Flap assisted take off

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Flap assisted take off

Postby Richgj3 » Fri Jan 29, 2021 9:26 pm

So it’s blowing 25 KTS gusts to 40 here and 20 F so I’m sitting here watching YouTube videos of guys going in and out of small places in their 170’s. I notice several times they start the roll with one notch and then when the tail comes up they pull on the flap handle and then reduce the flaps when off. Anybody here do that and have any tips? I don’t normally fly in and out of anything less than 2000 feet and I’m a sea level pilot for the most part. I flew a 58 172 taildragger for years and never tried that then. Or had any reason to.
I’m wondering your thoughts on the technique.
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Re: Flap assisted take off

Postby brianm » Fri Jan 29, 2021 9:40 pm

I don't do serious short-field work, but my normal short or soft field technique is to start clean, pull in full flaps around 35-40 mph to help "pop" the plane into ground effect, accelerate, reduce one notch once I'm into the green arc, then take all the flaps out once I'm climbing away. I'm not exactly sure of the speed where I put the flaps in, it's by feel. And I fly a ragwing. I suspect an A model would work similar, a B maybe not.
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Re: Flap assisted take off

Postby Bruce Fenstermacher » Fri Jan 29, 2021 10:42 pm

Rich, there are extremely practiced pilots so proficient in applying flaps and reducing them at the exact correct time all while perfectly controlling their aircraft, all in order to squeeze the best effort the aircraft can do. And then there is the rest of us.

Most of us can not control our aircraft proficiently enough while we fool with flaps to gain anything. Fact is we often lose performance over that which we would get if we just set the flaps and left them and proficiently flew the plane.

When I was operating out of at 1800ft grass strip with a 2 story house at the end I had to clear we spent some time trying to figure out how to consistently get the best performance. Our conclusion, set the flaps and forget them till we crossed the house. We were flying an A model but it is the same for a B.
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Re: Flap assisted take off

Postby GAHorn » Sat Jan 30, 2021 2:04 am

IF there was a “LIKE” emoticon.... I’d definitely add it to Bruce’s post!

The performance data which Cessna’s Test Pilots produced in the Owner’s Manual and duplicated in the AFM did NOT “Play games” with the flaps.

SET them for takeoff.... use the techniques in the book.... retract flaps after the obstacles are cleared. Simple, and meets the book performance data.
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Re: Flap assisted take off

Postby rnealon1 » Sat Jan 30, 2021 3:16 am

This subject is interesting to me as I have been learning the aircraft and teaching my son for his PPL. Youtube videos often show lifting the tail first and then "popping" the flaps, but as George and Bruce stated we are unlikely to improve on the book procedure without a lot of practice and some scary mistakes.

I do notice that in the Performance section it states to climb at 67 MPH with flaps 20 to clear obstacle; in the Operating Details Minimum Run Takeoff section it says to climb at 62. I am using 67 because that performance section is part of the AFM; Operating Details are not.

This is all assuming I can nail that speed on the climb out...

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Re: Flap assisted take off

Postby bgiesbrecht » Sat Jan 30, 2021 6:32 am

As mentioned I doubt there is much (if anything, less) benefit for popping the 170 off with flaps. Carbon cubs and extreme STOL birds, different story. That said, it is a heckuva lot of fun to do, and I do it half of the time just for kicks. It's not too difficult to manage, you raise the tail up as usual on the takeoff roll, and I pop in 20* and it floats up into ground effect. Accelerate & climb out, and reduce your flaps as needed. I never pop the flaps past 20* though.
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Re: Flap assisted take off

Postby IA DPE » Sat Jan 30, 2021 1:02 pm

I see Jughead pops his from 10 -> 20 momentarily to get it off, and he’s the best STOL 170 pilot that I’m aware of. He has practiced a lot to get that performance.

https://www.youtube.com/watch?v=tj_4Tsbkz5c

I’m with Bruce. Average pilots don’t have the experience or practice enough to benefit from such techniques. I have little desire to land in places that require such effort to get out of. My goal is to never be the subject of a news report or a thread in these forums. :lol:
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Re: Flap assisted take off

Postby GAHorn » Sat Jan 30, 2021 2:30 pm

Subjective matters aside... it takes little thought to get to the truth in this subject.

