LSA Flight Testing - Spins

[bway]

I decided to move flight testing stuff here for my LSA.

On Friday and Saturday, I did some more experimenting with spins, but before doing so, I did take the airplane all the way to 140 mph. There was absolutely no hint of a problem at that speed, but I was at a smooth altitude. I had the power pulled back to idle as I wanted to be sure I did not overspeed the engine. The MGL recorded a descent rate of 4,640 fpm and I was in a steep dive.

I then worked with spins from level flight as well as spins from approach and departure attitudes. At this point I am limiting myself to one complete turn, but I wanted to share my findings thus far.

From level flight, with the ball centered, my LSA tries to just nibble at the stall, at first, but if I use both hands on the stick and hold it for a bit it will eventually stall and drop a wing. Which wing is not predictable, however. Probably I have the ball out of center ever so slightly, causing the wing drop. Spin recovery is simple, if started right away, and can be done with little loss of altitude. On the other hand, if the stick is held back and centered, the airplane will finally stall, quickly swap ends and go very nose down. If that is allowed to progress, and the rudder is kept centered with the stick back, the spin converts to a tight, nose down spiral in my airplane. At that point I have quickly released back pressure, rolled the wings parallel to the horizon and then applied sufficient back pressure to recover to level flight at a reasonable G load. It is important that I be sure to not pull significant G's and roll out at the same time as that would put a twisting moment on the wing greatly reducing its ability to handle the G load. Maximum airspeed with a relatively gentle recovery was less than 120 mph.

Again, from level flight, I did intentional spins both ways and recovered after one turn. Recovery was conventional with airspeed not exceeding 105 mph in the recovery. Again, I was careful to stop the spin completely and not convert to a spiral while recovering for the reason mentioned above. I am now confident that a one turn spin can be recovered by just application of opposite rudder and release of back pressure (it would probably recover just by releasing the controls, but I did not try that). I did not pull a lot of G's as I was ok on airspeed. The loss of altitude with the one turn spins was about 500 feet. However, I could have safely recovered to level a bit quicker by pulling more G's.

I explored approach stall/spins next and I found no real surprises given what I already knew. Even in a simulated steep turn to final with the ball centered. the airplane is quick to recover without entering a spin just by easing off back pressure on the stick at the first sign of a stall. On the other hand, a stall in a skidding turn does result in a quick roll to inverted. If that happened at a low altitude, recovery in time would be doubtful. However, even in a relatively steep turn, the nose is high enough that the danger should be obvious to any pilot. Stalls out of turning slips are quickly recoverable just by reducing back pressure, but if back pressure is held, the airplane does roll over the top and will go inverted.

I next tried skidding left turn departure stalls at full power. The nose was so high at the stall that any pilot should recognize the danger, but the airplane does roll inverted quickly if one is so ham handed as to do this. I experimented with placing my feet flat on the floor and stalling out of a full power left turn, as well with essentially the same result, but just a little slower. I saw no reason to do this to the right as the left turn is the more critical.

My conclusion at this point is that the LSA is a very safe airplane for any careful pilot. It is not like a Cessna or Piper with wash-out in the wings, though. One result is that there is less buffeting prior to the full stall than with those airplanes, but the LSA wing hangs on for an incredibly long time before stalling. Once stalled, though, the LSA will spin easily but recovery is very conventional. I have a hard time seeing how anyone could stall and spin my airplane accidentally because it gives so many cues that it is outside the normal flight envelope before departing. That said, people do stall and spin Cessnas and Pipers accidently, so I'm sure it can be done.

I would emphasize that these results are applicable to my airplane only, and these tests were done with the back seat and the baggage compartment empty. I'll work on this some more later with weight in the back seat and baggage compartment. Also, as Mark so rightly suggests, if one is not very experienced with spins, this could be dangerous as inappropriate recovery could exceed design limits and break the airplane with disastrous results. Right now, I think I have done everything I think I need to do with spins for the test period, and I know my airplane is safe in terms of handling characteristics. BTW, all of this was done at high altitude (over 6,000 feet) and over very uninhabited areas of farm land.

I would be interested to know if what I have experienced with the LSA is similar in the Patrol, the LSA's big brother.

Bob

[Mark Goldberg]

Very interesting Bob. Thanks for the flight testing and detailed report. Mark

[Flygirl1]

Not sure if I have the nerve to do that or not in my newly built Patrol. I did spins in the Citabria often, (airbatic spelled backwards), but if any amount of time had passed it always would take me by surprise me at how much the earth spins on the first one. Being a glider pilot myself, I was wondering if you noticed what your yaw string was doing. Donna

[bway]

Donna, that is a good question. I was focused on stopping the spin at one full turn and did not notice. I'll check next time out though. Also, not being familiar with using an AOA instrument, I did not pay much attention to it but I do plan to repeat departure and approach stalls to see what it tells me. I'm particularly curious about its accuracy in hard slips and whether the accuracy is the same in left slips and right slips as the pickup is in my pitot tube assembly mounted a couple of feet from the left wing tip.

