C172 down at KMYF?

Mikey52 said:
It depends on the plane. In a DC-3, if you needed maximum braking, retracting the flaps did two things that helped. It reduced lift, and shifted the cg slightly aft. Those were the split flap configuration.
Click to expand...
It does WHAT? It moves the center of lift, but I can't believe it will appreciably change the CG. Anyhow, the CG was already behind the main gear; putting more weight on the tail wheel isn't going to make things better.
 
You're correct I meant the center of lift. The braking is done with the tail in the air. So it helps balancing the brake forces to keep you from noseing over.
 
ionel said:
Why would you dump the flaps directly after rotate? You aren't even out of ground affect. You've prob got runway ahead to land on if engine sputters etc.
Click to expand...
As with many things, it is always airplane dependent. Believe it or not, the 1948 Cessna 170 owners' manual for a short field takeoff actually tells you to start the takeoff roll with flaps up and then add full flaps with the johnson bar right at the point of 'rotation'.

The original Navy TBF/TBM Avenger guidance for short field (land that is) also called for full flaps selected/applied at the point of liftoff.

In both of those airplanes, you get to liftoff speed (not really rotation because those airplanes will fly off the ground when ready with no back pressure on the elevator required) faster and when you apply full flaps the airplanes literally leap off the ground.
 
Fearless Tower said:
As with many things, it is always airplane dependent. Believe it or not, the 1948 Cessna 170 owners' manual for a short field takeoff actually tells you to start the takeoff roll with flaps up and then add full flaps with the johnson bar right at the point of 'rotation'.
Click to expand...
Yes, same in the Super Cub owner's handbook.
 
Fearless Tower said:
As with many things, it is always airplane dependent. Believe it or not, the 1948 Cessna 170 owners' manual for a short field takeoff actually tells you to start the takeoff roll with flaps up and then add full flaps with the johnson bar right at the point of 'rotation'.
Click to expand...
Pretty sure the 48 C170 was a taildragger and you are talking about short field. The form drag with takeoff flaps would be quite different until the tail is flying vs a tricycle C172. To assess one would need to calculate the increase in form drag from takeoff flaps vs the decrease in main from early lift and then there's induced drag. Don't you think Cessna compute all this and test pilots confirmed best procedure? Even if on a 4k paved runway it made 10' of difference does it really matter, are there other ways to save 10', what are the tradeoffs of making a mistake vs all takeoff setting in place before taking the active runway and focus of speed, climb rate etc. This would make a bigger difference at faster departure speed but do we see said procedure in jets.

Side note spent first couple hundred hours in 65 C150 with Johnson bar flying off home grass strip. Short field takeoff and landings were fun.
 
Fearless Tower said:
As with many things, it is always airplane dependent. Believe it or not, the 1948 Cessna 170 owners' manual for a short field takeoff actually tells you to start the takeoff roll with flaps up and then add full flaps with the johnson bar right at the point of 'rotation'.

The original Navy TBF/TBM Avenger guidance for short field (land that is) also called for full flaps selected/applied at the point of liftoff.

In both of those airplanes, you get to liftoff speed (not really rotation because those airplanes will fly off the ground when ready with no back pressure on the elevator required) faster and when you apply full flaps the airplanes literally leap off the ground.
Click to expand...
ionel said:
Pretty sure the 48 C170 was a taildragger and you are talking about short field. The form drag with takeoff flaps would be quite different until the tail is flying vs a tricycle C172. To assess one would need to calculate the increase in form drag from takeoff flaps vs the decrease in main from early lift and then there's induced drag. Don't you think Cessna compute all this and test pilots confirmed best procedure? Even if on a 4k paved runway it made 10' of difference does it really matter, are there other ways to save 10', what are the tradeoffs of making a mistake vs all takeoff setting in place before taking the active runway and focus of speed, climb rate etc. This would make a bigger difference at faster departure speed but do we see said procedure in jets.

Side note spent first couple hundred hours in 65 C150 with Johnson bar flying off home grass strip. Short field takeoff and landings were fun.
Click to expand...
Anyone aware of an owner’s manual for an airplane **not equipped with manual flaps** that actually calls for flap deployment post take-off roll?

