift coefficients; in other words, more camber is for airplanes that go slower, and less camber is for airplanes that go faster. With a well-faired plain flap, a few degrees of downward deflection can at least in theory reduce wing drag at the lift coefficient-typically around .6-used for climb. But leakage through the slot and discontinuities at the ends of the flap exact a drag penalty that increases rapidly with greater flap deflection. That number .6-the climbing lift coefficient-is important. The maximum lift coefficient of an unflapped wing is usually somewhere around 1.3 to 1.5; so the climbing lift coefficient is way below the maximum. Deflecting a flap increases the maximum lift coefficient, meaning that the airplane stalls at a lower speed, but it does not affect lift at higher speeds. Lift coefficient still varies at the same rate-around 0.1 per degree of angle of attack-but the angle of attack at which a given lift coefficient, .6 for example, is developed is lower. In other words, putting the flap down a bit makes the airplane fly in a more nose-down attitude."

[Ed Cesnalis]

There is much discussion and confustion on this subject.  Here is a technical publication from the civil aviation authority of New Zealand.  https://www.caa.govt.nz/FIG/basic-concepts/climbing-and-descending.html

 

It contains this gem: 

Flap

Increases lift and drag and alters the Lift/Drag ratio. Since drag opposes thrust, any increase in drag will reduce the rate and angle of climb.

If you can't trust the (New Zealand) government to give you the correct answer, who can you trust?

[FastEddieB]

The general rule is called a "general rule" for a reason. It doesn't apply to all cases!

 

CTs tend to level out quite a bit with flaps, so I'm betting there might be some cross section component affecting drag.

 

 

That would fit my theory that the small increase in induced drag may be offset by less form drag from the fuselage.

 

But would not that hint at a possible design flaw? If the flaps have to go down to bring the fuselage into its lowest drag state, might they not have a faster plane just by repitching the fuselage - or designing "partial flaps into the wing shape?

[FastEddieB]

Increases lift and drag and alters the Lift/Drag ratio. Since drag opposes thrust, any increase in drag will reduce the rate and angle of climb.

If you can't trust the (New Zealand) government to give you the correct answer, who can you trust?

That pretty much backs up what I was saying, though I think that's more true discussing the rate of climb as opposed to the angle of climb - the angle may in fact be greater due to the slower forward speed.

[FastEddieB]

More good stuff from Ed's linked article:

 

"Once in a steady climb, the airplane is raised higher and higher by power, not by lift. Therefore, the higher maximum lift coefficient obtained by deflecting the flaps has nothing whatever to do with climbing. The only thing that matters is the drag of the complete airplane...

 

...Flaps do increase drag, and the more you deflect them the more drag you get. Some airplanes can't climb at all with their flaps fully deflected...In my opinion, it is unlikely that any flap deflection has a discernible positive effect upon rate of climb, but I would be interested in seeing evidence-from actual flight test, not anecdote-to the contrary."

 

Ed,

 

You might send the results of your test to Peter Garrison. Though the article is 10 years old, he might still find it interesting, and/or provide feedback.

[Ed Cesnalis]

Eddie,

 

I think you are lumping at least 2 issues together.  

1. extending flaps reduces climb rate (unless it doesn't)
2. extending flaps therefore reduces service ceiling (unless it enhances it)

Addressing issue #2, in my test at the negative 6 setting I was close to stall.  Stall is the only enforced ceiling where service and absolute are just book values.  My limits at negative 6 setting were fast approaching, my IAS numbers where in the forties. 15 degrees gave me a lot of margin from stall and permitted me to go much higher.

[Ed Cesnalis]

Here's the math: There are four forces that act on an aircraft in flight: lift, weight, thrust, and drag. The motion of the aircraft through the air depends on the relative size of the various forces and the orientation of the aircraft. For an aircraft in cruise, the four forces are balanced, and the aircraft moves at a constant velocity and altitude. On this slide, we consider the relations of the forces during a gradual climb. We have drawn a vertical and horizontal axis on our aircraft through the center of gravity. The flight path is shown as a red line inclined to the horizontal at angle c. The lift and drag are aerodynamic forces that are defined relative to the flight path. The lift is perpendicular to the flight path and the drag is along the flight path. The thrust of the aircraft is also usually aligned with the flight path. Some modern fighter aircraft can change the angle of the thrust, but we are going to assume that the thrust is along the flight path direction. The weight of an airplane is always directed towards the center of the earth and is, therefore, along the vertical axis.

Forces are vector quantities. We can write two component equations for the motion of the aircraft based on Newton's second law of motion and the rules of vector algebra. One equation gives the the vertical acceleration av, and the other gives the horiozontal acceleration ah in terms of the components of the forces and the mass m of the aircraft. If we denote the thrust by the symbol F, the lift by L, the drag by D, and the weight by W, the vertical component equation is:

F * sin© - D * sin© + L * cos© - W = m * av

where sin and cos are the trigonometric sine and cosine functions. Similarly, the horizontal component equation is:

F * cos© - D * cos© - L * sin© = m * ah

We can simplify the equations a little by using the definition of excess thrust Fex:

Fex = F - D

The resulting equations of motion are:

Vertical: Fex * sin© + L * cos© - W = m * av

Horizontal: Fex * cos© - L * sin© = m * ah

For small climb angles, the cos© is nearly 1.0 and the sin© is nearly zero. The equations then reduce to:

Vertical: L - W = m * av

Horizontal: F - D = m * ah

The resulting simplified motion is described on another slide. The horizontal equation is integrated on another slide to give the velocity and location as functions of time.

For more moderate angles, high excess thrust can provide an important contribution to the vertical acceleration. The next time you visit an airport, notice the high climb angles used by modern airliners. This flight path is possible because modernturbine engines develop high excess thrust at takeoff. The pilot climbs sharply to get the aircraft as high as possible within the confines of the airport which produces the least noise for homes near the airport.