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  #16  
Old 07-05-2007, 08:39 PM
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Now we move on to defining the tail feathers.

It is much harder to find design information about the airfoils used on the tail. If you can't find any data, or clear pictures to help guess, a Flat Balsa Slab can be used for the tail surfaces on most RC planes. If you are modeling a scale aircraft pick a symmetrical 8-10% airfoil as a default.
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Old 07-05-2007, 09:37 PM
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Go back and add control surfaces to the wings, horizontal stabilizer, and vertical stabilizer. If a control surface is split between two (or more) sections, add it to all sections that are applicable. Don't forget to assign servos and frame components. If you set up the radio & servos ahead of time (like this tutorial recommends), you'll be able to confirm that the motion of the control surfaces is correct and will find that the right aileron is moving in the wrong direction. Fix that by reversing the servo(s) throw for that aileron.
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Old 07-05-2007, 11:33 PM
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Let's give this bird some power. Judging by the wing-span of about 5 1/2 ft, this plane is a .60 sized plane. If you don't know how big an engine the plane will require, pick up an RC catalog and find a similar aircraft and look at its specifications.

A more exact method is to calculate the actual power requirements of the aircraft. Power is measured in Watts, so the calculation needed is Watts/pound. The general performance characteristics of a model can be predicted as follows:

< 50 Unable to take off
50-60 Take off & perform simple aerobatics
60-75 Loops from level flight
75-100 Aggressive climbs, Fighter-type performance
100-150 Extended vertical runs, Unlimited aerobatics
> 150 Missile-type performance, Wings are an option

Watts output is a easy value to find for electric motors, but can be more difficult to find for glow engines. Most glow engines, however, have a Horse Power rating and this may be converted to Watts. 1 HP is approximately 746 Watts.

For example, you get a warbird kit that the manufacture says should weight 6-7 pounds. Picking 80 Watts/lb. as a performance goal and 7 lbs as the target weight (you pick the high end of the weight range because you are going for a high performance level and that usually equates to a bigger engine), tells you that 640 Watts of power will be needed. Dividing that value by 746, and you need a engine that can develop .86 HP. So a .40 should do nicely.

Getting to our plane here; I like the sound and performance of four-stroke engines, so I'm going to use an OS .91 four-stroke.

Select ~CS_ENGINE1 as the component frame and a prop will spawn and the spinner start rotating.

Change Engine to Show to 2STROKE and the RC engine will appear. Note: I don't have a four-stroke engine modeled in this 3D model, so you can either live with a two-stroke glow head sticking out of the side or not have it show at all.

Move the engine to be over the center of mass of the modeled motor (just a little forward of the center of the cylinder).
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  #19  
Old 07-06-2007, 01:29 AM
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Aeronautical Side Note

Before we select a prop, I want to talk about airplanes and their props.

Selecting the right prop for a plane is probably the biggest factor in improving its performance. On the other hand, the wrong prop is best way to ensure you have a dog in the air.

All prop manufactures have a chart to select the correct sized props to match engines. Below is a copy of Top Flite's. Notice anything that is omitted from the chart? If you said the type of plane, you're right on the money because the chart only addresses the engines.

As an example, say we have two scale planes both with .90 engines. One plane is a scale bi-plane with full rigging and guy wires and the other plane is a low drag pattern aircraft with retracts. Even if by some wild stretch of luck these two planes weighed the same, you would not expect them to fly anywhere near alike. So why would you expect the same propeller to work for both airplanes? The answer is, of course, it won’t.

The way out of this dilemma is to use a ratio of diameter to pitch to help rate propellers to determine the type of aircraft it will be best suited on. For best aircraft performance use a prop that is in the power range for your engine, but match the prop ratio to the type of aircraft that the engine & prop is going on.

Draggy/3D - 3:1
Trainer - 2:1
Sport - 1.5:1
Racing - 1:1

Using the above ratios, let's put the same .40 engine on several different planes. The engine manufacture says you can use a 10x4 to a 11x6. Well a biplane would do well with something in the draggy range, so it would take a 10x4 (ratio of 2.5:1). Now a hot pattern plane needs a prop in the racing range, so it would get a 10x8 (1.25:1). Sitting in the middle, a trainer could use a 11x6 (1.8:1).
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  #20  
Old 07-06-2007, 01:52 AM
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Now that we've beat the subject of props to death, let's pick a prop for our Spit. I said that I put a OS .91 4-stroke on it, so the engine manufacture recommends an 11x11 to a 16x6.

