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  #1  
Old 06-01-2018, 12:38 PM
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uk_figs uk_figs is offline
 
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Location: Tulsa, OK
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Default Loop technique input request

Looking for some technique input from the aerobatic experts, doing some gentlemens aerobatics today in my -7 and in the loop on the backside as I pulled through I got significant buffet, after easing the back pressure it smoothed out and then as I pulled again it happened a second time. Entry was at 142 mph and recovery was close to initial altitude, G meter was 3+.

I tried to make the loop round but as I do not have inverted systems and am carburated I might have pulled over the top to prevent going negative and losing oil pressure.

Suspect I might have pulled too hard on the down line and mushed out or stalled the wing but not sure.

Appreciate any inputs for technique and things to watch for.

In addition in the RV what is used for horizontal level reference against the clear blue sky when you are going over the top waiting for the ground to come around.
Thanks
Figs
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  #2  
Old 06-01-2018, 01:11 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Other who fly competitively can probably help more, but I do do aero with a carb......

To make a loop round, you do need to easy off the pull while transitioning through the top. A carb. just requires positive G's. It doesn't have to be + 1.0.
It will still work fine at +0.5. With practice you can learn to easy up on the pull but still have more than enough G's to keep everything working like it should.

Horizontal ref. can be acquired with a fast glance at each wing tip prior to reaching the fully inverted point.

Sounds like you were nibbling an accelerated stall on the down line.
Try a little bit higher entry speed (160 -165 MPH)

With proper technique on the first half you should be passing over the top at about 85 MPH or so. This will still give you a safe margin to avoid over speed on the down side (be sure to pull power back if you have a fixed pitch prop).

Best advice though, is get someone experienced to fly with you.
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  #3  
Old 06-01-2018, 01:48 PM
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uk_figs uk_figs is offline
 
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Scott
Thanks for the input, yes I am fixed pitch and do power back over the top, (I still remember how fast the RV accelerated pointing straight down when I flew off the maneuvers in phase 1)

I have taken an aerobatic course in a great lakes and some acro instuction in a -6 with 200HP but both has CS props and each in their own way handled quite a bit differently than my -7.

The higher entry speed and maybe not pulling so hard is a good tip, I will try that.
I suspect you are right about the accelerated stall, if it was just one buffet I might have attributed it to flying the perfect loop and ending up back where I started but that would be highly unlikely
Thanks
Figs
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  #4  
Old 06-01-2018, 01:49 PM
rv7charlie rv7charlie is offline
 
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As another non-competitor, I agree with Scott about the area at the top. I had a tendency to ease off on g loading too soon on the way up; I was taught (by a Swift airshow pilot) to maintain the same g load until close to the top of the loop (less than 45 degrees) and then release almost all back pressure until the nose started down again (near 45 degrees on the other side). This was supposed to keep the loop closer to an O and not a ! . Never seen one of my own loops, so I can't say how well I do. :-) But I did fly lead in a Swift for the airshow guy to practice on, and he seemed happy with my flying.

I've flown quite a bit of 'fun' acro in two different 160 hp fixed pitch -4s, and I've never paid any attention to rpm while maneuvering. For rolls, loops, slow rolls, barrel rolls, cuban 8s, wingovers, and "sissie's" hammerheads (not quite vertical uplines & early kick), I just set up moderate cruise power (2400-2550 rpm with maneuvers starting at around 2000-2500' MSL) & leave the throttle alone.

The only times I've had carb related issues is trying a true hammerhead (quits every time on the up line) and an extreeemly slow roll, which can stay at 0g long enough to make the motor cough a bit.


Hope that's worth something...

Charlie
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  #5  
Old 06-01-2018, 01:55 PM
sandifer sandifer is offline
 
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Hey Figs, first of all don't worry about making what you think is a round loop. It won't be. Never seen anyone come close to an actual round loop who hasn't had a good bit of help from the ground. Nobody does it on their own. Everyone pulls too softly to enter, and then VASTLY underestimates the amount of time required spent floating over the top around zero G with very little pitch rate. So just do what pleases you.

But still, a few basics - don't pull power off on the backside. I've seen a lot RV videos where guys pull the power to near idle on the backside for fear of overspeeding the airplane and/or engine. This is unnecessary and makes for a terrible loop even by recreational standards. RVs will easily loop from cruise power, cruise speed. 3-3.5G works fine for "L" shaped recreational loops, but not so much for actual round loops. So your G pull is fine for what you're doing. A loop is a symmetrical figure, and so should be your G loads up and down. They constantly change through the figure. Low on top, high on the bottom. On the backside, you just have to gradually increase your pull and accelerate the G load, feeling for about the same load on the bottom that you felt during the initial pull. You feel for the G acceleration rate that avoids stalling/buffeting on the backside, but is also sufficient to avoid finishing at a lower altitude than you started with. At a constant power setting, you'd have to break the laws of physics to finish a loop at the same altitude but with a higher airspeed. So forget pulling power off on the back. You can either set it and forget it, or add full power on the way up and return to your entry power setting on the backside. You just need to practice the acceleration rate on the backside to avoid stalling or finishing low. It's all done by feel. Sounds like you're just pulling too hard too soon on the backside. If you're pulling a lot power off, that makes matters worse.

