sheetyfadi

Greeting All,

I have built the Cloud Dancer from plans, powered it with Magnum XL52 which i thought will be sufficient for vertical flight (but it stalls).

I have been flying it for few days now and the first thing i noticed is its tendency to climb once i increase throttle. Once i researched a little more i found that this tendency to climb is designated as Ballooning (I am not sure if it is the correct term, there seems to be a disagreement on it).

So to explain what happens, i trimmed the plane for level flight at half throttle and when increasing the throttle, it tends to climb fast. Now it is not extremely climbing but it still climbs. But when decreasing the speed for landing, it made it harder to land especially with the elevator trimmed for level flight at half throttle. This was at very low wind speeds, nothing significant. This effect made it hard to conduct nice fast low passes over the runway.

I did some research and found that the ballooning effect can be caused by one of three factors or any two or three combinations:
1- The ailerons could be set below the wing when at neutral.
2- The engine could need some down thrust.
3- The wing is fixed at an angle with respect to the stabilizer/elevator (i.e. Angle of Attack).

So after reading this, i found that my ailerons were really below the wing at neutral. I fixed it and it did reduce the ballooning effect, and improved the landing. But it still exist.
As for the engine, i don't remember if i did add a 1 degree of down thrust or not. But since the plans call for it, i would assume that i did. But i can still shim the mount for an additional 1 degree.
Finally, i can't really check if the wing is at an angle of attack with respect to the tail. I know of a tool that was featured in the AMA magazine called !QUOT!Killer Incident!QUOT! or something like that, that can be used to measure this angle. Any other suggestions to measure this angle? easier and faster than building a tool?

What makes it hard in the Cloud Dancer is that the wing is a low wing and bolted to the fuselage on the TE and fixed with two dowels on the LE which makes it hard to shim the LE as a test.

From your experience, should i always expect this ballooning effect? Or should i keep on trying to find the fault in my plane and try to fix it?

Thank You

Fadi

P.S. Check my profile for some pics of the Cloud Dancer.

dbacque

My Feedback: (3)

Fadi,

No you shouldn't always expect changes in pitch trim like this but don't be alarmed, most planes need minor adjustments after construction.

It sounds like your suspicion of needing additional down thrust is correct. Add a washer or two on the upper motor mount bolts between the motor mount and firewall. Alternately you can put washers between the engine and motor mount on the rear engine bolts.

Once that's done, take off and trim the plane out at full speed. Make a level pass and when the plane is in front of you, chop the throttle. What you're looking for is for the trim to be uneffected by the throttle position. If it climbs when you chop throttle, you need more up thrust. If it dives when you chop throttle, you need down thrust. Then continue adjusting as necessary. When you get the proper amount of thrust offset it should hold trim at any throttle setting.

To help with landings on your test flights, slow down and readjust your pitch trim before landing.

Dave

sheetyfadi

Thank you Dave for your advice. I will add a washer between the mount and firewall. and Test fly it.

But i am concered that i am trying to correct the problem by other than finding the real source. Shimming the engine might seem to work but the real problem could be the angle of attack of the Wing. How can i make sure that i am square with respect to the angle of attack before porceeding with correcting the engine down thrust angle?

But probably due to the ease of adding washers, i will go for that first.

Thank you again for your response and input.

Fadi

dbacque

My Feedback: (3)

There could be an incidence misalignment but the problem you describe sounds more like thrust angle. Again, trim for level flight at full speed. Fly a level pass at full speed and chop the throttle when in front of you. Does the plane dive? If so, it needs down thrust.

You should start with the most obvious needs and the easiest fixes. In the case of pitch trim changing with throttle setting, the most likely culprit is thrust line and it's the easiest to change too. Once you get the plane to where it's easier to fly, then you should go on to more advanced trimming, including flight testing for CG and incidence. By the way, when you change either CG or incidence, you'll need to recheck your thrust line. with the test mentioned above.

Check out some of the trimming charts on the web. They will tell you how to test your plane and how to trim based on the results of the tests. Start with these http://www.scootworks.com/rdrc/trim.html http://www.nsrca.org/TrimA.htm There are lots more if you look. Just right now I can't find the one I usually use but someone will speak up with more sites.

