tree

I have a 40 size scratch built stick, wings flat bottom the plane is alittle noseheavy, wings have 2deg. dehederal the plane climbs at full thottle I put down in the elevator and it flys ok.What can I do to make the plane fly with out putting down in it......THANKS

cappio777

My Feedback: (2)

Check the incidence of the wing and horizontal stab. if they look ok then put about 2 degrees of downthrust to the engine.

BMatthews

The nose heavy condition requires that you add up trim to balance the dive you would have otherwise. You probably did that already and now adding speed by adding throttle makes that extra up trim work to make the plane climb. You can add down trim or take out some of the up trim but then the model will want to do a shallow, or perhaps not so shallow, dive at idle.

You may need some donethrust as well but the best plan at this point is to get the balance point back to the spot it is supposed to be at or a little further back if you can handle the extra elevator sensitivity. Then you can take out a lot of the up trim to make the model fly level and the climb at full throttle should only be a slight amount.

It's quite normal for any model to climb a little when it's trimmed for a cruise setting and you then give it full throttle. Real planes do this too. I deal with it by using my elevator trim often during a given flight or trim the balance and downthrust so that it's trimmed for about 2/3 rpms. Full rpms gives me a shallow climb that I can control with a bit of forward stick pressure. Or if I'm going to fly that fast for long I'll add a couple of clicks of down trim until it's time to throttle back to cruise.

The best trim for elevator and downthrust is when the model glides smoothly but not too fast or too slow at idle, cruises level at about 2/3 to 3/4 rpm and climbs in sort of a high speed uphill glide at full throttle all at the same elevator trim setting. You can play with the CG and elevator trim until you get that and then play with the trim for speed if you want to rip around at full throttle.

MikeSell

My Feedback: (2)

If your plane climbs when level at full throttle does it also climb when sped up in a dive? If it only climbs when throttled up not when sped up then you definitely need down thrust. If it pulls up whenever it speeds up it needs longitudinal dihedral. A degree or two of positive incidence on the rear stabilizer should neutralize the tendency. I have to re-figure this on every oldtimer or free flight conversion. I use realflight to test the results before putting a new one in the air.

CaptKAOS

My Feedback: (7)

With a flat bottomed wing....speed creates lift.....so when you trim for straight and level at 1/2 throttle....it should climb when you open the throttle.....descend when you close the throttle.... Most trainers with flat bottomed wings have some down thrust built into the firewall to stop them from climbing drastically under full throttle....

Even with semi and full symmetrical wings the same thing applies....but not as noticeable.....

DipStick

CaptKAOS,

How dare you make such a statement..[>:]...me think you are a little bit of a heretic and have been hanging around them to long...

Just remember that there was a time not to long ago that making a statement like this could get you disband, disbarred, beat up, or for real crazies like you we would just burn you at the stake. So be careful, the intelligency are watching you and they know were you live.

Maybe, you would like to share your theory on the world being round too?[sm=angry.gif]

Happy Flying!!!!

Steve

Tattoo

http://www.rcuniverse.com/forum/m_72.../tm.htm#723406

MikeSell

My Feedback: (2)

What you do when you reflex the ailerons is reduce lift and therefore reduce drag. The flat bottom wing becomes "semi symetrical." This would explain the more axial rolls and less tendency to climb when speed rises. Less lift=less drag=more top speed.
Longitudinal dihedral controls pitch by balancing the increased lift with increased stabilizer authority. The plane may still rise with speed but not pitch up. Stability is a balancing act...an increase in one force has to be countered with another. I would rather use as little trim as possible when changing the speed of my aircraft.
I find it strange that one would build a stick with a flatbottom airfoil unless they were using it for a trainer. If this plane is to be used as a trainer it is doing just what it should. As a trainer dives it should automatically pull up. As it climbs it should drop in pitch. In that case down thrust is the correct solution.

In my opinion..

cappio777

My Feedback: (2)

I agree with Mike, all that was accomplished was change the airfoil shape and decrease the incidence of the wing. That defies the purpose of a flatbottom wing. Its correct to bring the ailerons up so they follow the chord line but the moment one moves it past that point you are changing the airfoil.

