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LSA CG testing

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  • LSA CG testing

    Yesterday, I worked my way up to 135 lbs in the rear seat using 45 lb sand bags. At each stage, I confirmed positive stability and did limited stall and slip testing before landing to add more weight. So far, I'm very pleased with the results. While the plane is more sensitive to elevator inputs, it is still has a way to go before reaching the practical most rearward CG. It is possible to go much deeper into the stall, however, as "up" elevator is more effective. The plane still trims well for slow cruise of 100 to 102 mph (20 in MP, and about 2400 rpm). For landing, I did not trim as I wanted to see if significant back pressure was required in the flare. It was, so that also tells me I can safely add more weight. More to come. Bob

  • #2
    I have not had a chance to fly since the last post, but I thought I would share a bit more about changes in flight characteristics as the CG moves aft in the LSA. I am no where close to being finished with this, but this is what I have observed so far. With 135 lbs in the rear seat, there is much more elevator authority, so it is possible to go much deeper into the stall. When I do so, I pretty quickly start to roll off into a spin but it is quickly recoverable by just easing back pressure. If I hold the back pressure, I can pick the wing up with rudder only at 1/4 turn, which is as far as I have gone so far. The spin rate increases with any attempt to hold the back pressure and pick the wing up with just aileron, however. If back pressure is released, the wing can be picked up with aileron. I tried that improper technique just to see what would happen as pilots who are inexperienced with spin recovery will often do it. With proper technique, recovery is virtually instaneous.

    In hard slips, there is much more elevator authority and the wing can be stalled to the point that there is a much more pronounced "shudder". I am also seeing some evidence that it may be possible to stall the wing to the point that the airplane will start to roll toward the high wing, but easing back pressure a bit stops that tendency instantaneously and completely. I'll explore that more later.

    At at least to this point with the CG testing this airplane seems to be very well behaved at slow speeds. I hope this good behavior continues as I add more weight in the rear.



    • #3
      The improved elevator effectiveness at a less-forward CG is something that applies to all airplanes that I know of. I found in the 4-place testing that in my solo configuration, no matter how hard I pulled, I couldn't get more than 2gs. On the first flights with ballast in the back, that changed for sure! Having a little weight in the back helps provide more elevator use in the landing flare, and also reduces the need for pitch trim changes at various airspeeds. Folks often forget the first half of the old adage of "nose heavy airplanes fly poorly, tail heavy planes fly once."


      • #4
        That is correct, so I'm determined to be careful as I move the CG rearward. Maybe I'll get to fly more this week. Bob


        • #5
          Because of weather and travels, I was unable to continue my CG range testing until this last weekend. I was very careful with this process as I added sand bags to the rear seat, gradually. I ran out of allowable gross weight before I reached a point where the airplane exhibited any sign of instability, so I'm going to gradually move some of the weight to the baggage compartment in the next phase. For the last test flight, I only had about four gallons of usable fuel on board for landing, so my airplane will always limit out on gross weight before it becomes unstable, it seems, at least with nothing in the baggage area.

          At each increase in weight in the rear, I performed a test series, as follows:
          1) trim for low cruise and check for positive stability
          2) straight ahead stall, power off
          3) slow turns left and right
          4) simulated approach and flare
          5) slips left and right
          6) departure stalls out of left and right turns

          When landing, I left the airplane trimmed for low cruise so that I could feel the degree of back pressure required to flare in ground effect. The landings continued to be easy.

          At all of the weights, I did maximum slips including turning slips in both directions. While it is possible to be so extreme with the elevator and rudder as to cause the airplane to roll opposite the slip (over the top), the rate is very slow, and the airplane is buffeting very heavily, so it is not likely anyone would have a problem with that, at least in my airplane. Frankly, I was surprised at this result as I expected a faster roll rate.

          I have also been surprised and pleased with performance at my maximum gross of 1320 lbs. I think performance would still be excellent at the design gross weight of 1500.

          I want to emphasize that my results are for my airplane only. Everyone should do their own testing, of course, as all of these airplanes are slightly different.

          More to come. Bob
          Last edited by bway; 03-07-2017, 08:08 AM.


          • #6
            I look forward to reading your flight testing reports. I dont know a thing about test flying. Do you use some kind of a Flight test evaluation Manual for your testing? Does the FAA, EAA, or someone else publish a document that provides builders with "Best Practice" techniques for post production test flying?

            Brooks Cone
            Patrol #303
            Brooks Cone
            Southeast Michigan
            Patrol #303, Kit build


            • #7
              Brooks, I'll respond more fully later, but print out this publication from the FAA web site. I think you will find it interesting.

              I decided to just add add some comments to this rather than add a new post.

              The techniques i use use are self developed over 50+ years of flying, but I certainly do not pretend to be any sort of expert on flight testing. Pretty much everything I do is aimed at exploring the edges of the aircraft envelope so that when I'm using the airplane I'm confident I will not get an unpleasant or worse surprise. I make sure I have plenty of altitude to recover from an unusual attitude, or spin, for each maneuver, and I don't rush things. For example, when exploring spin recoveries, I will start with just an extended stall and use the rudder to prevent a spin by recovering in the incipient stage (falling leaf). Then I will proceed to 1/4 turn, then 1/2 turns, full turns, etc. with my limit being what I can recover from at a safe airspeed and G limit.

              A couple of of comments about spins and recoveries. I do believe that it would help most pilots if they would get real spin training, including recoveries from fully developed spins (6 or more turns). Practicing spins safely really does help, not only with recovery skills, but also with the confidence factor. The primary danger from a spin comes from poor recovery techniques, i believe, as a proper recovery is not necessarily intuitive. Most pilots will initially use aileron in the recovery and it takes practice to surpress that tendency. Also, a spin may convert to a spiral in which case the wing needs to be leveled prior to pulling much G to avoid twisting the wing while it is subjected to high G loads. Again, counter intuitive for most of us when in a hurry. I used to teach spins and recoveries in Cessna 150's and I still think it is one of the best for that purpose, as it is easy to spin, and responds well to proper recovery technique.

              I did did a quick search on Amazon and found a couple of books on flight testing, one of which was highly rated. I would bet that it would be much better than anything I can offer on the subject.

              Good luck,

              Last edited by bway; 03-07-2017, 09:15 AM.


              • Bcone1381
                Bcone1381 commented
                Editing a comment
                I Printed the advisory circular, read it, and I can see that your testing far exceeds the FAA's advisory circular's recommendations. Great Job. I also am in your camp when it comes to spin training...

            • #8
              I have debated about whether or not to even mention where I have tested the most aft CG to for my LSA. My concern is that all of these airplanes are slightly different, and the measurement techniques available to me are not precise. Also, sandbags are not human bodies, so that can introduce an error in measurement, as well. I would have had less reluctance had I actually been able to weigh the airplane in level flight attitude when loaded to the most aft CG I have tested, as that would give me weights at points that are more definite and measurable. In the end, I finally decided that pilots who are building these airplanes are aware of all of the above and will test their own airplanes carefully, so here is what I found.

              As best I can tell, the most aft CG I have tested to date is about 20.1 inches; 1.5 inches aft of the published limit of 18.6 inches behind the wing leading edge. At this aft CG, my airplane flies fine and does not exhibit any bad characteristics. I tested landing characteristics by trimming for about a 100 mph cruise and then landing using that trim setting. There was still significant back pressure required in the flare so there might be a little more to go, but I'm at gross so I'm not going farther.

              I hope everyone will be careful with this testing.