It it is certainly appropriate to be careful about spins and, in truth, we can learn about everything we need to know to fly safely by just exploring spin entries from various attitudes and stopping at 1/2 turn. That is what I did at first. The rest is more about curiosity than anything else. I do believe that for anyone who intends to fly their plane on the edge of the slow flight envelope it is prudent to become very familiar with spin entries while simulating departures and approaches, but obviously at a safe altitude.

When I was a young pup many years ago, I had a J 3 Cub. I learned to fly in that Cub and did a lot of spins in it. As everyone knows, that airplane is soloed from the rear seat. One summer day when the Cub was very low on fuel, I decided to see how high I could climb. I got to about 12,000 feet before I ran out of fuel. In my youthful enthusiasm, I decided to spin down and see how many turns I could do. I had spun the plane more than 6 turns many times, but never with the fuel tank empty. Being young and stupid, I never gave a thought to the fact that I was at the very back of the CG envelope, if not outside it. Well, after a lot of turns (maybe 10) the nose started to bob up and down and the stick became very "soft", so I decided to recover. To my surprise, I had to use full rudder and snap the stick forward when the nose bobbed down to break the spin. That experience scared me enough that I have always been careful about CG since. When I get around to working on determining the rear of the CG envelope in my LSA, I will be very careful and approach that limit gradually. I have gotten over being young; maybe stupid too. Bob

[Flygirl1]

Bob, I think you just dis-proved the old saying "There are no old bold pilots"! I will not admit to all the stupid things Ive done many years ago, but I will admit I've learned my limitations doing them!! As you've stated i can't imagine a take off stall due to the extreme high nose attitude. But I have been on the edge during a low, slow base to final turn,which in my opinion is the worst possible place to do that. Many have not survived. The high speed stall was the most interesting. I never really understood that until during one-- the unmistakable buffeting was an eye opener. Fly safe!! Donna

[bway]

For the reason you mention, Donna, I prefer a continuous turn to final from downwind when I'm trying to keep the landing spot in sight and make a really short landing. That allows me to establish a continuous partial slip all the way around the turn. If I were to stall while in the slip, the airplane will depart over the top giving me time to recover. Also, when landing in marginal conditions, I set my trim for full power climb and just muscle the stick back for approach and landing. That way, if I have to add full power to go around, I don't have to hold heavy forward pressure while trying to trim the airplane. This may not work well for everyone, but it simplifies things for me. I first started using the slip technique in that old J 3 so that I could see my touchdown point from the back seat when on final. Only later did I decide that a slight slip when low and slow might be safer for me. Have fun! Bob

[Mark Goldberg]

Bob, I forwarded your comments about spinning your LSA to Bob Barrows. The only comment he had was his surprise at something you just said again here - that a stall while in a slip "going over the top". Bob Barrows does stalls while slipping and does not see his LSA go over the top. It stays more controlled than that and recovers just lowering the nose a little. Just thought I would pass along Bob's comment. Mark

[JimParker256]

I wondered about that, as well. So I went back and re-read a bunch of stuff about slipping and skidding turns. Here's the web site that seemed to have the clearest explanation (and illustrations) of what happens and why: http://www.boldmethod.com/learn-to-f...ip-skid-stall/

Most of the articles I read state that slipping turns are preferred over skidding turns because slipping turns are naturally "spin resistant". All the articles seem to say that in a slipping turn, the "high-side" wing stalls first. That causes it to drop, which tends to reduce the angle of attack somewhat, and pretty well stops (or reduces) the turn rate. So not only is the angle of attack reduced, and the wing loading is reduced, but the airflow across the wing is restored to a "straight and level" type flow. All those factors tend to make a slipping stall somewhat "self-recovering", assuming the pilot doesn't do anything to make it worse...

In a skidding turn, on the other hand, it is the low-side wing that is operating at a slightly slower airspeed, causing the aircraft to "over-bank" into the turn. The pilot's natural reaction is to counter the over-banking tendency with opposite stick pressure (which would require right-stick in our left-turn case). Now that low-side wing, which was already going slower, has its angle of attack increased further by the downward deflected aileron, resulting in a low-wing stall. Since the high-side wing is still producing lift, the aircraft rolls in the direction of the bank, and a spin often results.

