FLAP RETRACTION AFTER TAKEOFF

[donkaye]

11 minutes ago, PeteMc said:

CAN sink sure, especially if you're a little to fast with the flap retraction and not paying attention (which may be the last time that happens....)  But if discussing a normal take-off, do you really think you actually SINK when you retract the flaps?  Plane is climbing AND accelerating and yes you do change the coefficient of lift as you said.  But do you actually think for that moment you're retracting the flaps you're behind the curve and loose all lift resulting in a momentary SINK?  (Again, assuming you don't do it right at the edge of stall.)

I agree with the others that have said you get a pitch change and I agree there is some REDUCTION in lift as the flaps are retracted.  But I don't think you loose all your lift and actually start sinking, aka actually loosing altitude.  But this slight slowing in the climb rate and change in pitch angle could make people think they are sinking.

Might be an interesting flight to hold specific climb angle and see what happens to speed and rate of climb as you raise and lower the flaps to TO position.  (But I bet the real numbers are in some flight test report at Mooney. )

 

My real concern would be for high DA takeoffs with quick gear and flap retraction.  If you're always quick on the draw for gear and flap retraction (as I've seen many times) there's the likelihood you'll do the same at a time when it is inappropriate, therefore my 50' or no more runway for gear retraction and 300' for flap retraction.

[PT20J]

Recall, that lift is constant in unaccelerated flight whether climbing, descending or level. So, if you abruptly change lift you will create an acceleration. Humans are sensitive to that. The sinking feeling comes from sensing the downward acceleration. It's only a small change in lift and a thus a small acceleration, but it can feel more significant when close to the ground.

Also recall that lift is proportional to the square of airspeed, so the loss of lift from the flap retraction can be countered by increasing airspeed which is what happens if the flaps raise slowly while the aircraft is allowed to accelerate. The primary reason not to raise the flaps until clear of the obstacles is because you may already be at a low airspeed (Vx) so you don't want to increase pitch to increase angle of attack (and lift) and slow further, and you don't want to lower the nose to increase airspeed (to increase lift) because that will lower climb angle. So you are between a rock and a hard place.

The M20J has electric flaps and with King autopilots, the manual electric trim speed is well matched to the flap motor speed. I find that during takeoff, if I maintain about a 7.5 deg pitch attitude and raise the flaps at 80 KIAS while holding nose down trim that the airplane accelerates nicely, stays in trim and doesn't sink. With hydraulic flaps you have to learn to match pitch forces and/or manual trim wheel movement to accomplish the same result.

Skip

 

[DCarlton]

To me it feels more like a bump but a bump that falls away from "the seat of your pants"?  Not a sluggish sink.  Don't know if you can see it on the altimeter.  So... it does feel like a brief downward acceleration as mentioned above.  

Edited July 9, 2021 by DCarlton

[PT20J]

2 minutes ago, donkaye said:

My real concern would be for high DA takeoffs with quick gear and flap retraction.  If you're always quick on the draw for gear and flap retraction (as I've seen many times) there's the likelihood you'll do the same at a time when it is inappropriate, therefore my 50' or no more runway for gear retraction and 300' for flap retraction.

This is an excellent point. I believe in standardization, but not to the point of ignoring the differences in airports or conditions. An excellent example is KMMH (Mammoth Lakes, CA). KMMH has a 7000' runway at 7135' MSL oriented east and west. I'd always be off well before mid field, and with high density altitude, you'd think raising the gear and flaps as soon as possible would be in order. But when there is a crosswind out of the south, it funnels down over Convict Lake and hits the airport midfield creating a heck of a wind shear. You probably don't have enough power to climb out of it but you can usually fly through it if you give up altitude for airspeed. And, with the gear and flaps still down it is easy to land on the remaining runway if necessary.

Skip

[Shadrach]

Just now, DCarlton said:

To me it feels more like a bump but a bump that falls away from "the seat of your pants"?  Not a sluggish sink.  Don't know if you can see it on the altimeter.  

That would require a loss in altitude which I would bet dollars to donuts is not happening. What we “feel” and what is actually happening is not always the same thing.

[DCarlton]

5 minutes ago, Shadrach said:

That would require a loss in altitude which I would bet dollars to donuts is not happening. What we “feel” and what is actually happening is not always the same thing.

