• Ratty's Ramblings - Short Takeoffs

    Ratty's Ramblings

    Ratty's Ramblings - Short Takeoffs

    By Ian Radcliffe

    With all the scenery packages and individual airfields available in freeware and payware, there are innumerable enticing little aerodromes dotted around the flightsim world these days. Occasionally, if I see an interesting looking airfield on my travels, I'll break my trip and drop in to check it out. I'll usually look up the runway length, to make sure that whatever I'm flying will fit, but even if it's not on the chart, my experienced eye can generally tell whether it's too short. So I'll land, and taxi around to explore the scenery - and then remember what I seem to forget every time.

    Most airplanes take a lot less room to get down and stopped than they do to get up, especially if there are obstacles. A Cessna 172 at sea level and a temperature of 20 degrees Centigrade can clear the quintessential 50-foot obstacle, land and stop in 1,350 feet. Its takeoff, however, will require 1,750 feet to get to fifty feet. And to get even that performance demands a different technique than normal takeoffs.

    This is not just true of "short fields". Heat and altitude can make even a long runway short. The same 172 landing at an elevation of 5,000 feet needs 1,535 feet to clear the obstacle and come to a stop, but a whopping 2,920 to take off to fifty feet - almost twice the distance. And at 8,000 feet and twenty degrees, the takeoff roll alone requires 2,525 feet, and distance to fifty feet is 5,315 feet!

    Short takeoff techniques vary little from plane to plane. Mostly, the difference is whether flap is used, and how much. If you have a Pilot's Operating Handbook for your craft, the information will be in there. Cessna seems to recommend ten degrees of flap for their 172. In the Cherokee book, Piper suggest "up to 25".

    A short field takeoff checklist for the 172 looks something like this:

    1. Flaps - 10 degrees
    2. Use all available runway
    3. Hold brakes
    4. Apply full power/check engine instruments
    5. Brake release/rotate at 51 kts
    6. Climb at Vx, 56 knots, until obstacle is cleared, then raise the flaps and climb at Vy

    "Use all available runway" means start as far back as possible. Many fields have displaced thresholds, which means you can't land on the first bit of the runway, but nothing precludes you taking off from it.

    There is still debate over the efficacy of running up the engines against the brakes; some claim that cavitation makes the propeller less efficient, and advancing the throttle on the roll is the way to go. It does sound good, though, and I find the roaring and shaking sets the right frame of mind for the task ahead.

    Ah, yes - Vx and Vy. If you do have a POH, in there you'll probably find two speeds for climb: Vx and Vy. Vx, the slower of the two, gives you the best ANGLE of climb, and therefore the best obstacle clearance. Vy gives the best RATE; for the 172 that's around 77 knots.

    And remember: just as temperature and altitude affect performance, there are other considerations to factor in; wind, slope, and surface should all be taken into account. A tailwind that is 10% of your takeoff speed will add about 20% to the distance required, a 2% slope will add 10%, and grass adds up to 30%.

    Ratty's Ramble


    7 Comments
    1. lnuss's Avatar
      lnuss -
      Many fine words of advice, Ian. You've done a nice job of stating the principals involved and providing some examples.

      But for some clarification, I'll add a few things. For instance, you didn't mention your weight (which obviously has a big effect), but I presume the figures you quoted were at max gross.

      The same 172 landing at an elevation of 5,000 feet needs 1,535 feet to clear the obstacle and come to a stop, but a whopping 2,920 to take off to fifty feet - almost twice the distance.
      At what temperature, Ian? I ask because the "standard" temperature at 5,000 feet is 41º F, not the 59º F that is "standard" at sea level and, as you know, that makes a LOT of difference. And so the airplane will need an increased distance for higher temperatures than 41º F. I mention this mostly because I can "see" many folks thinking 20º C at both sea level and at 5,000' elevation.

      I'd also note that different version of the same "model" aircraft can have different performance. For example, the C-172 "owner's manual" that I have on hand is for a 1968 C-172, which has a 2300 lb max gross weight and Lycoming O-320-E2d engine rated at 150 HP at 2700 RPM, while your quote was probably from a much newer version with a 160 HP engine. Also, the 1968 model has a full 40º of those barn doors they call flaps, while your later model probably was one that had only 30º available. The manual was printed prior to the GAMA standard POH, so has less performance information. So I'll mention a few figures below that are different, but that come from this manual.

      The performance section doesn't give any information for a "short field" takeoff, only a standard takeoff from a hard surface with no flaps, but it states that at max weight (2300 lbs) at 59º F and sea level and 0º flaps, the takeoff run is 865' but to clear a 50' obstacle is 1525'. Landing at SL and 59º takes 520' but over the 50' obstacle is 1250'.