Tattoo

I'll be the first to admit that most of this aerodynamic theory stuff is way beyond me. I'm a try it and see what happens person. When I was a kid I would fold up and tweek hundreds of paper airplanes just to see what they would do. I never really cared why...how the plane performed was more important. I'm the same way with R/C. When I had one of my favorite designs turn into a totally better performer with some aileron reflex, I started taking note of this. Amazing how many people, especially beginners, show up at the field with drooping ailerons. Amazing what lengths guys will go to to trim a plane, without even checking aileron neutral position. Amazing how many seasoned "vetrans" don't even realize or consider this. I'm not talking from a design standpoint. Where I'm coming from is the view of the plane is already done, and someone is having a crappy day at the flying field because he is having trim problems. I really don't care what kind of give and take aerodynamic trade off or airfiol rules I'm using or breaking. If a clevis turn or two and a degree or two of aileron neutral setting will change someone's enjoyment of how the plane flies, I'm all over it. It's also not that drastic...just a degree or two can make all the difference in the world. Also, one of the places this has made the biggest difference is smaller scale airplanes with higher wing loading. A 1/12 scale Spitfire or P-51 that is a tip stalling dog can become greases lightening with a little aileron reflex. Also, I have yet to see this effect lift or or any other areas of the aircrafts performance in a negitive way, if anything, the entire flight envelope of the aircraft is more stable. It one of those things that I'm glad I didn't know the aerodynamics behind it or else I may not have tried it. Now I wouldn't handle pitch trim problems any other way on a properly built airplane, and the proof is in the air.

cappio777

My Feedback: (2)

Same concept as before, if the small model will tip stall very easily and aileron reflex helped it...basically you altered the airfoil once again and gave it less incidence therefore it will not stall at the same AOA as before. The question is....were any of those models that have benefited from the aileron reflex properly built? I am not there to say yes or not but an airplane that is built correctly shouldnt need modifications like aileron reflex. In reality you are converting a flat bottom or semisymetrical airfoil into a symetrical one in a very inefficient way. If the airplane is not behaving like it should then the builder made some errors in the construction. I have built hundreds of airplanes including some for engineering research projects and each time an airplane doesnt fly like its supposed to we can find out the mistake in the design.....not because it needs more elevator trim, aileron reflex or chewing gum in the carburator. If a boat has a lot of people near the bow and its a little under the water level the solution is not to put pieces of foam under the bow to raise the floatation. Although that would solve the problem at that point in time doesnt necesarily mean that is the right way to fix that condition.
There are proper ways to asses the solution and it always follows a systematical approach.
My .05 cents........keep the change.

michpittsman

My Feedback: (11)

The world is round? I did not know that! Next you will tell me it revolves around the burning object in sky. Yeah, right.

cappio777

My Feedback: (2)

Everybody knows there are monsters and dragons at the edge of the world........thats why I dont travel by boat....[sm=lol.gif]

DipStick

Tattoo,

I agree that this works having tried it myself. I'm not sure how to explain it but there most be a good aerodynamic explination for why reflexed rons creates a more stable plane...(changing the thrust line and incedence for one).....I've even built planes that reflexing the rons has had the effect of creating MORE lift than before reflexing(this is very counter to what I would expect)....I know it was creating more lift because the plane could fly a lot slower in a power off condition. I suspect that the reflexed rons were helping the boundry layer stay attached longer or maybe it just improved a poor foil shape to a better foil shape (same difference).

I believe that science is a very powerful tool....but can be mis-applied and become untested dogma which is not science at all. The prof is often in the pudding. On the other hand experience alone without scientific understanding also runs the same risk. One example would be attacking the following statement: "all planes have a tendency to climb with added power".....You may not like your plane to climb with added power but it is a natural aerodynamic tendancy and a sound scientific statement. Anyone that would attack the person that made a statement like this because they don't understand the science behind the statement is a ignorant ass (Oh that was me oopps).

