Hey - help me settle a bet with my buddy (sorry if this topic has been thrashed to death already, I did a search but didn't find a definitive result as easily as I hoped...)

Q:
Will using best-lift flap settings for climbing out (say 15-20 degrees) after a max-performance takeoff typically improve obstacle clearance, overall for the whole takeoff sequence, compared to lifting off with flaps and cleaning them away once you get positive rate of climb?

I know this varies hugely with prop / aircraft / etc. So if all other aspects are held constant, the crux of the question is whether your average high-camber wing is more efficient in terms of Vx performance with flap down or up.

I would go test it, but I can't afford to fly into obstcales!

Yes.

MTV

Yes, in general, with a normal airplane of a standard configuration type (Cessna 100 for example), you can clear a TYPICAL obstacle a little shorter by using flaps. By typical I mean an FAA spec 50 foot tree at the end of a short runway.

If your obstacle is a tall mountain quite a ways away from the runway, then it often becomes a function of whether your airplane has the power to sustain a "best angle" climb for that long before overheating, or the drag buildup of a given flap setting catches up with the engine thrust (A tired old 150 will have a fair amount of trouble with that, and a PPONK'ed 180 will do a lot better).

So on a lot of airplanes, in my opinion there are two "best angle" scenarios. Sustained best angle is usually quoted as no flaps, and is a repeatable, flight-test-able, define-able, teach-able condition, that takes drag buildup and cooling into consideration.

Initial best angle, which is much more relevant to most "obstacle" takeoffs where you are clearing something close to the runway, is usually a lot better using flaps. The takeoff from start until first 20 or 30 seconds after liftoff can benefit from flaps much more, because you can usually get up and over the obstacle before the drag really catches up to the thrust, and by then you can retract the flaps and/or lower the nose to accelerate to best rate. This is why you often see 150's and 172's leaping off the ground for a few moments and then the climb slows down once they reach 100 feet.

Now the reality of this is that you can also make measurable gains by using one flap/takeoff technique over another, ESPECIALLY where obstacles and short strips are concerned. But I can hear some of the natives getting restless already, and I'll have to wait for an invitation to discuss that further

depends on the plane.......

EZFlap wrote:Yes, in general, with a normal airplane of a standard configuration type (Cessna 100 for example), you can clear a TYPICAL obstacle a little shorter by using flaps. By typical I mean an FAA spec 50 foot tree at the end of a short runway.

If your obstacle is a tall mountain quite a ways away from the runway, then it often becomes a function of whether your airplane has the power to sustain a "best angle" climb for that long before overheating, or the drag buildup of a given flap setting catches up with the engine thrust (A tired old 150 will have a fair amount of trouble with that, and a PPONK'ed 180 will do a lot better).

So on a lot of airplanes, in my opinion there are two "best angle" scenarios. Sustained best angle is usually quoted as no flaps, and is a repeatable, flight-test-able, define-able, teach-able condition, that takes drag buildup and cooling into consideration.

Initial best angle, which is much more relevant to most "obstacle" takeoffs where you are clearing something close to the runway, is usually a lot better using flaps. The takeoff from start until first 20 or 30 seconds after liftoff can benefit from flaps much more, because you can usually get up and over the obstacle before the drag really catches up to the thrust, and by then you can retract the flaps and/or lower the nose to accelerate to best rate. This is why you often see 150's and 172's leaping off the ground for a few moments and then the climb slows down once they reach 100 feet.

Now the reality of this is that you can also make measurable gains by using one flap/takeoff technique over another, ESPECIALLY where obstacles and short strips are concerned. But I can hear some of the natives getting restless already, and I'll have to wait for an invitation to discuss that further

Bill, you remind me of Alton Brown on the Food Network

Battson wrote:Q:
Will using best-lift flap settings for climbing out (say 15-20 degrees) after a max-performance takeoff typically improve obstacle clearance, overall for the whole takeoff sequence, compared to lifting off with flaps and cleaning them away once you get positive rate of climb?