From a purely mathematical standpoint, I don’t see how the ideal flap-extension speed would ever be zero. If extending the flaps increases drag, there’s no benefit to carrying that extra drag from a dead stop when you don’t yet need the immeasurable lift. Somewhere later in the takeoff roll, the benefit of the added lift begins to outweigh the drag penalty, and that would be the theoretical optimum point to extend them. Exactly where that point falls would obviously vary with the airframe, flap design, surface, weight, density altitude, wind, etc.

I suspect the bigger reason things generally standardized around setting the takeoff configuration before rolling has more to do with reducing pilot workload and the opportunity for error, along with the increased likelihood of mechanical failure when the controls aren’t manually driven. If your Johnson bar feels too stiff, you’re probably not going to pull so hard that you snap your Johnson off. An electric or hydraulic system, however, may happily keep trying until something gives.
 
jesse said:
Anyone aware of an owner’s manual for an airplane **not equipped with manual flaps** that actually calls for flap deployment post take-off roll?

From a purely mathematical standpoint, I don’t see how the ideal flap-extension speed would ever be zero. If extending the flaps increases drag, there’s no benefit to carrying that extra drag from a dead stop when you don’t yet need the immeasurable lift. Somewhere later in the takeoff roll, the benefit of the added lift begins to outweigh the drag penalty, and that would be the theoretical optimum point to extend them. Exactly where that point falls would obviously vary with the airframe, flap design, surface, weight, density altitude, wind, etc.

I suspect the bigger reason things generally standardized around setting the takeoff configuration before rolling has more to do with reducing pilot workload and the opportunity for error, along with the increased likelihood of mechanical failure when the controls aren’t manually driven. If your Johnson bar feels too stiff, you’re probably not going to pull so hard that you snap your Johnson off. An electric or hydraulic system, however, may happily keep trying until something gives.
Click to expand...
At what speed do you think parasite drag becomes an imortant factor?
 
Fearless Tower said:
As with many things, it is always airplane dependent. Believe it or not, the 1948 Cessna 170 owners' manual for a short field takeoff actually tells you to start the takeoff roll with flaps up and then add full flaps with the johnson bar right at the point of 'rotation'.

The original Navy TBF/TBM Avenger guidance for short field (land that is) also called for full flaps selected/applied at the point of liftoff.

In both of those airplanes, you get to liftoff speed (not really rotation because those airplanes will fly off the ground when ready with no back pressure on the elevator required) faster and when you apply full flaps the airplanes literally leap off the ground.
Click to expand...
Johnson bar on a high wing? Was it overhead? If not, that would be some complicated linkage.
 
jesse said:
Anyone aware of an owner’s manual for an airplane **not equipped with manual flaps** that actually calls for flap deployment post take-off roll?

From a purely mathematical standpoint, I don’t see how the ideal flap-extension speed would ever be zero. If extending the flaps increases drag, there’s no benefit to carrying that extra drag from a dead stop when you don’t yet need the immeasurable lift. Somewhere later in the takeoff roll, the benefit of the added lift begins to outweigh the drag penalty, and that would be the theoretical optimum point to extend them. Exactly where that point falls would obviously vary with the airframe, flap design, surface, weight, density altitude, wind, etc.
Click to expand...
True, but this reminds me of the old joke with an engineer and a mathematician who are put at one side of a room with a naked woman on the other side, and told they can go half the distance to her every 10 seconds. The mathematician says "This is dumb, we'll never get all the way to her!" and the engineer says "No, but we'll get close enough!"

At zero speed, there is zero drag. As speed increases, drag increases but so does lift - And with the 10 degrees used for short/soft on an average Cessna you're getting more lift than drag. I doubt the drag is significant at speeds where the lift isn't significant, especially when you consider pilot workload and technique (can they hit the switch at the perfect time?) so they just leave it as is. Plus, with electric flaps it's going to take a few seconds to actuate them anyway, so chances are that the difference in overall drag is small enough that it doesn't matter. I'd be surprised if it makes a difference of more than single-digit feet.

