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Background: Two things affect how long your plane will stay in the air; gravity pulls it down all the time and the energy released from the rubber motor makes it go up. To go up, the motor and propeller must make the airplane go forward through the air so the wing can produce lift. The amount of lift produced by the wings depends on the angle at which the wing strikes the air. That angle is determined by the location of the center of gravity along the wing chord. Setting the wing position to get the right center of gravity location is an important part of trimming your airplane for best flight time. We find that location by trial.
Directions: You will study how the wing location will affect how high a plane will fly and its time aloft. Work with your partner or group and choose one plane to study. Adjust your wing position to establish a reasonable center of gravity (CG) for good flights. One way to do that is to see what has worked for others. Another way is to glide the plane, moving the wing until you get a slow, steady descent. Mark the wing leading edge position on the stick with a pen. Mark four more positions, 1/8″ and 1/4″ ahead of and behind that first position. Use a 12″ loop of 3/32″ rubber for your motor. Lubricate and stretch wind it, per good practice. Make five flights with 1,000 turns wound into the motor for each wing position. Record the flight times with a stopwatch. Estimate the maximum height of the airplane. You can estimate the number of feet altitude or you can estimate height as a percent of height to the ceiling if you are flying indoors. Show which method you used; record feet like this: 18′ and record % like this: 45%. Also catch the plane right as it lands and unwind the motor, counting the turns remaining on the motor.
1. Start by trimming the plane to make smooth circling flights, with the wing in the middle position and with the largest circle you can safely fly. Use the rudder to set the circle size. If the plane zooms up and tumbles, try banking the plane a bit at launch, with the inside wing lower than the outside wing. Practice until you can get consistent flights.
2. List three things to keep the same every time you fly the plane for these tests.
a.
b.
c.
3. Set the wing with its leading edge at the most forward of the five marked points on the stick. Use a winder to put 1,000 turns into your motor and fly the plane. Make five flights and record the time of flight, estimated maximum height reached by the Alpha and note the number of turns remaining on the motor. Note anything significant about how the plane flies.
4. Set the wing with its leading edge at the second most forward of the five marked points on the stick. Use a winder to put 1,000 turns into your motor and fly the plane. Make five flights and record the time of flight, estimated maximum height reached by the Alpha and note the number of turns remaining on the motor. Note anything significant about how the plane flies.
5. Set the wing with its leading edge at the third most forward of the five marked points on the stick. Use a winder to put 1,000 turns into your motor and fly the plane. Make five flights and record the time of flight, estimated maximum height reached by the Alpha and note the number of turns remaining on the motor. Note anything significant about how the plane flies.
6. Set the wing with its leading edge at the fourth most forward of the five marked points on the stick. Use a winder to put 1,000 turns into your motor and fly the plane. Make five flights and record the time of flight, estimated maximum height reached by the Alpha and note the number of turns remaining on the motor. Note anything significant about how the plane flies.
7. Set the wing with its leading edge at the fifth most forward of the five marked points on the stick. Use a winder to put 1,000 turns into your motor and fly the plane. Make five flights and record the time of flight, estimated maximum height reached by the Alpha and note the number of turns remaining on the motor. Note anything significant about how the plane flies.
8. Average the times recorded for each of the five wing positions. Average the estimated heights recorded for each of the five wing positions. Average the number of remaining turns recorded for each of the five wing positions. To average, add the five numbers together and divide the sum by five.
9. Did one wing position produce the highest average flight time?
10. If the wing position with the highest average time was the first or last in the series, move the wing an additional 1/8″ beyond that point and make another five flights. Continue until the position with the highest average time has at least one position on each side of it with a lower average time.
11. How did changing the wing position affect the average time?
12. How did changing the wing position affect the height?
13. How did changing the wing position affect the turns remaining? What is the significance of those remaining turns?
14. Which wing position would you use in a contest for best time? Do you think it would be worthwhile to test other wing positions?
Hi Gary
I am confused about trimming and CG. Approach 1: shifting the wing to change CG as you mention and as Darcy also mentions for Squirrel. Approach 2: keep the wing at a fixed location (where?) and use ballast to change CG by minor distance to achieve best glide; shift wing if CG needs to be shifted too much.
In approach 1 which you mention, what I can understand is that all parameters are changing all at once- CG, NP, Tail volume, Lift moment, Static margin.
In approach 2, NP and Tail Volume remain constant; Lift moment,CG and Static Margin change.
Now an airplane also needs to have a minimum static margin depending on its decalage and flight characteristic. For small decalage and fast-steep climb with 1/8″ motor at high power, like the Squirrel with 1.5mm shim at LE, I need a relatively aft CG which also has a lower static margin that allows to climb steeply and transition to glide. If FOR THE SAME MODEL, I decide to fly it on low power with 3/32″ motor so that it climbs slowly, I increase decalage by making the shim 3mm and a relatively forward CG which also increases the static margin. So the flight trajectory, motor and stability of both models will be different. Now my question is, for changing my 1.5mm shim model to 3mm shim model as described, what is a good approach: keeping the wing at original position and using ballast to change CG, or shifting the wing? If I start shifting the wing for trimming every model, don’t I run the risk of changing the stability and even having an unstable or less stable airplane? Which approach is better?