P

For my Bonanza at 2800 pounds a takeoff with flaps will be higher AGL until approx 3400 feet down range. At that distance the no flap takeoff will be passing thru the flap takeoff altitude and climbing faster. So the question is where is the obstacle to be cleared?

Jr.CubBuilder wrote:Your POH will reflect the best performance of the plane on a concrete runway at various density altitudes. You will see that as your altitude goes up the climb advantage with flaps begins to drop off. However in my experience, when you are operating on unpaved strips, the more quickly you can get the plane off the ground the more quickly you can gain airspeed and climb. Consequently when I'm on unpaved strips I will use flaps at altitudes higher than the POH indicates will be advantageous.

Thanks all for the kind responses so far, that's been really helpful!

It was a general question, but for the Bearhawk we have to write the POH - so not too much help there

The Cessna / Piper POH's for the types I fly all suggest limited flap is best, seldom over 10 degrees anyway.... But as has been said - repeatable, instructable technique wins out sometimes despite the reality I guess.

Jr.CubBuilder wrote:Your POH will reflect the best performance of the plane on a concrete runway at various density altitudes. You will see that as your altitude goes up the climb advantage with flaps begins to drop off. However in my experience, when you are operating on unpaved strips, the more quickly you can get the plane off the ground the more quickly you can gain airspeed and climb. Consequently when I'm on unpaved strips I will use flaps at altitudes higher than the POH indicates will be advantageous.

Thanks all for the kind responses so far, that's been really helpful!

It was a general question, but for the Bearhawk we have to write the POH - so not too much help there

The Cessna / Piper POH's for the types I fly all suggest limited flap is best, seldom over 10 degrees anyway.... But as has been said - repeatable, instructable technique wins out sometimes despite the reality I guess.

Flying my 8GCBC (160 hours only):

Wing loading: 11.9 lb/ft², 180 HP, MT prop, Power/mass: 11.9 lb/hp

I keep flaps in for short obstacle (under 150 feet) cleareance (20 degrees) on dirt and water. But, for long haul climbing no flaps.

Seaplane:
* Flaps good for getting off the water (20 degrees)
* Flaps not good for Vx, Vy (0)
* Flaps great for water touchdown (20-30 degrees)
Bush 31x16x6:
* Flaps good for getting off rough ground (20 degrees)
* Flaps not good for climbing Vx, Vy (0)
* Flaps great for landing (30 degrees)

I evaluate HP/thrust vs Lift for every aircraft.

* low HP/thrust and high lift wing (slow flying) = no flaps or little flaps for Vx, Vy
* high HP/thrust and low lift wing (fast flying) = flaps, Vx, Vy for sure

-- 8GCBC

Battson wrote:

Jr.CubBuilder wrote:Your POH will reflect the best performance of the plane on a concrete runway at various density altitudes. You will see that as your altitude goes up the climb advantage with flaps begins to drop off. However in my experience, when you are operating on unpaved strips, the more quickly you can get the plane off the ground the more quickly you can gain airspeed and climb. Consequently when I'm on unpaved strips I will use flaps at altitudes higher than the POH indicates will be advantageous.

Thanks all for the kind responses so far, that's been really helpful!

It was a general question, but for the Bearhawk we have to write the POH - so not too much help there

The Cessna / Piper POH's for the types I fly all suggest limited flap is best, seldom over 10 degrees anyway.... But as has been said - repeatable, instructable technique wins out sometimes despite the reality I guess.

From POHs I have at hand:

1976 Cessna 206: Maximum performance takeoff: flaps 20 degrees.
1985 Cessna 185: Maximum performance takeoff: flaps 20 degrees.
Husky Model A-1B: Maximum performance takeoff: flaps 30 degrees.
Piper PA-18: Maximum performance takeoff (and best angle of climb configuration): flaps full
Piper PA-28: Maximum performance takeoff: flaps 20 degrees.

The original question was related to obstacle clearance. As Bonanzaman says, if you're trying to clear an obstacle that's a few miles from takeoff, flaps up may indeed be your best bet, but most aircraft I've been around recommend use of flaps for "obstacle clearance takeoffs" which MOST folks relate to something right off the end of the runway, not a few miles away.