Deployed flaps create drag but add lift. The logic behind “popping” flaps is to begin the takeoff roll without the drag of deployed flaps during the acceleration-phase...but gain the benefit of lift of deployed flaps at the moment of having achieved flying speed. This is a theory which has little, if any, practical benefit because we also know that drag increases as the square-of-speed. This means that drag at the beginning of the takeoff roll is nil.... and that as speed increases and the (Un-measurable) drag develops.... so does the LIFT develop which decreases weight-on-wheels... and it’s that weight-on-wheels which generates the g r e a t e r drag on un-improved strips. (The whole idea of a soft/rough-field takeoff technique is to get up-and-flying in ground-effect to rid the airplane of terrain-drag. So, this means that drag produced by deployed flaps only means lift is being generated by deployed flaps and they cancel out. I suspect the thrill enjoyed by the apparently sudden flight which occurs when the flaps are “popped” simply disguises the fact that all that technique has no real gain.

A similar thing occurs when the tail is “forced up” prematurely. Unnecessary drag is created by the downward-forced elevator. (The idea being that tail-wheel-drag is avoided by forcing the tail Up.) Unfortunately that technique places all weight onto the MAIN wheels and increases the drag from those....while simultaneously dumping the lift normally generated by the wings angle-of-attack (because the AOA was reduced in the forcing-up of the tail.)

In my opinion, a takeoff with the elevator in the neutral position will allow the tail to fly when it receives sufficient air-flow in that neutral position...which lifts the tail at the same moment the wings are also generating the most lift from their natural 3-pt AOA. (This is the technique used by the test-pilots when documenting performance for minimum ground-run. If such techniques provided the desired shortened takeoff distance you can be certain the test pilots would have used it and it would be a recommended technique in the manuals.)

But such simple techniques lack the “moment of thrill” and therefore mask the fact that, once again, we are measuring it with a micrometer, marking it with a grease-pencil, and cutting it with a hatchet .... because we do not use scientific methods in a consistent environment to prove the difference is more imaginary than real.

(And, while you are concentrating on your pop-the-flaps technique... did you happen to notice you left the carb-heat ON?). :oops:
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Re: Flap assisted take off

Postby cessna170bdriver » Sat Jan 30, 2021 4:18 pm

IIRC, the A-model (and I would ASSuME the ragwing also) owners manual recommends “popping” flaps toward the end of the takeoff roll for a short of soft field takeoff, while the B-model recommends starting the roll with 10 or 20 degrees. Also, the B-model manual notes, and the performance tables bear out, that deployed flaps above about 6500 feet density altitude actually hinder takeoff performance.
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Re: Flap assisted take off

Postby Richgj3 » Sat Jan 30, 2021 5:04 pm

Thanks guys.

Originally asked because I fly into a grass strip with a displaced threshold for landing. It is marked with white bricks in the ground. It is exactly in the wrong place for departing full length. It’s like hitting a rut just before lift off. (I use 10 degrees). I could start the take off in front of it but every time I do I hear my instructor from 52 years ago telling me the three most useless things in aviation are:

1. The air in your tanks

2. The sky above you.

AND

3. The runway behind you.

So, it sounds like all in all, this technique is not worth the trouble, so I may try it for fun but I get what you’re saying. I’ve got tons of time in both PA18-150 and Cub Crafters PA18-180 and never tried it in them either. Never had to, especially in the 180.

Thanks again
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Re: Flap assisted take off

Postby GAHorn » Sat Jan 30, 2021 5:21 pm

The A-model Owner’s Manual states for takeoff : “Set flaps in second notch before takeoff, maintain approximately three point attitude, and “fly off”. Slowly raise flaps as climb is established.”

