Newbee with landing flap questions.

[FlyingMonkey]

Unless the angle of attack was zero.

 

That's what I meant, but Doug's point is well taken.

[Andy]

I'm going to take issue with the idea of "impact lift".  Really, the "hand out the window" concept is more of an illustration of drag, especially when you're talking subsonic aerodynamics..  See my blog "Only One Form of Lift"(June 20, 2016) at:  http://theandyzone.com/flightblog/

[Roger Lee]

http://sciencelearn.org.nz/Contexts/Flight/Science-Ideas-and-Concepts/Wings-and-lift

 

 

There are as many studies supporting impact lift as there are groups that don't believe. So all those people can do the arguing since they have the tools and research facilities to work it out.  To me it doesn't really matter since it keeps us in the air. It could be gremlins holding me up and that works so long as they don't drop me.   

 

 

Impact lift is all over the web by respectable organizations and scientist. 

 

 

 

[gbigs]

The negative flaps are intended to lower air pressure above the wing by diverting air on the trailing edge of the top of the wing upward in cruise flight (0 or near 0 AoA).  This should have the effect of increasing velocity because lift is enhanced without increasing the throttle (power).  And therefore the plane should fly slightly faster at any given throttle position in cruise.

[ralarcon]

I wouldn't try a -6 flap landing unless you REALLY wanna have a white-knuckle experience.

Done it several times. Use -6 regularly on high cross winds landings. Used it with John Hurst on board, and is recommended by him on high cross wind landings. No white knuckles whatsoever.

 

Cheers

[sandpiper]

Done it several times. Use -6 regularly on high cross winds landings. Used it with John Hurst on board, and is recommended by him on high cross wind landings. No white knuckles whatsoever.

 

Cheers

 

Agree. A pilot should be proficient landing with all flap settings. Sure, -6 looks and feels different than 15 or 30. That is why we maintain proficiency using all settings. If you were to do a flight review with me, you would be asked to do at least one landing at -6, and other configurations as well. 

[FlyingMonkey]

I'm going to take issue with the idea of "impact lift".  Really, the "hand out the window" concept is more of an illustration of drag, especially when you're talking subsonic aerodynamics..  See my blog "Only One Form of Lift"(June 20, 2016) at:  http://theandyzone.com/flightblog/

 

Andy, would you say that "impact lift" is actually a form of drag?

 

Let's say that we have an airplane, whose wings were flat sheets, set at some non-zero angle of attack as the airplane sat on the ground.  If the airplane added power and rolled down the runway, would it fly at some (probably very high) speed, or would it simply tool along on the ground at some speed slower than it would be if the wings had zero AoA?  

 

I have no idea, but if it would fly that argues for lift, if not that argues for drag.  

[FlyingMonkey]

The negative flaps are intended to lower air pressure above the wing by diverting air on the trailing edge of the top of the wing upward in cruise flight (0 or near 0 AoA).  This should have the effect of increasing velocity because lift is enhanced without increasing the throttle (power).  And therefore the plane should fly slightly faster at any given throttle position in cruise.

 

I don't think it's possible for lift to be equal at -6 and 0.  If that were the case the airplane would stall and land at the same speeds in both settings.  It would also mean reflex flaps would be a "free" way to add speed, and every airplane would have them.

 

I always read/learned that you can't reduce drag *and* lift at the same time, since induced drag from an airfoil is a function of lift.

[Andy]

Hi Mr. Flying Monkey,

 

Yes, I was saying that what some folks are calling "impact lift" is really drag acting in a vertical direction.  The analogy you used is spot on; it's debatable that you could ever generate enough lift to fly with flat plate wings which is why we don't use them.  The CT flies rather well on only 100 horsepower because the airfoil generates enough lift to get us in the air while still leaving us some excess horsepower to climb with.  For your flat plate-wing airplane, you could get it airborne if you had enough excess thrust so it was more of a rocket than an airplane and you generate an exception.  (You know, the old joke about the F-4 being an illustration of the ability to fly a barn door if you put enough thrust behind it.)

 

Thanks for the comments.  I was thinking along the same lines about how to illustrate the argument better, and maybe if I ran some numbers, it might show it. (Assume the lift coefficient is zero and use only drag and then see what happens as you change only the wing area and the angle of attack.) 

[gbigs]

Done it several times. Use -6 regularly on high cross winds landings. Used it with John Hurst on board, and is recommended by him on high cross wind landings. No white knuckles whatsoever.

 

Cheers

 

Such a configuration is dangerous at low speed.  You get no help with lift at near stall speed so must use power - presumed you did this with strong upwinds on the runway.  But the POH still calls for 15 flaps in xwinds. 

[gbigs]

I don't think it's possible for lift to be equal at -6 and 0.  If that were the case the airplane would stall and land at the same speeds in both settings.  It would also mean reflex flaps would be a "free" way to add speed, and every airplane would have them.

 

I always read/learned that you can't reduce drag *and* lift at the same time, since induced drag from an airfoil is a function of lift.