So like Bob (and apparently Mark), I'm a bit surprised that a slipping turn would result in an "over the top" spin entry without further "encouragement" on the part of the pilot. But then, I'm not an aerobatic pilot, nor do I play one on TV, and I didn't even sleep in a Holiday Inn last night... The few spins I did were many years ago, in Cessna 152s and Piper Tomahawks (back when they first came out, and were supposed to be the greatest thing for pilot training).

[bway]

After hearing from Mark that Bob Barrows' LSA did not seem to have any tendency to spin over the top when stalled in a slip, I decided that I needed to do proper testing of mine in smooth air. The first, and only, test I had done was in really rough air when the AWOS at the airport was reporting wind 16 kts, gusts 27 kts varying from 10 degrees to 70 degrees off the runway heading. I actually did the "test" because I knew that obviously I was going to need to slip the airplane aggressively to the upwind side at touchdown. I was about 1,500 agl and the updrafts and downdrafts were really bad. When I slipped the airplane to the right, as I would have to do for the landing, using about half rudder and enough aileron to hold heading while pulling the stick back to approach the stall, I got what I expected. The airplane rolled left about the time I had the stick full back, but immediately recovered when I released back pressure and rudder. So, nothing dramatic, much better than my old J 3, or PA 11, I thought. I went on to the airport and did a wheel landing on the upwind main gear and was able to get turned downwind and taxi to the hangar without incident, thankfully. BTW, the winds were only about 11 or 12 kts when I took off but increased during the two hours I was flying.

Yesterday, I took the airplane out again with my plan being to climb to smooth air ( 9,000) to do proper spin testing of the LSA out of slips. After about 30 minutes, I gave up. I was absolutely unable to get it to spin "over the top". I tried full rudder slips left and right, partial slips left and right, and aggressive slip entries that would have resulted in a quick spin entry "over the top" in my old J 3, or the homebuilt PA 11. To say I was surprised would be an understatement. My old J 3 would not spin with the CG forward (2 people aboard) without cross controlling it (not enough elevator), yet here the LSA was simply slipping, with a little burble, out of the very same maneuver. I guessed that I was banked so far in the slips that there was not enough elevator to get to a critical angle of attack, so I tried to stall out of a slip with only about one third rudder input. Same result except the airplane did start a wings level turn while burbling at the stall and finally entered a spiral. I had both hands on the stick pulling back but there was just not enough elevator to completely stall the wing. At that point I decided that I had tried about everything and so I just started flying around thinking about all this.

Obviously, I had experienced something very different from my experience of a few days before, as well as my prior experience in what I thought were pretty similar airplanes. The thing that came to mind about a few days before was the difference in conditions, and also the difference in fuel level. I was almost full on fuel where I was almost empty previously. I resolved to burn off fuel doing other stuff and then drop down into the rough air to try again. When I did, I was finally able to get a result like I experienced previously (after about five tries, but it was not as turbulent as before). This time, I just held what I had, about half right rudder, but I pushed the stick left when the left wing came up; still holding the stick full back. To my surprise, the wing went back down. If I had done that in the J 3 it would have probably spun, but not the LSA. I was very pleased, but confused. I decided to finish my three hour flight and think about this some more to see if I could come up with the "why" of it all.

Tomorrow, I will report on what I thought of, as well as what Bob Barrows had to say to me about why he thinks the LSA is so resistant to "over the top" spin entries compared to my other airplanes.

Bob

[bway]

Before I talked to Bob Barrows, I had thought about what the differences were between my old J 3, homebuilt PA 11, and the Bearhawk LSA that could account for the differences in spin characteristics out of a slip. After all, more things seemed to be similar than different. In a slip, all would have the air flowing diagonally over the wing effectively, it would seem, shortening the wingspan and increasing the chord. The ailerons seemed similar. The only thing that I thought of that was quite different (at least on my planes) was the dihedral, but I could not envision how that might cause the difference, so I called Bob to discuss the matter.

Bob told me that he thinks the primary difference that accounts for the better behavior with the Bearhawk is the airfoil used. Both of the rag wing Pipers use the same airfoil, essentially, which is very different from that of the LSA. I'll take Bob's word for this as he is the expert and I know for a fact that the airfoil used on the LSA far outperforms that of the Pipers in other areas such as climb and speed while still allowing impressive slow flight so I'll just add one more thing to my list of reasons that I think the Bearhawk is not just a great LSA; it is a great airplane, period.

