Flaps on take-off?

[Super Dave]

I think this is incorrect.  You assume the plane climbs faster without flaps, or to put it another way, that your rate of climb at Vy without flaps is greater than rate of climb at Vy with flaps.  Vy is actually achieved with 15 degrees of flaps, at least on some models.  I've attached the Vy charts from the POH of a 1968 C model, which specifies 15 degrees of flaps be used.

 

Interestingly, I just looked and the same chart in my current M20K model, also attached, which says "Flaps Up" for Vy.  So it's possible we have a difference between the short and mid bodies on this subject.  The same POH also specifies 10 degrees of flaps for best takeoff distance.

 

Also, it's not like using takeoff flaps results in standing the plane on its tail and roasting the engine at 450 dF.  We're talking about partial flaps until the plane is in the air and the gear is tucked up.  The quicker liftoff and higher initial climb rates very well might put your same theoretical plane well above its no-flap counterpart, at a higher airspeed, with cooler CHTs, at any given time during the first two minutes of flight.

I'd like to stop beating on poor old Trigger, but performance and aerodynamics are subjects near and dear to my heart and I can't bring myself to walk away.

It's not an assumption of mine that planes climb faster without flaps, it's an irrefutable law of aerodynamics.

Climb performance is determined strictly by excess power or excess thrust. In propeller aircraft, best rate of climb (Vy) occurs at the speed which yields the greatest margin between power required and power available. In jet and propeller aircraft, the best angle of climb (Vx) occurs at the speed which yields the greatest margin between thrust required and thrust available.

Extending flaps to any degree increases the coefficient of lift. This means that a wing with flaps extended can produce the same amount of lift at a lower speed than the clean wing does. This reduces stall speed and allows us to takeoff or land at a slower airspeed, making for shorter takeoff or landing rolls. Unfortunately there is no free lunch because flaps also increase the coefficient of total drag. Total drag is the sum of the coefficient of drag at zero lift (a measure of parasite drag), and the coefficient of induced drag. This is where the penalty lies - the coefficient of induced drag is proportional to the SQUARE of the coefficient of lift. Despite your flight instructor telling you that such and such a flap angle "increases lift more than drag" (my first flight instructor told me the same thing), extending flaps to any degree will always increase the coefficient of drag proportionately more than the coefficient of lift.

One other note; because extending flaps increases the coefficients of lift and drag, the airspeeds at which maximin excess thrust and power occur will be reduced. The POHs for some aircraft will list Vy or Vx for various configurations, and the airspeed values will always be somewhat lower with flaps or gear extended. As will be the rate or angle of climb.

As for the POH of your '68C, I was puzzled, but I found a complete POH for a '66 C on Delta's website and it has the same information (although in a graph instead of a table) specifying "gear up-flaps15"on the graph. However, just above the graph it says "CLIMB CONFIGURATION: Gear Up - Best Power Mixture - Cowl Flaps Open - Flaps Up". Mooney built great airplanes, but Cessna, Piper and Beech were way ahead on POHs.

[Shadrach]

I'd like to stop beating on poor old Trigger, but performance and aerodynamics are subjects near and dear to my heart and I can't bring myself to walk away.

It's not an assumption of mine that planes climb faster without flaps, it's an irrefutable law of aerodynamics.

Climb performance is determined strictly by excess power or excess thrust. In propeller aircraft, best rate of climb (Vy) occurs at the speed which yields the greatest margin between power required and power available. In jet and propeller aircraft, the best angle of climb (Vx) occurs at the speed which yields the greatest margin between thrust required and thrust available.

Extending flaps to any degree increases the coefficient of lift. This means that a wing with flaps extended can produce the same amount of lift at a lower speed than the clean wing does. This reduces stall speed and allows us to takeoff or land at a slower airspeed, making for shorter takeoff or landing rolls. Unfortunately there is no free lunch because flaps also increase the coefficient of total drag. Total drag is the sum of the coefficient of drag at zero lift (a measure of parasite drag), and the coefficient of induced drag. This is where the penalty lies - the coefficient of induced drag is proportional to the SQUARE of the coefficient of lift. Despite your flight instructor telling you that such and such a flap angle "increases lift more than drag" (my first flight instructor told me the same thing), extending flaps to any degree will always increase the coefficient of drag proportionately more than the coefficient of lift.