 

The negative flaps are intended to be used when AoA is zero or near zero as is the case in level cruise.   Remember lift is a function of Newton not Bernoulli.  The down force below the wing increases pressure (the more AoA the greater this force) and produces the equal and opposite lifting force.   The airfoil shape above the wing diverts air to create a low pressure 'vacuum' of air making less resistance to the upward movement.  In level flight AoA is small so the down pressure on the wing is small but some lift can be scavenged by changing the trailing edge of the wing to fling air upward as is the case with negative flaps and thus lower pressure above the wing a bit more.

[Doug G.]

Other than it blurring the definition of lift and drag, I agree. A kite provides a good example I think. They fly with only drag and AoA in a sense. Although there is still some impact lift (very small).

Any particle hitting an object imparts force, right?

[Andy]

The negative flaps are intended to be used when AoA is zero or near zero as is the case in level cruise.   Remember lift is a function of Newton not Bernoulli.  The down force below the wing increases pressure (the more AoA the greater this force) and produces the equal and opposite lifting force.   The airfoil shape above the wing diverts air to create a low pressure 'vacuum' of air making less resistance to the upward movement.  In level flight AoA is small so the down pressure on the wing is small but some lift can be scavenged by changing the trailing edge of the wing to fling air upward as is the case with negative flaps and thus lower pressure above the wing a bit more.

 

You're incorrect in saying lift is a function of Newton and not Bernoulli.  Suggest you read my blog I mentioned above.  Applying Newton's Third to understand and evaluate lift is a more involved process than you might think, while Bernoulli's principles much more easily explain it which is why they are more routinely used, especially in a wind tunnel. Lift is generated by the total flow field around the airfoil, and the major pressure differential is caused by the negative pressure area above the wing being typically larger than the positive pressure below (Bernoulli) ; lift can be found by integrating and summing the force contribution of the two areas.  There is no downforce below the wing per se; the momentum change of the airflow is caused by the circulatory field around the airfoil and the aircraft and can be analyzed to find lift (which shows up in downwash behind whichever one you're looking at); but to do that practically means measuring the total downwash around the entire aircraft (Newton).  

 

Haven't seen or pursued any technical details about the actual effects of (change in pressure distribution or resultant impact on the total lift vector) associated with negative flaps but suspect it doesn't affect lift as much as drag and also suspect it's due to a slight forward shift of the orientation of the lift vector,i.e, a decrease in induced drag.  (The negative flaps slightly reduce the effective angle of attack of the wing which does reduce induced drag; there is a slight lift decrease but I believe it to be quickly recovered by the increase in speed that occurs.)

[FlyingMonkey]

Hi Mr. Flying Monkey,

 

Yes, I was saying that what some folks are calling "impact lift" is really drag acting in a vertical direction.  The analogy you used is spot on; it's debatable that you could ever generate enough lift to fly with flat plate wings which is why we don't use them.  The CT flies rather well on only 100 horsepower because the airfoil generates enough lift to get us in the air while still leaving us some excess horsepower to climb with.  For your flat plate-wing airplane, you could get it airborne if you had enough excess thrust so it was more of a rocket than an airplane and you generate an exception.  (You know, the old joke about the F-4 being an illustration of the ability to fly a barn door if you put enough thrust behind it.)

 

Thanks for the comments.  I was thinking along the same lines about how to illustrate the argument better, and maybe if I ran some numbers, it might show it. (Assume the lift coefficient is zero and use only drag and then see what happens as you change only the wing area and the angle of attack.) 

 

Thanks Andy, I think I agree with everything you said.

[FlyingMonkey]

Other than it blurring the definition of lift and drag, I agree. A kite provides a good example I think. They fly with only drag and AoA in a sense. Although there is still some impact lift (very small).

Any particle hitting an object imparts force, right?

 

My friend's Avid Flyer is another example.  The tail is completely "Newtonian"...it has no airfoil shape, just straight metal frames covered in cloth.  The elevator works completely by the force of wind hitting it when deflected.  This has the advantage of simplicity, but I imagine two disadvantages:  high drag, since it essentially works by creating asymmetric drag, and secondly it probably loses effectiveness faster at low speed because there is no airfoil and the force of air hitting it drops off with the square of the speed.  In other words, at 50mph, the force that the tail can generate is one fourth of what it can generate at 100mph.  Airfoils maintain better forces until near their stall AoA, at which point the lift (tail force) they can generate falls to near zero.

[FlyingMonkey]

Of course, the horizontal stabilizer exerts a downward force to offset the CG forward of the center of lift.  That's why airplanes pitch nose down when you slow down.  It's also why an airplane loaded toward the rear will fly faster than an airplane loaded toward the front.  The horizontal stabilizer doesn't need to exert as much downward force.

 

All true, but you can certainly run out of pitch authority at low speed, and my point is that that will happen sooner in a tail that is not an airfoil,  Unless the horizontal stab is made very large, in which case it will become twitchy at speed due to excess pitch authority.