MTV

mtv wrote:From POHs I have at hand:

1976 Cessna 206: Maximum performance takeoff: flaps 20 degrees.
1985 Cessna 185: Maximum performance takeoff: flaps 20 degrees.
Husky Model A-1B: Maximum performance takeoff: flaps 30 degrees.
Piper PA-18: Maximum performance takeoff (and best angle of climb configuration): flaps full
Piper PA-28: Maximum performance takeoff: flaps 20 degrees.
MTV

MTV,

For the examples mentioned, do those settings equal the angle of maximum aileron deflection for those makes/ models? Or is that mostly a Cessna thing?

Thanks,

-DP

Several years ago in the annual fun in the box we have to do we had run out of boxes to check for our checkride but still had some time to kill. The instructor said he had always wanted to see what the real world difference was between keeping takeoff flaps throughout the climb and going by the book and retracting flaps at 1500' was. So he set up a scenario that involved taking off followed by an immediate 90 degree turn climbing to 5000' then a 180 turn back to a VOR on a ridge. The total flying distance was about 15 miles depending on how fast we got to 5000'. Using the magic of the simulator he could then see how high we were over the VOR. And of course we would lose the proverbial engine on takeoff. Damn simulators almost never have everything working. The transport category procedure following an engine failure is to hold V2 and takeoff flaps to 1500 AGL, lower the nose to accelerate to flap retract speed then hold that as it approximates Vx in the clean configuration. Going by the book got us about 1000' higher over the VOR compared to holding onto takeoff flaps even thought the IAS was higher without flaps. The same flap setting and other parameters were used on both takeoffs.

Imagine a graph with horizontal distance along the bottom and altitude on the vertical axis. Graphing the two methods shows, for example, the takeoff with flaps has a horizontal run of 800' followed by a climb of 300 ft per mile while the second method without flaps has a run of 1000' followed by a climb of 350 ft. per mile. which is better depends on where the obstacle is.

Basically to recap what other have already said, every airplane and every situation will be different. Flaps will get you airborne quicker and likely over a fence or short trees better but even a minimum amount of flaps create drag which will slow your angle of climb down. Where those two lines cross will vary from one day to the next.

The POH for a 1974 C172 says, "Normal and obstacle clearance take-offs are performed with wing flaps up. The use of 10 degrees of flaps will shorten the ground run approximately 10%, but this advantage is lost in the climb to a 50 foot obstacle."

Conversely, from a 182 manual, "Using 20 degree flaps reduces the ground roll run and total distance over an obstacle by approximately 20%." It goes on to say if you use them, keep them down until clear of the obstacle and above a certain speed.

Like has been said, different for different plane and situations.

denalipilot wrote:...do those settings equal the angle of maximum aileron deflection for those makes/ models? Or is that mostly a Cessna thing?

I don't think the max aileron deflection on my C150 matches any of the flap settings. Yup, just checked the TCDS-- aileron travel up 20 degrees, down 14.
I generally use 20 degrees for takeoff, it seems to do more than 10. Actually, I pretty much just use 0, 20, & 40. Sometimes pull on 10 degrees for 80 mph observation orbits, or 30 for landing when it's gusty, but otherwise 20 for takeoff, and for landing 20 downwind abeam & 40 on final. Nice to have other options available I guess, but I've never thought that something like the Pacer was handicapped by only having two notches.

hotrod150 wrote: I generally use 20 degrees for takeoff, it seems to do more than 10. Actually, I pretty much just use 0, 20, & 40. Sometimes pull on 10 degrees for 80 mph observation orbits, or 30 for landing when it's gusty, but otherwise 20 for takeoff, and for landing 20 downwind abeam & 40 on final. Nice to have other options available I guess, but I've never thought that something like the Pacer was handicapped by only having two notches.

The main situation where I use all the detents is milking the flaps off. Usually after pulling full flaps to get into ground effect, say if it's a rough ski departure.
Normally I try to avoid abruptly going from one detent to the next. The handling is smoother when I ease it from one to the next.

denalipilot wrote:Normally I try to avoid abruptly going from one detent to the next. The handling is smoother when I ease it from one to the next.

