learn2turn

The two loops in Sportsman are probably my most inconsistent move. Sometimes I get it pretty good and sometime my plane wanders at the top deviating from the line almost to the point where it's pointing across the box.

I started to think about what goes on in the physics of a round loop. Compare to what I call a roller-coaster loop, i.e., pull back on the stick hard and the radius decreases at the top of the loop and gradual increases again come out of it. With a loop like that, the plane pulls positive Gs all the way through. Now compare that to a round loop where you ease up on the elevator over the top. During the top, I'm thinking could the plane be pulling negative for a bit, so the inverted wing is providing up lift? Now if the plane is actually flying inverted for a bit, ailerons are going to turn the opposite direction. So if I'm trying to correct the plane to move it a bit further away from me but it goes inverted and then the plane flies toward me instead of away.

I'm trying to figure out what to do to correct this issue if it occurs. I could correct with rudder. Or maybe the answer is speed management so I fly after at the top and don't really go much into negative Gs.

Thoughts?

Jeff Worsham

My Feedback: (28)

Having flown Sportsman last year, I know your pain with those blasted consecutive loops. We can all be thankful it's only two consecutive instead of three! My thoughts are, if the loops are truly round, they are positive G only. Your ailerons don't cause the plane to roll any different when inverted than they do when upright. Your rudder is opposite when inverted. Loops are tough to trim for Sportsmen fliers. If you are getting consistent corkscrewing, it may be caused by a heavy or warped wing panel, or by elevator halves that don't travel exactly the same amount. Quick test for heavy wing panel is to pick up the prop (balancing on tailwheel) several times to see if one wing consistently falls heavy. Another good test is to fly straight away from you on a very calm day (or downwind if very light) and carefully pull back on the elevator (don't induce roll) and watch for consistent corkscrewing.

Once you've resolved any trim issue causing corkscrewing, a well trimmed plane should be able to maintain the proper crab angle thru the loop with a little rudder help here and there. Use the rudder to establish the vertical plane for the first loop, and continue using it to keep the second loop in that same vertical plane. Resist the urge to fly a wing low in trying to compensate for crosswind. Wings need to stay level throughout the loop. I know- easier said than done. Good Practice!

apereira

The best way will be to get help from an experienced flyer at the field so he can identify where the problem is during the maneuver and help you work it out.

regards

MAVROS

Hi there,
Ailerons act the same way in any attitude even during negative g loading unless you induce supersonic airflow..........Somehow I don't think this is your problem with the 2 consecutive loops.
Energy management or speed control is the answer .Try carrying more speed into the start of the first loop and control the speed so that it doesn't bleed off during the manoeuvre
If you bleed too much energy during the first loop then other factors like trim prop or secondary effects of controls will upset your manoeuvres

A good engine that maintains fuel pressure to the carby is also very helpful for energy management during aerobatics

regards

Mavros

Jetdesign

My Feedback: (8)

As others have mentioned there could be a few things going on. One thing I was taught is to use plenty of power and fly over the top of the loop. It really helps to make the loop round.

A typical airplane bleeds off a lot of power by the time it gets to the top and will want to nose down. Imagine a full scale Cub flying a loop - it usually has to dive to gain enough speed to start the loop, uses all it has on the way up, and falls over the top into the downward leg. And that's why typical sunday flyer loops have a tight radius at the top. So you need to use more power over the top. Most of our pattern planes have enough power to compensate. If I'm flying a loop I use the most power during the top 1/4 approaching the top of the loop. I back off power a little over the top but still keep plenty so I can keep carving the round shape. And don't forget to add power before you complete the loop so you can control the radius at the bottom.

Ailerons would be the same regardless of positive or negative G's. My first reaction was to suggest using rudder for the correction. We all know we should be using rudder in pattern. But then I realized that after flying Sportsman many of us start 'hiding' our corrections in loop segments with ailerons. But really you shouldn't have to work that hard to keep the airplane straight. I know I heard that a lot when I started flying but didn't really get it. Especially because I had airplanes that all had twists in the fuse! This year I got a 2M airplane and spent a TON of time setting it up. Every time I fly it I am blown away at how much easier it is to fly. It goes where you point it, and if you get off the sticks it just stays put (Thanks Verne! )
If the plane is not tracking well through a loop, #1 make sure your elevator halves are moving the same amount and at the same speed, and make sure your tail section and rudder are straight. You can try flying directly away from you and pull into a loop, then push back over the top and see if your plane keeps its wings level. And don't forget about your mixes! If you have a mix to correct something, it will burn you somewhere else. It's happened more than once where I chased something funny going on with the airplane only to realized it was the result of a mix I put in for something else. I don't have a problem with people using mixes but I personally try not to use them for that reason. I don't use a single mix with my Integral (I would add a condition for a spin if I had to, but it would be on a switch).