M20K said:
Forgot to ask earlier, which App or Apps alerts you to aviation crashes?
Click to expand...
I think this one came from the ATC app. It's a new one that's kind of like LiveATC except it doesn't appear to have any Center frequencies, and it adds the capability of entering a flight number and seeing a transcript of all communications with that flight from any frequency they have, plus it shows you where the flight is at the same time. If you enter an airport and frequency, when a plane starts talking it jumps right to where that plane is on the map. And, of course, it listens for unusual things like "crash" and "fire" and "emergency" and puts out alerts. Pretty cool, if you're not the one having a bad day.
 
flyingcheesehead said:
True, but this reminds me of the old joke with an engineer and a mathematician who are put at one side of a room with a naked woman on the other side, and told they can go half the distance to her every 10 seconds. The mathematician says "This is dumb, we'll never get all the way to her!" and the engineer says "No, but we'll get close enough!"
Click to expand...
I always viewed this analogy a bit different. From my perspective an engineer will trip over a pile of naked virgins to screw a mechanic. :biggrin:
 
ionel said:
At what speed do you think parasite drag becomes an imortant factor?
Click to expand...
I don't think there's a particular speed where parasite drag suddenly becomes "important." That's not really the comparison I'm making.

During the ground roll, roughly:
acceleration = thrust - drag - rolling resistance

and rolling resistance is roughly:
rolling resistance = tire friction * (weight - lift)

So when you add flap, the part that matters is basically:

benefit from extra lift = tire friction * extra lift
vs.
penalty = extra drag

That's the comparison that matters. The extra lift reduces rolling resistance, while the extra drag is working directly against acceleration. And since both lift and aerodynamic drag are basically tied to V squared, both start at zero and increase as speed comes up. There isn't some magic airspeed below which flap drag doesn't count.

The interesting question isn't "when does drag become important?" It's whether:

tire friction * extra lift > extra drag

On pavement especially, tire friction is pretty small, so the increase in lift has to be many times larger than the increase in drag before unloading the wheels actually makes up for the added aerodynamic drag.
 
Last edited:
flyingcheesehead said:
True, but this reminds me of the old joke with an engineer and a mathematician who are put at one side of a room with a naked woman on the other side, and told they can go half the distance to her every 10 seconds. The mathematician says "This is dumb, we'll never get all the way to her!" and the engineer says "No, but we'll get close enough!"

At zero speed, there is zero drag. As speed increases, drag increases but so does lift - And with the 10 degrees used for short/soft on an average Cessna you're getting more lift than drag. I doubt the drag is significant at speeds where the lift isn't significant, especially when you consider pilot workload and technique (can they hit the switch at the perfect time?) so they just leave it as is. Plus, with electric flaps it's going to take a few seconds to actuate them anyway, so chances are that the difference in overall drag is small enough that it doesn't matter. I'd be surprised if it makes a difference of more than single-digit feet.
Click to expand...
Yes, at zero airspeed there's zero aerodynamic drag. There's also zero aerodynamic lift. Once you're moving, both increase roughly with speed squared.

But saying "10 degrees gives more lift than drag" doesn't tell us whether it's helping acceleration on the ground. The extra lift doesn't propel the airplane forward...all it does during the roll is unload the tires. So the comparison is basically:

extra lift * tire friction
vs
extra drag

For an easy example, say the flap gives you 100 lb of additional lift and only 10 lb of additional drag. That's ten times as much lift as drag, which sounds great. But if rolling resistance is .02, that 100 lb of extra lift only saves about 2 lb of rolling resistance, while you just added 10 lb of aerodynamic drag.

100 * .02 = 2 lb rolling resistance saved
2 lb saved - 10 lb added drag = 8 lb worse

Grass obviously changes that because rolling resistance is higher.

So early in the roll, you’re worse off.

But that obviously doesn’t mean flaps are never useful. As you get closer to flying speed, the value of that extra lift changes completely. At some point the added lift lets the airplane get off the ground sooner, and now you’re no longer just saving a small percentage of rolling resistance - you’re eliminating the ground-roll portion altogether.

That’s the whole reason I think there has to be some optimum point somewhere above zero speed. Early on, the drag penalty can outweigh the small benefit from unloading the tires. Later on, the lift benefit becomes much more valuable because it can actually end the ground roll.

It’s just too damn hard to calculate the ideal scenario for each takeoff, and too hard to deploy the flaps at the perfect time, and too risky to mess with configuration while distracted. Hence why we set them before takeoff. I’m just saying the reason isn’t the math.
 
Last edited:
Is it true if you over inflate the mains by 30 psi and bounce it at the same time as pulling 30 degrees of Johnson bar you can save 10 feet of takeoff roll? :rolleyes:
 
ionel said:
Is it true if you over inflate the mains by 30 psi and bounce it at the same time as pulling 30 degrees of Johnson bar you can save 10 feet of takeoff roll? :rolleyes:
Click to expand...
If you start your takeoff roll parallel to the fence in a square pasture, roll up on one wheel to make the turn parallel to the fence on the next side, lift off into ground effect, and bounce just before you get to the fence on the third side, you can takeoff in a pasture that’s too short in any one direction.
 
flyingcheesehead said:
and told they can go half the distance to her every 10 seconds.
Click to expand...
I remember that one from a logic class where you had to walk across a room to the wall on the other side. Prof was not happen when I said, yea, you can do it that way, but I chose to walk TO the wall.
 