This is for a fighter, so we want a prop ratio in the sport range - about 1.5 to 1. so let's compare some props from Top Flite's chart. TF doesn't have an 11x11, so the smallest we'll look at is a 12x6.
12x6 - 2:1
12x8 - 1.5:1
13x6 - 2.2:1
13x8 - 1.6:1
14x6 - 2.3:1
14x8 - 1.75:1
15x6 - 2.5:1
15x8 - 1.9:1
16x6 - 2.7:1

From this we see that the "best" prop would be the 12x8, but I'd rather have a little bit bigger prop so I'm going to pick the second best at 13x8.
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  #21  
Old 07-06-2007, 02:20 AM
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The last portion of the wire-frame is to add the landing gear and intake scoops. The scoops are symmetrical fuselage sections attached to the roots of the wings. By now, you shouldn't need my help to do this.

The wire-frame is now complete and the end result should look like the attached picture
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  #22  
Old 07-06-2007, 09:25 AM
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The next step is to adjust the weights of all the components. So far everything has the default weights that RF assigns.

The most accurate way is to do this as you are building a real model. weigh each piece and use those weights. When you don't have that luxury, use a ratio of the overall weight to adjust each component.

So far our model weighs 6.15 lbs. and the target weight (which you can get from a plane manufacture or estimate from similar sized planes) is between 8 & 9 lbs. I'll take the higher end, because I'm using a heavy four-stroke power plant. Picking 8.8 lbs. as the target, you get an adjustment ratio of 1.4:1.

Another method for coming up with a goal weight is to calculate the wing loading based on the type of aircraft. Wing loading for models is measured in oz/sq.ft. Common wing loading values are:

10 Gliders
15 Trainers
20 Sport planes
25 Fighters

Don't trust the wing loading calculations shown in RF. These calculations use all "wing" surfaces, including the tail feathers. The traditional method only uses the area of the main wings, so do the calculations yourself.

There are two ways to set the weights in RF, the first is to go through the editor and multiply all of the weights by 1.4 (except the engine). The second is to just go through the editor and look at all the weights and adjust them until they look right and you get to the target.

Which is the best, I don't know, but a 3.5oz fuselage doesn't make sense to me so I fiddled with the weights individually. I put in better estimates for the retracts and wheels, the wing (because I used three sections) seemed pretty close so I just added 1.5 oz for the servo in each side, the Horz Stab was heavy (because again I used two sections here), the Vert Stab on the other hand seemed a bit light for its size, the scoops were about twice as heavy as they should be for their size, and lastly I made the fuselage 24 oz (which is more reasonable than 3.5).

This gave me a corrected weight of a little under 8 lbs. , closer but still a bit under the target. At this point I looked at the CG (the green cross) for the first time and saw that it was farther aft than it should be.

Well the plane was still a little light and the CG needed adjustment, so I took care of it the same way I would in one of my real RC models. I added a hunk of lead...
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Last edited by dhk79; 07-10-2007 at 01:27 PM.
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  #23  
Old 07-06-2007, 10:12 AM
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Thanks for putting in the effort and time to make this tutorial. Eventually I hope to contribute with some of my own models and this will be a necessary tutorial for me.

Can we get this stickied possibly?
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  #24  
Old 07-06-2007, 11:03 AM
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Aeronautical Side Note

Let's talk about the initial placement of the CG for a minute. It's not as critical in the sim as you have the "little red button", but in real life it can make a big difference on your maiden flight.

If you are building an ARF or a kit, always start out with the manufacturer's recommended CG location. Most plans/designs show a CG range. For initial test flights set the CG somewhere in the forward half of that range (i.e. slightly “nose-heavy”). Easy enough, but how do you tell where it should be on that beautiful scratch built scale job that you just spent the last nine months putting together; and you don't have any plans with the CG nicely marked on them?

In this case a little wing theory is necessary.

A typical safe starting point for almost any plane is to have the CG at 25% of the Mean (average) Aerodynamic Wing Cord (MAC). Look at the attached diagram. The MAC is the line shown going across the various wing shapes. The initial CG is shown as being 25% of the total MAC back from the leading edge. A setting at this point should ensure that you at least get your bird back on the ground in one piece after its maiden flight.

Further adjustments are probable, but keep these limits in mind: The farthest back the CG usually gets on a typical trainer is 33%, flying wing and tailless models typically fly with the CG at 15%-20% of the MAC, and few sport or 3D models ever have the CG more than at 40% of the MAC.