Think of it this way, if you add the washers and the plane trims out at all speeds, hasn't that fixed the problem?

Dave

LouW

All airplanes that are longitudinally stable will behave as you describe. The first step should be to check the location of the cg. The more forward the cg, the more pronounced the “ballooning� behavior. As the cg is moved rearward, the pitch up tendency with added power becomes less until a point will be reached where it no longer pitches up with added throttle. This is the neutral point and while most aerobatic airplanes are balanced close to this point, they require constant attention and skillful thumbs to fly. If the model is built according to the plans you shouldn’t have to mess around with incidence thrust line, controls, etc. Just move the cg rearward until it flies like you want it.

dbacque

My Feedback: (3)

No, they wont. They'll only behave like that if the thrust line is off. Think about what happens if a stable plane with the perfect CG has the engine mounted with a lot of down incidence. When flying at full throttle, the engine is producing forward thrust as well as pulling the nose down. That will have to be compensated for with lots of up trim to get level flight. Now chop the throttle and see what happens. You've no longer got all that down thrust trying to pitch the nose down but you've still got all that up elevator trim so the plane climbs.

Now take the same plane and put the engine at a large positive thrust angle. The results will, not surprisingly, be just the opposite.

Both cases had the correct CG but both trimmed poorly.

CG and thrust line are both important adjustments that can be made to improve flight trim. But you are right that CG location should be the first step in the process.


Plans can be wrong. Construction errors can be made. Tolerances can gang up against you. Nothing is ever perfect. Thrust line adjustment is very commonly required on a new plane.

Let's take the same hypothetical plane we were discussing above and try to correct it's trim problems by moving the CG back. Eventually you'll have a plane that still won't trim at different speeds and it'll also be so pitch sensitive that you won't be able to fly it. The correct adjustment for thrust line problems is thrust line adjustment. The correct adjustment for CG problems is CG adjustment And the trim charts are developed to help you determine which part of the airplane's set up needs to be adjusted. That's why I described how to flight test for thrust line determination. Flight testing for CG is different. Again, see the charts.

Recommendations for things like thrust line and control throws are only starting points. Every plane is slightly different and for best performance each one needs to be tested and tweaked.

Dave

sheetyfadi

Thank You LouW for your input. It is exciting to see back and forth discussion on the aerodynamics of the plane.

As for the CG, I think it is on the spot. The way i found out is through landing, I have been told by trainers that cg is best checked as it flares for landing. The cloud dancer has been the easiest plane to land. It flows so nicely at idle and makes a perfect 2 wheel touch down with barely any impact.

I believe in Dave's theory, nothing is ever perfect. I think of myself as a very exact and perfectionist and yet i did mistakes with this plane.
Thank you Dave for the Links they will help tremendously. but let me update you on the outcome of the recent flight.

I added a pair of washers under the engine mount (since the engine is mounted side ways). The washer thickness is 0.75mm which contributed to approximately 0.9 degrees. I know that is not much, but still i was doubtful of adding too much.
The Result:
The plane still had some ballooning effect. but again slightly less.
I check for throttle line alignment slightly different than your process but i think they both give the same result. I trim it at half throttle and then crack up the speed and observe the plane. In this case it is going up: "ballooning effect"
But i noticed something really important. As i cracked the throttle up, it didn't just go up but it rolled slightly to the left. After landing it, i noticed that the wing and the stabilizer are not parallel. So first thing i will do is shim one side of the wing to have it parallel to the stabilizer. This will most probably fix the slight left roll.
Another thing, My landings were not as perfect anymore. It could be me. since i am only a rookie, or the wind, or the added down thrust in the engine. The landings were acceptable but with faster descent that made it look a much harder landing than i am used too.

Next, I will fix the wing level to the stabilizer and then go through the Guide that you sent me and try to identify the possible reason.


A side issue: My knife edge is horrible, the plane holds for a second and then the nose dives fast downward. Even with full rudder and full throttle. I might be doing it wrong, but not that wrong. But yet again maybe it is too early to start with such maneuvers when i don't have the plane properly trimmed. And the engine can not lift the plane into a vertical flight. Could that be the reason?

