If you can perform perfectly a full stall landing with full flaps and no flaps then you can do it anywhere inbetween so why teach partial? Instead I personally teach full and none (even on windy days) and teach the dynamics explained in my previous post #69, ignoring the last two paragraphs.
Full flaps
- Thread starter Dazzler
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If you can perform perfectly a full stall landing with full flaps and no flaps then you can do it anywhere inbetween so why teach partial? Instead I personally teach full and none (even on windy days) and teach the dynamics explained in my previous post #69, ignoring the last two paragraphs.
Correct me if I'm, wrong, and I'll look it up after work, but isn't the PRIMARY purpose of flaps not to decrease stall speed, but rather to steepen the approach w/out increasing airspeed? In a 172 it does give you nearly 10kts less stall speed. But step into anything that doesn't have barn doors for flaps, and the stall speed reduction is minimal, say a seminole, or a bonanza where you get 5 kts less. I don't have the performance data in front of me right now. But I will after work.
P.S. When you set down at 40-50 kts, touch down at the slowest possible speed is not necessary. Touching down when the airplane is done flying is (i.e. stalling speed). And a max performance landing is only necessary when you have 500ft of space to land, which will be a scenario where you take the wings of a 172 and truck it back to the airport. Just because you can get it down on that spot right there, does not mean you are going to get it back off.
More to come later, I'm gonna be late for work.
Dazzler
Well-Known Member
tgrayson
New Member
Flaps were invented to allow airplanes to have smaller wings, because smaller wings allow the aircraft to fly faster. But smaller wings means a higher stall speed, which means longer runways because the aircraft must fly faster approaches and touch down at higher airspeeds. I'm aware that the FAA may promulgate merely the "steeper" approach explanation, but I am providing the explanation in light of what the aerodynamics books say about flap design, both the current engineering goals and the history of the technology. If all you want is a steeper approach, there are simpler technologies, e.g., spoilers.
It's true that in something like a C172, the 40 degree setting provides minimal stall speed reduction, maybe half a knot, and the practical reason that Cessna provided it was to enable steeper approaches, but in aircraft with leading edge flaps of some sort, large flap settings are much more effective in reducing the stall speed.
But regardless of the effectiveness of the final setting, using it sends a broader message: use everything you have and land at the slowest possible airspeed. This lesson provides the student with a goal by which to judge any technique or procedure. And if you really want to drive a lesson home, you've got to reinforce it every flight.
Well, no, it's only 5 knots in a C172S...48 vs 53 CAS. If you look at IAS, it's about 8 knots, but "IAS" is meaningless.
The two terms are synonymous.
My view is that the pilot should always be striving to be the best that he can be, regardless of whether the situation demands it.
Engineers also had a few other reasons I didn't see that I think are very important to get across to every student.
Is full stall safer in general? I don't think so but that is my opinion. I believe this deal with fast landings and not stalling is coming from airline landings and peoples knowledge of them. You simply can't full stall land a 747 safely because delta wing stall propagation is tip to root so they lose full lateral control before the wing is even stalled. Where as our aircraft, typically with rectangle wings, stall from root at the leading and and fan back and out which is why even in almost a full stall we still have lateral control. I agree teach them the aircraft limits particularly on the low end.
IMO learning aircraft control up to and including full stall especially for every landing (for stalled flight practice) with the exception of meeting PTS requirements is far better than any single or small grouping of stall lessons. Not because the stall lessons are bad, but purely because with this they get to practice stalled flight daily which a single or small group of lessons can't compete with. I have my students fly the flare at varying speeds (slower as they build experience) nearly every lesson just to build comfort and outside visual control since inside the aircraft here might get you tangled on the wind sock.
- Less work on the brakes and tires
- Safer in the case of a ground loop, remember the original design engineers were in taildraggers
- Less overall stress on the aircraft
Is full stall safer in general? I don't think so but that is my opinion. I believe this deal with fast landings and not stalling is coming from airline landings and peoples knowledge of them. You simply can't full stall land a 747 safely because delta wing stall propagation is tip to root so they lose full lateral control before the wing is even stalled. Where as our aircraft, typically with rectangle wings, stall from root at the leading and and fan back and out which is why even in almost a full stall we still have lateral control. I agree teach them the aircraft limits particularly on the low end.
