Full flaps

[tgrayson]

how much lift and how much drag you get with each: 10/20-deg give much more lift relative to drag as compared with 30 (or 40) where you get more drag than the relative gain in lift.

You will find language like this in many pilot publications and I find it to be meaningless. An aircraft with full flaps has the same amount of lift as an aircraft with no flaps, so saying that a particular flap setting provides more lift than drag says absolutely nothing to me.

The language does have some meaning in a wind tunnel where you can change the flap setting, but leave the AoA and airspeed the same, which results in an increased lift coefficient and increased drag coefficient. At some point, the drag coefficient starts increasing faster than the lift coefficient.

But that has no relevence in an airplane where L must equal W. Deployment of flaps requires a lowering of the AoA for the same airspeed to keep from ballooning, so the lift coefficient will not change at a particular airspeed, no matter what the flap setting. The end result is that flap deployment only results in an increase in drag coefficient, which is particularly large at the higher flap settings.

 

[mshunter]

When turning final.



Needless disburbance of a stabilized approach, IMO. If you lose an engine, raise the flaps.



Gusts are irrelevant with regard to flaps or partial flaps. Only real issue is rudder authority, which flaps will decrease at the performance limits. But even with 40 degree flaps on a C172, you can handle about 30 knots direct crosswind. I see no reason to decrease the flap setting until the wind is stronger than that. Even then, you can increase rudder authority with a faster approach or keeping a trickle of power in.

So I essentially teach full flaps on every landing, which ensures the aircraft handles the same way every time.

WOW! Raising the flaps when the engine quits is probably the worst thing to do IMHO. The reduced drag that you will get will be negated by the loss in lift and resultant increase in sink rate. I think I'm going to try this today with one of my students and see what happens. And full flaps on every landing. Come on out to So Cal when the Santa Anna's are blowing at 30 G45. I gaurentee that you'll have a hard time getting a 172 on the runway with full flaps. If I have a 20kt head wind, I fly the exact same pattern, power comes to idle on final, and flaps are used to slow the airplane to not over shoot the aiming point.

My patterns are as follows.... Downwind abeam carb heat and 15, 10 deg. inside the white arc and descend. Hold 85kts with pitch. Base at 75kts, no flap change. Final at 65-70 depending on flap setting and engine at idle. Don't use power as a crutch, because if you do loose the engine, and are carrying power to touch down, when the prop stops, you are gaurented not to make the runway.

There are very few airplanes that I fly that I carry power to touchdown, and none of them have have less than two engines. In a single engine airplane, you have to always expect your only engine to quit at any second. So be prepared to make a power off landing EVERYTIME. In a multi engine airplane, you have more options, thus more outs should one engone quit.

 

[mshunter]

Your stabilized approach is already screwed up by the engine failure.

Not if you fly a power off approach.

 

[tgrayson]

Raising the flaps when the engine quits is probably the worst thing to do IMHO. The reduced drag that you will get will be negated by the loss in lift and resultant increase in sink rate. .

As long as you increase your AoA to compensate, you won't see any sink at all. In the extreme, one fun thing you can do is after you flare the airplane, flip the flap lever to the "up" position. As the flaps retract, increase your back pressure and you can hold the airplane one foot above the runway as the flaps come up. This sorta brings home the fact that you don't *necessarily* get a sink when you retract the flaps.

 

[wjmiller3]

As long as you increase your AoA to compensate, you won't see any sink at all. In the extreme, one fun thing you can do is after you flare the airplane, flip the flap lever to the "up" position. As the flaps retract, increase your back pressure and you can hold the airplane one foot above the runway as the flaps come up. This sorta brings home the fact that you don't *necessarily* get a sink when you retract the flaps.

Ground effect will play a big part in that senerio. I will have to try that at altitude. The few times I have pulled up the flaps at idle I have had lots of sink, granted I was on approach and pitching for an airspeed, so holding altitude was not the idea. Seems like pulling flaps at idle, then increasing AOA to cancel a sink rate, could possibly put you in a clean power off stall senerio. But I will give it a try next week....

 

[at1024]

Glide ratio depends on the L/D ratio; drag depends on IAS and L = W, and neither of those are affected by density altitude.

Oh okay I see what you're saying here. I was just thinking of the lift equation where density is a factor. But I had forgotten about the effects of drag.

 

[tgrayson]

Oh okay I see what you're saying here. I was just thinking of the lift equation where density is a factor. But I had forgotten about the effects of drag.

That's the proper way to think about it, but consider that your TAS will be higher at a lower density (higher density altitude) for a given IAS. It's easy to draw the wrong conclusions from the lift equation unless you keep that in mind.

 

[tgrayson]

The few times I have pulled up the flaps at idle I have had lots of sink, granted I was on approach and pitching for an airspeed, so holding altitude was not the idea.