Good question on the visual reference going up and over. I see lots of folks look straight ahead the whole time and then crane their neck back as far as possible trying to pick up the horizon on top. No need to contort and strain. This can also miss key information on the way up. If you have interest in flying with precision, I suggest an alternate technique on where to look and when - starting the loop, look straight ahead until you lose the horizon under the nose. Be sure you are perfectly wings level as you lose the horizon. Then glance to your left and notice the position of the wingtip relative to the horizon. You need to learn exactly where the horizon must bisect the wing in order for the airplane to be perfectly vertical in yaw as the airplane is vertical in pitch. Ground critiquing helps, but you can figure it out by yourself by flying some vertical uplines just long enough to glance left and right until you make the relative position of the horizon and wing the same on both sides. Once you've figured this out, every time you pull through vertical on a loop (or a vertical upline), you need to put the wingtip in this position. And once you've learned the sight picture, there is no need to bobble your head around looking left and right on the way up. I see lots of folks do that and there's no need. One wingtip tells you everything you need to know. This concept is fundamental to precision competition style flying should you want to get into that at some point.

So point being, if you get perfectly vertical in yaw on the way up, you will know the sight picture of the wingtip relative to the horizon such that you are automatically set up for a perfect wings level attitude by the time the horizon will come back into view over the nose over the top, and then you shift your view back over the nose for the rest of the loop without any neck craning. If you find that you must make rudder corrections to get the wingtip into this position, this means you made a slight inadvertent aileron input during the pull, which is causing an error. If you can pull to this perfect yaw attitude with no rudder corrections, you're pretty much set up for a perfect level attitude over the top if you just keep it going.

Well I guess I went overboard, but for me the little details are what make acro interesting and worth working on....at least for those of us who care to progress beyond "flopper" status.

Last edited by sandifer : 06-01-2018 at 02:07 PM.
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  #6  
Old 06-01-2018, 01:56 PM
rv7charlie rv7charlie is offline
 
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Figs,

Just read your reply. If you fly a loop in a vertical plane, you'll hit your upline wake as you pull back to horizontal. It won't be a buffet though; it will be a bump. If you're just slightly off, it will be a bump on one wing. If you're off a lot, no bump.

If you're pulling power over the top, I'd agree that you're probably getting stall buffet on the way down because you're too slow for the g's you're pulling.
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  #7  
Old 06-01-2018, 02:40 PM
rvsxer rvsxer is offline
 
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Quote:
Originally Posted by uk_figs View Post
Scott
Thanks for the input, yes I am fixed pitch and do power back over the top, (I still remember how fast the RV accelerated pointing straight down when I flew off the maneuvers in phase 1)
When I had my fixed-pitch Sensenich (180 hp, carb) I would enter at 145-150 kts, 3-3.5g pull, ease off over the top, then squeeze the "g" back on through the recovery. Usually recovered to level at about the same speed. Never changed the power. The increased g-loading is what keeps the speed in check. If you accidentally relax the pull on the way down you'll be surprised how fast the speed builds. I know it's a leap of faith to be pointed at the ground with cruise power but it works great.
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  #8  
Old 06-01-2018, 03:05 PM
rv7charlie rv7charlie is offline
 
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For visual reference, you'll have to find what works for you. My 1st few loops were in the back seat of a Decathalon. I was taught to look to the side & keep the wing tip on the horizon. Worked great. But in the Swift and the RVs with low wings, I found that I was doing modified barrel rolls using that technique. What works best for me is to make sure I start the upline with wings level and the ball centered. As long as the ball stays centered, I've found that once I lose sight of the horizon I just look up and wait for it to come into view again. I almost always hit my wake.

Again, you have to find what works for you. But watch your roll input while pulling g's; it's easy to pull sideways while you're pulling back.
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  #9  
Old 06-04-2018, 02:10 PM
precession precession is offline
 
Join Date: Apr 2006
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Default Left rudder on the initial pull in an RV or not?

Quote:
Originally Posted by sandifer View Post
If you have interest in flying with precision, I suggest an alternate technique on where to look and when - starting the loop, look straight ahead until you lose the horizon under the nose. Be sure you are perfectly wings level as you lose the horizon. Then glance to your left and notice the position of the wingtip relative to the horizon. You need to learn exactly where the horizon must bisect the wing in order for the airplane to be perfectly vertical in yaw as the airplane is vertical in pitch. Ground critiquing helps, but you can figure it out by yourself by flying some vertical uplines just long enough to glance left and right until you make the relative position of the horizon and wing the same on both sides. Once you've figured this out, every time you pull through vertical on a loop (or a vertical upline), you need to put the wingtip in this position. And once you've learned the sight picture, there is no need to bobble your head around looking left and right on the way up. I see lots of folks do that and there's no need. One wingtip tells you everything you need to know. This concept is fundamental to precision competition style flying should you want to get into that at some point.