IMO learning aircraft control up to and including full stall especially for every landing (for stalled flight practice) with the exception of meeting PTS requirements is far better than any single or small grouping of stall lessons. Not because the stall lessons are bad, but purely because with this they get to practice stalled flight daily which a single or small group of lessons can't compete with. I have my students fly the flare at varying speeds (slower as they build experience) nearly every lesson just to build comfort and outside visual control since inside the aircraft here might get you tangled on the wind sock.
unclenobby
Well-Known Member
Interesting per the C152 Information Manual (Amplified procedures) - "When landing in a strong Xwind, use the minimum flap setting required for the field length....." ..... and the warrior PIM pretty much says that same thing (partial or no flaps for strong xwind)
Since Max demonstrated xwind in the 152 is 12kts - you would have to assume Cessna are saying partial flaps for xwind > 12kts unless short field?
tgrayson
New Member
The reason is that flaps tend to increase the directional stability of the aircraft. This is counter-productive because you're trying to maintain a sideslip to compensate for the crosswind, and the strong directional stability of the aircraft is what you're trying to neutralize with the rudder.
The net result is that you can handle less crosswind with full flaps than with lesser settings, all things being equal. But the limit is far greater than 12 knots. I've landed a C152 in a 24 knot crosswind, but it required full rudder.
There are a variety of techniques to increase the crosswind capability:
- Keep some power in to increase rudder authority,
- Faster approach speed to increase rudder authority.
- and yes, a lower flap setting.
tgrayson
New Member
Well, ok, but the AFH is a very unreliable source of any sort of engineering/scientific information, if you're anticipating using its description of the use and function of flaps. The section is riddled with misconceptions.
SpiraMirabilis
Possible Subversive
tgrayson,
Flying into Denver in the Dash-8 I can reasonably land and get off at the reverse hispeed or 90' (on 12,000ft runways) but in doing so I have to be a little aggressive with the disc and brakes which is uncomfortable for the passengers.
Additionally our ref speed slows down to something like 89 KIAS with flaps 35 vs flaps 15, which sometimes puts a wrench in approach control's plans. Our flap speed is 148kts for 15', lately I have been flying the approach at around 140-145kts then at the fence bring the power to idle and intentionally float to the edge of the touchdown zone before touching down, this allows me to make the first highspeed quickly and smoothly and exit the runway rapidly without jerking the passengers into the seat in front of them. I always do touchdown in the touchdown zone and on the centerline.
Would you say this technique is incongruent with trying to be the best a pilot can be?
Flying into Denver in the Dash-8 I can reasonably land and get off at the reverse hispeed or 90' (on 12,000ft runways) but in doing so I have to be a little aggressive with the disc and brakes which is uncomfortable for the passengers.
Additionally our ref speed slows down to something like 89 KIAS with flaps 35 vs flaps 15, which sometimes puts a wrench in approach control's plans. Our flap speed is 148kts for 15', lately I have been flying the approach at around 140-145kts then at the fence bring the power to idle and intentionally float to the edge of the touchdown zone before touching down, this allows me to make the first highspeed quickly and smoothly and exit the runway rapidly without jerking the passengers into the seat in front of them. I always do touchdown in the touchdown zone and on the centerline.
Would you say this technique is incongruent with trying to be the best a pilot can be?
SpiraMirabilis
Possible Subversive
152 can do about 35kts crosswind before you run out of rudder, landing with a lot of extra power in.
So is about every other book on aviation, it seems unless you buy a book geared towards flight dynamics you will pick up a lot of miss information.
Edit: Just thought I would add, Flight Theory for Pilots by Charles E. Dole is a great book to clear up much of the misconstrued content found in many other aviation books on engineering. The A&P book for 20 bucks on engines will keep you from thinking vapor lock happens only when a tank runs dry and many other faulty descriptions on engines. We can thank Jeppesen for this wonderfully wrong description of vapor lock.
On your other post about directional control going up with increase in flaps (I know it is right from my testing in ME training) will you give more detail? Some equations or a practical example of airflow change or something to help me better understand this, thank you.
unclenobby
Well-Known Member
My post was not a "what can it handle" question , it's more along the lines of why teach only full flap landings since the manufacturer recommends partial/no flap landings in the PIM. -
If it is recommended then surely it should be taught and also at what point would you recommend to a student to use partial flaps per the PIM? The PIM just states "strong xwind" so isn't it prudent to apply the partial/no flap technique above max demo xwind?
unclenobby
Well-Known Member
So do you recommend to your students to use partial flaps beyond a specific xwind compoment (maybe 24 as stated above)?
tgrayson
New Member
I make sure they understand that it's an option, and I want them to understand that this is only an issue at the limits of the airplane capability. In this part of the world, crosswinds of that sort of strength are rare. Locally, my observation is whenever crosswinds are greater than 20 knots, most pilots aren't going to fly, and that includes instructors.
