Why bother with flaps on takeoff?

Discussion in 'Flight Following' started by Salty, Oct 7, 2017.

  1. Salty

    Salty Pattern Altitude

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    You misspelled POA. Which means of course you do something randomly different every time. ;)
     
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  2. luvflyin

    luvflyin Final Approach

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    This guy seemed pretty responsible. I doubt if he teaches things like that to primary students. It was just a rental checkout for me, I have a commercial certificate
     
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  3. denverpilot

    denverpilot Taxi to Parking

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    Do that at our altitude and you’ll probably get to see how crashworthy the airframe is into the trees at the end of the runway.
     
  4. Clark1961

    Clark1961 Touchdown! Greaser!

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    Where are there trees at the end of the runway out here?
     
  5. denverpilot

    denverpilot Taxi to Parking

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    LXV. North end. :)
     
  6. Stewartb

    Stewartb En-Route

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    Why’s that?
     
  7. denverpilot

    denverpilot Taxi to Parking

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    Performance loss. Engine and airfoil. You won’t climb out of ground effect. Especially at 40.

    You can simulate it there. Take the aircraft up to 6000’ and go into slow flight with flap 40 and see if you can power up and climb. Or get enough speed to retract the flaps and go back to cruise climb.

    The colder out it is, the better your chances. But try it on a day when it’s at least 90F at the surface.

    My runway is at 5885’. DA over 9000’ is common in summertime. Getting cooler out now, though. So our much better performance season is coming.
     
  8. Clark1961

    Clark1961 Touchdown! Greaser!

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    Never have departed to the north there...
     
  9. denverpilot

    denverpilot Taxi to Parking

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    Wind was weird that day. That was also the day with the ancient CFI on board who pointed out the gap in the treeline to the left and said he was just pointing it out because he’d had to use it before...

    He continued and said if you’re having that much trouble there on any particular day, a very shallow wide banked turn to the left until you’re headed south and then just wait until you get to Buena Vista to land... don’t bother trying to do a lap at LXV if it won’t climb. Just go downhill. Less banking. :)
     
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  10. midlifeflyer

    midlifeflyer Final Approach

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    Definitely.
     
  11. denverpilot

    denverpilot Taxi to Parking

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    By the way, for @Stewartb - grab any POH and see if you can do the performance calculations from it for a departure from KLXV at 70F.

    Doesn’t get that warm up there very often, but that north takeoff I am talking about above was done in those exact conditions.

    Half tanks and two people in a 182. And we were climbing slowly enough that other CFI pointed out the gap in the trees. :)
     
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  12. Aaronk25

    Aaronk25 Pre-takeoff checklist

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    I took off in my J at 63f with 1/2 tank and 2 people. What a dog. Tip toe off the runway. The turbo would have been nice.


    Sent from my iPhone using Tapatalk Pro
     
  13. Eric Gleason

    Eric Gleason Pre-takeoff checklist

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    Procedures and speeds specified in the POH are often listed for reasons other than performance. Cessna's never going to tell you that you can reduce your takeoff roll if you save flap deployment towards the end because they're worried that increasing pilot workload will cause an accident, and that a jury will find it's their fault for making things too complicated for the pilot. But people who have tried it find that it does indeed result in a shorter takeoff in most aircraft.

    Best glide speed is another one to be careful of. In most planes, the speed listed is not best L/D (furthest glide), and it's not minimum sink speed (longest time in the air). Both of those speeds will change with the weight of the airplane, and few manufacturers list different engine out glide speeds for different weights.

    They're also not going to tell you that your longest glide can be accomplished by diving for ground effect and using the excess energy to stretch your glide where there's less induced drag, but it works quite well.
     
  14. Fearless Tower

    Fearless Tower Touchdown! Greaser!

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    You mean ‘modern Cessna’.

    The short field takeoff technique described in the owner’s manual for the 1948 Cessna actually tells you to start the takeoff roll with zero flaps and as you approach takeoff speed, put the flaps full (Johnson bar).

    And that’s in a tailwheel airplane.
     