bjr_93tz

You're on the right track with thinking about the rudder. Ideally you'll keep the wings "level" around the loop with aileron and correct any in/out using the rudder. Rudder works opposite (cringe) when you're upside down in the same way ailerons work opposite when you're flying towards yourself, some people use tricks to remember which way to push things other people just learn to push them the right way without having to think about it. Either way, wrong rudder shows up real quick to you and the judges :-)

You're also pretty close on that if your plane is banked with the outside wing low and it's pulling positive G at the top it will turn towards you a bit, but if it's slowed down and generating negative G it'll turn away from you a bit.

Keeping the airspeed up and tightening the loop helps by keeping the plane less at the mercy of wind gusts and engine thrust issues at the top with high power and little airspeed.

The key is, find a size that your plane/power combination is happy to fly at and stick to that size. A bigger loop will (but shouldn't) score higher than a smaller loop but a big wobbly loop won't score as well as a smaller round loop will..

lonestoner

Hi all, stoner here
Finally ready and practicing pattern flying sequences at the field just wanted to say hello and introduce myself.
This coming winter I shall be embarking on my first pattern build and everything that goes along with it.
Airplane will be a kaos 90 from eureka aircraft co, and engine is either a Jett 90 with a pipe or a Saito 125 any thoughts or ideas ??

drac1

My Feedback: (4)

If you keep the speed as constant as possible, the plane will be less likely to veer off track.

Plenty of power as you pull up into the loop and then gradually ease off the power as the plane goes over the top and back to idle as you go past 3 o clock. Then gradually increase the power again and have plenty of power on again as you start to pull up into the second loop and so on. You will also need to constantly adjust elevator to maintain the correct geometry.

The best way to correct for coming in or going out on loops or part loop maneuvers, is to use the ailerons. If the plane is coming in, then lower the outside wing slightly and if it is going out, lower the inside wing. This will be less noticeable to the judges and eliminates the chance of wrong rudder input.

If the inside wing is down and you use opposite rudder to correct, the plane will get all crossed up and look terrible. So concentrate on keeping your wings level and use rudder to wind correct in level flight and verticals.

viva_peru

Hi -

When I started flying sportsman I tried to break each loop into two parts: first half on the way up, second half on the way down. I would normally look at the plane at the bottom and top of the loop and correct my wings if they were not level. This helped. As I got better, then I started using a little bit of rudder to correct the airplane's track.

The other thing to consider is to make sure that your plane is laterally balanced; ie. a wing is heavier than the other. If this is the case, as you start applying G's, the plane may have a tendency to roll. Also, check your elevator halves and make sure that they are both moving in sync.

Good luck,

Teo

HarryC

My Feedback: (1)

The physics of a loop is interesting.

Assume a perfectly circular loop.

If load at the bottom is 3G, 1G of that is normal gravity, so the loop itself is causing 2G. If the speed was constant all the way around, the load would be 3G at the bottom and 1G over the top, because the loop is causing 2G and inverted you have to deduct, not add the 1g of gravity. To make the wing pull 1G, the plane is already trimmed to fly at that, so the elevator would be at neutral over the top of the loop.

But keeping a perfectly constant speed is near impossible.
Let’s see what happens if the speed at the top of the loop is half what it is at the bottom of the loop, remember we are keeping the plane going around a perfect circle.
Again 3G at the bottom, of which 2G is the loop and 1G is gravity. The G load of the loop part is velocity squared/radius. For a perfect circle the radius is fixed, so G load varies with the square of the speed. Since the speed at the top is 1/2, the G load is 1/4 what it was at the bottom. 1/4 of 2G is 1/2G, so at the top, the loop part causes 1/2G load, but it is inverted so deduct the 1G due to gravity, and the G load is minus 1/2G.

So in a perfectly circular loop which has 3G at the bottom and where speed over the top is half what it was at the bottom, the G load varies between plus 3G at the bottom and minus 1/2G at the top.

To get minus 1/2G the elevator needs to be pushed forward.

