speedracerntrixie

My Feedback: (29)

Change the trim for landing??? You must be kidding! Do you two guys actually fly R/C at all? An airplane with the correct CG will have NO repeat NO trim change from full speed down to just above landing speed. Who on earth is going to want to re-trim their airplane twice every flight?

hugger-4641

My Feedback: (6)

I don't know what planes you fly, or what you think the "correct" cg is. "Those two guys" you refer to know what they are talking about.

First of all, there actually isno such thingas"The" correct cg. There is a "range" for cg location that should be maintained when setting up a plane. Within this range there is quite a bit of difference in the performance and trim adjustments that may be needed. CG should alsobemeasured with the tank empty of fuel. Depending on the plane and location of the fuel tank, cg will change significantly as the fuel level drops. Depending on the plane and location of the fuel tank, this doesn't always require trim adjustments, but on many planes it does. Wing incidence, dihedral,and other structural features can also cause a need for different trim settings at different speeds. If you don't believe this, you need to fly in some full scale aircraft, or talk to some full scale pilots. I don't think you will find many who will tell you they don't make trim adjustments to their planes.

Back to the OP's problem.
Didmoving the CG back help enough, or does it still try to climb at idle when you have nose up AOA?

speedracerntrixie

My Feedback: (29)

hugger-4641

My Feedback: (6)

If the wing incidence and/or dihedral is not correct in the first place then it can affect PITCH at different speeds Notice I said pitch and not trim. It's not the trim changing, its the lift is increasing at the higher speeds because the wings AOA in relation to stab is off. We are not talking about full scale, we are talking about models. You can spout all the full scale theory you want, most of it does not apply to models because the power levels, wing loading and much more do not translate down from full scale to model usage. Try flying a model with a scale wing loading and you will see what I mean


I will admit that the comment about " You two guys was directed to you. When one gives advise one should have the experience to do so.


I don't care who your comment was directed at.
Giving helpful advice is one thing, ridiculing the help of others is what you were doing. No exprerience neededwhen you have to ridiculeothers to make yourself sound like an expert.

Though I certainly can debate your statements,I don't care to highjack this thread todebate theories about full scale or models, or to tout my own credibility. However, I have taught dozens of people how to fly models, and Ihave plenty of experience to dispute your statement about "a plane with proper cg not needing trim adjustments". As I said before, some planes do require trim adjustment for lower speed, especially if there is a flaw or design characterstic concerning dihedral, wing incidence, or wing loading, or CG. Any one who flies high wing models or certain low wing models knows this.

The OP is obviously not as experienced as you claim to be, and hisGP Shoestring is not a perfectlyengineered pattern plane or sail plane.The problemhe is having could be due to his flying, or it could bedue to a problem with the plane, this is what we are trying to help him determine since we can not stand right there with him and help him.
Ifyou have nothing helpful to say,all your "expertise" does him no good,. My advice to you is keep your negative advice to yourselfwhile others are trying to help him.Feel free to start your own thread to attack me and others who are trying to helphere, but don't highjack this one.

speedracerntrixie

My Feedback: (29)

My intention was not to ridicule anyone. I did not list what my R/C experience is until I was questioned about it. I was simply pointing out that in my opinion telling someone to trim the elevator on his airplane was poor advise. Imagine him coming in to land and have to for any reason needing to do a missed approach and powering up with a bunch of up trim. That's a tip stall waiting to happen. The better thing to do is to teach him to make adjustments to the airplane in order to make it fly correctly. This way he can apply this knowledge to future airplanes. Like the old saying " give a man a fish he eats for the day, teach him to fish and he eats for a lifetime"

cfircav8r

My Feedback: (1)

Trimming for landing will allow you to have a stablized approach from down wind all the way to final. As far as the go around, in full scale aircraft you can also easily stall if you jam the throttles and don't correct yet every pilot is trained this way. I do it with some of my models and in full scale and they react the same, so no there is no difference with size, only design. The key is don't trim for near stall just a slower safe speed, and practice at a safe altitude. It is true I don't fly every model this way, but some do benefit from this method, and it can help a new pilot when taught correctly. Learning to set up a good easy stable approach gives you the opportunity to learn the airplane so that you can then begin to adjust correctly to non standard approaches.

speedracerntrixie

My Feedback: (29)

My concern with this technique is that I feel it can and eventually cause a stall. I am aware that for full scale pilots this is standard. A full scale pilot has more time to put in and take out trim. CG is not a constant on a full scale aircraft either. Passenger weight and baggage are the variables. Yes a model does burn off fuel but the CG moves slightly aft, the direction I am advocating.

