Flap assisted take off

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Re: Flap assisted take off

Postby TFA170 » Sat Feb 13, 2021 1:57 am

GAHorn wrote:JOe, it is my view that thrust from the engine/prop results in a forward “pull” at the front of the crankshaft.
I “get” that prop-blast might be “felt” at the flaps. This prop-blast would result in one thing as far as flap is concerned: relative wind against the flaps...which would result in Lift and drag.

Since ALL prop-blast is felt by ANY portion of the airframe ...then it is UN-avoidable. But a benefit of that relative wind created by the prop would also be LIFT... over by the flaps as well as the wings. To imagine flap deployment as a detriment to acceleration (and therefore to obtaining flying speed) would be equivalent to seeing the much larger and blunter airframe as being such an impediment. The greater resistance to acceleration (and therefore the generation of lift by the wings) would be the drag caused by rolling-resistance, especially on a rough-field/soft-field situaiton.... in-which the lift simultaneously created by deployed-flaps would reduce said drag. That is why soft-field/rough-field techniques recommend take-off flap deployment on rough/soft fields.

If your suggestion (that deployed flaps inhibited acceleration as compared to retracted flaps) then the Cessna performance charts would not reflect shorter takeoff distances with flaps deployed versus undeployed. :wink:



You would be 100% accurate if the chord line was perfectly aligned with the thrust line, but since the chord line is a few feet above the thrust line, not all of the prop blast goes across the wing generating lift and much of it strikes only the lower half of the deployed flap. This is pretty normal for any high wing, fowler-type flap setup where the thrust-line is not aligned with the chord of the wing. Unlike the C-130, where the thrust line is almost aligned with the chord line and virtually all of the thrust from the prop is divvied up evenly between both the top and bottom of the wing generating effective "blown lift", the Cessna and most other GA high-wing STOL aircraft do not benefit from this phenomenon because of the misalignment of the thrust line and the chord line. Most of the lift during takeoff is generated by forward speed, not blown thrust across the wing.

Further, since the thrust line is lower than the chord line by a few feet, much of the thrust is indeed only striking the lower portion of the deployed flaps and also being directed downward by the deployed flaps instead of directly rearward. Consequently, it is more detrimental to acceleration vs flaps up.

I think the confusion lies in the conflating of acceleration with takeoff distance. In any fowler-type flap setup, there is a point of diminishing returns - low flap settings will tend to generate more lift than drag and high flap settings will tend to generate more drag than lift and the trick is to find the "sweet spot" - this is often where the flaps begin to track downward more than rearward due to camber changing more than chord. Regardless, this is why we don't take off with 40* of flaps - too much drag. It's also why takeoff with flaps 10-20 is shorter than flaps up, but not necessarily quicker from an acceleration point of view. And you become airborne at a lower speed with flaps deployed than with flaps up. Because acceleration and takeoff distance are not necessarily linked and distance is more a function of climb and how quickly you get off the ground vs how quickly you get there.

So, if you put two identical 170Bs side-by-side and had one deploy flaps to 20 and another leave the flaps up and they both took off - you might find that the flaps-up will accelerate quicker, but take longer to get off the ground distance-wise, whereby the exact opposite would be true with the aircraft with flaps set at 20*.

So, Joe's observations in no way invalidate or contradict the Cessna performance charts.
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Re: Flap assisted take off

Postby GAHorn » Sat Feb 13, 2021 5:13 pm

I don’t have any “confusion” regarding the amount of flap deployed... or when they should be deployed.... and the thrust line consideration which I was addressing has little to do with if that line is above/below/or at the chord-line. While that’s an interesting matter...it’s not what I was discussing... If THAT argument were valid then helicopters would not fly. (Hmmmmnnnn .... maybe so...??? :lol: ). LIFT is defined by the air-pressure-contrast between that Above versus Below the surface. Prop-blast will increase that differential regardless of whether or not it exists above/below/at the thrust-line. (Hold a piece of loose-leaf paper by the edge and blow above it and you will see it Rise into the relative wind.... Blow BELOW it ...and you will also see it rise.)
And it is not confusing as to how much flap is to be used for takeoff.... “take-off flap settings” are defined already by Cessna and that does not include settings greater than 20-degrees in this airplane.

The rate-of-acceleration over time is also unimportant to this discussion, IMO. The takeoff distance is what we are discussing... which is defined as from the point of brake-release to 50’ of altitude (the presumed obstacle).... and can be further partitioned to consider “ground roll” ...which Cessna estimates at between 38 and 40% of total distance for any given takeoff condition.

Flaps deployed at the beginning of the TO...even Before Brake Release...Before Acceleration....are ALREADY providing LIFT....purely due to relative-wind and prop-blast.... that deployment aids and assists shortening the takeoff distance for the reasons already mentioned.