As the CG moves aft, the plane becomes less stable in pitch, less sensitive to airspeed changes, and more maneuverable. On the other hand, if the plane is too tail-heavy it’ll become uncontrollable. As a plane gets close to being tail-heavy, the first sign is that the elevator gets touchy and feels as though the elevator trim is inconsistent. For semi-symmetrical/symmetrical wings, if it takes too much down-elevator to fly inverted, the model is likely to be nose-heavy. If it takes no down-elevator, or even climbs sometimes, it is definitely tail-heavy.
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  #25  
Old 07-06-2007, 11:16 AM
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OK, the plane is now ready for its first flight. But let's pause a moment and talk about what to look for on that first flight. It is not enough to say it flies like &#!^. You need to figure out why it does, so that it can be corrected.

Most of the problems common to many RC aircraft are actually capable of being corrected with minor adjustments (i.e. trim settings). Experienced pilot’s skills may often cover up many of these problems, but they will make it more difficult for a novice. It’s really better, however, for all skill levels to eliminate the problems. After all, even good pilots can look better with a plane that does exactly what it should do.

The kinds of problems that can be fixed:
1) Airplane drastically changes pitch trim with changes in throttle and airspeed.
2) Airplane does not settle into a predictable glide slope when throttle is reduced.
3) A tendency to veer off in one direction (usually left) when climbing or when full power is applied.
4) Poor aileron control response (especially at low airspeed) and directional trims that change with airspeed.

Since aerodynamics is a constant balancing act of several forces (lift, gravity, and engine thrust), there is often more than one potential cause for any particular problem. The real task is that you have to figure out what needs to be adjusted.

Over the next few test flights, I'm going to show you a systematic method for setting up and trimming a model plane. This method not only works in the sim but it will also work for you in real life.

At this point you may use the AV you've been working on, or fly along on the one I've been building for this tutorial.
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Last edited by dhk79; 07-06-2007 at 11:22 AM.
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  #26  
Old 07-06-2007, 11:51 AM
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On the first flight test, we'll focus on pitch adjustments.

Once the plane is trimmed for level cruise (at about 3/4 power), do a couple of simple tests.

Test 1: Smoothly advance the throttle to full. Without making elevator corrections, but still keeping the wings level, watch the climb that results.

Is the climb too shallow & fast? This might be fine for a sport plane, but for a trainer the climb should be solid with adequate airspeed.

Is the climb too steep? Watch to see if the climb is so steep that the airspeed is decayed.

Has aileron control become sloppy and it is difficult to promptly correct for wind effects? That is a sign that the airspeed has become too slow due to the steepness of the climb.

General Solution: To reduce the steepness of the climb and make the airplane less speed sensitive you can either move the CG aft and add down-elevator trim or add down-thrust to the engine. Now if the plane climbs too shallowly, you'd want to do the opposite.

To decide whether to change the down thrust or CG point, we'll go to the next test.

Test 2: Perform a low-throttle glide test. Set up a straight and level pass, parallel to the runway and roughly 100’ up. Trim for cruise power level flight and with your hand off of the elevator stick, quickly reduce power to one or two clicks above dead idle (final approach speed) just before the plane passes. Watch the glide slope that results, again keeping the wings level but making no elevator corrections.

Does the model settle into a nice glide, or does it come down like a rock, or is the glide slope so shallow that the plane wallows along at a near stall?
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  #27  
Old 07-06-2007, 12:00 PM
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Now that both pitch tests have been performed, take what you have observed to make changes to the plane's setup.

Look at the trim settings you had to dial into your controller. If the elevator had to be trimmed level or slightly down for level flight, the model has insufficient down thrust. Alternatively, if the elevator had to be trimmed up, the plane is nose-heavy.

If the model needs more down-thrust, at full power it will climb too steeply. It will also glide too steeply when the nose-up engine thrust is removed and the down-trim or nose–heaviness takes over.

It is also possible that the plane climbs too steeply under full power and glides OK or even a little steep if the model is nose-heavy. To tell if this is the case, again look to see if the elevator was trimmed up for level flight (even a little bit), showing that the plane was nose-heavy and that trim was needed to counteract it.
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Old 07-06-2007, 12:47 PM
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Putting the results of the first flight test all together, to determine which pitch adjustment to make:

The climb or glide is too steep and the elevator trim is up.

The trick to telling the difference between nose-heaviness and insufficient down-thrust in a model that climbs too steeply under full power is to look at the elevator trim. If the plane carries up-trim, move the CG back 1/4" and retrim the elevator for level cruise and do the tests again.

Once the climb and glide rates are acceptable, even though there may be a bit of down-trim, stop there.