And I agree the idea isn't to hold altitude, but it will be a side effect. If you keep the lift coefficient the same, then your airspeed will remain the same and your lift will remain the same and there will be no net change in your flight path. I think you can see that theoretically, this *must* work.

I've encouraged students to perform the exeriment, dump flaps on short final and then keep airspeed or pitch attitude the same, and it's worked nicely, so this isn't a high skill maneuver.

 

[shdw]

I've encouraged students to perform the experiment, dump flaps on short final and then keep airspeed or pitch attitude the same, and it's worked nicely, so this isn't a high skill maneuver.

If you don't mind me stepping on your toes here is how I explain this.

If we look at the lift formula we can derive this (it is for practical purposes and does not work, only shows relationship in easy math): Lift = Speed + AOA, from this we can determine a few truths:

• If speed goes up AOA must go down to keep lift the same
• If speed goes down AOA must go up to keep lift the same
• If either speed or lift go up/down without the other changing to compensate then total lift will go up/down with it

AOA = angle between relative wind and the chord line
Chord line = line drawn straight from the leading to trailing edge of the wing.

So draw a chord line or imagine one on the wing at the airplane, then lower the flaps and imagine the new chordline for the sections of the wings that has flaps. You will see that the chord line changes which would result in an increased angle of attack

As we lower the flaps the AOA increases on about 50 percent of the wing (the student doesn't need to know the percentage crap IMO) and since our formula says that AOA goes up and our speed is still the same our total lift has gone up by some amount on 50 percent of the wing. This is what gives us that "lift" we see when we lower the flaps which is why tgrayson said that when you lower the flaps if you slightly nose down you can put the angle of attack back down (with pitch) so that lift remains approximately the same. All of this happens in reverse when you retract the flaps.



Now for the confusing side that I haven't seen yet and would like some opinions on explaining. All of what I stated above is typically true for high wing aircraft, however lowering the flaps in many low wing aircraft produce quite the opposite effect (nose down when lowered and little to no change when raised). Anyone have a good way to explain this?

Here is my explanation; because the flaps are below and behind the CG they can create an upward and backward acting force when lowered which is stronger then the lifting force applied ahead of the CG resulting in a nose down from lower the flaps in a low wing. Why wouldn't this be the same in a high wing though is where this explanation confuses me.

 

[tgrayson]

Now for the confusing side that I haven't seen yet and would like some opinions on explaining.

Flaps inherently produce a nose down moment. More flaps, more nose down moment that must be trimmed away. On a high wing, this nose down moment is masked by the increase in downwash on the horizontal stabilizer, which produces a net nose up moment.

Pipers seem to have much less nose up tendency than Cessnas, but it's still there. A Mooney and Navion have a strong nose down moment, as you'd expect.

 

[Roger, Roger]

Now for the confusing side that I haven't seen yet and would like some opinions on explaining. All of what I stated above is typically true for high wing aircraft, however lowering the flaps in many low wing aircraft produce quite the opposite effect (nose down when lowered and little to no change when raised). Anyone have a good way to explain this?

In a high wing, the downwash from the flaps hits the horizontal stabilizer, increasing downforce on the tail. This causes a pitch up motion when flaps are extended.

 

[Dazzler]

In a high wing, the downwash from the flaps hits the horizontal stabilizer, increasing downforce on the tail. This causes a pitch up motion when flaps are extended.

True, although the pitch up motion is only momentary - as airspeed bleeds off, the nose will pitch forward again as the aircraft seeks to regain its trimmed airspeed; this time at a steeper angle. Got to love that positive static stability!

This is turning into a great thread. Thanks for all the responses!

 

[shdw]

In a high wing, the downwash from the flaps hits the horizontal stabilizer, increasing downforce on the tail. This causes a pitch up motion when flaps are extended.

I am asking about the pitch up moment in in low wing, and the pitch up moment in high wing is encountered because lift is increased and the lift vector acts on the CP which is ahead of the cg causing the nose up rotation. Though I have heard this flap discussion and agree it is probably part of the total equation for why we pitch up in a high wing.

Still on the table, why does a low wing pitch down when lowering the flaps?

 

[Roger, Roger]

I am asking about the pitch up moment in in low wing, and the pitch up moment in high wing is encountered because lift is increased and the lift vector acts on the CP which is ahead of the cg causing the nose up rotation. Though I have heard this flap discussion and agree it is probably part of the total equation for why we pitch up in a high wing.

Still on the table, why does a low wing pitch down when lowering the flaps?

Increases the lift of the wing (momentarily). The CP of the wing is located behind the CG, so an increase in lift causes a pitch down moment.

 

[tgrayson]

because lift is increased and the lift vector acts on the CP which is ahead of the cg causing the nose up rotation.

No. Having the CP ahead of the CG would result in an aircraft without longitudinal static stability. (In reality, we should be talking about the aerodynamic center, not the CP.)

why does a low wing pitch down when lowering the flaps?