One other note; because extending flaps increases the coefficients of lift and drag, the airspeeds at which maximin excess thrust and power occur will be reduced. The POHs for some aircraft will list Vy or Vx for various configurations, and the airspeed values will always be somewhat lower with flaps or gear extended. As will be the rate or angle of climb.

As for the POH of your '68C, I was puzzled, but I found a complete POH for a '66 C on Delta's website and it has the same information (although in a graph instead of a table) specifying "gear up-flaps15"on the graph. However, just above the graph it says "CLIMB CONFIGURATION: Gear Up - Best Power Mixture - Cowl Flaps Open - Flaps Up". Mooney built great airplanes, but Cessna, Piper and Beech were way ahead on POHs.

So what you're saying is that L/D ratio always goes down with the addition of take-off flaps? You're also saying that flaps cannot be designed it improve L/D ratio at lower speeds?  If this is the case, why does the AC lift off sooner with take off flaps? 

[Super Dave]

So what you're saying is that L/D ratio always goes down with the addition of take-off flaps? You're also saying that flaps cannot be designed it improve L/D ratio at lower speeds?  If this is the case, why does the AC lift off sooner with take off flaps?

Yes, L/Dmax will always go down with the extension of flaps. L/Dmax will also occur at a lower airspeed with the extension of flaps.

The aircraft lifts off sooner with takeoff flaps because the flaps increase the coefficient of lift. This means that the wing can produce the required lift at a lower speed than would be required with no flaps. Since the airplane needs less speed to lift off, it can accelerate to that lower speed in less distance. Once airborne, the airplane will not climb as well (in rate or gradient), but if we are concerned about a short runway or a 50' tree at the end of a short runway, we want takeoff flaps because the decrease in takeoff roll outweighs the penalty we pay in climb. If the obstacle were high enough and far enough away (we don't know how high or far because Mooney didn't supply the data) it would be better to leave flaps up because the improved climb performance will outweigh the longer takeoff roll.

We could shorten our takeoff roll even further by using full flaps (I'm not suggesting anyone actually tries this); The airplane would not climb very well at all once airborne, but you could easily beat book numbers on the takeoff roll. There is nothing magical or particularly efficient about takeoff flaps. Takeoff flap setting was determined to be the "right" amount of flap by company test pilots because that particular setting resulted in the least distance required to clear the industry standard 50' obstacle while still allowing acceptable climb performance. If the industry standard was a 25' obstacle, we would all be setting more flaps for takeoff, if the industry standard was a 100' obstacle, we would all be setting less flaps at takeoff.

As for designing a flap that increases coefficient of lift while also increasing L/Dmax, you may as well set out to invent a perpetual motion machine.

[gregwatts]

Dats what Im talkin bout!

[Bob_Belville]

Super Dave> Let's draw an elevation view superimposing two take offs. We all agree the takeoff using flaps will leave the r'way after a shorter roll and it will be able to climb at a lower IAS and move slower toward the "50' obstacle". The plane rolling without flaps will not rotate until it moves past the point where the first plane left the ground. Consider the flight paths in profile. At some point, the path of the second plane might cross the path of plane one, but maybe not if the first plane, after reaching 51' agl dumps flaps and trims to Vy (0 flaps). ISTM that while plane 2 will move down range faster, it is not at all clear plane 2 will get to cruise altitude in less time. Granted that flaps increase drag -- at the same speed -- since drag is proportional to the sq of speed, there is less induced drag on the relatively short flight segment flown with flaps. Finally,consider the extreme case... a five foot obstacle located at the point where the zero flap flight path rotates... Only one of the flights works.

[Super Dave]

Super Dave> Let's draw an elevation view superimposing two take offs. We all agree the takeoff using flaps will leave the r'way after a shorter roll and it will be able to climb at a lower IAS and move slower toward the "50' obstacle". The plane rolling without flaps will not rotate until it moves past the point where the first plane left the ground. Consider the flight paths in profile. At some point, the path of the second plane might cross the path of plane one, but maybe not if the first plane, after reaching 51' agl dumps flaps and trims to Vy (0 flaps). ISTM that while plane 2 will move down range faster, it is not at all clear plane 2 will get to cruise altitude in less time. Granted that flaps increase drag -- at the same speed -- since drag is proportional to the sq of speed, there is less induced drag on the relatively short flight segment flown with flaps. Finally,consider the extreme case... a five foot obstacle located at the point where the zero flap flight path rotates... Only one of the flights works.