  15. Lachlan

    Lachlan Pattern Altitude

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    If you dive for ground effect around here, well, the ground effect you’re going to get is not gonna buff out. Unless you’re flying over a flat, relatively smooth surface, how in the world does your last sentence make sense? I’m imagining someone losing an engine at 3000’ agl and deciding to dive to ground effect. How steep of a dive? Should it be configured clean so that there’s a chance of exceeding Vne? What’s the proper technique for this so I can practice it next time I’m out? (I think there’s definitely a reason that, as you said, “(t)hey’re also not going to tell you that your longest glide can be accomplished by diving for ground effect...”) ;)
     
  16. Eric Gleason

    Eric Gleason Pre-takeoff checklist

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    That's interesting! Which model? I wonder if they did that for all models, even though the flaps in the 140 are far less effective than the bigger models.

    The oldest Cessna manual I've seen was for the early era of electric flaps, mid-late 60s. It was around the time of Superbowl I, so I guess that makes it the "modern" era :)

    I wonder if it's a procedure that was taken out when they changed to first style of electric flaps, where you have to turn the flaps off when they've moved as far as you want them to, rather than set-and-forget like the modern ones. Putting the flaps out late is quick and easy with the Johnson bar or with the newest ones, but not a good idea
     
  17. Fearless Tower

    Fearless Tower Touchdown! Greaser!

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    Sorry I left it out. That was the 1948 Cessna 170. Flaps very small, and created a lot more lift than the drag of the later 170s.
     
  18. dell30rb

    dell30rb Final Approach

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    Adding flaps increases the angle of attack at a given aircraft fuselage attitude (called angle of incidence), because the shape of the wing is changed. The angle of attack is the relationship of the chord of the wing to the relative wind as the wing goes through the air. The chord is defined as an imaginary line between the leading edge of the wing and the trailing edge. When you add flaps, you lower the trailing edge of the wing, changing the chord.

    2 reasons for needing to add nose up trim in a Mooney. Adding flaps can move the center of pressure of the wing aft, changing the relationship to the aircraft's CG (making it feel nose heavy). Flaps can also disrupt airflow over the elevator. As we know the horizontal stab produces downforce. By blocking some of the airflow over the stab/elevator, a higher negative angle of attack is needed to produce the same downforce. Hence the need for more nose-up trim.
     
  19. Eric Gleason

    Eric Gleason Pre-takeoff checklist

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    It's a technique that many advanced glider pilots know quite well. My late uncle did it on his commercial glider checkride after losing some lift in the late afternoon, and he passed with flying colors.

    I believe the technique is to dive at a relatively high speed, near Vne. And, yes, of course you'd be in a clean configuration. If the goal is to reduce drag with ground effect, why would you do it any other way?

    My point was that the procedures in the POH don't necessarily reflect the best performance of the aircraft. They're a compromise between performance, pilot workload, company liability, and whatever other issues come up in the meetings where they discuss these things.
     
  20. FormerHangie

    FormerHangie Pattern Altitude

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    I never bothered with them on my 170, either.

    [​IMG]

    (That would be a Wills Wing Falcon 170, not a Cessna 170).
     
  21. Tantalum

    Tantalum Pattern Altitude

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    Was this somewhere on short approach where he ducked down and made it to the strip? Or was he 10 miles from the field at 3K and he dove into ground effect and buzzed along fields? Genuinely curious. I also imagine winds and lift could be different lower down. My brother in law had a few outfield landings up in Canada and there are a lot of sailplane tricks they had up their sleeves that I'm not sure necessarily all translate to powered flight (wasn't Gimli glider and Hudson pilots glider pilots?)

    Theoretically I get that ground effect gives you less drag, so getting into ground effect fast makes sense.. and maybe with a 50' wingspan the dynamics are different.. but I can't imagine cruising along in a Skyhawk somewhere over San Diego at 5K and then diving down to get into ground effect as a means of rescue

    Chances are the drag dynamics are also different in a sailplane vs a conventional plane. For, say, a typical 172 I am picturing something like the graphs below. I could be wrong, but the drag at least feels like it goes way up with speed.. so while you're getting down into ground effect faster, you're ultimately just sacrificing more energy that you will not fully recover in ground effect

    First curve is dive into ground effect, second is establish and fly best glide

    upload_2018-4-16_12-36-52.png
     
  22. Eric Gleason

    Eric Gleason Pre-takeoff checklist

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    I'm not sure what you mean by "drag dynamics," it's not a term I've heard before. This particular glider was a Schweizer 2-22, a ship that looks like it was designed with little regard to aerodynamics and only manages a 17:1 glide ratio. For comparison, something like a 172 is about 9:1.