With experencies and time someone applies down thrust and it seems to help.....so from there on out the wisdom becomes that this is the right way to correct this problem.....or maybe someone reflexs the rons and this becomes the "right" method.....but there is no reason why other methods might not be equally able to address this tendancy. The problem with aerodynamics is that usually by doing one thing it effect a lot of other things and without a real scientific understanding of what these effects are then often we are just grasping at dogmatic veiws. There may be reasons why one method is better than another.....I think you have given some stong reasons why your method has merrit.....but there may be other methods for other applications or other reasons that are sound.....in any case I will not be happy until I have a scientific explination for why it works and what the trade off are...even if I know it does.

Happy Flying!!!! Long Live SPAD!!!!

Steve

LouW

I accept the fact that reflexing the ailerons a bit cures the tendency to climb under power. I will try to explain why it happens.

First I need to correct a comment regarding stability. An airplane that exhibits sharp pitch change with added power (or increase in airspeed) is quite stable while one that “goes where it’s pointed� is much less so. Reflexing the ailerons makes the airplane a little less stable and that’s why it works. Consider an airplane with a well cambered airfoil (flat bottom). This airfoil has a relative large negative (nose down) moment, which must be resisted by a down load on the tail. With the cg near the ac, a lot of decalage is required to balance, or the cg must be moved aft. With a less cambered airfoil, there is less negative moment, so less decalage is required, or balance is achieved with a more forward cg.

The neutral point (the cg location that results in neutral stability) is more forward on the airplane with the less cambered wing. This means that if nothing else is changed, and you remove camber by reflexing the ailerons, the neutral point moves forward, and the airplane is less stable. You could achieve the same results by leaving the ailerons alone and moving the cg a bit aft. However as you said, turning a clevis is simpler than moving the cg.

The effect on the small warbirds you mentioned is a little different. In addition to the reduced stability, since the ailerons on them are not full span, you have simply provided effective washout that is the old standby for eliminating tip stall.

You haven’t discovered some new aerodynamic principle, but have come up with a simple fix that can sometimes correct badly built (or designed) models.

As to a possible down side, the small changes you describe, although they have a dramatic effect on handling, have a pretty negligible effect on performance

MikeSell

My Feedback: (2)

I like your explaination!
I'm not disagreeing with your expalination only one term. I know it is often incorrectly used even on instruction sheets.
Decalage is a term correctly used to describe the difference between the incidence of the upper and lower wing on a biplane (like toe in on a car). Longitudinal dihedral is the term to describe the difference in incidence of the wing and horizontal stab.

cappio777

My Feedback: (2)

Guys, its the same thing, just like saying fuselage or the body of the aircraft. Decalage, (longitudinal dihedral)is the term used to describe the relationship or angle between a wing and a horizontal control surface on the tail or between biplane wings. Decalage is a french word just like fuselage , canard and monocoque.

DipStick

Louw,

Thanks for the reply. I will watch my sloppy use of the term stability.... A couple of questions. On low Reynold's, low AR wings, built for sport flying how much of a concern is foil effeciency? Would a plane that requires down thrust also be a 'badly build (or designed) plane'?

As I see it >>>

Down thrust creates a dynamic c.g. that shift forward with added thrust, so some of the thrust is wasted to create this dynamic weight.

Down thrust also creates a more draggy plane.

Down thrust also effectively reduce the pitch of the prop so it will probably reduce the top speed of the plane.

Happy Flying!!!!!

Steve

LouW

Many airplanes, especially those with the wing on top, fly better with a little downthrust. That dosen't mean they are badly designed. A small amount of downthrust has a negligible effect on efficiency and is often the simpest way to control high power climbs.

The effect of power on pitching moment has two components, the pitch up due to longitudinal stability described in this thread, and the moment caused by the thrust vector being above or below the center of drag. With a high wing, the center of drag is relatively high and a low thrust line causes a nose up pitching moment in addition to that produced by stability. For a low wing aircraft, the center of drag is relatively low and the thrust vector is frequently above the center of drag, producing a nose down moment with addition of power which acts opposite the pitch up due to stability. For this reason low wing aircraft usually don't require down thrust and seem to fly well even with the cg well forward of the neutral point. Adding a little down thrust to a high wing aircraft moves the thrust vector closer to the center of drag reducing the thrust induced pitch up tendency. It's really not bad design practice.