Thank you Both Dave and LouW for your input .... keep them coming

Regards

Fadi

dbacque

My Feedback: (3)

Hi Fadi,

Glad to hear the update.
Okay, that sounds like good news. The pitch trim difference with throttle setting has been reduced. I'd add another set of washers and try again.

Sort of amounts to the same thing. But you are only seeing the pitch differential between half throttle and full throttle. That's fine for now. But when you get to where you see less difference, try the way I suggested it. Since you're now testing full throttle against idle, instead of half throttle to full throttle, you'll be able to do finer tuning. Remember, you want to reach the point where you don't have to retrim at any throttle setting so there really shouldn't be any difference between trimming for half throttle and for full throttle anyway.

Actually having the wing and stab out of alignment won't cause the effect you describe. What it will cause is a coupling between elevator and roll, the plane won't loop with the wings level. This is something that you'll want to fix but the roll problem should be next on your hit parade. Read on...

What will cause a plane to roll as speed changes is a twist in the wing. I just fixed that very same problem in my Hots. The reason it happens is that if a wing is twisted you can correct for it with ailerons. But when you change speeds, the aero effect of the twist is greater than the aero effect of your aileron setting.

This is still easy to correct but it requires an incidence meter (which can be purchased or constructed, see http://www.rcuniverse.com/forum/m_4983611/tm.htm) Put the meter on the the wing right next to the fuselage. Block the plane up so the meter reads zero. Now move the meter to the tip of the wing. If it's the same, great. If not, the wing is twisted. But it's easy to fix. Grab your heat gun, twist the wing until the covering wrinkles and reshrink the covering on both sides. Now check the incidence at root and tip again. You'll find that you changed the twist in the wing. It may take several cycles to get rid of it all or you may go too far. Keep trying until you have the same reading at the root and the tip. Repeat for the other wing. Of course, the readings should match from one wing to the other too!

When I did my new Hots, it took me about 3 cycles to get all the twist out and next time I flew it all of the roll trim problems were gone.

You really might have just had an off day. I was having one Saturday and kept slapping my pattern plane on the runway even though the conditions were perfect. Of course, you're also landing the plane with different trim than you're used to on it. Give it a little time and it should be no problem.

I'm not familiar with that particular plane. Not all planes knife edge well. But have you tried knife edge on both edges? Does it hold better on one edge than on the other? If so that could indicate a side thrust issue. But you're right. I'd wait until the pitch and roll trim concerns are taken care of.

By the way, there are a couple of different ways to flight test for CG. In a steep turn, if the nose drops, it may be nose heavy. If the tail drops, it may be tail heavy. Or after trimming for level flight (level at all speeds!) put the plane in a dive, like from a stall turn. If the plane holds the down line the CG is good. If it tends to pull out of the dive the CG may be a little forward. If it tucks under, the CG may be a little aft.

Welcome to the world of flight trimming. I have planes that I've flown for years and still continually make smaller and smaller changes. As we said before, nothing is ever perfect. But boy does it feel great when you make it a little better!

Best wishes,
Dave

BMatthews

There's no perfect CG point since the final CG point is related as much to the comfort zone of the pilot as the design of the airplane.

Having said that and provided you do not have any serious angular setup oddities in your model I would suggest that Lou hit the nail on the head with his description. The CD is an aerobatic model and such models should not need to have more than one or at MOST two degrees of downthrust compared to the wing airfoil's centerline. If you have issues with ballooning at that point then they ARE CG related.

By adding downthrust to excess you're actually masking the true culprit which is the CG issue. The best way to test this is with some dive testing. Set your throttle for about 1/4 to 1/3 of the max RPM (just a little above idle in fact) and trim for level flight with the elevator trim. Now push the model into a 45 degree dive and let go of the stick and watch the model. If it has a very abrupt nose up reaction then your CG is too far forward, if it has a noticable but not fast nose up then it's likely just right for general sportflying, If it stays in the dive or wants to tuck the nose harder then the CG is too far back. For something aerobatic like a CD the altitude needed to pull out of the dive should be large. A slow pullout that uses 250 to 300 feet is not untypical. Often pilots will set the CG and resulting elevator trim so the model doesn't pull out or tuck at all. At dead neutral in other words.

