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Thread: Trim/CG configuration for Landing?

  1. #1
    Utah-Jay's Avatar
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    Trim/CG configuration for Landing?

    I am sure this will illicit a bunch of responses... but I am looking for some help here. I have read various posts over time about setting trim for landing and some planes like aft CG.

    Here is my struggle, I am about 10 hours into transitioning from the Rans S-20 to the S-21 and although I made a ton of progress yesterday (two flights all pattern work) I still have a long way to go. The S-21 has been wanting to float on me. I will admit that because I could spot land ( I was hitting the zebra strips before the numbers every time) in the S-20, I was trying to make the S-21 touchdown where I wanted, not when it was ready. With the much larger tail surfaces and a more efficient wing my old stick movements are/were way too aggressive and I was getting ballooning. But this wing is just a lot more efficient and really wants to fly were as the S-20 when I got it low and slow with a bit of flare it planted itself on the ground. The 21 just wants to go back up and fly and then it balloons up and plops down. So yesterday I went back to early flight training and just tried to hold it off and was getting nice touchdowns. But.... I was wasting 600-800’ of perfectly good runway when I need to be able to hit my spot.

    So after all that bloviating, would adjusting trim or adding some aft CG help? Or am I just not doing it right? Currently I rarely touch the trim.

    Crappy weather here today, so no flying and I am bugging you guys.
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    akavidflyer's Avatar
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    That equates to excess speed that you are scrubbing off when your floating down the runway. Only way to loose that is to slow it down on short final. Elevator controls speed, power controls rate of decent.

    There is a trim for a reason, why would you not use it to make your life easier?
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    Utah-Jay's Avatar
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    Quote Originally Posted by akavidflyer View Post
    That equates to excess speed that you are scrubbing off when your floating down the runway. Only way to loose that is to slow it down on short final. Elevator controls speed, power controls rate of decent.

    There is a trim for a reason, why would you not use it to make your life easier?
    I did slow it down yesterday, I was about 5-7mph above stall on short final and it was better. I am clueless about trim helping for landings, hence my post.... out of ignorance asking the questions. I am open to help

  4. #4
    akavidflyer's Avatar
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    If its still floating a lot your still coming in too fast.
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    mvivion's Avatar
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    What sort of trim system does the S-21 have? If it has a jackscrew or other type trimmable stabilizer, trim will make a difference in landing speeds. I believe, however that the S-21 has a trim tab on the elevator. In that case, trim just makes it easier to fine tune your approach, if you trim off the stick pressures, it just makes life a bit easier, but it really won't slow the airplane down any, unless you add back pressure.

    As to CG: Every "standard configuration" airplane (ie: Not a canard type, or ??) like the S-21 will stall at a slightly slower speed at an aft Center of Gravity. The simple explanation of that is because the stabilizer and elevator don't have to provide as much down force with the load aft.

    So, yes, loading the plane further aft will allow you to land a bit slower.

    But, I'm not sure that'll help much.

    What is the source of the speed you are referring to as "Stall speed"? Have you actually gone up to safe altitude and performed a BUNCH of stalls in this airplane in all configurations? If you're using the manufacturer's listed stall speeds, you need to go up and find out where YOUR plane stalls. There is a plethora of different things that can cause one plane to indicate a different speed than a seemingly identical plane. Just a tweak of the pitot mast makes a huge difference.

    MTV
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    Utah-Jay's Avatar
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    Quote Originally Posted by mvivion View Post
    What sort of trim system does the S-21 have? If it has a jackscrew or other type trimmable stabilizer, trim will make a difference in landing speeds. I believe, however that the S-21 has a trim tab on the elevator. In that case, trim just makes it easier to fine tune your approach, if you trim off the stick pressures, it just makes life a bit easier, but it really won't slow the airplane down any, unless you add back pressure.

    As to CG: Every "standard configuration" airplane (ie: Not a canard type, or ??) like the S-21 will stall at a slightly slower speed at an aft Center of Gravity. The simple explanation of that is because the stabilizer and elevator don't have to provide as much down force with the load aft.

    So, yes, loading the plane further aft will allow you to land a bit slower.

    But, I'm not sure that'll help much.

    What is the source of the speed you are referring to as "Stall speed"? Have you actually gone up to safe altitude and performed a BUNCH of stalls in this airplane in all configurations? If you're using the manufacturer's listed stall speeds, you need to go up and find out where YOUR plane stalls. There is a plethora of different things that can cause one plane to indicate a different speed than a seemingly identical plane. Just a tweak of the pitot mast makes a huge difference.

    MTV
    MTV

    Last question first, I have done a lot (over 20) of stalls and it stalls at 43-44mph every time with either 3 or 4 notches of flaps. 48 with 2 notches, and 51 with 1 notch. I am basing my approach speeds on my IAS tape, and I am very comfortable on very short final at 49-53mph. The stall in the 21 is just a mush, nothing at all difficult to control. I do have GoPro video with gps speeds, but of course they are not the same as IAS especially at my altitude. I can post one if it is helpful

    You are correct, the trim is not a cub like jack screw, it is a trim tab on the elevator so I guess it is not much help. The stick pressures are super light and the plane is very easy to maneuver.

    I will put a bit of extra weight in the back of the baggage area, of course increasing if need be incrementally so as not to over do things.