Here's another observation: during one pattern session with a student with crosswinds gusting to 30 knots, we landed just fine with 40 degree flaps in a C172. However, during takeoff, we slid almost uncontrollably towards the side of the runway. We saw this happen over and over.
tgrayson
New Member
Unfortunately, it's an experimental result, rather than a theoretical one. There is at least one NACA report that documents an increase in directional stability with flaps, but they don't really explain it, only say that it exists. Another data point is the reduction in Vmc with flap deployment, which implies an increase in directional stability. You can see data for that on the website of Boundary Layer Research, a vortex generator company. And regarding Cessnas, the book "Cessna, Wings For the World" has lots of flight testing stories from one of their former test engineers and in one case he remarks that Cessna's recommendation about partial flaps during crosswinds is due to rudder authority.
So it would be presumptuous to say that this is a universal truth, but there are enough data points to suggest that it may be so.
Why might this be? I suspect the reason is that when sideslipping, the downwind wing root is shielded by the fuselage, reducing the drag of that flap. The drag differential of the upwind vs downwind flap would be stabilizing. Speculation of course.
tgrayson
New Member
My main point is that when a pilot demands less of himself in situations where high skill isn't needed, he isn't being the best that he can be. You know this is true.
tgrayson
New Member
Here are some extracts from the Airplane Flying Handbook, Chapter 11, that impinge on the subject at hand. My view is that the authors have a deep misunderstanding of some aspects of an aircraft's interaction with its environment that I hope will be obvious once I point them out.
Ok, I'm already wondering whether I should read any further. The authors reveal a fundamental misunderstanding about flight mechanics. Airplanes, while flying through the air, cannot “feel” steady wind. The airplane is flying in a body of air which happens to be moving relative to the earth, but the airplane doesn't know that. The only wind it feels is that generated by its own motion.
If you're coming in for a landing using the wind low method, the aircraft is sideslipping in the direction of the wind, so the relative wind that it “feels” is produced by its own motion through the air. If the airplane is sideslipping to the left to counteract a left crosswind, the relative wind is produced by its sideslipping motion, not the crosswind itself, and this relative wind would be felt anytime the pilot applies his flight controls in the same fashion. (i.e., sideslips are idential aerodynamically to forward slips.)
Huh? If the airplane is crabbed, there is no sideslip and the airplane is completely unaffected by the existence of the crosswind, particularly since the aircraft cannot feel wind at all. This language “eliminated to a slight extent” (in the vein of being slightly dead or a little bit pregnant) suggests, once again, a complete misunderstanding of how wind affects airplanes.
Lowered wing blanketing the upwind flap? Where exactly do the authors think the relative wind is coming from? They seem to envision that it moves parallel to the horizon and hits the lowered wing from the top, which would indeed blanket the flap...and the entire wing, resulting in a rapid fall from the sky. In reality, the relative wind strikes the wing from below, like always, but just slightly from the left or right, depending on the direction of sideslip.
Sounds a bit vague, but overall wrong; flap deflection reduces the dihedral effect, according to all the aerodynamics books, so that makes lateral control easier with flaps deflected. Less stability equates to more control. And remember, there is no crosswind, only sideslip.
There is no crosswind to the airplane, there is only sideslip and how could that change as you get closer to the ground, assuming your flight controls remain positioned the same? You probably will encounter increased mechanical turbulence, but also a slower wind speed, which requires less sideslip and thus increases the controllability of the airplane. With strong winds, it's often impossible to achieve enough sideslip to maintain runway centerline at higher altitudes, but becomes possible the closer you get to the ground, which belies the authors conclusions, much less their explanation.
I'm assuming that the author is talking about after touchdown. NOW we have a crosswind, since the airplane is now on the ground, and the air is moving relative to the ground, rather than due to the aircraft motion. It's quite true that the airplane will attempt to weathervane...that's what the vertical stabilizer does. The flaps will likely contribute to this, since they increase directional stability. Moreover, since the airplane is now at a constant AoA, the flaps increase lift and thus reduce the controllability due to differential braking or nosewheel steering. Raise them! This is the only significant and accurate thing the authors have stated in this section. Raise the flaps after touchdown!