First, the question of time to altitude, or climb rate: As you described, plane 2 will take a couple more seconds to reach flying speed, but once it does, it is immediately gaining on airplane 1 in height. This is nothing but a guess on my part, but I'd wager that airplane 2 reaches 50' before airplane 1. I am certain however, that airplane 2 will be higher than airplane 1 by the time airplane 1 dumps flaps and accelerates to Vy. Any advantage that was gained in the takeoff roll will have to be payed back at flap retraction, in addition to the penalty of carrying those flaps to 51'. Extended flaps will always increase the coefficient of drag proportionately more than the coefficient of lift.

Secondly, the question of climb angle: If airplane 2 accelerates to Vy after takeoff, it's path will never intersect with the path of airplane 1.

And yes, if airplane 1 just squeaks over the 50' obstacle, airplane two will not survive the experiment.

[Hank]

And that, Bob, is why you are supposed to THINK about what to do. With an obstacle 1000' away, you have selected a poor place to takeoff in your Mooney. Can the obstacle be reduced/removed? How about taking off in another direction, leaving cargo behind, etc.? Or taxi around the close obstacle and then takeoff . . . A final option would be drive home and part out the plane.

[Bob_Belville]

And that, Bob, is why you are supposed to THINK about what to do. With an obstacle 1000' away, you have selected a poor place to takeoff in your Mooney. Can the obstacle be reduced/removed? How about taking off in another direction, leaving cargo behind, etc.? Or taxi around the close obstacle and then takeoff . . . A final option would be drive home and part out the plane.

Hank> or use flaps? I stated 

 

First, the question of time to altitude, or climb rate: As you described, plane 2 will take a couple more seconds to reach flying speed, but once it does, it is immediately gaining on airplane 1 in height. This is nothing but a guess on my part, but I'd wager that airplane 2 reaches 50' before airplane 1. I am certain however, that airplane 2 will be higher than airplane 1 by the time airplane 1 dumps flaps and accelerates to Vy. Any advantage that was gained in the takeoff roll will be have to payed back at flap retraction, in addition to the penalty of carrying those flaps to 51'. Extended flaps will always increase the coefficient of drag proportionately more than the coefficient of lift.

Secondly, the question of climb angle: If airplane 2 accelerates to Vy after takeoff, it's path will never intersect with the path of airplane 1.

And yes, if airplane 1 just squeaks over the 50' obstacle, airplane two will not survive the experiment.

Thanks for the reply Dave. ISTM when you concede that path 2 will never intersect 1 you are verifying the reason to use flaps. Terrain clearance is one reason. path 1 is better. Taking off behind a heavy airliner is another.

 

Most Mooney pilots want to get their gear up pretty quickly after takeoff but path 1 will get rid of that big drag sooner. With gear up, lowering the nose a little to get from Vx (~70k) to Vy (~90k) path 1 will have to raise flaps pretty quickly since Vf is under 90k. My suspicion is that path 1 will be in the same climb configuration as path 1 at at least as high an altitude as path 1 which will mean time to cruise altitude for path 2 is not less than 1. In any case we're talking about a very some difference.    

[Bob_Belville]

And that, Bob, is why you are supposed to THINK about what to do. With an obstacle 1000' away, you have selected a poor place to takeoff in your Mooney. Can the obstacle be reduced/removed? How about taking off in another direction, leaving cargo behind, etc.? Or taxi around the close obstacle and then takeoff . . . A final option would be drive home and part out the plane.

Come on Hank, I outlined an extreme case so even the Texans on MS could get the picture. But what if it's not 1000', it's 1800' but it is a nice warm day, 85F, and the winds are shifting and... well just go back to my earlier post on this thread and read what actually happened in those conditions to N2566W (the tail number was later changed, twice, it is now N943RW!) August 17, 1968 @ O27 (Oakdale CA). 