All of the testing I have done so far has been with a forward CG. I plan to test the spin characteristics of my plane with weight in the back seat and gradually move the CG toward the rear until I can establish my limit. I felt very confident going into my testing that the LSA would behave well in spins with a forward CG, but I want to be very careful while moving the CG to the rear. I will use sand bags in an army duffle bag that I can strap in the rear seat. That way I can increase weight gradually. It will be interesting to see if the airplane will still be as well behaved when stalled in a slip with the CG to the rear. I hope it will be, but the only way to find out for sure is to try it. It will be a while before I do that, but I will post results when I can. Bob

[Bcone1381]

I am enjoying this thread, Bob, and look forward to the reports.

Brooks

[Mark Goldberg]

I also commend you for the elaborate testing you are doing Bob. Most builders (myself especially) do nothing like the testing you are doing. It is good info for everyone. Thanks. Mark

[Battson]

Interesting to hear the LSA spins so quickly after the stall begins (if you continue to hold back-pressure). Have I understood that correctly?

We can literally fly our 4-place with normal control inputs during a stall, if we keep the ball centered and the power low, ailerons & rudder still work completely normally and you can hold maximum backpressure for as long as you like. All the while descending at some steep angle, but still in full control and flying (but lift < weight). It's one feature I really appreciate about the design, it's so very safe.

Even with a wing drop and more power on, we've never had a situation where it felt like it might enter a spin - but we always release back pressure when the wing drops sharply.

[lsa140]

bway Thank you for sharing your flight testing results in a thoughtful and careful manner. This is excellent information and much appreciated!

We can literally fly our 4-place with normal control inputs during a stall, if we keep the ball centered and the power low, ailerons & rudder still work completely normally and you can hold maximum backpressure for as long as you like. All the while descending at some steep angle, but still in full control and flying (but lift < weight). It's one feature I really appreciate about the design, it's so very safe.

Battson, do you have any flaps deployed in this configuration? Do you have VGs installed?

I know some builders of Zenith high wing designs (701, 750, etc.) remove the slats and replace them with VGs. It sounds like the VGs accomplish much of what the slats provided and are quite a bit less draggy.

Some aircraft designs that don't have any wing washout use some type of small sharp edge on the inboard wing to induce airflow separation away from the ailerons and provide warning to the pilot with a elevator buffeting. The Wittman Tailwind has a sharp point on the leading edge where the wing tapers to the fuselage that induces the stall near the root.

[Battson]

bway Thank you for sharing your flight testing results in a thoughtful and careful manner. This is excellent information and much appreciated!



Battson, do you have any flaps deployed in this configuration? Do you have VGs installed?

I know some builders of Zenith high wing designs (701, 750, etc.) remove the slats and replace them with VGs. It sounds like the VGs accomplish much of what the slats provided and are quite a bit less draggy.

Some aircraft designs that don't have any wing washout use some type of small sharp edge on the inboard wing to induce the stall away from the ailerons and provide warning to the pilot with a elevator buffeting. The Wittman Tailwind has a sharp point on the leading edge where the wing tapers to the fuselage that induces the stall near the root.

Yes we have full flap deployed, it seems less stable without? We have STOLspeed VGs installed per manufacturer's recommendations (denser spacing at the wingtips), at the 3rd line of rivets on the top skin.

[lsa140]

Yes we have full flap deployed, it seems less stable without? We have STOLspeed VGs installed per manufacturer's recommendations (denser spacing at the wingtips), at the 3rd line of rivets on the top skin.

In that configuration I would imagine the increased angle of attack the inboard wing is seeing from flap deployment is causing some degree of airflow separation/stall while the aileron portion of the wing remains "flying". Wing lift is greatly reduced by the inboard separation and contributing to a steep descent. While the outer wing's lift is greatly reduced from the slower airflow it is remaining attached and allowing the ailerons to remain effective.

When the LSA wing is held at the point where airflow is beginning to separate the entire wing is seeing the same angle of attack. The large wing area/light wing loading/higher camber airfoil allows the plane to achieve very low airspeeds but when but when it is held deep enough into a stall the separation/drop can be abrupt and roll control lost.

Flap deployment provides a large degree of effective wing washout (maybe 8 degrees with full flaps?) Denser VG spacing at the outer wing helps keep the airflow attached? The higher camber airfoil and very low wing loading gives the LSA very low speed capabilities but it does so in a distinctly different way than a wing with flaps. Increasing the low speed capabilities using flaps also provides the advantage of being able to add/remove lift/drag very quickly with deployment or retraction.

All of the above can be read as me thinking out loud and is as much a question as it is a statement. I'm just fascinated by all of this and grateful you guys are sharing your observations.

[Nev]

All of the testing I have done so far has been with a forward CG. I plan to test the spin characteristics of my plane with weight in the back seat and gradually move the CG toward the rear until I can establish my limit.

This is a very interesting discussion and enjoyed reading your testing reports and experience.
Did you end up testing the spin characteristics with an aft CG ? Be very interested in the results.