Hello to all and thank you for your contributions.

I have a question regarding elevator trim on my Global RCM 40 (high wing trainer with flat bottom airfoil), fitted with an OS 46 FX.

The plane generally flies well and has been a good learning tool. However, I notice that the elevator trim appears to be throttle dependent. For example, suppose that I trim the plane for level flight at 2/3 throttle. If I increase to full throttle, the plane gains altitude. If I decrease to half or 1/3 throttle, the plane loses altitude. So,

Q1. Is this behavior normal for this type of plane? If so, why? Is it because the amount of lift generated by the wing is changing with the change in air speed? Does this happen with planes with semi or fully semetrical airfoils? Would a 4*60 do this (I am considering this for my second plane)?

Q2. If the answer to Q1 is yes, then do others who have a similar situation with their plane reset the elevator trim when they change throttle settings, at least when they are going to fly with a particular throttle level for a long time?

Q3. Do others re-trim for landing? Right now I compensate with up elevator as I am approaching for landing at low throttle.

Regards,

Travis

BlueMax I have an RCM trainer and it did just as you explained the first time I flew it. Mine was a combination of things. First: needed to change the thrust just a hair downward (small washers under the back two bolts of the engine). Second: I needed up incidence in the elevator (added washer on the back bolt of the elevator). This kept my plane level at all throttle positions. There is a chart somewhere on the internet for flight trimming a plane. The exact location escapes me now, but you may do a search on google or someone here may be kind enough to post it. It tells you what manuvers to do and what to look for to tell if your plane needs change in thrust, incidence and so on. If you need further help, feel free to pm me and I'll be glad to help in any way I can.

When I say all throttle positions I left out dead slow. Your plane is going to start sinking when it slows to a certain speed reguardless of what I stated above. I'm talking about level flight at full throttle, half throttle, and low throttle (til you lose enough air speed). Keep in mind it will never be exactly perfect as your CG changes somewhat as you use fuel. Did I start to ramble again? Sorry I like the plane and get to running the mouth sometimes.

Just my two pennies

Greetings and welcome to the best hobby on earth BlueMax3: I believe the site trailing edge is referring to is rcfaq.com once in that web site scroll down the left side of the page to flying faq and click on it. Then scroll down the right side of the page to trimming your plane. You'll be glad you did I know I was, my planes are different animals now, though still not tame, unless someone else flies them. Take the time to explore the whole site. There's lots-o-good stuff. Always glad to help if I can. Garry

www.nsrca.org
www.geistware.com/rcmodeling
these two sites will help

Q1. Yes, all aircraft whether full size jumbos or tiny models do it. An engine is what adds the energy to raise a mass, lack of it allows a mass to come down. If you want to climb, increase power, if you want to descend reduce power. The engine is your up/down control, the elevator is your attitude control and through that it controls your speed. There are 2 reasons for the climb though. The first one you hit on yourself, as the speed tries to change, the lift from the wing changes, so the plane goes up or down. The second reason is pitching caused by the tailplane and this is responsible for the very rapid response to throttle by trainers and the more gradual response by aerobatic planes. It is not affected by the wing section, it is affected by the force, if any, produced by the tailplane to keep the plane level at that speed, and that is heavily dependent upon the balance of the plane. If the plane is nose heavy, the tail produces a down force to balance that. As the throttle is opened the airflow over the tail speeds up and so the tail's downforce increases, pitching the plane up. We tend to fit flat bottomed wings to nose heavy trainers, and symmetrical wings to aft balanced aerobatic models. People can't physically see the balance, they see the wing section and mistakenly think that is why the trainer pitches up. But it would pitch up just the same if you fitted a symmetrical section, because it is the balance that causes it. Ignore any aerobatic "whizz" who claims his model's throttle controls speed and does not make it climb or descend. Remember that models fly in a straight line for such a short time that a slow reacting model will not have had much time to show its reactions before the pilot lifts the nose up again in a turn. If anyone tells you his aerobatic model does not climb/descend on altering power ask him to close the throttle and carry on flying.

Q2. Yes, re-trim if you are going to fly at a new throttle setting for some period of time and don't want it to climb or descend. Remember that altering the elevator or its trim is the speed control.

Q3. No. What you are doing is generally correct, though there is nothing wrong with re-trimming if you want to. In full size we re-trim constantly but there the stick forces are much bigger, and a landing approach can take a lot longer. A model's landing sequence is a very short time and the stick forces are tiny. The disadvantage to not re-trimming is that for beginners it can be hard to find the correct neutral again if you have had to move the stick, but it is a skill that you should learn so persevere. The disadvantage to re-trimming is that it will be nose up trim and if you open up the power to go around the model will pitch up violently, possibly resulting in a stall.

Harry

Thanks for all your help. You've given me a lot to study and think about.

Trailingedge: I believe you posted about modifying your landing gear. The wire landing gear on mine were so springy that it was extremely difficult to land without bouncing. I also had the sag problem. Last week I replaced the wire gear with a single piece (plastic or fiberglass) main gear that works fantastic. Yesterday I had at least 10 perfect landings with no bounce that I must attribute to the new landing gear. What a difference!

Harry: In my plane the fuel tank is forward against the firewall and I balance with an empty tank. So during flight, my plane is certainly nose heavy. Based on your comments, if I trim for initial level flight at say 1/2 throttle, my plane should have a tendency to climb more near the end of my flight than at the beginning, as the balance point moves back. Does this sound right? Thanks for your sage advice!