The CG descriptions seem to be at odds with the results but they aren't. In the first case (and what it sounds like you have) the CG is strongly forward (I don't care what the plans say). A forward CG is more stable but it requires more up trim to compensate. In a dive where the speed builds up the elevator trim will become stronger than the excess nose heaviness and pull up the nose sharply. Similarly when you add throttle the speed builds and the strong elevator up trim needed to hold up the heavy nose at slow speeds becomes too much and again the nose comes up.

To get a proper handle on the issue you need to measure all the angles of the model's setup to ensure that something is not out of whack. This includes the engine thrust angle ( measure the prop disc and take away 90 degrees), wing airfoil center line angle and stabilizer center line. Set all controls to dead neutral to blend the airfoils perfectly before you do this. The results may find a problem that you need to deal with. You're looking to have the stabilizer at 0 to -1 degree to the wing and teh engine at 0 to -1 or -2 at most compared to the wing. From there you just need to work on that CG.

Yes your landing charactaristics will alter a bit but it won't be that much. You may have to slightly change how you approach and set the model down but you should still be able to land it fine.

By the way, part of your "problem" is the engine. You've overpowered it and that will also tend to make this ballooning issue that much more noticable. WIth the extra power the need for properly setting the angles closer to 0-0-0 and the CG towards a more neutral setting becomes that much more important.

sheetyfadi

Thank you guys for your input.

Very Helpful.

Checking for the ballooning effect by using the 45 degree dive is a great and smart approach to settle this problem.
If my estimates and Dave's are not correct with respect to the engine thrust line, then this maneuver should show that since the speed of the plane will increase without adding engine thrust.

As for over powering the plane, I don't know about that. If it is overpowered, how come I can't go straight up? I thought by having an overpowered plane, i will be able to go straight up. It will go but for a short distance until it looses its momentum and then stalls.

I was thinking of building an incidence angle but i don't think i have the time and i will keep on putting it off with out really measuring the angles of the plane. Any suggestions of what to buy with a reasonable price?

Thank you again guys ..... and be patient for my next flight. I am in Maine, and here the season is really hard for flying during the winter. I was lucky to fly last weekend.

Fadi

dbacque

My Feedback: (3)

Yep, the dive test is my preferred method for testing CG, although I usually use a vertical dive. As I mentioned before, any time you move the CG, you should retest for thrust line.

I never suggested that thrust line adjustment was all that should be done. Just that it's a good starting place. Test, test, trim, retest, retrim, ... And after you make and adjustment, go back through all the previous tests again.

Dave

da Rock

There is also a very simple CG test that takes almost no effort. Simply fly the model past one way at speed, then turn it around and come back the other way. Only on the 2nd pass, fly it inverted. If it takes appreciable down elevator to hold the inverted flight elevation constant, there is a good probability that the model's CG is too far forward.

You don't have to trim the sucker for slow speed or anything. You've already got it trimmed for that "normal" speed and the elevator trim is either carrying a forward CG or not. And if you roll it over and fly that trim upside down, what does it do? It tries to lift the nose toward the ground and you have to push the elevator "down" to get the elevator to carry that forward CG. That forward CG is going to pull the nose toward the ground whether the airplane is right side up or upside down.

Simple and quick.

da Rock

Are there overpowered airplanes that can't climb out of sight, don't have unlimited vertical? You betcha. Put the wrong prop on any engine/plane combination and the vertical is the first to suffer.

There's a guy at one field who overpowers everything he flies. He also uses props that are too small for his engines. Most of his stuff has the right prop for the engine that ought to be on the airplane in the first place, if you can follow that. And since most of his stuff doesn't have vertical worth spit, he also uses higher nitro fuel. Go figure...... But by god, his engines scream!!!

BTW, he also loves APC props because "they're more efficient" because he gets higher revs with them.

da Rock

Actually the vertical is the 2nd to suffer. The takeoff is the first.