In-flight performance indicators for a better setup?

[VHO]

I predominantly fly by myself, with contests thrown in a few times a year, so am on my own when setting up. Usually the setup works out fine using factory settings or those gleaned from forums here, but I can miss things at times.

My Xpro (vtail, 1970g) handles well in light lift, but has a habit of greatly increasing in speed when in big thermals. It doesn't slow down enough to establish a good thermalling pattern (moderate bank angle, smooth flight etc). I have been unable to slow down using a small amount of flap or up elevator. My CG is at 105mm which is at the rearward end of the range. The model does not pull out of a dive (but doesn't tuck under either) so the CG should be close to neutral point.

Any suggestions about what I need to do to set up to reduce the major speed increase in big lift, without degrading performance?

Hutton

[Tuomo]

My Xpro (vtail, 1970g) handles well in light lift, but has a habit of greatly increasing in speed when in big thermals. It doesn't slow down enough to establish a good thermalling pattern (moderate bank angle, smooth flight etc). I have been unable to slow down using a small amount of flap or up elevator. My CG is at 105mm which is at the rearward end of the range. The model does not pull out of a dive (but doesn't tuck under either) so the CG should be close to neutral point.

When you hit a powerfull thermall, sink rate is not critical. What is important, is to stay in raising air - if you are succesfull, subtle differences in glide performance quickly become irrelevant.

To stay in raising air column, you have to have positive control all the time. Keep the plane moving and turn as cleancly as possible. Maybe you could also try how little more conservatice CG works for you?

When air is active, rearly use any thermal flaps, but it is very depandant to your flying style and plane. As I said, I try to keep speed up. My plane (Aspire) has also a very wide speed range - if needed, it will float quite well even with trailing edge neutral.

[O.L. Adcock]

Hutton, I'm not familiar with the plane you are flying but I've never seen a plane where the speed it flies has anything to do with the "air" you are in?? You should fly certain speeds in different conditions but those are your choice, not the planes.

"I have been unable to slow down using a small amount of flap or up elevator." So full up elevator doesn't result in a stall??

"My CG is at 105mm which is at the rearward end of the range. The model does not pull out of a dive (but doesn't tuck under either) so the CG should be close to neutral point."

Is that in your "thermal" camber setting, cruise setting, or speed setting?? Cause the "neutral" CG will be different for every one of them. Pick one and live with what it does to the rest. Personally I set my CG the way I like it in the thermal setting cause that's where I hope to spend 99% of my time! :)....O.L.

[Robglover]

If a plane is greatly increasing speed when entering a thermal the pilot is not controlling the plane properly. Ask the pilot to pull back on the elevator stick more. If the plane still doesn't slow down kick the pilot and ask him to pull back on the elevator stick some more. If the plane keeps speeding up then walk away and find a more responsive pilot. Watch the plane closely because it will probably crash soon and you don't want it to hit you.

[OVSS Boss]

Any ship should speed up in lift if the CG is right, like Rob says. With a rearward CG, the rear end is rising from the up air, and as long as it is speeding up, that pilot had better translate the speed to altitude, all of it is a matter of energy and what to do with it.

Marc

[Bryan Quick]

Here's a synopsis of what Dr. Drela has to say about center of gravity location and aircraft behavior on the Charles River RC site at http://www.charlesriverrc.org/articles/supergee/CGMarkDrela.htm:


1) CG at the aft limit... glider continues in a straight line or tucks in very slightly when put into a dive from a slow glide.

Pro: Glider will strongly pitch up and slow down, or "balloon", when flying directly into the slightest lift, and dive down and speed up when flying directly out of lift. Obviously this behavior is a nice weak lift indicator, especially when very far away or directly overhead where you can't easily tell if the glider is going up or down. This type of "lift ahead" or "lift behind" indication via pitch change and airspeed is a good addition to the usual "lift to the side" indication via banking.

Con: Glider will not hold pitch trim, but will tend to stall or dive at the slightest disturbance. Lots of work is needed to constantly twiddle with the elevator. In turbulent thermals you have to fly faster than ideal to avoid occasional stalls.

2) CG well forward of aft CG limit... glider pulls out very quickly when put into a dive.

Pro: Glider will solidly hold pitch trim and airspeed, even in turbulence. When in a thermal, this allows flying nice and slow at minimum sink, with little risk of stalling from an upset.

Con: Glider has little tendency to pitch up and slow down when flying straight into lift, but just plows steadily along. It will still directly rise in lift while staying level, but again this is not as obvious as a pitch change when directly overhead or very high up. The "lift to the side" indication via banking is unaffected, and should not enter into the CG position decision.

Choose your poison.

Bryan

[VHO]

Any ship should speed up in lift if the CG is right, like Rob says. With a rearward CG, the rear end is rising from the up air, and as long as it is speeding up, that pilot had better translate the speed to altitude, all of it is a matter of energy and what to do with it.

Marc

Now we are getting somewhere. Its all too easy to turn that lift into altitude, but it very quickly turns into flying a fast dot around the sky. I'm not really concerned when flying by myself, but in MoM competition it can be like flying a fighter plane amongst bombers..

Kick the pilot is good advice except if you are the pilot (who is doing his best to slow down whilst being kicked).

1-2deg additional camber (thermal settings) has little discernable effect. Will try mixing elevator and flap.

Keep the ideas coming guys.

Hutton

[lincoln]

If you pull up enough, it will slow down. However, you may consider trying the cg a bit more forward and seeing if you like it better.

I don't seem to be able to "read" a long tailed glider when the c.g. is too far back. Not sure why, but it's happened to me with my Ava and with a DLG called the Cyberdyne.