Travis

Yes, and it applies to almost every model. However the difference is tiny and not something to bother about in most cases. As it loses weight it will also contribute to a tendency to climb since lift now exceeds weight. Competition aerobatic fliers who use pumped engines will site the tank at the CG so that there is no change in trim caused by weight distribution through the flight. As the model loses weight it will start to climb, but the difference is so extra tiny that even competition fliers can ignore it.

Full size airliners are flown at the most economic attitudes, i.e a particular AoA in the cruise. As the tons of fuel burn off, lift now exceeds weight. If the pilot does nothing, the plane will slowly gain height, this is known as cruise climbing. If he has to stay at that flight level he can either lower the nose to reduce AoA, or reduce power to stop the climb. But he can't lower the nose for economic reasons, as the plane would go faster but be at a less efficient AoA and he must keep it at the best AoA. So he is forced to reduce power and by the end of the flight the result of reducing power and holding up the nose to maintain best AoA can mean the plane is flying quite a bit slower than it was at the start of the cruise. Flying is fascinating dynamics!

Harry

A flat bottm wing will be effected more by speed changes than semi-symetrical or full-symetrical. The faster you go the more lift is generated. An OS .46 is a lot of power (not a bad thing) and will give higher speeds than needed for a trainer. You will find more aerobatic planes with more symetrical wings will generally not be effected this way. If you are going fast with a trainer, maybe it's time to move up in planes! The RCM Trainer is a good plane, training someone with one now. Keep the speed down some and enjoy it. Good luck.

The value for lift is found from the formula 1/2.rho.v^2.A.Cl. For any 2 aircraft, one with a flat bottom wing and the other with a symmetrical wing, in the same place, the 1/2 and rho terms are fixed so don't come into play when looking at differences. The wing area is fixed so the A term is of no interest. Co-efficient of lift is different for each plane since they are using different sectiosn at different AoAs, but the value for Cl is fixed at whatever AoA is being used and that is determined by the tailplane/elevator which we will leave unchanged. The only variable then is the velocity^2 term. If both start at the same velocity and increase to some new, same velocity, then the increase in the V^2 term is exactly the same so the increase in the lift generated is the same.

Wings with camber, i.e flat bottom wings produce a nose down pitching moment related to speed. The faster they go the greater is the nose down pitching force. Symmetrical sections do not produce a nose down pitching moment. If everything else was equal between two planes except for the wing section, the flat bottomed one would actually pitch up less with a speed increase because it's wing is producing an increasing nose down force.

You must be rigorous in differentiating lifting from pitching. Lift is determined purely by the formula given above. Pitching, which affects climb and descent, is determined by many other factors not related to the amount of lift or to the wing section being used.

Harry

If I remember my physics correctly, lift is generated when air traveling over a wing (not symetrical) has a longer distance to travel over the top then the bottom thus creating a low pressure area above the wing. With this being the case, wouldn't the wing traveling faster create more of a low pressure area giving more lift thus causing the plane to climb (not pitch up)? Now this is with the understanding that all incidences and thrust angles are at 0. Every trainer (flat bottom wing) I have seen has some major down thrust in the firewall. Also, since a fully symetrical wing with the top and bottom distances from LE to TE being the same and the lifting forces cancled out be less likely to pitch? If a symetrical wing were more pitch sensative in relationship to speed, why are they the standard on aerobatic aircraft with a wide range of speeds? I am no physics major and may be way out in left field here.

You are probably remembering the lesson perfectly, but sadly that lesson is wrong, even though it is repeated ad nauseum in some books and magazines. How can the symmetrical section that you refer to generate any lift then? Or flat plate wings? Or flat bottom wings flying inverted? Lift has nothing to do with the lengths of the top and bottom of the wing. NASA is so tired of it and some other very widespread stories about lift that they have web pages explaining why these are wrong. Take a look at http://www.grc.nasa.gov/WWW/K-12/airplane/short.html and under the heading LIFT look at the link for "incorrect theory of lift #1 - equal transit". While you are there also take a look at the explanation of why the cherished half venturi/Bernoulli theory is wrong, under "incorrect theory #3".

Trainers pitch up when power is applied because they are nose heavy, it is nothing to do with the wing section. The pitch up is related to the balance point's static margin ahead of the Neutral Point, and none of the equations for that calculation include the wing's section or co-efficient of lift. To stop the pitch up you can move the balance back, which trainers don't want, or you can use downthrust. That's why trainers have downthrust. One advantage of a nose heavy plane pitching up with an increase in speed is that if the beginner gets into trouble like a dive, leave the model alone and it will pull up out of the dive. That pitching up is the beginner's friend. It's only a problem with some models whose pitch up on power is too violent, and that is what the downthrust is there to reduce. The worst case I know of for pitching up is the Precedent Hi-Boy, which is about 80% symmetrical, whereas the Thunder Tiger Trainer Mk1 barely pitched up at all despite having a flat bottomed wing.

You are falling into the trap I mentioned in an earlier post. Trainers pitch up more than aerobatic planes. You can see a different wing section so you are ascribing the pitch up to the wing section. But that is false connection, there are many other factors which you can not see, and as it happens it is one of them that causes the pitch up.

Flat bottomed wings do not produce more lift than symmetrical wings. If a plane weighs 10lbs and is in unaccelerated flight, the wing is producing 10lbs of lift, whether it is undercambered, flat bottom, flat plate, symmetrical, or a flat bottomed section flying inverted. Sections have different co-efficients of lift which affects stall speed, but once in flight the wing produces lift to the weight of the plane whatever section it is.

Harry