MauleSkinner said:
If you start your takeoff roll parallel to the fence in a square pasture, roll up on one wheel to make the turn parallel to the fence on the next side, lift off into ground effect, and bounce just before you get to the fence on the third side, you can takeoff in a pasture that’s too short in any one direction.
Click to expand...
Are you advocating for circular runways?
 
GoFly said:
Are you advocating for circular runways?
Click to expand...
Nah…you’ve gotta get some speed up before you can bank. Gotta be square.

I will note, however, that Minneapolis International was originally an oval racetrack, so there are some compromises available.
 
I saw a guy perform a takeoff like that from a straight long, paved runway. And he didn’t even over inflate the tires


…
 
Last edited:
tlglenn said:
Ninth approach. I can imagine the student fatigued past the point of being able to learn.
Click to expand...

I don't recall doing that many missed approaches in my training. I'm sure I must have done one, but really cannot recall it. It really isn't a complicated procedure. The report makes this even weirder given the statements by witnesses reporting they saw the aircraft flying "erratically" and even stalling before the fatal stall. What the hell was the CFI doing?
 
jesse said:
That’s the whole reason I think there has to be some optimum point somewhere above zero speed. Early on, the drag penalty can outweigh the small benefit from unloading the tires. Later on, the lift benefit becomes much more valuable because it can actually end the ground roll.
Click to expand...
Oh, I absolutely agree there... I'm just not sure it makes a significant difference. Six inches, six feet? Naah, who cares, that's inside the likely margin of error for technique and not-brand-new airplanes. 60 feet, maybe. It'd be interesting to know just how much of a difference it would really be.

jesse said:
It’s just too damn hard to calculate the ideal scenario for each takeoff, and too hard to deploy the flaps at the perfect time, and too risky to mess with configuration while distracted. Hence why we set them before takeoff. I’m just saying the reason isn’t the math.
Click to expand...
Well, it isn't the main reason. It could be *part* of the reason, though - If the aero-Es at Cessna determined that it made a six-foot difference in the takeoff roll, then the math indicates that there's not a significant enough reason to not do it the easy way.
 
Den60 said:
I don't recall doing that many missed approaches in my training. I'm sure I must have done one, but really cannot recall it. It really isn't a complicated procedure. The report makes this even weirder given the statements by witnesses reporting they saw the aircraft flying "erratically" and even stalling before the fatal stall. What the hell was the CFI doing?
Click to expand...
Based on the witness descriptions, it sounds like the CFI may not have known when to quit. Like he was determined to get the student to do it properly and the student was either having a bad day or just not getting it.
 
Den60 said:
I don't recall doing that many missed approaches in my training.
Click to expand...
Had dinner with a CFII friend of mine last night. We discussed how I've done 6 approaches on one flight to get current, he said he thought he'd done maybe 7 max himself. Then as we continued the discussion and that for a student trying to remember everything and being so focused on trying to keep the needles EXACTLY centered, it would be an exhausting strain to try and do 9 approaches and the full Missed on a single flight.
 
M20K said:
Had dinner with a CFII friend of mine last night. We discussed how I've done 6 approaches on one flight to get current, he said he thought he'd done maybe 7 max himself. Then as we continued the discussion and that for a student trying to remember everything and being so focused on trying to keep the needles EXACTLY centered, it would be an exhausting strain to try and do 9 approaches and the full Missed on a single flight.
Click to expand...
Several posts use language that implies that these were instrument approaches (“full missed, needles centered, etc”). The preliminary and ADS-B track indicate that they were just beating up the pattern. Just want to clarify in case this is going down the wrong rabbit hole.
 
Flying Keys said:
The preliminary and ADS-B track indicate that they were just beating up the pattern.
Click to expand...
Good to know, I missed that.

Pending the kind of day they both had prior to the flight, I suppose it could still have been a factor. But I doubt anything like a newbie Inst pilot with multiple repeat approaches.
 
You must log in or register to reply here.
Back
Top