Ain’t this a Fun Topic? I like the discussion and Thank you for the comments.
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Re: Flap assisted take off

Postby Vertical » Sat Feb 13, 2021 8:27 pm

GAHorn wrote: The takeoff distance is what we are discussing... which is defined as from the point of brake-release to 50’ of altitude (the presumed obstacle).... and can be further partitioned to consider “ground roll” ...which Cessna estimates at between 38 and 40% of total distance for any given takeoff condition.


The technique to clear a 50' obstacle is different than simply getting airborne in the shortest possible distance (wheels leaving the ground). In my experience, wallowing in ground effect with full flaps after leaving the ground as soon as physically possible will delay the ability to climb vs not inducing quite as much drag with a less dramatic flap use and a slightly longer ground roll.

The composition of the ground surface, aircraft loading, DA, wind etc. will further change things.
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Re: Flap assisted take off

Postby Joe Moilanen » Sat Feb 13, 2021 11:09 pm

I'm not in any way claiming to be an aeronautical engineer of any sorts, but here is how I came to my method. When I realized that there was 17 acres available for purchase that was adjacent to my 17 acres, I started plotting out whether I would have enough total room for an airstrip if I bought it ( Purchase price was 20K and I logged 43K worth of timber off it by myself with a 2U D-8 cable blade cat that I purchased for 4.5K and sold for 6.5K when I was done building the strip so it was a win-win, strip or no strip). I deducted that I could build a 650' strip with 30' trees 250' from the departure end. The downhill slope dictates the departure and landing direction.

At my local airport (KKLS), the first turn-off when landing on 30 is exactly 650'. The grass between the taxiway and the runway at that point in time has become MY runway. Our new Airport manager started sniveling about pilots using the grass, but none-the-less I would beat myself up pretty hard if I EVER passed that turn-off whether taking off or landing. I reasoned that I if I could take off from KKLS in the TALL grass, without the downhill advantage of my own strip, and be comfortably above 30' , 900' from beginning of takeoff roll, it was feasible. I even did it with full fuel and wind/temperature conditions that I would NEVER try at my house. I have never taken off from my home strip with more than 1/4 tanks. KKLS is only 10 minutes away. I would practice for hours and hours, 20 degrees flaps from the beginning, and grabbing 20 degrees at various points in my takeoff roll. I even experimented with the rolling start from an angle. I found that in my experience the rolling start from an angle didn't work for me as the opposite rudder/brake to straighten out was a negative factor for acceleration. To be the highest possible above the 30' trees, this is what works best for me: Tailwheel at the first inch of runway and straight. ENGINE FULL WARM! I can't stress that enough. Full power run-up, tweak mixture max RPM, check engine gauges, say prayer, release brakes. Steer with rudder only, keep it perfectly straight with minor inputs, the larger the inputs the more rudder drag. Elevator neutral once rudder become effective. Regardless of airspeed, roll in 20 degrees of flaps at the 500' mark on strip (passing my pump-house for my reference point). I let the plane balloon off the ground as I let it seek VX which I already had it trimmed for pre-takeoff. Once I'm over the trees the terrain descends rapidly so I'm home free with as much forward yoke as I wish for airspeed gain. Starting out with 20 flaps from beginning gets me off the ground much quicker but watching the trees get bigger holding VX makes my toes curl and I forget to breath for a while...and I do not clear the trees by a better margin. It's kind of like the adage that if you are worried about whether or not you will clear the trees ahead of you, you should fly straight towards them, picking up airspeed and then balloon over them bleeding off airspeed as you crest them, instead of trying to climb all the way to them.

I don't claim to be any kind of expert or even anywhere close to the greatest pilot around, but this works the best for me on my strip. I believe when you start pushing these planes to their limits on sketchy strips, each strip can require it's own method. As of the 29th of this month, I will have owned my B model for 32 years, flown most of the Idaho back country, and have the advantage of a Sportsman STOL and an 8043 prop. I'm not bragging, just saying that I do have the feel of my plane down pretty good. This is the kind of flying that I love the most. Never-the-less, my ears are wide open to anything that I can learn procedure-wise, for there are back country pilots out their that can fly circles around me. I've learned more about back country flying sitting around camp fires drinking beer in the outback then I ever could reading books! In all of the crazy hobbies that I do, it seems like as soon as I start getting a big head and think I'm pretty cool I get a quick and scary lesson!
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Re: Flap assisted take off

Postby Kevin Pearce » Sun Feb 14, 2021 7:44 pm

Joe

Thank you for going into detail about how you operate at your strip. The focus on obstacle clearance is very relevant to me as my strip is 1120ft with 40ft trees at one end and similar trees 200ft from the other end and 250ft amsl. I am used to my strip but am fairly new to the 170B with its 180hp and STOL kit. Can I ask what speed you fly your final approach? When you cross your trees do you have any power on and then close the throttle or are you set up on a glide approach?