I can see that if I keep the wing with 3mm shim at the original position for 1.5mm shim to keep the NP and Tv same, it might require a lot of ballast at nose to move the CG forward. So after a point, it might be better to shift the wing back so as to not need too much ballast; but as I said shifting changes the NP and Tv. I guess stabilizer size is designed for a particular Tv, so doesn’t shifting the wing disturb this original design consideration? Because if the same stabilizer size is working for various positions of wing, it means that despite change/reduction in Tv the airplane is still flying, and so probably the stabilizer was originally made too large and could have been made smaller? So again, which approach should be followed and why?
Also, what static margin (%) is good enough? And if shifting the wing doesn’t have any downsides, then why are tail volume, tail arm, tail area, static margin etc of any importance?
I used this excel file for getting the NP value of Squirrel:
http://rcaerobase.ipjdev.co.uk/index.php/in-flight/in-flightflight-powered/44-neutral-point-and-static-margin-stability-in-the-pitch-or-lateral-axis
With my glide tests for 1.5mm shim Squirrel, I got a good slow glide at 46mm (78%) cord. However, as per sheet, that CG location is behind NP and it is unstable! (I did not put area of the 2 winglets in wing area). So is it possible to get a good glide even for an unstable airplane? Or should I conclude from my glide tests that the NP calculation is wrong and NP would DEFINITELY be behind CG?
I hope I could articulate properly. I am all mixed up! 😛 🙂
Thanks!
Ashutosh,
I like your style, but you are way overthinking this. The planes discussed on EndlessLift were not designed with a formal engineering process, they were designed with a TLAR principle; That Looks About Right. The sense of what is right is honed by years of observing what has worked. Even Charlie Grants “scientific” design formulas are rule of thumb. What worked was determined by many trials.
Adjusting CG is made necessary because the balsa wood in our planes can vary quite a bit, causing the relative position of the CG to vary quite a bit. Getting the plane to perform as we want requires having the CG in the right position on the wing chord. The wing shouldn’t have to be moved much to get the right CG and our TLAR design has considerable latitude built in. You will have adequate stability over quite a range of wing positions. Yes, this may sometimes mean more stability than necessary, meaning slightly more drag than necessary. For competition designs, eliminating that drag might be important and there are ways of doing that. For our designs, that minimal amount of drag is not important. The plane will fly fine and that is all that is required.
The AMA Alpha is intended to be flown for duration in a gradual climb, not for high power climbs.
The difficulty with a formal engineering design process is that we do not have the numbers that are required. We do not have aerodynamic coefficients for wings in our scale. The performance of an airplane depends on the aerodynamic characteristics of the entire airplane, not just the wing. Many design formulas make simplifying assumptions that ignore important effects. For example, CG formulas often ignore the important effect of down wash from the wing to the tail plane, which significantly affects the balance of the lift. The NP formula is based on assumptions about aerodynamic coefficients that may not be valid for our little airplanes.
There are important differences between glide conditions and powered flight. When you add the typical right handed tractor propeller, you have motor torque producing a left roll. The left roll produces a left bank which produces a left side slip, a left turn and less lift. The plane will now require more speed and power. The circular airflow will alter the longitudinal balance. The prop slipstream will alter the aerodynamic characteristics and the down wash. The thrust line will most likely not pass through the center of gravity or the center of resistance, producing moments that will force changes to the attack angle, further altering the aerodynamic characteristics and balance. We don’t have the numbers for a formal evaluation of any of this. We are forced to rely on flight testing to trim our models. We trim for glide because the rubber model may glide down when the motor runs out. That is a good place to start, but more needs to be done for powered flight.
We may not be able to use formal engineering methods in quantitative analysis, but understanding the underlying principles can provide us qualitative guidelines in flight testing. We are told that when flight testing to change just one thing at a time. We decide what to change based on our informed judgement of which thing will have the strongest effect on the thing we want to change. As you indicate, changing one thing will necessarily change others. One change will probably require others to mitigate any undesirable side effects of the first change.
Trim must be different for different performance requirements. For greatest distance, the aircraft should be flown at maximum lift to drag ratio, L/D. For greatest duration, the plane must be flown at minimum sink speed for minimum power required. For greatest altitude, the plane must be trimmed for zero lift and minimum drag in the climb, but slow sink in the glide. Each of these will require different angles between propeller thrust line, wing chord, and tail plane chord and CG location. Each will have different test criteria. Each of these has different stability requirements. The high power model must meet one set of stability conditions under power and another in the glide. Fortunately the critical lateral stability requirements in the fast climb and the longitudinal stability requirements in glide are somewhat independent of each other. The longitudinal requirements are set for the glide phase. Your power wing may appear to have inadequate stability margin, but you are flying with a significantly different set of assumptions than used in the calculation. The fast climb will likely be optimized with much less decalage than the duration model. The proper test is in the flying.
I generally avoid adding ballast. It increases the sinking speed. It detracts from distance, duration and altitude performance. I would move the wing. If stability can not be achieved in the power model, consider increasing the tail area or decalage. Most model trim assumptions are for minimum sink horizontal duration flight. Your power model will likely have less decalage than assumed in these formulas.
Model airplanes teach us an important engineering principle. We often find conflicting requirements that prevent us from optimizing each parameter and we must seek an optimum balance.