The actual numbers for your model will depend on initial G load, how much speed is kept or lost, but the point you can see is that it is perfectly possible for G to go negative over the top of a genuinely circular loop. It also shows you why most loops are not circular, because people are still pulling back over the top, when they should be pushing forward!

Another factor of loops is that on a prop driven plane, the rudder is only trimmed at one speed and throttle setting, a change in either will cause the plane to yaw. With the way our props rotate, high power and low speed causes a yaw to the left, so right rudder is required to stay straight. Loops involve high power and low speed so some right rudder will be required as soon as speed starts to reduce and will reach a peak at the slowest part of the loop.

Someone said ailerons reverse if supersonic, not true, if it were true it would make it near impossible for people to fly supersonic planes!

n233w

Learn2turn, I always love to see your threads because your questions lead to info that is helpful for those of us who haven't been around years and years. Some of us don't have the benefit of having an experienced pilot on hand and rely on this forum to help us.

I wanted to key in on the aileron comment in the OP - the ailerons don't reverse when inverted as others correctly note, but THEY DO reverse when you stop flying away from yourself and start flying toward yourself - that is, when you pull through the vertical after the 1st 1/4 of the loop. As an older beginner in RC and a teacher by profession I analyzed this stuff for my own benefit, some of which is hashed out in this thread, if you're interested in more. Thanks.

lonestoner

Really, it's just a loop .

n233w

I like this kind of thinking which examines the relationships between physics, flight dynamics and geometry. However, a) I don't think that with our models, generally, the speed at the top is as low as 50% of that at any other point of the loop, b), isn't the highest G;s experienced at about the 7/8ths point of the loop?, and c) isn't where did you get the 3 G's as a starting point?

learn2turn

Useful stuff I got from this thread (so far) --

I think I have to fly faster near the top, 10-2 o'clock. I think it might also help to start a little slower and hit the throttle as the plane get to vertical. I think I'm pulling a lot of G's in the first 1/4 loop and that portion should be slower and then I should up the throttle so I don't lose speed, it's more constant.


I really like the G calculation above. When I make loops reeeeeaaallly big, I find I do need a tad of down elevator over the top to keep from pinching the top.


Maybe ailerons reverse wasn't the right term but I still think something reverses when you go from positive to negative Gs. Consider a plane flying straight and level. Roll the plane 20degrees right. Pull on the elevator and the plane turns right and goes up. Push hard on the elevator, the plane goes left and down. Now think about the loop. Right wing is down 20degrees and you pull into the loop, the loop corkscrews to the right. But suppose you pull net G's at the top. A good was to think about it is a 1/2 loop to inverted flight. Enter the loop flying left to right with the right wing, the wing closet to you, 20 degrees down. During the 1/2 loop, the plane will cork screw closer to you. But then at the top entering the inverted flight, the right wing is now 20 degrees up with respect to the ground. In inverted flight the plane will now move away from you. That is the reversal I'm talking about.


Yeah, having a mentor would be great but no one else around my area flies pattern. Maybe I'll get to a Friday practice day sometime. The one contest I did get to this year had rain Friday.

-l2t

n233w

Now I get what you mean by reversal. You could think of it as a reversal but what the reversal is, is the fact that if the wings aren't level the elevator acts like a rudder the more away from level the wings get. Until they're at 90 degrees and the elevator is 100% the rudder and vice versa.

drac1

My Feedback: (4)

Why do you want negative G's at the top? The only time you need to push down elevator, is if the first part of the loop wasn't done properly and you need to get back to center.

Enter the loop at 1/2 throttle. As you start to pull up, gradually increase the throttle to maintain speed. When you reach around 10 o clock, start easing off the elevator and power. Just before you reach the top, elevator will be neutral and due to gravity the model will gradually fall continuing the loop. You may need to start pulling a bit of elevator at 1 o clock and at around 2 o clock, pull more elevator to hit the 3 o clock position at 90 degrees. At this point, you will need to pull start pulling more elevator and start adding power. You will need to pull more than you think to maintain the correct geometry and then at around 04:30, start easing off the elevator to hit the bottom at center with the model level.

HarryC

My Feedback: (1)

It was just an example, with the numbers specially chosen to show how it is possible for the G to go negative at the top. I did say "The actual numbers for your model will depend on initial G load, how much speed is kept or lost, but the point you can see is that it is perfectly possible for G to go negative"

The point of highest G can't be stated absolutely, as each loop and each plane's characteristics and each pilot's use of the throttle is different. At the bottom of the loop, gravity adds its full 1G to the loop so that point is a contender for max G. Speed also affects G and if the throttle is constant the rate of increase in speed due to gravity will reduce between the vertical down and being level but speed will still be increasing right up to the point that the plane achieves level, so in that case max speed and max G will be at the bottom, not at 7/8ths.