Or it's just a brief downward acceleration with no significant change in altitude as mentioned above.  I'm going to start feeling and watching more closely when I fly.  

[Shadrach]

42 minutes ago, DCarlton said:

Or it's just a brief downward acceleration with no significant change in altitude as mentioned above.  I'm going to start feeling and watching more closely when I fly.  

Think this through with me. You’re on climb out from lowland airport at a relatively conservative 700 ft./min. That’s a vertical speed of 8mph (think fast jog). You raise flaps and in a second or two the vertical vector not only stops but reverses direction and then reverses again back to climb? Sorry, but I don’t think that’s the case. What’s more plausible is that a momentary change in both climb rate, speed and pitch attitude are combining to give you the sensation of sink. 

[GeeBee]

2 hours ago, PeteMc said:

CAN sink sure, especially if you're a little to fast with the flap retraction and not paying attention (which may be the last time that happens....)  But if discussing a normal take-off, do you really think you actually SINK when you retract the flaps?  Plane is climbing AND accelerating and yes you do change the coefficient of lift as you said.  But do you actually think for that moment you're retracting the flaps you're behind the curve and loose all lift resulting in a momentary SINK?  (Again, assuming you don't do it right at the edge of stall.)

I agree with the others that have said you get a pitch change and I agree there is some REDUCTION in lift as the flaps are retracted.  But I don't think you loose all your lift and actually start sinking, aka actually loosing altitude.  But this slight slowing in the climb rate and change in pitch angle could make people think they are sinking.

Might be an interesting flight to hold specific climb angle and see what happens to speed and rate of climb as you raise and lower the flaps to TO position.  (But I bet the real numbers are in some flight test report at Mooney. )

 

An accelerating airplane likely will not sink because the aerodynamic pressure is increasing faster than the loss of coefficient of lift.

We would do well to review the lift equation. L=Cl x q x S where CL is the coefficient of lift, q is the aerodynamic force and S is the wing area.

q is calculated as 1/2 x p x V2 where p is the air density and V is the velocity. Notice the velocity is squared so a little bit of acceleration goes a long ways towards negating the loss in lift from a drop in CL

This is why an accelerating airplane will not sink, but a non accelerating airplane will sink. The numbers do not lie unless you want to repeal the laws of aerodynamics.

[GeeBee]

2 hours ago, donkaye said:

My real concern would be for high DA takeoffs with quick gear and flap retraction.  If you're always quick on the draw for gear and flap retraction (as I've seen many times) there's the likelihood you'll do the same at a time when it is inappropriate, therefore my 50' or no more runway for gear retraction and 300' for flap retraction.

Obviously, you have a low aerodynamic force because you have a low p which results in low q. Your only choice is to increase V or Cl. Increasing CL brings on a whole host of problems including getting on the back side of the power curve.

[steingar]

Perhaps I'm a dullard, but I have never experienced sink from raising takeoff flaps.  I can't see why you would, since the wings go from making more lift to less in an already dynamic environment.  Sounds like an old wives tale to me.  Might do things different in high DA, but I might do a lot of things different, like only take off first thing in the morning.  One thing for certain, I'll know it when I see it.

 

[MooneyTunes]

Altitude is your friend. Gain it as fast as you can. Specially in densely populated surroundings. 

[steingar]

4 minutes ago, MooneyTunes said:

Altitude is your friend. Gain it as fast as you can. Specially in densely populated surroundings. 

A good way to get more altitude is to make your wings more efficient.  Still, I'm not pointing my nose straight up in an aircraft notorious for engine cooling issues.  you do as you like.

[thinwing]

18 hours ago, donkaye said:

My mentor instructor, who was an aeronautical engineer, said that approach flaps (takeoff flaps) provide ⅔ as much lift as drag while full flaps provide ⅔ as much drag as lift.  So, gear up at 50' or no more runway to land on and flaps up no less that 300' (to account for sink when flaps are raised) or after I'm done with a takeoff turn (for added safety margin).  That's the way I teach it.

and that is the way I fly it...at least in a long body

[carusoam]

Does it help to look at it a different way?

