Physics for true-to-life performace; a Tutorial

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dhk79

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We've had tutorials on modeling, texture mapping, importing to RF, and doing colorschemes. This one is going to focus on setting up the model's physics, such that it flies just like a real RC model would fly or "true-to-life". There are a few people who like to adjust the physics so that a biplane can do 400+ MPH. Well that is simply ludicrous, and we're not going to go there.

There is a further refinement known as "scale" performance, where the aircraft is supposed to fly exactly like a miniature version of the full sized aircraft. Some minor adjustments have to be made to achieve true "scale" performance. This is because while you can scale down the aircraft, you can't scale down the air in which the scaled plane flies. The airflow across the model's airfoils, therefore, will not separate and become delaminar at the same point along the surface. We may touch on some of these adjustments, but the primary focus will be to just get your model to correctly fly like a RC plane.

For those that would like to follow along as the steps in creating the physics model unfold, here is a freshly imported copy of the plane that we're going to work on. No changes have been made to the physics, this is how it shows up when first imported to RealFlight.
 

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dhk79

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As you can see in the above picture, nothing in the default wire frame lines up with the 3D model. At this point you can either edit what's there or get rid of it and start over. After getting burned a couple of times by missing a setting in the default model, I usually prefer to just start over.

Get started by deleting all aircraft frame components, so that only the Airframe and battery remain (the default model is an electric).
 

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dhk79

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Now open up the electronics section. Go down through the default servos and see what you want to keep.

I like to use two servos for the ailerons, as it gives better response and allows you to use differential throws (we'll discuss this later) and/or define the surfaces as flaperons. The normal servo speed for a standard servo is about .2 seconds for 60 degrees of rotation (for example a Futaba 3004 is .23 seconds), set all of the servos to an appropriate speed.

In this plane, we're not going to use a separate servo for the steering but it will have a mechanical retract servo. So rename the steering servo to a retract servo and change its speed to .58 (Futaba S136G).
 

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dhk79

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Lastly open the Software Radio section. We're going to use channels 1 & 5 for the ailerons. As a starting point for the ailerons change the channel settings for 1 & 5 to:
Low Rates - 50%,
Expo During Low Rates - 30%,
Expo During High Rates - 60%.
 

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dhk79

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Also under channels 1 & 5 there are some other factors that may need adjustment.
In the category Low Rates When, change the Activated When to "Conditional Channel below or equal to value #1". The Conditional Channel should be Channel 5 - Dual Rates. This sets your low rates to the forward position of the dual rate switch. Note: This is a personal preference, as I like to have my real radio setup with a default take-off configuration that has all switches in the forward position. One glance and I know my radio is configured properly for take-off.
Exponential when should be set to Activated Always.
No changes are needed to the input as it is set to Channel 1 - Roll and is active always.
 

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dhk79

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Output Channel 2 should be set up the exact same way as Channels 1 & 5, with the exception that Input should be Channel 2 - Pitch.

Output Channel 3 will be a little different. You probably do not want to have either dual rates or exponential throws on the throttle. The settings are therefore:
Low Rates - 100%
Expo During Low Rates - 0%
Expo During High Rates - 0%
Low Rates When - Always
Exponential When - Never
Input - Channel 3 - Throttle
Input When - Always
 

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dhk79

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Output Channel 4 will be very similar to channels 1, 2, & 5; with one exception. I normally like to have a little more rudder authority during low rates, so I set the Low Rates to 70%, but keep the same expo settings.

All the other settings are the same; except for the input, which is Channel 4 - Yaw.
 

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dhk79

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The last channel that needs to be defined is for the retracts. This is going to be Output Channel 6 and will be either up or down. Opening Channel 6, we see two inputs in the default physics model. It looks like it was set up for a conditional input using the controller's knob and top right switch. We're not going to use these inputs, so delete both of them.

Set the following:
Low Rates - 100%
Expo During Low Rates - 0%
Expo During High Rates - 0%
Low Rates When - Always
Exponential When - Never

Add a new simple input feed for the channel, with the following settings:
Input Channel - Channel 7 - Smoke (this is the top right hand switch)
Percent - 100%
Reverse - No
Logic - Add to other feeds
Input Feed When - Always
 

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dhk79

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The last thing that needs to be cleaned up is the settings under the Airframe. The CG adjustment needs to be zeroed out, and the Mass Scaling Factor set to 1.

Notes:
The CG adjustment should only be changed to over-ride RealFlight's calculated CG if you have an unusual distribution of weight. An example of this might be when you model a sail plane with a long tear-drop shaped fuselage. In this case, most of the fuselage's weight will be in the front and RF may not get the CG right.
The Mass Scaling Factor is an adjustment for very light or very big airplanes, this will be neither so set it at 1.
 

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dhk79

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Move one step up in the hierarchy to the RootFrame, if you are using the demo file I posted it will be called "Tutorial". Otherwise it'll be the name of your plane.

Change the Power Plant Type to "Unpowered" and make sure the Graphical Scale and Physics Scale are both 100% (these settings are where you would change the size of an aircraft for an AV. They change the size of the 3D model and the wire-frame respectively, but that is all they change).

Note: This will just be a temporary setting for use as we are defining the physics wire-frame for the fuselage.
 

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dhk79

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Now, we can get started on really setting up the wire-frame without having all of the trash from the default physics model.

Under the Airframe add a Single Fuselage. It'll initially show up as a long rectangle.

Change the Component Frame to RootFrame and the new fuselage will take on the approximate shape of your 3D fuselage.