The straight 170 states: “For unusually short field takeoffs, the application of full flaps will be of assistance, applied just before the airplane is ready to leave the ground.”

Flaps on the straight and A-model are not great lifting devices. Better than nothing...but they are better as drag-devices. For this reason, “popping” the flaps just at achieving flying-speed may help “pop” the airplane into the air...but will allow the airplane to sink if retracted AND they will penalize any climb towards an obstacle. So I would think popping them would be a better technique for getting clear of a muddy, rough, or high-grass field to get into ground-effect but not a good idea for true “short field” performance. Remember, Total takeoff distance includes the clearance climb to obstacles. Take off “ground run” or “ground roll” is only part of the takeoff distance calculation.

As for forcing the tail up early, the manual states “The shortest takeoff run can be obtained by keeping the tail low during the whole procedure.”

Also, the Owner’s Manuals are NOT approved documents. They were likely produced with approval of the marketing department.

Miles’ reminder of density altitude versus flap-use is also good. The B-model shows that any flap use above 4,000 feet at temps above standard will penalize the takeoff distance.
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Re: Flap assisted take off

Postby hilltop170 » Sat Jan 30, 2021 5:29 pm

Another couple of thoughts.

The 170B flap track brackets in the wing are made out of fairly flimsy sheet metal riveted to the wing skins and rear spar and are made for steady flap loads. Jerking the flaps puts higher stresses into the brackets. Over time I have seen the flap brackets crack out a few times on airplanes flown that way. It is very hard to inspect that area and not an easy or fun repair job to replace those brackets and definitely not cheap $. Let your pocketbook be your guide.

The 170A hinged flaps are probably less susceptible to problems b/c they are smaller and a simple hinge. However, 170A flaps are much less effective, takeoff and landing, than the semi-Fowlers on the 170B.

Having flown 170s since 1973 and 180s since 1983 in Alaska, I have found setting the flaps to 20° on either model gives the best overall performance. If you are in a place where max performance is required, you are probably better off paying attention to proper precise takeoff technique than being distracted by lifting the tail and jerking the flaps hopefully precisely at the right time, at a time when you need to not make any mistakes. You cannot “horse” these airplanes off, they don’t have enough power with stock engines, they won’t fly until they are ready. If you try to rotate too soon, the tailwheel will just plunk back down on the ground and you can pull back all you want and nothing happens until it finally staggers off, using excessive runway.

These airplanes consistently take off shortest when the tailwheel is held 1/4” above the runway (or just out of the snow, rocks, sand, or grass when off-field). They know when they are ready to fly and will at the earliest opportunity if held in that position, no matter what the load or field condition is. Any other technique usually lengthens the takeoff. I’m not saying popping the flaps doesn’t work if done correctly, I just choose not to do it. YMMV.
Last edited by hilltop170 on Tue Feb 02, 2021 3:13 pm, edited 6 times in total.
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Re: Flap assisted take off

Postby counsellj » Sat Jan 30, 2021 6:31 pm

“Subjective matters aside”as stated by George. Much of what George, Bruce and others say about techniques and POH/AFM procedures is correct if you are operating off of a smooth, consistent longer surface. I call anything over 500’ “longer.”

Bruce’s statement,

“Rich, there are extremely practiced pilots so proficient in applying flaps and reducing them at the exact correct time all while perfectly controlling their aircraft, all in order to squeeze the best effort the aircraft can do. And then there is the rest of us.”

.
is very accurate. But ask those gentlemen how much experience they have operating off of 300’ rockbars, clump grass mountain sides and sandbars. Better yet, ask them if they are so sure of their “correct” techniques if they will meet me at a short, rough off-airport location and they can demonstrate how their techniques are better. Or ask them why their techniques aren’t the preferred, most common technique of STOL contest performers. This technique is of much greater value in the B-Model, than other models. The main reason for starting with flaps 10, is that it is easy to reach the lever to pull additional flaps without excessive upper body movement detracting from practiced, skilled aircraft control.