 

When putting flaps down…

We know the center of lift moves back…

So we trim like crazy to keep the nose from falling…

While under low power…

Speed is changing slowly…

Very little trim required after that…

 

Now, we are under full power…

We take the flaps up…

Center of lift moves forward…

Speed is changing rapidly…

Plenty of downward trim change is expected as the speed increases…

The need for changing trim while raising T/O flaps is often masked by the larger need to trim for increasing speed…

 

Once above 90kias… it is hard to feel T/O flaps being retracted in my plane… they are motorized and move pretty slowly…

 

Use caution when setting T/O trim too high… the act of taking out flaps… while the plane is nicely trimmed with them in…  may cause the nose to bob upwards…(?)  gentle pull on the yoke makes it easy to get the stall warning…

 

There is a lot going on in the T/O phase…  changes occur quickly…

We have had three MS planes stall on T/O…

Chance of survival is less than 50/50…

M20E, M20R, and M20J… no preference for body length.

Military pilot, aviation college pilot, and a GA pilot… no preference for pilot training type.

No method to define who is next…

So… be on your toes.  use all of the runway. Know your DA. Do your WnB. Check the charts. Follow the procedure…. Stay alive…   
 

PP summary of experiences gained around here…

Best regards,

-a-

 

 

[jlunseth]

1 hour ago, carusoam said:

Use caution when setting T/O trim too high… the act of taking out flaps… while the plane is nicely trimmed with them in…  may cause the nose to bob upwards…(?)  gentle pull on the yoke makes it easy to get the stall warning…

Yes, true. However, my plane and many of our Mooneys have very forward CG’s, especially with two guys sitting in the front seats and little or no luggage. They have a tendency to wheelbarrow during the takeoff, that is, lift off one or both mains and run on the nose tire. A little up trim and a little back pressure during the roll will keep the aircraft on the mains until liftoff, where it should be. Certainly the big engines would want to take care in how much up trim, if any, they put in.

[larrynimmo]

On 7/8/2021 at 11:10 AM, PT20J said:

Flaps shorten the takeoff roll but decrease climb rate. Generally, the reduced takeoff roll is more advantageous than the improved climb to clear the FAA’s standard 50’ obstacle. Many light airplane POHs say to raise flaps after the obstacles are cleared because after that they are hurting rather than helping. I generally raise the flaps in my M20J accelerating through 80 KIAS. I don’t like to do it too low because I use the same procedures IFR and VFR. There is a significant trim change with flap retraction and I don’t want to be distracted unnecessarily at low altitude on an IFR departure. 

Skip

Skip,

I like to be fully configured for an ifr climb before I enter the clouds…so I will raise my flaps a little earlier than normal 

[Shadrach]

14 hours ago, carusoam said:

Does it help to look at it a different way?

 

When putting flaps down…

We know the center of lift moves back…

So we trim like crazy to keep the nose from falling…

While under low power…

Speed is changing slowly…

Very little trim required after that…

 

Now, we are under full power…

We take the flaps up…

Center of lift moves forward…

Speed is changing rapidly…

Plenty of downward trim change is expected as the speed increases…

The need for changing trim while raising T/O flaps is often masked by the larger need to trim for increasing speed…

 

Once above 90kias… it is hard to feel T/O flaps being retracted in my plane… they are motorized and move pretty slowly…

 

Use caution when setting T/O trim too high… the act of taking out flaps… while the plane is nicely trimmed with them in…  may cause the nose to bob upwards…(?)  gentle pull on the yoke makes it easy to get the stall warning…

 

There is a lot going on in the T/O phase…  changes occur quickly…

We have had three MS planes stall on T/O…

Chance of survival is less than 50/50…

M20E, M20R, and M20J… no preference for body length.

Military pilot, aviation college pilot, and a GA pilot… no preference for pilot training type.

No method to define who is next…

So… be on your toes.  use all of the runway. Know your DA. Do your WnB. Check the charts. Follow the procedure…. Stay alive…   
 

PP summary of experiences gained around here…

Best regards,

-a-

 

 

Haven’t most of stalls been a result of trying to out climb obstacles? Or lifting off in ground effect on short runways.  I’m not trying to be controversial but I think the danger of a departure stall from retracting flaps is often overstated. The pitch up can feel dramatic but I think it’s more a function of the change in the effective cord line as the flaps come up. I’d bet AOA does not change that much. Of course there’s additional fuselage drag from the increase in pitch so I can’t say for sure.  If I ever finish annual, I’ll do some testing at altitude. 