If you have to move the CG back far enough that down-elevator trim becomes necessary for level cruise, move the CG forward the last step and start adding down-thrust.

The climb or glide is too steep and the elevator trim is near neutral.

If the plane had no noticeable up-trim to begin with, add down-thrust.

If at any point the model gets touchy in pitch, the plane is too tail-heavy, move the CG back to the last location where the elevator control was predictable. If the plane still needs a great deal of elevator trim to fly level, change the wing incidence. This may require some time in the shop for a real RC plane, but it's an easy thing to do with the simulator. That's one of the reason I like to model each plane I build for real in the sim, before I finish assembly. If it needs a lot of up-trim, shim the LE of the wing (high-wing). If it needs a lot of down-trim, shim the TE of the wing (also high-wing). Use small steps to avoid drastic or unpredictable consequences. In the sim, increase the incidence of the main wing 1/2 to 1 degree at a time and retest.

The climb or glide is too steep and the elevator trim is down.

The most likely cause of this is that the wing and/or stabilizer incidences are wrong and creating a strong nose-up tendency, which gets worse at high airspeed. This is a sign that the plane has excessive pitch stability and/or excess horsepower.

Trainers are intentionally stable but do not tolerate overpowering well. In this case the cure is not to have less power, but put the plane in “low gear” with a prop that limits the top airspeed. A larger-diameter, low-pitch prop or a three-bladed one of the same diameter and lower pitch will help limit the excess airspeed while harnessing the same power.

The climb or glide is too shallow and the elevator trim is down – even a little.

If the model climbs well (or even a bit shallow) at full throttle and then glides nicely (or a bit shallow), and you want the plane to change trim with airspeed more than it does. Look at the elevator trim to tell whether or not you should reduce the down-thrust or push the CG forward. Start by moving the CG forward to get rid of the down-trim, and then reduce the down-thrust. Move the CG forward, retrim for level cruise and retest. Continue making adjustments until the elevator trim is nearly level, then move on to adjusting the down-thrust.

The climb or glide is too shallow and the elevator trim is level, or even a little bit up.

If the elevator was not trimmed down, the CG is not the issue. Reduce the down-thrust.

Alternate test for Sport Planes:
Set up a hands-off level pass about 50’ off of the deck, suddenly pull the power to idle. For a second or two the plane will still be zipping along at cruise speed. The down-thrust will have been taken out of the equation and aerodynamic forces will predominate.

If the nose twitches up and the model slows into a glide, you have too much down-thrust.

If the nose abruptly drops a tiny bit and the plane instantly assumes a fast, nose-down glide, you need more down-thrust. This is because the trim has been fighting down against an engine induced climb.

If the down-thrust is correct, the plane will continue straight for a second or two and then gradually fade into the glide angle.

It is also possible to see the effect of incorrect down-thrust when power is applied. In cases where much more down-thrust is needed, the plane may abruptly nose-up when power is fire-walled for a go around.
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  #29  
Old 07-06-2007, 12:58 PM
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In flight testing my Spit, it seems that the CG and thrust were pretty much perfect. But I've been doing this for a very long time.

No adjustment to the CG, trim, or prop were necessary. The math and diligence in picking airfoils paid off...
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Old 07-06-2007, 01:12 PM
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Now that the plane has been trimmed for pitch and has a proper glide, we'll move on to looking at directional control.

One of the most common problems observed at the flying field is planes that take off and immediately veer to the left.

Test 3: Set the plane up so that it is flying straight away from you and headed either directly into the wind or directly downwind. A crosswind in this case will hide the turn you are looking for. Add full throttle and smoothly pull into a climb, at the same angle as a steep post-takeoff climb. In all likelihood the model will start to turn.

If the plane deviates to the left, more right-thrust is needed.

If the plane has too much right-thrust (fairly uncommon), it will deviate to the right.

If the right-thrust is close you may have to repeat the test a couple of times to see which direction it tends to.

Most model airplane pilots never really master accurate and independent control of the rudder, so right-thrust is a compromise to counteract an airspeed-dependant problem.

At low and partial throttle the effect of right-thrust is minimal. So we set the right-thrust to straighten out a full-power takeoff climb and accept the small, unwanted influence it has at cruise. So engine right-thrust corrects for the engine’s torque, which makes the plane turn left during climb.

It is best to adjust the right-thrust angle one degree at a time and repeat the process. Most planes will require 2°-3° of right-thrust, although a rare few need much more.

On the next flight, retrim for straight and level (probably just a click or two of rudder) and test again. Repeat until the plane climbs straight.
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