Thanks

Kevin
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Re: Flap assisted take off

Postby Joe Moilanen » Sun Feb 14, 2021 9:41 pm

Kevin Pearce wrote:Joe

Thank you for going into detail about how you operate at your strip. The focus on obstacle clearance is very relevant to me as my strip is 1120ft with 40ft trees at one end and similar trees 200ft from the other end and 250ft amsl. I am used to my strip but am fairly new to the 170B with its 180hp and STOL kit. Can I ask what speed you fly your final approach? When you cross your trees do you have any power on and then close the throttle or are you set up on a glide approach?

Thanks

Kevin

Hi Kevin,

I usually set up a partial power approach at about 50-55 indicated and pull the power back when I cross the trees so that I can touch down at the beginning of the strip. The beginning of the landing portion of the strip has an uphill to it so I usually have to give it a slight blast of power to arrest me decent just before touchdown.

Joe
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Re: Flap assisted take off

Postby Kevin Pearce » Mon Feb 15, 2021 2:26 pm

Hi Joe

Thank you for the reply, pretty much what I am doing. I seem to float a bit and think that the ASI was slightly under reading. It is on its way back from overhaul so am keen to try again in the Spring.

Kevin
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Re: Flap assisted take off

Postby mmcmillan2 » Mon Feb 15, 2021 4:47 pm

Joe Moilanen wrote:
Kevin Pearce wrote:Joe

Thank you for going into detail about how you operate at your strip. The focus on obstacle clearance is very relevant to me as my strip is 1120ft with 40ft trees at one end and similar trees 200ft from the other end and 250ft amsl. I am used to my strip but am fairly new to the 170B with its 180hp and STOL kit. Can I ask what speed you fly your final approach? When you cross your trees do you have any power on and then close the throttle or are you set up on a glide approach?

Thanks

Kevin

Hi Kevin,

I usually set up a partial power approach at about 50-55 indicated and pull the power back when I cross the trees so that I can touch down at the beginning of the strip. The beginning of the landing portion of the strip has an uphill to it so I usually have to give it a slight blast of power to arrest me decent just before touchdown.

Joe


Flap setting when crossing over trees?
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Re: Flap assisted take off

Postby Joe Moilanen » Mon Feb 15, 2021 8:16 pm

mmcmillan2 wrote:
Joe Moilanen wrote:
Kevin Pearce wrote:Joe

Thank you for going into detail about how you operate at your strip. The focus on obstacle clearance is very relevant to me as my strip is 1120ft with 40ft trees at one end and similar trees 200ft from the other end and 250ft amsl. I am used to my strip but am fairly new to the 170B with its 180hp and STOL kit. Can I ask what speed you fly your final approach? When you cross your trees do you have any power on and then close the throttle or are you set up on a glide approach?

Thanks

Kevin

Hi Kevin,

I usually set up a partial power approach at about 50-55 indicated and pull the power back when I cross the trees so that I can touch down at the beginning of the strip. The beginning of the landing portion of the strip has an uphill to it so I usually have to give it a slight blast of power to arrest me decent just before touchdown.

Joe


Flap setting when crossing over trees?


40 degrees

Joe
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Re: Flap assisted take off

Postby Joe Moilanen » Mon Feb 22, 2021 1:21 am

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Re: Flap assisted take off

Postby Vertical » Tue Apr 27, 2021 2:33 am

It's weird. No one ever seems to comment on videos here. That was a nice take off. A guy could get it off the ground quicker, but strikes a good balance of minimizing drag (not pulling full flaps) to maximize climb out.
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Re: Flap assisted take off

Postby Joe Moilanen » Tue Apr 27, 2021 10:55 pm

Vertical wrote:It's weird. No one ever seems to comment on videos here. That was a nice take off. A guy could get it off the ground quicker, but strikes a good balance of minimizing drag (not pulling full flaps) to maximize climb out.

Thanks!!
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Re: Flap assisted take off

Postby mmcmillan2 » Thu Apr 29, 2021 12:33 am

Finally saw the video, very cool! I play around with popping 20 degrees after tail raises, mine will levitate at that point. 170b
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Re: Flap assisted take off

Postby Joe Moilanen » Thu Apr 29, 2021 11:27 pm

mmcmillan2 wrote:Finally saw the video, very cool! I play around with popping 20 degrees after tail raises, mine will levitate at that point. 170b

That's what works best for me, at least on my airstrip!

Joe
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