Yea... but that 'leap' that it does ripping on full flaps and getting your bushwheels up off the pavement is sure a good feeling

crazedpilot wrote:

denalipilot wrote:Normally I try to avoid abruptly going from one detent to the next. The handling is smoother when I ease it from one to the next.

Yea... but that 'leap' that it does ripping on full flaps and getting your bushwheels up off the pavement is sure a good feeling

I should know better than this by now... but I'll do it anyway

On a STOL takeoff, if you are EVEN ABLE to retract your flaps from one notch directly to another, then by definition you are not getting the most climb or obstacle clearance performance out of your airplane.

Said another way, if you were squeezing the maximum possible theoretical STOL takeoff performance out of your airplane (you're at a televised STOL competition with money at stake, and the winner also gets a hot date with two nymphomaniac Playboy Bunnies), and you retract the flaps ten or fifteen degrees in one quick movement, your airplane should either stall and fall out of the air, or settle back downwards into terra firma. If it doesn't, then you had a little more lift available that you weren't using.

Most pilots are forced to retract the flaps faster than they know they should. So in order to safely allow retracting the flaps too fast (like from one notch directly to another), most pilots add another three or four miles an hour of airspeed. This "safety margin" makes it so the airplane doesn't settle or stall when you have a sudden reduction in available lift.

If you can manage to retract the flaps very very very slowly, over the course of 30 seconds as one slow smooth motion instead of three or four jerky steps... then you can put that extra three or four miles an hour on your rate of climb indicator instead of your airspeed indicator, with no loss in safety margin.

...my comment was only in reference to reducing ground roll and that vertical launch you get Retraction is an artform, I definately prefer my manual flaps using the technique described above.

Cheers for that - to answer the recurring question we were originally talking about a short-distance obstacle - most of the bush strips around here are in the mountains, so there are a few with 20ft scrub / 80-100ft trees / or rising terrain which you have to beat. Typically strips are between 1000ft to 500ft long for the tricky ones, ususally grass or tussock.

So there's not many that challenges the might Maule, C185, 206, Super Cub, etc... but if you're plodding about in a C172 / PA-28-150 / high performance LSA as we used to once upon a time, all it takes is a change in wind on takeoff or an unusually hot afternoon and things started getting more uncomforable. Not really dangerous, it just reduces the rate of climb enough that you are watching the obstacle more closely and thinking... "hmmm.... " and maybe leaning forward over the dash a little more than usual - that seems to help

As grassstrippilot said, in the 172 POH I remember it says no flap / 10 degrees, but then you look at something else and it says full flap... and so did my buddy. Which is how the conversation got started.

EZFlap wrote:Said another way, if you were squeezing the maximum possible theoretical STOL takeoff performance out of your airplane (you're at a televised STOL competition with money at stake, and the winner also gets a hot date with two nymphomaniac Playboy Bunnies), and you retract the flaps ten or fifteen degrees in one quick movement, your airplane should either stall and fall out of the air, or settle back downwards into terra firma. If it doesn't, then you had a little more lift available that you weren't using.

Most pilots are forced to retract the flaps faster than they know they should. So in order to safely allow retracting the flaps too fast (like from one notch directly to another), most pilots add another three or four miles an hour of airspeed. This "safety margin" makes it so the airplane doesn't settle or stall when you have a sudden reduction in available lift.

If you can manage to retract the flaps very very very slowly, over the course of 30 seconds as one slow smooth motion instead of three or four jerky steps... then you can put that extra three or four miles an hour on your rate of climb indicator instead of your airspeed indicator, with no loss in safety margin.

From a purely theoretical point of view (because it would be dangerous to try) could you not just pull back as you raise flaps, increasing the angle of attack for the whole wing as the incoming flap reduces it? (Assuming you have the spare thrust). If you know what I mean?
Obviously you'd need to bleed the flaps off, you couldn't drop a whole notch and jerk the stick back all at once..

I tend to think of flaps as increasing the effective chord's angle of attack in a local part of the wing, or a smaller increase of the average chord's AOA over the whole wing. Helps me picture it aerodynamically.