 

The point I am trying to make is that if you want the safer SOP for taking off in an M20, you should be in the habit of using 15 deg flaps, rotate at ~ 60k and put some distance between yourself and mother earth. If for no other reason than because Murphy Rules the World. Or you can throw that scarf around your neck, pull down those goggles, and spit in the eye of fate and that old saw about old/bold pilots. 

[Glenn]

It may be safe to assume that the specification to use partial flaps on take-off in the POH was the result of controlled testing by well qualified professional test pilots, who wanted to get the maximum performance from the airplane.  That to me would constitute a good reason to follow their advice unless presented with very compelling evidence to the contrary.  

[201er]

The gear vs flaps retraction is a very interesting point. Since the drag of the gear is many times more than the drag of the flaps, the few seconds sooner that the flap-takeoff plane gets to a safe gear retraction point may nearly make up for the entire net drag of the time flying with flaps extended. Also less wear on the tires, better visibility, short field takeoff practice (to consistently know how much distance your plane actually needs to takeoff), good SOP that can someday save you, and a slightly slower impact if that flight is terminated prematurely. I really see no good argument not to use flaps on takeoff all the time.

 

Also I suspect (but perhaps someone could confirm) that flaps make a bigger difference when loaded heavy?

 

Oh and PS, to take the guess work of what flap extended Vx or Vy are, just get an angle of attack indicator!

[Shadrach]

Yes, L/Dmax will always go down with the extension of flaps. L/Dmax will also occur at a lower airspeed with the extension of flaps.

The aircraft lifts off sooner with takeoff flaps because the flaps increase the coefficient of lift. This means that the wing can produce the required lift at a lower speed than would be required with no flaps. Since the airplane needs less speed to lift off, it can accelerate to that lower speed in less distance. Once airborne, the airplane will not climb as well (in rate or gradient), but if we are concerned about a short runway or a 50' tree at the end of a short runway, we want takeoff flaps because the decrease in takeoff roll outweighs the penalty we pay in climb. If the obstacle were high enough and far enough away (we don't know how high or far because Mooney didn't supply the data) it would be better to leave flaps up because the improved climb performance will outweigh the longer takeoff roll.

We could shorten our takeoff roll even further by using full flaps (I'm not suggesting anyone actually tries this); The airplane would not climb very well at all once airborne, but you could easily beat book numbers on the takeoff roll. There is nothing magical or particularly efficient about takeoff flaps. Takeoff flap setting was determined to be the "right" amount of flap by company test pilots because that particular setting resulted in the least distance required to clear the industry standard 50' obstacle while still allowing acceptable climb performance. If the industry standard was a 25' obstacle, we would all be setting more flaps for takeoff, if the industry standard was a 100' obstacle, we would all be setting less flaps at takeoff.

As for designing a flap that increases coefficient of lift while also increasing L/Dmax, you may as well set out to invent a perpetual motion machine.

I never mentioned L/Dmax. I was talking about L/D ratio at rotation.  If D is increased proprotionately more than L by adding take-off flaps at rotation, why does the airplane lift off sooner? It would seem that the additional drag would lengthen the amount of time and distance required to get to lift off speed even though that lift off speed is somewhat lower.

[Bob_Belville]

I never mentioned L/Dmax. I was talking about L/D ratio at rotation.  If D is increased proprotionately more than L by adding take-off flaps at rotation, why does the airplane lift off sooner? It would seem that the additional drag would lengthen the amount of time and distance required to get to lift off speed even though that lift off speed is somewhat lower.

ISTM that what Dave's physics application ignores is that drag is not constant but increases with speed and that drag from flaps is not as high at the lower speed required to lift off with the higher lift the flaps creates. Our main objective at that critical stage of flight is maximizing our position AGL.

[Mooneymite]

Our Mooney POH's are very basic when it comes to the fine points of performance planning, but just to add some murkiness to the already muddy waters, I would point out that in the 121 world of airport analysis, the where and when of flap retraction is integral to the selection of the amount of flaps in the climb profile.  The airlines have performance people who do nothing but look at such.  We, on the other hand, very often have to make decisions on very little hard data.

 

Without detailed performance data, the trial and documentation required to make all the determinations would be onerous.  I suspect that's why Mooney kept it so simple.

 

OTOH, we don't have to consider loss of the critical engine in our calculations!   