    He's not around to ask the specifics, but it was a technique the pilots in the club would use somewhat regularly. I think he learned it when he indulged in a week of high-performance glider flying in Colorado and was trying to earn his cross country distance badges. Off-field landings are not uncommon with gliders, so sometimes you're trying to get yourself into the best field to land in. In this situation, I think he was probably at about 1000' AGL a few miles from the airport.

    This isn't one of those theory things that can only be duplicated in lab conditions; it's a pretty straightforward application of the science. Your total energy state is the combination of your kinetic energy (determined by your speed) and your potential energy (altitude). Assuming you keep your airspeed constant, your total energy is only affected by drag. So you get yourself into the region of substantially reduced drag quickly, and you get to keep that potential much longer.
     
  23. denverpilot

    denverpilot Taxi to Parking

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    There’s a few videos of glider folks stretching glides to runways by getting down into ground effect. It’s pretty impressive on the higher performance ships that are already over 30:1.

    In power airplanes if you have a constant speed prop, pulling the prop all the way back is going to have a lot more impact. I practiced a power off 180 yesterday and pulled the prop all the way back... just to see how my airplane behaves. It added over 500’ to my touchdown aiming point.

    If I ever had to do both (prop back and ground effect flight) I’ve screwed up really bad and I’m likely to take out a runway end identifier light or two.

    Of course under controlled conditions aiming well down a nice long runway, the results of both can be eye opening for someone and give them a couple of tools in the bag for an “Oh ****!” day. Have to be real careful if a go around is needed too. Bringing the prop up will re-add the drag.
     
  24. TK211X

    TK211X Pre-takeoff checklist

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    I personally like to set one notch of flaps and retract after takeoff while passing a good altitude and speed. Usually that means passing 400 - 600 ft and 5-10 knots over my chosen VY or VX
     
  25. Chip Sylverne

    Chip Sylverne En-Route

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    Fear is the poison of our lives.
    You wanna leap off the runway in a hershey bar Cherokee? Full power, accelerate to 50 mph with slight back pressure. At 50mph, pull in two notches of flaps, count to 2, then pull back and hold Vx. Art Mattson developed this technique. Climbs like a homesick angel.
     
  26. Tantalum

    Tantalum Pattern Altitude

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    I was taught that ground effect happens at about an altitude that is half of the aircraft's wingspan.. so in most GA planes that's around 10-15' off the runway.. which makes sense, as that's where you can feel the plane behave differently

    So, are there glider pilots cruising along for miles at 20' off the ground? How do the dynamics work? Do they crank it up to Vne then dart over the landscape?

    The drag profile of an aircraft is not constant, unless the airspeed stays constant (as you allude), so, honest question then:
    how do you keep your airspeed constant and yet quickly get yourself into a reduced drag environment, IE, ground effect? If you "dive" down to ground effect you'll naturally pick up airspeed, no? The reason I mentioned "drag dynamics" is because it is well know that total drag increases with parasitic drag as speed goes up. Yes, induced drag goes down, but at a lower rate, while parasite drag goes up at an increase rate. So if you "dive down" into ground effect you've dramatically increased your drag coefficient and sacrificed a ton of energy by doing so, and hence glide distance, just to get down to a lower altitude, IE, closer to terrain and obstructions, and have also reduced your glide range. The unknown to me is, how much does ground effect reduce your total drag? Perhaps it reduced a ton, and so the tradeoff is in your favor.. but this doesn't really pass the sniff test to me. Maybe glider dynamics are different.. but this sounds a little bit like the whole "getting up on step" thing with cruise speed, were people overshoot their altitude by 100' feet then dive down to get a bump in airspeed (not to open another Pandora's box though!)
     
  27. denverpilot

    denverpilot Taxi to Parking

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    Nah the demos I’ve seen are all at normal approach speeds and no big dive or anything. Just calculated demos of it that they can make it with their hyper high glide ratios and the reduced drag.

    Longest I’ve seen was about two miles.
     
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  28. Tantalum

    Tantalum Pattern Altitude

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    Two miles! Man. I went up a few times but it's been years. Over due!
     
  29. denverpilot

    denverpilot Taxi to Parking

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    The video I saw was to a little glider strip in the middle of freakin' nowhere... just flat ground in the video going by and some sagebrush-y lookin' stuff...

    You'd dunk yourself in the lake and/or crash into the trees at Boulder trying that... if you didn't hit a car on 119... :) :) :)
     
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