Other people say they can "read" lift better with an aft cg. I suspect that we look at different characteristics when deciding what lift is. (Mostly not a conscious process for me, though there are certain signs I can recognize explicitly, like the speeding up thing.)

[AMBeck]

I think you’re seeing two different things going on here. Assuming the CG is reasonably close to the right place, when you first hit lift, the sailplane will pitch up and slow down. As long as your CG is anywhere ahead of the neutral point, when you roll into a turn, the nose will drop and your plane will try to speed up no matter what the lift is doing.

The first part: Unless the CG is very far forward, a sailplane (full size or model) will pitch up as you hit lift. With the CG comfortably near the neutral point, that pitch-up is strong enough to be a clear signal of lift. With the CG further back, you do a lot of elevator rowing just to keep things under control, so it’s hard to tell when a pitch up is caused by lift, and when it’s just the very low pitch stability. This is especially true if you have a smallish elevator (TVC less than ~0.4)

The second part: When you turn, part of the wing’s lift is used to pull the airplane around the turn. That means that there is less available to hold up your bird. The plane will automatically compensate for the loss by speeding up. Your plane is trimmed to maintain a constant wing angle of attack, so it will naturally increase speed in order to make the lift required to both support and turn you model while maintaining the trimmed angle of attack. To keep from speeding up, you need to increase angle of attack by pulling back on the stick. This is true in any turn, not just in lift.

You don’t need thermals to practice keeping your turn speed down. In fact, lift will often mess you up. When you’re thermalling, the air will be rough and it’s harder to tell what you are doing, and what the air is doing to you. Make a bunch of calm air launches and just spiral back down. Use shallow to medium banks. Notice that with no up elevator your turns quickly become large and fast. Slow down using back pressure on the elevator stick. Try sneaking up on stalls in the turns. If your CG is right (and your model isn’t prone to tip stalls), you should be able to recover by relaxing back pressure and loose very little altitude. Try different flap settings. Increasing camber is like pulling back on the elevator. However, the most efficient camber setting still might not change the trim enough to slow down the way you’d like. You could still need some up elevator. Get to know how your plane behaves in calm air and it will be a lot easier to separate the plane’s responses to you from its responses to lift.

Another issue may be elevator travel. If your throws are on the short side, it will seem like you are pulling way back on the stick in turns. I like the elevator throw to be enough to sneak up on a stall in a medium banked turn with about ½ stick travel, or a little less.

[KevinSharbonda]

Nothing noted yet is the weight of the ship! A heavy plane will fly fast and not respond to what you described as wanting to do! A lighter ship or less heavy will be very different and might!

[Robglover]

Light planes will react to lift more obviously, as will planes with aft cg. Both of these things also make the plane harder to fly smoothly, so like everything else it's a trade off. The only way to find out what works best for you is to try it out and see for yourself.

You must fly your plane a lot in order to read lift well. You can read lift with a heavy plane, a light one, and forward or aft CG. It takes practice. Try to predict where the lift will be and then put the plane there and see what happens. A vario will help shorten the learning curve. Hand launch gliders also help.

When you find a thermal use it for a couple circles to find the center and figure out which way it's moving. Once you have it figured out then leave it and try to go straight to the next one, which you have been watching for while you were in the first one. Whenever you see a sign go check it out. You won't learn as much if you just park in the same thermal that everybody else showed you and sit there all day.

[VHO]

Thanks for the advice. I have some things I can try now.

First, I will shift the CG forward a bit and see what happens.

Second, I will get used to putting in more up elevator in thermals than I have been with the Xpro.

Strangely when I fly my home brewed Supra I am much more comfortable cranking in up elavator in thermals than I am with the Xpro. The paradox is that with the Xpro, the down part of the zoom at top of launch is much more pronounced than with the Supra. i.e. the Supra is less sensitive to elevator. Hmmm... :confused:


Some of the comments above give the impression that thermals are relatively small most of the time. I haven't found this, and have built and flown DLGs over the past year to improve my confidence in flying low and picking thermals. Further, when I do fly the Xpro in comparatively small thermals I don't find the extreme speeding up phenomena. It is mainly an issue when the whole sky is going up i.e. the area of lift occupies a significant arc of available friendly air space. The extreme speeding up is not an issue with the Supra, which suggests that it is in the setup of the Xpro and is not always my incompetance.

I will report back after I try out the above 2 strategies over the weekend. In the meantime I welcome further insights. :)

Editted after Tuomo's inputs below. Just realised there may be some confusion about the model. It is an Xperience Pro V tail, under 2kg flying weight.

[Tuomo]

In the meantime I welcome further insights. :)

Practise?

Don't take it too theoretically. Consentrate on staying in lift and avoiding sink ;) When air is active, thermals are often rough and turbulent - you have to fly the plane to keep it there. But when you do it well, small differences in sink rate quickly become insignificant.

Typically a glider sinks 0.5m/s or better. Strong thermals are often 3-4-5m/s.

Trim the plane so that you can consentrate on weather. Place CG by the best overall handling. In floating conditions, I sometimes remove 10-20g of nose weight.

[davidjensen]

You have not told us if you use camber when in the thermal? If so how much? You may have to add some up elevator trim in your thermal mode. My X-Pro was very responsive to CG location. I think all current moldy's are this way. Moving it 1/8" (2mm) makes a big difference in the way it feels and I'm not talking about tucking or pulling out of a dive. I initially set my CG using the dive test then play with moving it forward and backward by less than 1/8" until it feels right. My X-tail CG'd at 106mm. I also found that with the CG at 106 if I dove to the ground there would be some pull out (about a 1000' radius) after it got up to speed and adding reflex would eliminate the pull out. Let us know how you configure the trailing edge of the wing for all modes.