The drag associated with rolling a tailwheel over rocks, thru sand/snow or over other rough surfaces results in 50% greater takeoff rolls in many off-airport situations. Not to talk about the forces being applied to the tailwheel, tailwheel tire, airframe etc. As Richard discussed, the wear on the flap tracks I feel is of very little concern at the low speeds we are operating. I worry more about wear from pulling flaps above 80mph. I personally use 70mph as my preferred "top of the white arc."

If you are going to use the technique to your advantage, the key is that the flaps have to be pulled in conjunction with an aggressive “rotation” or application of back-yoke pressure to put the tailwheel on the ground, or just above the ground, depending on the surface, to maximize the AOA while the flaps are being added. I don’t pull 10 to 20 degrees. I pull 10 to “whatever degree of flaps” I’m at when the airplane lifts off. I aim for 30, but if I’m a half to full second late, it’ll fly with 20. If I go to early, it may not fly at all, or at least not for another second and another 100-150’ of takeoff roll. If I’m operating for a location that I know I need 80-90% of the length to get airborne, I’ll wait till the last second, utilize all the distance and ensure a clean liftoff at the end of the bar or takeoff area. If it’s rough and longer, I’ll pull as soon as I think I can get airborne. Also realize, the POH/AFM numbers are all based on Max Gross Weight, the lighter you are, the more advantages I feel the "popping technique" is. Maybe it is time for me to put a video together and discuss/demo different techniques.

George and Bruce are a wealth of valuable information to our Assoc and the 170 community. But in this narrow field of operation, I feel they are under-informed/educated/experienced. I don’t mean any disrespect by that statement. It is simply my opinion. There are areas of discussion that I stay out of, because I know I’m not the most experienced, or knowledgeable about. This isn’t one of them. I enjoy the friendly, respectful debate.

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Re: Flap assisted take off

Postby GAHorn » Sat Jan 30, 2021 7:38 pm

counsellj wrote:...

George and Bruce are... under-informed/educated/experienced. ...

Jughead


I agree ...when it comes to producing flying videos.

But anyone who’s seen Bruce’s demos at Petit Jean will not soon place a money-bet against him. And although not what I’d call “current” in proficiency (more important than ad-hoc personal techniques) ... I’m no stranger to short/rough and “off-airport” operations having spent thousands of hours landing on pipeline right-of-ways, oil-field roads, and Mississippi-Delta levees in my pipeline days.

Jughead, the suggestion to “meet...at a short, rough off-airport... demonstrate how their techniques are better”. ... as you know is not likely to happen....not because it wouldn’t be fun and informative...but because it would require we all use the SAME AIRPLANE under exact same conditions. ...and that is not likely either.
But I’ll bet the Cessna test-pilots knew what they were doing and they’re probably the better authorities.

As you wrote, WEIGHT has a lot to do with successfully operating off short/rough/unimproved fields. Leaving the unnecessary 200-300 lbs of stuff on the ground makes a standard 170 a nice short-field airplane. And having the “right” propeller subtracts hundreds off the factory distances.
The STOL-demo guys you praised are using specialized equipment and are practiced / proficient-in-the-extreme at this... They are NOT operating the typical “fleet” Cessna 170s and their narrow techniques do not apply to those pilots who do.

Keep the fun videos coming tho’! :P
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Re: Flap assisted take off

Postby Bruce Fenstermacher » Sat Jan 30, 2021 8:28 pm

Bruce Fenstermacher wrote:there are extremely practiced pilots so proficient in applying flaps and reducing them at the exact correct time all while perfectly controlling their aircraft, all in order to squeeze the best effort the aircraft can do.


Jughead, you are the person I thought of writing the my first sentence.

The rest if my comments pertain to the group I described in my second sentence.

And then there is the rest of us.


Jughead, its nice to read your technique should I ever find my self on a sand bar, it is info I'd like to have. In that dream however I'll be flying a helicopter, then I'll show you what short is. :D
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