    Edited to add: https://youtu.be/t0ePHrmoS1s

  7. #7
    cubdriver2's Avatar
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    Go up high and do a bunch of stalls. It will teach you what speed works best

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"
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    Quote Originally Posted by cubdriver2 View Post
    Go up high and do a bunch of stalls. It will teach you what speed works best

    Glenn
    That is the plane for tomorrow, crappy is weather today with 12-20 crosswinds

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    Go fly and do maneuvering at minimum controllable airspeed, where you start to feel the burble. Dial your approach speed back to just above what you can do a 15 degree bank turn at. Should be around 47 if your straight ahead stall is 43.


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    skywagon8a's Avatar
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    Quote Originally Posted by Utah-Jay View Post
    I am basing my approach speeds on my IAS tape, and I am very comfortable on very short final at 49-53mph. The stall in the 21 is just a mush, nothing at all difficult to control. I do have GoPro video with gps speeds, but of course they are not the same as IAS especially at my altitude.
    You need to fly with an instructor learning to fly by visually referencing the wings and nose in relation to the horizon. It sounds as though you were taught to fly by reading numbers on an instrument. A bit more "seat of the pants" input will help you a lot. Lacking an available instructor, completely cover your instrument panel with something so that you can not see the instruments. Leave the altimeter uncovered. Fly around, practice some stalls. Learn what the airplane is telling you, look at the wings and nose during the stalls. What are their attitudes? What does the sink rate feel like? These are basic things which apply to all airplanes.
    N1PA
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    DJ's Avatar
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    Agree with what's been said. Floating means too fast on final. At altitude you can set up a simulated approach and find the lowest speed/ highest AOA for short final that still gives you a flare to 3 point attitude or just beyond, before it stalls.
    Beside the wing differences, how do the props compare between the two? Length and pitch can make a big difference in sink rate and drag.

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    Quote Originally Posted by DJ View Post
    Agree with what's been said. Floating means too fast on final. At altitude you can set up a simulated approach and find the lowest speed/ highest AOA for short final that still gives you a flare to 3 point attitude or just beyond, before it stalls.
    Beside the wing differences, how do the props compare between the two? Length and pitch can make a big difference in sink rate and drag.

    Sent from my SM-G965U1 using SuperCub.Org mobile app
    And engine idle speed, if your idle is just a little high, you will have more of a tendency to float.


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    You might try comparing indicated speed in ground effect with airspeed at altitude
    Skywagon has a point, you simply need to feel the plane..burn enough go juice til you wear it


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    BC12D-4-85's Avatar
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    What is your weight versus gross and CG when practicing?

    Gary

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    Quote Originally Posted by DJ View Post
    Agree with what's been said. Floating means too fast on final. At altitude you can set up a simulated approach and find the lowest speed/ highest AOA for short final that still gives you a flare to 3 point attitude or just beyond, before it stalls.
    Beside the wing differences, how do the props compare between the two? Length and pitch can make a big difference in sink rate and drag.

    Sent from my SM-G965U1 using SuperCub.Org mobile app
    Prop diameter and pitch are very similar, the 20 had a Rotax 912, the 21 has a 915.

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    Utah-Jay's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    What is your weight versus gross and CG when practicing?

    Gary
    I am practicing well under gross, guessing about 400# under gross

  17. #17
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    Firstly, thanks for all the help!

    I went out early this morning and worked on stalls (again!!!!) and then as suggested extremely slow flight turns. My slow flight turns were right at 50mph. I knocked my power off stalls down to 42mph, so was very comfortable making 15* turns at 50mph. I also did a fair bit of level flight at 47mph IAS prior to practice landings.

    I then re-calibrated my AOA. This helped on the landing practice as it was beeping pretty good which I like.

    I was at or below 55mph right after my turn to final, and at mid-final I was at or below 52mph. On short final I was not really looking at my air speed indicator as I was just flying, but I am guessing I was in the 47-48mph IAS.

    I was nose up with a bit of power all the way on final, this really helped me slow down. I was so comfortable with nose down and 4 notches of flaps in the S-20 that this was totally new to me. It was actually very easy to maintain and surprisingly comfortable.

    I had 3 of my 5 landings stopped in under 600’, and that is measured from the start of the asphalt not my touchdown point. The PAPI’s are 600’ so it is easy to gauge.

    I did nail a 250’ short-field takeoff which was my shortest so far.

    All in all, I made a lot of progress today. Bottom line, I have a freaking long way to go.

    Thanks to all who offered suggestions.

    I will say my cousin and I had a long talk and he too helped a ton. He was a long time A-10 pilot and now with Delta.
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    Utah-Jay's Avatar
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    Here is a video with the fruits of my “labor”. Got up early to practice a bit. Last touchdown was at 43mph IAS which is my stall, the other two were both at 46mph IAS. Did three landings and all of them are in the video.
    https://youtu.be/L_3bYqwdec4

  19. #19

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    The last two landings have minimal float and tw touching first. So good energy and CG. If I can get my 912 to idle smoothly at 1600, a 915 should be able to.
    What's a go-around?
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  20. #20
    Utah-Jay's Avatar
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    Quote Originally Posted by Skywalker View Post
    The last two landings have minimal float and tw touching first. So good energy and CG. If I can get my 912 to idle smoothly at 1600, a 915 should be able to.
    Thanks.

    The first landing was the 1st one trying some new stuff so I was not really dialed in. I learned a lot form the 1st landing and improved a lot on the last two. I have some power in all the way on final and a bit more at the very end to get rid of some sink rate. Next move is to change my aim point short of the runway instead of the start of the asphalt. I will also be moving over to the grass to the right, but we had a lot of rain (for us) on Monday and it would have been messy yesterday.

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