What I am having difficulties explaining and I hope this will help is that with a model that is correctly set up none of this is nessesary. If your model has a bad habit then adjust the habit out of the airplane. How many times do you shift CG until you are happy with where it is? It usually takes me 3 to 4 adjustments. Then all the other trimming can start. On average with an aerobatic model it may take me 20 flights to get all the trimming adjustments where I want. The point I am trying to make is don't fly an airplane once or twice and decide if it flys good or poorly or worse yet just decide to live with a bad tendancy. Learn what is causing it to do certain things and how to correct it. This really appies to all airplanes. Yes there is a higher expectation for a pattern airplane but a sport flier like this GPShoestring will benifit too.

cfircav8r

My Feedback: (1)

Yes and after all of the trimming pilots still have trouble slowing down for landing. What we are saying is that if after the CG adjustments and the triming if you still are having trouble you need to look at how you are flying it and these are examples of how to do that. I have flown both for years (models 30 and full scale 18) and they are not at all different except for our perception. They respond to control inputs the same.

HighPlains

My Feedback: (1)

Quite often it is the thrust line that causes problems in regard to needing a lot of trim change when you slow a model. But in general, a more advanced model like the Shoestring needs no elevator trim change to land when you get everything else correct.

As to full size aircraft, they are setup more like a typical trainer, and are very speed sensitive thus requiring trim changes for each speed.

cfircav8r

My Feedback: (1)

What does trim control? The elevator position. Elevator controls pitch, or more correctly AOA. AOA controls airspeed, to go slower you need a higher AOA. Without changing configuration, you cannot change speed without changing AOA. If you trim for level at full throttle, to climb you will need to trade some speed for altitude by increasing the AOA which slows you down but allows the plane to climb. If you want to slow down you need to reduce throttle which will cause it to descend at about the same airspeed so you must increase the AOA to maintain altitude which causes you to slow down. To slow down for landing you have to increase the AOA and reduce throttle. If you release the elevator during the landing it will drop the nose to resume the trimmed AOA, and therefore airspeed, so you will have to keep constant pressure on the stick throughout the entire landing pattern unless you add some trim to keep the new AOA. Most experienced pilots that are comfortable enough with their plane so have no problem with this, but if you are still learning the plane it can make landings difficult. Hence the idea of trimming for landing. As far as the pitch up on go around it is worst with a trainer style and far less with a sport plane such as this, so is really not a huge issue if you learn how it will react.

Lnewqban

Just a beautiful summary!

speedracerntrixie

My Feedback: (29)

Sorry but we are going to dis-agree again. All good theory for full scale but like I said previously, very little translates down. Again if you doubt this think about the following. Most 40% Extras weigh around 40lb. Much heavier then this and performance really falls off. I hope we can all agree on that at this point. So lets do the math, a full scale Extra weighs just shy of 2,000 lbs. Scale that down and at 40% thats about 800lbs. We all know the outcome if you was to build a 40% Extra at 800 lbs. So lets talk about model airplanes here please and not full scale.



A nose heavy airplane will land fast. Why? Think of the airplane as a reeter totter. The pivot is the airplanes CG. To keep the teeter totter level , equal amounts of force fore and aft of the pivot (CG) But remember we are noseheavy so that force at the tail has to come in the form of up elevator trim. Now as speed falls off what happens to that trim? It becomes less effective. So when on final as the speed decreases even more, that trim is even less effective so two things are needed. Either more elevator trim or back stick IF available or guess what? MORESPEED! So if one is flying a nose heavy sport plane that is reasonably fast he will first have to reduce throw so the airplane isn't too sensitive at full throttle. So when it comes time to land as speed drops he instinctivly keeps the speed up because he feels he is using a fair amout of backstick and does not want to run out of elevator. The next question is why not just flip the dual rate and have nore elevator throw? Good way to stall the stab and corkscrew into the ground.

cfircav8r

My Feedback: (1)

First of all when you scale size down weight does not scale down 1:1, it is a volume (factor of 4) and density change. Therefore a full scale plane at 2000 lbs would weigh closer to 125 lbs at 1/4 scale. It is true the amount of weight an airfoil can carry does not change at the same rate as weight and wing area due to fluid dynamics, however that does not change the way an airplane works. That has nothing to do with tail or nose heavy issues. A plane needs to land faster when nose heavy due to the increased load added by the tail down force required. This increases the speed at which a plane stalls, the same as adding weight. If trim is not changed when power is reduced it will drop the nose until the same airflow over the tail is re-established. This will happen at close to the same speed as before the power reduction (slightly less due to the reduced prop wash.) So if you don't change trim you will have to hold more elevator to keep the nose from dropping. No plane, model or otherwise is going to be so neutrally balanced as to not need some trim change, (elevator position change) when speed is changed.