[coppercity]

Of course, the horizontal stabilizer exerts a downward force to offset the CG forward of the center of lift.  That's why airplanes pitch nose down when you slow down.  It's also why an airplane loaded toward the rear will fly faster than an airplane loaded toward the front.  The horizontal stabilizer doesn't need to exert as much downward force.

 

We should have stuck with Canards like the Wright Flyer   Then we could have a lifting tail

[gbigs]

All true, but you can certainly run out of pitch authority at low speed, and my point is that that will happen sooner in a tail that is not an airfoil,  Unless the horizontal stab is made very large, in which case it will become twitchy at speed due to excess pitch authority.

 

Okay, now that we have solved why negative flaps scavenge a little lift when in cruise thus producing slightly more velocity for any given throttle position (saving fuel).  

 

Why are neg flaps useless in climb and landing configs.

 

In climb, lift is mainly due forces below the wing (high AoA) so negligible air deflection from negative flaps are useless.  In landing and slow flight, stall speeds increase due to less force below the wing.  Flaps obviously scoop and capture the air below the wing increasing below wing forces gaining back lift (and drag) and reducing stall speeds.  So anyone doing 0 or negative flap landings are toying with higher stall speeds in slow flight close to the ground....

[Tom Baker]

My friend's Avid Flyer is another example. The tail is completely "Newtonian"...it has no airfoil shape, just straight metal frames covered in cloth. The elevator works completely by the force of wind hitting it when deflected. This has the advantage of simplicity, but I imagine two disadvantages: high drag, since it essentially works by creating asymmetric drag, and secondly it probably loses effectiveness faster at low speed because there is no airfoil and the force of air hitting it drops off with the square of the speed. In other words, at 50mph, the force that the tail can generate is one fourth of what it can generate at 100mph. Airfoils maintain better forces until near their stall AoA, at which point the lift (tail force) they can generate falls to near zero.

Andy, when you have a separate stab and elevator that bends at a hinge line you have created an airfoil.

[FlyingMonkey]

Andy, when you have a separate stab and elevator that bends at a hinge line you have created an airfoil.

I agree, but it's a sort of "virtual airfoil" with the chord line drawn between the leading and trailing edges. This is how single surface ultralight wings work, but they are very draggy and it's much more efficient and higher performing to have a solid wing surface. Even higher performing ultralights have two-surface wings.

[ralarcon]

Such a configuration is dangerous at low speed.  You get no help with lift at near stall speed so must use power - presumed you did this with strong upwinds on the runway.  But the POH still calls for 15 flaps in xwinds. 

When you develop the skill of landing at all flap settings, the danger decreases it does not increase. Furthermore the ability to land at lower flap settings, increases rudder authority, thus increasing the safety margin , when landing in high cross winds. Having said that, I admit it is a skill that requires, first learning, and second practice, to remain proficient. It is a skill worth developing.

 

Cheers

[FlyingMonkey]

When you develop the skill of landing at all flap settings, the danger decreases it does not increase. Furthermore the ability to land at lower flap settings, increases rudder authority, thus increasing the safety margin , when landing in high cross winds. Having said that, I admit it is a skill that requires, first learning, and second practice, to remain proficient. It is a skill worth developing.

 

Cheers

 

Not to mention we should be practicing slow flight and stalls at all flap settings, so that we understand the behavior of the airframe at all configurations within the flight envelop.  Slow flight and stalls at -6 are no more scary than an 0, 15, 30, or 40.  In any config the CT has about the most docile handling imaginable.  

[ralarcon]

Not to mention we should be practicing slow flight and stalls at all flap settings, so that we understand the behavior of the airframe at all configurations within the flight envelop.  Slow flight and stalls at -6 are no more scary than an 0, 15, 30, or 40.  In any config the CT has about the most docile handling imaginable.  

Correct !

[gbigs]

When you develop the skill of landing at all flap settings, the danger decreases it does not increase. Furthermore the ability to land at lower flap settings, increases rudder authority, thus increasing the safety margin , when landing in high cross winds. Having said that, I admit it is a skill that requires, first learning, and second practice, to remain proficient. It is a skill worth developing.

 

Cheers

 

Landing without flaps is only recommended on long runways where you have the room needed to bleed off the excess speed.  No one needs to develop an unnecessary skill landing at higher stall speeds.  The POH states the plane can be landed near it's max xwind component using 15 flaps.   Landing with full flaps is also a rarely needed skill unless you know you will be landing on grass or soft fields from time to time.  But landing with full flaps is not dangerous, landing with zero or negative flaps is dangerous.

[ralarcon]

A pilot should know how to land with each and every flap setting.  You don't know when you may have an electrical failure, etc.  Each flap setting has a different stall speed.  If you know the stall speed and fly accordingly, landing with zero flaps is not dangerous.  You will use more runway, not bleeding off speed, but because your stall speed and landing speed will be higher.  There is an element of risk landing faster due to the increased inertia if you have a problem.  If you haven't learned to land your plane at every flap setting, you have not been properly checked out.

Agree.

 

Cheers