I explained that in post #70.

 

[tgrayson]

the nose will pitch forward again as the aircraft seeks to regain its trimmed airspeed

Except that you just retrimmed the airplane for a different airspeed; the downwash acts just as if you had dialed in nose up trim. I've never seen a Cessna regain the original airspeed after lowering any flaps without dialing in some nose down trim.

 

[shdw]

No. Having the CP ahead of the CG would result in an aircraft without longitudinal static stability. (In reality, we should be talking about the aerodynamic center, not the CP.)



I explained that in post #70.

Gah yes I knew that woops and ok thanks.

 

[mshunter]

As long as you increase your AoA to compensate, you won't see any sink at all. In the extreme, one fun thing you can do is after you flare the airplane, flip the flap lever to the "up" position. As the flaps retract, increase your back pressure and you can hold the airplane one foot above the runway as the flaps come up. This sorta brings home the fact that you don't *necessarily* get a sink when you retract the flaps.

An increase in AoA will also result in an increase in induced drag. Still gonna get you to the same spot, short of the runway. And increasing the AoA will also result in pulling back on the yoke, the wrong thing to do when your slow, on approach. If anything, you want to push. Bottom line, in a light single, full flaps are not necessary. We are not robots that fly in a perfect world under the same conditions every time. Having the use of 10-20-30 deg of flaps is all part of teaching ADM. Teaching "you must always use full flaps" takes ADM out of the equation. When my students become proficient, and they reach for the flap lever to add flaps, I smack their hand away and tell them "the circut breaker just pooped, you are now stuck at that flap setting, lets land." I would rather have taught a pilot to deal with every foreseeable circumstance, than one who thinks that they need 30 deg. of flaps when it's 16022G38 landing on rwy24 with full flaps to the wind like a sail, and the gust is 3kts above stall speed.


On a side note, I also take a student to a specific location that will set us up for "Cessna 1234x, make straight in rwy 12, report 3 mile final." When I hear those words, the yoke becomes "locked" and the have to get us down with power/trim/rudder. If you try and use flaps in that scenario, the resultant pitching moment will cause you to become de-stabilized.


I think what this comes down to, is CFI's teaching to the bounds of the PTS, and being narrow minded. I prefer to "teach outside the box." The PTS shows how well you have control of the aircraft under pre-determined circumstances. In real flying, when it hits the fan, it has not been pre-determined. You have to stay calm and think clearly to make things happen right, you have to be taught a fair amount of ADM.


I see what you are trying to accomplish (I think) and it's good tgray. I want my students to pass their check ride as much as the next guy. But I also want my students to walk away from their check ride with the examiner thinking "Wow, all of his students are really effin good." Not just "another one passed."

 

[tgrayson]

An increase in AoA will also result in an increase in induced drag. Still gonna get you to the same spot, short of the runway.

Your understanding is flawed. You think 65 knots with full flaps has the same glide angle as 65 knots clean? The lift coefficient is the same, but the drag on the flapped airplane is much greater due to the parasite drag of the flaps.

You're using a faulty understanding of the aerodynamics to argue that it is not possible to do what I do. You seem very resistant to new information, dismissing it without experiencing it.

I would rather have taught a pilot to deal with every foreseeable circumstance, than one who thinks that they need 30 deg. of flaps when it's 16022G38 landing on rwy24 with full flaps to the wind like a sail, and the gust is 3kts above stall speed.

Flaps don't work that way and it's one of the lessons I want to teach them.

. I want my students to pass their check ride as much as the next guy. But I also want my students to walk away from their check ride with the examiner thinking "Wow, all of his students are really effin good."

That appears to be a jab implying that I train for the checkride but you train "aviators". Pretty much every instructor thinks that way about themselves, so I will avoid posturing.

Regardless, you don't understand what I'm trying to do. I want them to

1) Not have this superstitious fear of flaps in windy situations
2) Understand that the purpose of flaps is to lower the stall speed in order to have the slowest touchdown speed
3) To be able to fly the airplane in the most demanding configuration
4) Remove one extra variable during landing to let them focus on flying the airplane.

I find this sort of discipline extremely effective in enabling the students to extract the maximum performance from the airplane with minimum effort, since every landing is (almost) a max performance landing. As I pointed out previously, landings at lower flap settings are childishly easy compared to the maximum drag configuration, so insisting that they use lower flap settings is a wasted lesson. (Other than no flap landings, of course.)

Bottom line: The discipline of using full flaps as primary landing configuration contributes to a long-term improvement in airmanship and increase in confidence. In contrast, inconsistency in flap usage often correlates with a flawed understanding of what flaps do and how they behave in various environmental conditions, and connotes a pilot's lack of confidence in his ability to control the airplaine.

 

[at1024]

partial flap landings = great practice. Nothing says those things can't fail on final.