There is no great benefit from efficiency in a model airplane except for the most serious of racing or endurance types. The typical R/C model is so overpowered that even fairly large changes in efficiency would go un-noticed. The main concerns are handling qualities and performance. By performance I don't mean fuel efficiency but ability to have unlimited vertical climb and to hover (for the 3D types).

cappio777

My Feedback: (2)

In addition to what LouW said, the combination of a high center of drag from a high wing with a flat bottom airfoil can become destabilizing and therefore an additional force must be applied to bring neutral stability.....downthrust.
Some aircraft can benefit from lifting tail condition, the only one that I cant think of right now is the Senior Telemaster, where the effect creates a very slow flying platform but very efficient in the amount of lift generated. As far as draggy aircraft, a lifting tail generates induce drag while a conventional tail has much smaller component of drag since it works with gravity.

LouW

Actually changing the thrust angle has no effect on stability. A very stable aircraft will pitch up sharply with an increase in speed and pitch nose downward with a decrease in speed. An aircraft with high longitudinal stability will try to maintain a constant airspeed (actually angle of attack). With reduced stability it will not pitch so much with speed change and as it approaches neutral stability it will just go where it's pointed at any speed with little pitch change. For a given configuration, the only thing that has a significant effect on stability is cg position. More forward cg more stable, more rearward cg less stable.

Whether a tail is lifting or not again depends on cg position and not on the airfoil shape of the horizontal stabilizer. For there to be an actual upward force on the tail requires a rather rearward location of cg and to be both balanced and stable requires a somewhat larger than normal tail area. During the 50's an airfoil shaped tail surface referred to as a "lifting tail" was popular on free flight models. It was an attempt to control the power in a near vertical climb and transition to glide without the option of radio control. It is not a particularly efficient setup and serves no purpose in a radio controlled aircraft.

A flat bottom airfoil is not inherently more unstable than a symmetrical one. It just requires a different cg location or tail setting to balance because of the negative moment at the ac.

cappio777

My Feedback: (2)

Read what I said, I never said flat bottom is unstable.

DipStick

I have one more question....everything that has been said so far implies that the reason why a plane climbs with power has to do with a change in the angle of attack, but what if I built a plane that would stay at the same angle of attack would the plane still climb with added power?

cappio777

My Feedback: (2)

Assuming you can keep the same AOA then all it needs to do is have a change in airspeed to generate lift. In reality thats how we are taught how to fly real aircraft. Control climb and descent with throttle and control airspeed with the elevator.
Kinda confusing when you apply it to R/C but you have to take into account we fly with an extreme amount of thrust for our models. But the principle apply just the same.

LouW

Dipstick, that’s exactly what a stable airplane tries to do, stay at a constant angle of attack. As power is applied, the airplane speeds up and the wing, at the same angle of attack, then produces more lift so the airplane climbs (which brings the speed back down to the trim speed). As power is reduced the airplane slows down and, the wing, at the same angle of attack, produces less lift and the airplane dives (bringing the speed back up to trim speed). To minimize this behavior (for instance with an aerobatic airplane) stability is reduced, usually by moving the cg aft. You can only go so far because at the neutral point and beyond, the handling becomes somewhat like a lawn dart.

When the airplane is being designed, there are a number of other things that can be done to produce desired handling characteristics. Some of the factors effecting handling are, tail moment arm, stabilizer area and location relative to the wing, wing loading, wing span and aspect ratio, thrust line, etc. Once an airplane is built, tweaking of trims (including flaps like described above) changing thrust angle, and moving the cg are about all you can do.

Of these moving the cg has the most powerful effect. A typical trainer can be made to fly like a different airplane by simply moving the cg aft. If this is combined with a reduction in dihedral, it will make a pretty good aerobatic trainer. Will it ever be as good as a purposely designed aerobat? Of course not, but it will have some suprisingly snappy moves. In a similar manner a fairly hot aerobatic ship can be tamed into a pretty docile little craft by moving the cg forward.