I picked 3G for the reason above, its an example, I never said that all loops are 3G at the bottom, once again may I remind you that I said "The actual numbers for your model will depend on initial G load,"
If you go in at 4G at the bottom and still have 2/3 speed over the top, and you are flying a perfect circle, the G load over the top is zero! Use your own numbers and you can see there is a massive variation in G load, and therefore how hard you pull and/or push, around the loop. But be realistic, unless you have airspeed telemetry, you are guessing about the loss of speed through a loop and it may be a lot more than you think.

Just a little aside on the loss of speed in a loop, I have flown the full-size P-51 Mustang a few times, going into a loop at 300knots and 3.5G, dropping to a mere 70knots over the top. I bet if you were watching from the ground you would never have estimated the speed had fallen by that much, and the same is true when you estimate model speeds!

HarryC

My Feedback: (1)

Do you not understand what an example is? If you do a prefectly circular loop with the conditions as stated, the load over the top will be minus 1/2G.
Let me repeat what I said since you didn't read it the first time, "The actual numbers for your model will depend on initial G load, how much speed is kept or lost, but the point you can see is that it is perfectly possible for G to go negative"

MAVROS

Harry C,

Mate if you care to google control reversal at high speed compressible air flow you may understand that it is real.
Yes transonic and supersonic flight is possible nowdays because engineers have proved the phenomenon and designed wing structures acordingly

rgds
Mavros

HarryC

My Feedback: (1)

Ailerons do not reverse their effect, not does any control. Early structures had problems with the wing twisting in the opposite direction to the aileron but it is true at any speed if the wing is not stiff enough, it is not exclusive to supersonic flight. It is a phenomenon of a weak structure, not some magical reversal of the aileron's aerodynamic effect.

What that says is that ailerons, ALL ailerons, reverse their effect when supersonic and that just isn't true.

drac1

My Feedback: (4)

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drac1

My Feedback: (4)

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Jeff Worsham

My Feedback: (28)

Wow- I thought we were talking about making 2 consecutive round loops with a model airplane in Sportsman class pattern.

drac1

My Feedback: (4)

Well aileron reversal at supersonic speed must have something to do with round loops, surely.

learn2turn

The reversal thing is not aileron reversal in the aerodynamic sense. What I was trying to convey altering of flight path do to non-level wings and how that reverses when inverted.

Scenario--

1. Plane is flying straight and level left to right.
2. Pilot applies up elevator at center of box to begin loop.
3. The wind is toward the pilot so pilot uses ailerons to roll plane left, left wing down, right wing (nearest pilot) up.in an attempt to keep the plane on a straight path.
4. At top of loop, since the right wing was toward the inside of the loop during the beginning of the loop it is still toward the inside and therefore now down.
5. If the plane slows at the top of the loop, the wing will begin to lift while inverted (negative Gs).
6. With the wing lifting inverted and the wing toward the pilot now down, instead of preventing the plane for altering course toward the pilot, the wing has the opposite effect, adds to the displacement do to the wing and actually alters the path further toward the pilot.
7. Since the plane is slow and now out of a line, the loop is not completely on time. That lengthen's the top segment even more extending the time inverted and exacerbating the problem until the plane is out of path to the point of heading almost toward the pilot.

I think that sums up the issue.

I think the cure is --

1. Go into the loop slower.
2. Apply throttle from the 4 to 1 o'clock phase (flying the loop counter clockwise, to avoid neg Gs at the top).
3. Make the loop big but not so big that there is an extended neg G segment at the top. (If you want to do the math, I don't care how fast the plane is flying or how much power it is, there is a loop radius at which it will pull negative Gs at the top.0
4. Be more subtle about adjusting course in the loop.

I know someone said to avoid rudder correction but I would think that rudder correction would be what's needed here to compensate for wind. If the winds blowing straight at the pilot and the pilot applies a tad of left rudder through the entire loop (loop entered left-to-right), wouldn't that compensate for the wing and help keep the plane on a straight heading?

-l2t aka Ken

PS

Please drop aerodynamic control reversal from this thread. There is no need to debate how wing twisting or supersonic compressibility might or might not cause it as it is 100% not germane to this thread.