So point being, if you get perfectly vertical in yaw on the way up, you will know the sight picture of the wingtip relative to the horizon such that you are automatically set up for a perfect wings level attitude by the time the horizon will come back into view over the nose over the top, and then you shift your view back over the nose for the rest of the loop without any neck craning. If you find that you must make rudder corrections to get the wingtip into this position, this means you made a slight inadvertent aileron input during the pull, which is causing an error. If you can pull to this perfect yaw attitude with no rudder corrections, you're pretty much set up for a perfect level attitude over the top if you just keep it going.
Thanks for the excellent information/instruction.

Here's a question I've had for a while -- pertaining specifically to looping in an RV as compared to other aircraft. Regarding the effort to keep the wings perfectly neutral in yaw and roll when looping: Apply left rudder on the initial pull up or not?

I initially started doing loops without applying any rudder during the initial pull. Based on the sight picture I was getting looking at the left wing on the way up, it seemed like I was staying fairly neutral in yaw and roll. But I was never 100% sure, and lots of times I could tell I was exiting a loop somewhat off on orientation, for what reason I wasn't sure.

Then I was reminded (through reading aerobatics books or articles, I believe) that in many (all?) aerobatic aircraft you are supposed to apply some left rudder on the initial pull. I believe this is to counter gyroscopic precession caused by the pull. So I started throwing in a little left rudder with the pull. But I've been basically guessing as to how much to throw in, and doing it mostly because I thought it was probably needed. As it turns out, at this point I'm really not sure whether the initial left rudder is making things better or worse. (As I said in the other thread, I haven't yet had the chance to get ground critiquing - hopefully can fix that this summer.)

As everybody who flies RVs knows, RVs are sort of known for being able to do lots of maneuvers "with your feet flat on the floor" -- i.e., little or no rudder input required for turns, non-competition aileron rolls, etc. From reading prior comments on this board from Sandifer, I believe this is because RVs have differential ailerons, as well as frise ailerons, which I understand act to reduce adverse yaw.

So what I'm wondering is whether you guys are finding it helpful to apply some left rudder on the initial pull, or not really necessary in an RV? (And probably same question as to the pull on the exit.)
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Last edited by precession : 06-04-2018 at 04:31 PM.
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  #10  
Old 06-04-2018, 03:51 PM
sandifer sandifer is offline
 
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Quote:
Originally Posted by precession View Post
Then I was reminded (through reading aerobatics books or articles, I believe) that in many (all?) aerobatic aircraft you are supposed to apply some left rudder on the initial pull. I believe this is to counter gyroscopic precession caused by the pull.
There's a degree of gyroscopic yaw to the right any time you pitch toward the canopy in an airplane with a Lycoming turning motor. But the amount of precession of course depends on RPM, prop mass, pitch rate, and prop to airframe weight. Even with a metal prop, it's such a small effect in an RV that there's nearly negligible need to correct for it with left rudder. With a wood/composite prop, forget it. Even in something like a Pitts S-1 turning a big metal prop at 3000+ RPM and pulling 6G into a loop, the effect was very small. Before I switched to a composite prop in the Pitts, all I really told my left foot to do was breathe on the left rudder for hard pulls to vertical. Hardly even a real movement. Keep in mind that as soon as you pull, there's a degree of P-factor working in the opposite direction of the gyrocsopic yaw. Both are small forces.

Bottom line, I wouldn't bother trying to make a correction here. Just enter the loop ball in center, and do whatever it takes with the rudder to keep it there...which will likely be nothing, or virtually so. Where this gyroscopic effect is most apparent is doing pull Humptys at zero airspeed over the top with a metal prop. You will see the difference in rudder use required when you compare that to a push Humpty in the same configuration. But for a loop, airflow is so high, and pitch rates so low that it's not much of a factor.

By far the main contributor to yaw (or 'wing drag') issues during a pull is inadvertent aileron input rather than rudder mishandling. Due to arm ergonomics, it's easy to apply a very slight bit of aileron during pulls and pushes. Again, it's all about learning the sight picture and getting the wingtip into the right position on the way up. It's all a building block process that lays a good foundation for more advanced maneuvers. If you ever want to do decent vertical rolls, you have to be able to nail a perfectly vertical pitch and yaw attitude each time before you'll even be able to work on fixing the inadvertent elevator inputs that are screwing up your vertical rolls.

Last edited by sandifer : 06-04-2018 at 03:54 PM.
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