[Shadrach]

ISTM that what Dave's physics application ignores is that drag is not constant but increases with speed and that drag from flaps is not as high at the lower speed required to lift off with the higher lift the flaps creates. Our main objective at that critical stage of flight is maximizing our position AGL.

While true for parasitic drag; induced drag is higher at lower speeds due to increased AOA. Perhaps, his point is that induced drag will in fact be higher because the the airplane is flying slower and therefore at a greater AOA. I still do not understand why the aircraft is able to achieve take off speed in a shorter distance and in less time. Given that power is constant, it seems it should take longer in terms of distance and time to overcome the proportional increase in drag...

[Bob_Belville]

While true for parasitic drag; induced drag is higher at lower speeds due to increased AOA. Perhaps, his point is that induced drag will in fact be higher because the the airplane is flying slower and therefore at a greater AOA. I still do not understand why the aircraft is able to achieve take off speed in a shorter distance and in less time. Given that power is constant, it seems it should take longer in terms of distance and time to overcome the proportional increase in drag...

AOA on the ground roll? 

[PMcClure]

I am not sure about all this.  But I am sure about one thing, if making an obstacle or not depends on using flaps or not, I am in the wrong place. Do some of us really run that tight in a box? Shouldn't the margin of error be left large enough that flap settings don't determine if we live or not?

[Bob_Belville]

I am not sure about all this.  But I am sure about one thing, if making an obstacle or not depends on using flaps or not, I am in the wrong place. Do some of us really run that tight in a box? Shouldn't the margin of error be left large enough that flap settings don't determine if we live or not?

It's a relative thingy. But most of us do not restrict ourselves to 10,000' runways. Or even 4000' in an aircraft that is supposed to only need 760' ground roll or 1300' over 50' @ MGW at sea level on a standard day.

 

There are lots of variables, some plus, some minus, temp, DA, wind, engine and airframe condition, runway condition, slope, pilot skill... most of us try not to get too many thingies working against us at the same time. Intersection T.O.? Probably not.

 

I was taking off from Lancaster PA last July. Full fuel, 3 full size adults, luggage - right at GW, 99F.! I declined the tower's offer of rwy 31 (4102') and taxied an extra mile all the way to 26 (6933') I suppose I used less than half of the runway but I'd make that call every time.

[Marauder]

When we last left our flap castaways, they were discussing the merits of using or not using flaps. Here we are, pages later, and I am glad to see some things in this world never change. Let me know when you guys have all this figured out... Just looking for the Cliff Notes version.

[Shadrach]

I am not sure about all this.  But I am sure about one thing, if making an obstacle or not depends on using flaps or not, I am in the wrong place. Do some of us really run that tight in a box? Shouldn't the margin of error be left large enough that flap settings don't determine if we live or not?

 

I do not think I run on the ragged edge, but there are certainly airports that I operate in and out of that would become “no go” if flaps were inop. Anything under 1800ft or with fair condition turf, especially in the summer time... There is a little strip close by that is both less than 1800ft and has fair condition turf.  The faster I’m up and in ground effect the better.

[scottfromiowa]

I believe my POH says on a 100 degree day I need 2500 feet of paved to get airborne and clear a 50 feet obstacle. Now I am going to check again as your scenario sounds ragged to me.

[fantom]

When we last left our flap castaways, they were discussing the merits of using or not using flaps. Here we are, pages later, and I am glad to see some things in this world never change. Let me know when you guys have all this figured out... Just looking for the Cliff Notes version.

 

Just a little flap here, and much more educational than debating the Garmin loving dent

[Shadrach]

I believe my POH says on a 100 degree day I need 2500 feet of paved to get airborne and clear a 50 feet obstacle. Now I am going to check again as your scenario sounds ragged to me.

 

At sea level on 100df day, mine says 1670ft to clear a 50ft obstacle (1070 to get airborne and 600 to climb 51 feet ). However those numbers are for MGTOW. I am usually much closer to or less than 2300lbs. At that weight, the numbers are 690ft ground roll + 505ft air distance = a total 1195 to clear the 50ft.  The margins are not as close as you think....but plenty close for me.

[Marauder]

You know guys, if you keep playing with your flaps like this you are going to go blind.

[Marauder]

Just a little flap here, and much more educational than debating the Garmin loving dent

 

You think? He is in such a state of denial...