      That 5,000' elevation takeoff (41º F) is 1255' on the ground and is 2480' to clear the 50' obstacle. And for landing it needs 605' ground and 1385' over a 50' obstacle. And at 2,000 lbs weight the 5000' elev. takeoff is 905' with 1625' to clear 50'.

      One other comment for the pilots out there, sim or RW: The figures in the handbook/manual/POH are with a brand spanking new airplane with minimum protrusions (antennas, etc.) and was flown by a highly skilled test pilot. Few people in the real world can match those figures (and they leave NO margin for error) or have a brand new airplane to fly.

      Final word: The flight dynamics of many sim aircraft are a poor match for the real world figures found in handbooks, including excerpts included in the sim version of the manual.

      I really like what you are doing with your articles to help folks understand more about flying -- keep up the good work.
    1. zswobbie1's Avatar
      zswobbie1 -
      Hi Ian, I fly with a Yoke & pedals, & I agree that it becomes more immersive when not flying with a keyboard.. so what we did, at home & at our VA was to plug in a 2nd keyboard & remap the keys using FSUIPC (now free for FS2004, with registration) I turned the keyboard so it was in portrait mode, grouped the functions I wanted to map (radio together, space bar for brakes, & printed out the legends, color coded) & stuck then onto the keys. So, everything that could be pressed was now on the second keyboard, grouped & color coded. The immersion factor really soared. Doing it this way obviously duplicated the main keyboard, which was not used. Using FSUIPC, the mouse wheel became the trim wheel.

      Much fun was had.
    1. ianhr's Avatar
      ianhr -
      Quote Originally Posted by zswobbie1 View Post
      Hi Ian, I fly with a Yoke & pedals, & I agree that it becomes more immersive when not flying with a keyboard.. so what we did, at home & at our VA was to plug in a 2nd keyboard & remap the keys using FSUIPC (now free for FS2004, with registration) I turned the keyboard so it was in portrait mode, grouped the functions I wanted to map (radio together, space bar for brakes, & printed out the legends, color coded) & stuck then onto the keys. So, everything that could be pressed was now on the second keyboard, grouped & color coded. The immersion factor really soared. Doing it this way obviously duplicated the main keyboard, which was not used. Using FSUIPC, the mouse wheel became the trim wheel.

      Much fun was had.
      That's brilliant, and you didn't have to spend a fortune on peripherals to achieve that effect! Thanks for sharing. Comments like yours and Larry's help promote what I've come to realize is my main goal with these columns: to inform and inspire (and, I hope, entertain).
    1. North Central DC-9's Avatar
      North Central DC-9 -
      Excellent article Ian!

      I find that short fields are the most fun, even with virtual airliners (or riding in real ones).

      It’s fun to land a 737 on a 10,000 foot runway, but far more fun to somewhat violently land on a 6,500 foot runway with thrust revers and spoilers and brakes deployed, with luggage bins flying open! Burbank and Chicago Midway are good suggestions in the US.

      Steve
    1. DavidN16's Avatar
      DavidN16 -
      I added the Orbx freeware called “Global Airport Pack” to my set up, P3Dv3, which has 849 small airports that are very well detailed to visit. Please note that you don’t need any extra Orbx payware to enjoy these free airports in your sim.

      I downloaded the list of airports that are in the pack from here http://flightlessons.6te.net/listbx.txt

      Next I pick one state and use “Little Nav Map” (free) to plot a flight plan and entered all the airports within that state to visit. It’s just another way I keep the sim interesting.

      I also use “FS Earth Tiles” (free) to download photo real scenery for the current state I am flying in to make the flight even more eye popping. I download the tiles at level 3.
    1. dbauder's Avatar
      dbauder -
      OK, once I did something in the RW that I've neglected to do in Flight Sim, a short field takeoff in the DHC-6 Twin Otter. I was Copilot and we had no passengers, plenty of runway. It's been 40 years but as I recall, the manual said "20 degrees flaps, hold the brakes, yoke (elevator) full back, full power, release the brakes". As the nose started to rise, you started releasing the back pressure on the yoke.

      That great aircraft was airborne in seconds!
    1. ianhr's Avatar
      ianhr -
      Quote Originally Posted by dbauder View Post
      OK, once I did something in the RW that I've neglected to do in Flight Sim, a short field takeoff in the DHC-6 Twin Otter. I was Copilot and we had no passengers, plenty of runway. It's been 40 years but as I recall, the manual said "20 degrees flaps, hold the brakes, yoke (elevator) full back, full power, release the brakes". As the nose started to rise, you started releasing the back pressure on the yoke.

      That great aircraft was airborne in seconds!
      Wheeeeeee! Thanks for that. I just happen to have the Aerosoft Twin Otter . . .
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