speedracerntrixie

My Feedback: (29)

At this point all I can do is repeat what I have already said. Maybe this will help others who want to learn. Look at the video. It is me landing my 33% Laser on a friends driveway. No wind at all that day. I know it is difficult to see and you may have to pause but notice the slow landing speed and look closely at how little elevator deflection there is. Obviously this airplane was set up as per my methods.


http://www.youtube.com/watch?v=bpm4cwaFrAg

Bozarth

My Feedback: (15)

But in this case, the fulcrum point is not fixed. If it were, your argument would make sense. Teeter totters have a fixed fulcrum point, planes do not.

Kurt

speedracerntrixie

My Feedback: (29)

Other then the slight CG shift as fuel is being burned how is it not fixed? Suppose the tank is on the CG, how does the CG then change?

cfircav8r

My Feedback: (1)

That fulcrum is mostly fixed but the center of pressure moves depending on AOA. Your plane requires very little elevator due to the extreme pitch sensitivity from your CG being close to your COP. That does not change the need for a force on the tail to change AOA only a smaller force, and trim can still be used to maintain an ideal airspeed. That ideal airspeed is different for normal flight and approach to landing.

speedracerntrixie

My Feedback: (29)

At this point all I can hope is that the OP will try moving the CG back and report back to us weather it helped or not. All anyone here wants to do is repeat full scale theory and not listen to practical r/c experience.

Bozarth

My Feedback: (15)

Saying the CG does not move is different than saying it is fixed. Adding weight to the nose and making a plane more nose heavy drives the cg forward (i.e. it moves the location of the virtual fulcrum forward) - it does not force the nose down as it would with a fixed fulcrum ( i.e. teeter totter). You cannot ignore the resulting moments create by the aerodynamic forces.

Kurt

speedracerntrixie

My Feedback: (29)

Try adding 1/2 lb to thge nose of a .40 size sport plane, take off and relax on the sticks. I'm fairly sure it will drop it's nose and head for the ground.

cfircav8r, you agreed that my Laser requires less elevator during landing. Less trim equals less airspeed required to support the effectivness of trim

Bozarth

My Feedback: (15)

But where is the CG? I.E. Where is the fulcrum location required to now balance the model? It was forced to move forward - now the teeter totter is balanced. When one side of the teeter totter falls to the ground, the board isn't balanced around the fulcrum. You could either redistribute the weight or move the fulcrum. The fulcrum is thankfully not fixed on an airplane.

Kurt

cfircav8r

My Feedback: (1)

I don't think anyone said not to check the CG. You are the one saying not to use trim because models don't work like full scale. Wing loading, tail area, airfoil design and a few other fluid dynamic properties don't exactly translate to models, but aerodynamic principles do not change. Yes, check the CG and carefully adjust to suit your flying style, but make sure that you are properly flying it. CG does not solve every problem and can create more catastrophic problems than it will fix when not done properly. I understand the fact that many, including many full scale pilots, don't fully understand CG and how it relates to performance, and it can turn a mediocre plane into a real performer, but it is not the only answer just one piece of the puzzle.

Bozarth

My Feedback: (15)

cfircav8r - who are you referring to?

Kurt

speedracerntrixie

My Feedback: (29)

Agreed however I beleive that it will improve the landing issue that the OP has with this particular model. I also beleive that 90% of the models I see flying have thier CG set too far forward. I am simply attempting to convince people that the myth of a more forward CG is more stable is just that, a myth.

cfircav8r

My Feedback: (1)

Moving the CG back means you need less down force on the tail. There can come a point, just as with the wing, that the tail can run out of sufficient lift either through lack of airflow (speed) or stalling, if the CG is too far forward. At the point you have the CG you need very little down force at your landing speed, but you need even less at high speed. When it comes right down to it the percentage of travel for landing speed and high speed wont change, just the actual measurment of travel will change. So if at a more nose heavy setting you need 5% up for full speed and 25% up for approach speed, you will need that same percentage at a less nose heavy setting, just smaller travels. Trim is still very useful, but again you need to learn how to properly do it.