Using the viewing window to zoom and rotate the plane around, adjust the Dimensions and Location in Parent to superimpose the wire-frame over the 3D model.

The last thing to do to this component at this point is to adjust the airfoils to be used for the fuselage. The thing to remember here is that fuselages are lifting bodies and they are may also be effective from the sides (i.e. knife edge).

Looking at the aircraft from the top, the profile of the fuselage is symmetrical and the widest bulge is just a little forward of center, that looks like a standard NACA airfoil. To tell which one to use, divide the width of the fuselage by its length (you should have just figured this out, I got 7.3 & 54.6) and you have a 13% airfoil. So pick the NACA 0013 for the sides.

Looking at the fuselage from the side, you can see that this one will not be as straight forward. The bulges from the canopy and airscoop are not symmetrical. Dividing the fuselage height by the length, we know that we need an 18% airfoil. So we start searching through RF's airfoils for an 18% airfoil where the top bulge is behind the lower bulge. I found a NACA 63-3-418 that looked about right and choose that.

After confirming that the CG location looks about right for the fuselage, change the "Unpowered" to "Internal Combustion Engine". A fuel tank should show up near the tail of the aircraft. Don't worry about where it is, we'll move it after we add an engine.
 

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dhk79

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Aeronautical Side Note:

The above selections of airfoils for the top/bottom and sides of the fuselage were eye-ball estimates and are good enough for most applications. If you are going for true scale type performance, however, you might want to actually do the math to get a more accurate selection of airfoils. The numbers shown in the bottom of RF's airfoil selection window are key. These two numbers (expressed as percentages) define in rough terms how the airfoil performs.

The way to calculate these numbers for any airfoil is to first measure the cord of the airfoil. The cord is a straight line from the leading edge to the trailing edge. The lower number on the airfoil selection window is the basic class of the airfoil and is the easiest to calculate. It is a ratio of the thickest section of the airfoil divided by its cord.

Camber is the numerical characteristic of the curves of the upper and lower surface of the airfoil, and is a little harder to calculate. It determines the amount of lift that an airfoil produces as air flows around it. For example, a high-speed, low-lift airfoil has very little camber; while a low-speed, high-lift airfoil has a very pronounced camber.

The camber at every point along the cord may be calculated by taking the upper camber and subtracting the lower camber, then dividing the result by the cord (see the diagram below). The largest result (from all points along the wing) will be the upper number shown in RF's airfoil selection screen. A spreadsheet like EXCEL will be a big help with the calculations.

Armed with these two numbers, you can go through the available RF airfoils and pick out a matching subset that have characteristics that are close to the ones you just calculated. Then out of this subset, pick the one that best matches the needed shape.
 

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dhk79

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Getting back to defining the wire-frame, we now want to add some wings. For a single fuselage aircraft, these can be added to either the Airframe or the Fuselage. For consistency, however, let's add some symmetrical wings to the fuselage.

To most closely match the wing shape, you might want to do this in multiple stages. Set values for the location, size, and don't forget the dihedral. Don't worry about the weight of the airfoil at this point.

I used three sections to define the wing. Be sure to name the parts and assign the 3D model's matching components to the wind sections. This makes the 3D model break properly when the physics model crashes into something.

A good catch from opjose is that the default Stall Severity for all airfoils is zero. This value make the stalls in G3 unrealistically gentle and makes certain aerobatic maneuvers nearly impossible. In playing around with the effect of this setting, we have determined that all airfoils should have at least 50% (even the most docile). Mid-lift airfoils that are commonly used in most sport models in the 70-80% range and low-lift airfoils found on 3D craft closer to 100%.

You may have to try a bit of trial and error to find the correct setting, because the more aggressive the wing - the greater effect the value seems to have. Look at the lift curve shown in the performance graph for each airfoil, The more abruptly the curve drops at the high end should be a guide.
 

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dhk79

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Now let's go back and define the airfoils. The note on airfoil design still applies and you can rely on that if you can't find any other data.

Most common planes, however, are pretty well documented and you can find a lot of the data on the Web.

Here are a couple of links that may help you select the proper airfoils for you plane.
The Incomplete Guide to Airfoil Usage
UIUC Airfoil Coordinates Database

For those of you who haven't recognised this plane yet, it's a British Supermarine Spitfire. Looking it up in the Guide for Airfoil Usage, we find that it has a NACA 2213 at the root and a NACA 2209.4 at the tip.

Starting with the NACA 2213 first, we quickly find that RF doesn't have this exact airfoil in it's database (now that's a big surprise :rolleyes: ). So we go to the second link (the airfoil coordinates database) to see what a NACA 2213 actually looks like. Just our luck, neither airfoil is in there either. So now we turn to our truest friend Google.

We find that in NACA Four-Digit Series. The first digit specifies the maximum camber in percentage of the chord, the second indicates the position of the maximum camber in tenths of chord, and the last two numbers provide the maximum thickness of the airfoil in percentage of chord. With these values, we go to http://www.pagendarm.de/trapp/programming/java/profiles/NACA4.html and get a sketch of each airfoil.

Going back into RF's airfoils, we start looking for two airfoils with a 2.0% camber, one 13% thick and one 9.4% thick, that look to be close to the images we just created.
 

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dhk79

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Searching through RF's entire airfoil database, I selected the RAF 34 for the root and the S3016-095-87 for the tip. These are not exact matches, but were the closest to what I was looking for. I set the transition between the two airfoils to occur in the mid wing section
 

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dhk79

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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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dhk79

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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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dhk79

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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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dhk79

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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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dhk79

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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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