Well, we know that not all flap installations cause a nose down pitching motion. Although flaps alone do produce a nose down moment, downwash on the horizontal stabilizer can offset and exceed this nose down tendency.
Given that the aerodynamic center of the wing is behind the CG (or should be), the increase in lift should pitch the airplane down, rather than up. But there is also the propeller blast on the horizontal stabilizer to consider, as well as the Propeller Normal Force (caused by the change of direction of airflow through the propeller disk) which will produce a nose up moment that will assuredly exceed the nose down moment of the lift increase.
That's why when performing a go-around, you're supposed to increase the AoA as you raise the flaps. Didn't the authors read their own chapter on go-arounds?
Ok, I'm already wondering whether I should read any further. The authors reveal a fundamental misunderstanding about flight mechanics. Airplanes, while flying through the air, cannot “feel” steady wind. The airplane is flying in a body of air which happens to be moving relative to the earth, but the airplane doesn't know that. The only wind it feels is that generated by its own motion.
If you're coming in for a landing using the wind low method, the aircraft is sideslipping in the direction of the wind, so the relative wind that it “feels” is produced by its own motion through the air. If the airplane is sideslipping to the left to counteract a left crosswind, the relative wind is produced by its sideslipping motion, not the crosswind itself, and this relative wind would be felt anytime the pilot applies his flight controls in the same fashion. (i.e., sideslips are idential aerodynamically to forward slips.)
Huh? If the airplane is crabbed, there is no sideslip and the airplane is completely unaffected by the existence of the crosswind, particularly since the aircraft cannot feel wind at all. This language “eliminated to a slight extent” (in the vein of being slightly dead or a little bit pregnant) suggests, once again, a complete misunderstanding of how wind affects airplanes.
Lowered wing blanketing the upwind flap? Where exactly do the authors think the relative wind is coming from? They seem to envision that it moves parallel to the horizon and hits the lowered wing from the top, which would indeed blanket the flap...and the entire wing, resulting in a rapid fall from the sky. In reality, the relative wind strikes the wing from below, like always, but just slightly from the left or right, depending on the direction of sideslip.
Sounds a bit vague, but overall wrong; flap deflection reduces the dihedral effect, according to all the aerodynamics books, so that makes lateral control easier with flaps deflected. Less stability equates to more control. And remember, there is no crosswind, only sideslip.
There is no crosswind to the airplane, there is only sideslip and how could that change as you get closer to the ground, assuming your flight controls remain positioned the same? You probably will encounter increased mechanical turbulence, but also a slower wind speed, which requires less sideslip and thus increases the controllability of the airplane. With strong winds, it's often impossible to achieve enough sideslip to maintain runway centerline at higher altitudes, but becomes possible the closer you get to the ground, which belies the authors conclusions, much less their explanation.
I'm assuming that the author is talking about after touchdown. NOW we have a crosswind, since the airplane is now on the ground, and the air is moving relative to the ground, rather than due to the aircraft motion. It's quite true that the airplane will attempt to weathervane...that's what the vertical stabilizer does. The flaps will likely contribute to this, since they increase directional stability. Moreover, since the airplane is now at a constant AoA, the flaps increase lift and thus reduce the controllability due to differential braking or nosewheel steering. Raise them! This is the only significant and accurate thing the authors have stated in this section. Raise the flaps after touchdown!
Well, we know that not all flap installations cause a nose down pitching motion. Although flaps alone do produce a nose down moment, downwash on the horizontal stabilizer can offset and exceed this nose down tendency.
Given that the aerodynamic center of the wing is behind the CG (or should be), the increase in lift should pitch the airplane down, rather than up. But there is also the propeller blast on the horizontal stabilizer to consider, as well as the Propeller Normal Force (caused by the change of direction of airflow through the propeller disk) which will produce a nose up moment that will assuredly exceed the nose down moment of the lift increase.
That's why when performing a go-around, you're supposed to increase the AoA as you raise the flaps. Didn't the authors read their own chapter on go-arounds?
In cleaning up the airplane during the go-around, the pilot should be concerned first with flaps and secondly with the landing gear (if retractable). When the decision is made to perform a go-around, takeoff power should be applied immediately and the pitch attitude changed so as to slow or stop the descent. After the descent has been stopped, the landing flaps may be partially retracted or placed in the takeoff position as recommended by the manufacturer.