[exM20K]

Every once in a while, I retract the flaps after initial level-off when I see I’m 10-15 knots slow.

-Dan

[Shadrach]

1 hour ago, exM20K said:

Every once in a while, I retract the flaps after initial level-off when I see I’m 10-15 knots slow.

-Dan

This has never...ever happened to me but, I've heard from others that it only cost about 8KIAS in the 200hp birds.

[Rwsavory]

1 hour ago, Shadrach said:

This has never...ever happened to me but, I've heard from others that it only cost about 8KIAS in the 200hp birds.

What a coincidence! That’s never happened to me either.

[DCarlton]

23 hours ago, Shadrach said:

Think this through with me. You’re on climb out from lowland airport at a relatively conservative 700 ft./min. That’s a vertical speed of 8mph (think fast jog). You raise flaps and in a second or two the vertical vector not only stops but reverses direction and then reverses again back to climb? Sorry, but I don’t think that’s the case. What’s more plausible is that a momentary change in both climb rate, speed and pitch attitude are combining to give you the sensation of sink. 

Perhaps it is the pitch change that makes the bump feel like its downward in the pilots seat.  Next time I fly, I'm gonna go play with my flaps and observe the bump a little more.  

[carusoam]

On 7/10/2021 at 8:30 AM, Shadrach said:

Haven’t most of stalls been a result of trying to out climb obstacles? Or lifting off in ground effect on short runways.  I’m not trying to be controversial but I think the danger of a departure stall from retracting flaps is often overstated. The pitch up can feel dramatic but I think it’s more a function of the change in the effective cord line as the flaps come up. I’d bet AOA does not change that much. Of course there’s additional fuselage drag from the increase in pitch so I can’t say for sure.  If I ever finish annual, I’ll do some testing at altitude. 

two were trees that got really big in the windshield…

Patrick’s J mushed into the tree tops…

The short body didn’t get very far before fighting to recover from the stall…

The Long body used an unusually steep climb for an unknown reason…. (Security video showed a lot… and the ADSB-out data had some good info for a few seconds…) spiraled to the ground…

I don’t recall if any flap status is known for these three accidents…. For the Long body, many people were interested to know if the flaps and trim were reset after the previous flight, earlier in the day…

Fuzzy PP memory at best…. No harm meant for our lost Mooney brothers…  just good information to share…

Best regards,

-a-

[Ricky_231]

On 7/9/2021 at 8:04 PM, jlunseth said:

Yes, true. However, my plane and many of our Mooneys have very forward CG’s, especially with two guys sitting in the front seats and little or no luggage. They have a tendency to wheelbarrow during the takeoff, that is, lift off one or both mains and run on the nose tire. A little up trim and a little back pressure during the roll will keep the aircraft on the mains until liftoff, where it should be. Certainly the big engines would want to take care in how much up trim, if any, they put in.

I had the O2 bottle on my K taken out for a hydrostatic test and holy smokes - even with almost 50lbs of "ballast" in the baggage compartment, the first couple of take offs were very weird. Exactly what you're describing - the mains would pop off the ground and it took a brisk, firm pull of the yoke to get the plane pointing in the right direction (up). More trim definitely helped.

[Mooneymite]

On 7/8/2021 at 8:10 PM, donkaye said:

... or after I'm done with a takeoff turn (for added safety margin). ...

I think Don makes a great point here and one that rarely gets mentioned in these threads about flap retraction.  People argue about what speed and altitude to retract, but never seem to mention a departure turn.

The departure turn can be steeper and cover less ground after takeoff if the flaps are left alone until rolling wings level headed for the destination.  Simple technique.

[A64Pilot]

47 minutes ago, Mooneymite said:

I think Don makes a great point here and one that rarely gets mentioned in these threads about flap retraction.  People argue about what speed and altitude to retract, but never seem to mention a departure turn.

The departure turn can be steeper and cover less ground after takeoff if the flaps are left alone until rolling wings level headed for the destination.  Simple technique.

The Air Tractor guys won’t even consider a turn when loaded without flaps, to the point that the manufacturer puts the flap switch on the control stick.

What they are doing probably without realizing it is washing the wing out, an airplane with little washout can tip stall in a turn and that’s real bad, so by adding flap before the turn it ensures that a stall will begin inboard, where the flaps are.

Edited July 12, 2021 by A64Pilot