A Model Aircraft Design Case Study

To give you an idea of how I put my design philosophy into practice I will go through the steps I recommend to create a radio control model aircraft design.

The resulting model invariably differs from the original specification for a variety of reasons.  Small aircraft in particular have limited space for mounting everything.  The only way to know for certain that everything will fit as intended is to either draw a plan or build things larger to cope with unforeseen problems.

Additionally, something that seemed to work out ok in theory may not look right in practice.  A built model always looks larger than in a plan view.  I have a bad habit of drawing fins that are too small because at the "right" size they tend to look huge on paper.  As I'm building the model the fin starts to looks too small.  That may be a problem unique to me.  I have tossed out more than one fin and built one that is larger.

The point being you should be disciplined about creating a design specification and building towards it, but you should also be flexible to change things that aren't right.  If something looks wrong it very well may be.  Adding a few inches of area to a flying surface won't add appreciable weight.

Case Study 1

Background

My buddy Mike has an idea for a model that I think is very interesting.  He has been spending his time locked away in a secret location hashing out the details so I won't give anything away here.  This design is a cousin to his.

The airplane is intended to be very pure — sleek, semi-fast, very smooth and aerobatic but not touchy on the controls.

Specifications

• Aerodynamically pure and clean.
• Neutrally stable.
• Not compromised aerodynamically to improve aesthetics.
• Cruises efficiently at 50-70 MPH with a top speed of 90-110 MPH.*
• Strong to unlimited vertical performance.
• Crisp, smooth and predictable control response in all axis.
• Predictable stall characteristics.
• Minimal control coupling.  E.g. rudder does not cause pitch or roll.
• Controls and linkages designed and built to prevent flutter within airspeed envelope.
• Ailerons controlled by individual servos to reduce slop and allow flaperons.
• Flying elevators will be considered for feasibility.  Each elevator half may be controlled by its own servo to allow the incorporation of ailervators.
• Fast mini servos of adequate torque will be used throughout.
• Installed systems to be simple, accessible and reliable.
• Meets specifications powered by a strong sport .40 2-Stroke engine.  A racing engine or tuned exhaust system is not required, but the plane will have the best aerodynamics with a rear-exhaust piped engine.
• Capable of 10 minute flight times.
• Structure stressed to handle high speed maneuvering.
• Built utilizing conventional construction techniques and readily available materials.
• Wing panels remove for transportation.
• Landing gear is optional.  If used it should be retractable or extremely sleek.
• A color/trim scheme will be used that clearly differentiates the top from the bottom of the aircraft to prevent disorientation.

* Given speeds are what I believe them to be as I really don't have any idea how fast 100 MPH is.  I just know how fast I think it is and that's what the design should achieve.

Non Concerns

• Low speed flight
• Knife edge flight
• 3D aerobatics
• Crash resistance

Notes

This is simply a specification that I will strive to accomplish.  The end result will most likely differ somewhat for practicality.  For example, I probably don't want to hand-launch a .40 powered airplane.  A .40 can pull pretty hard and there will be a safety concern trying to hold the model while launching it into a desirable flight attitude.

Even though a landing gear may compromise the aerodynamic integrity of the design I may not have much choice about it.  I'll do the best I can.

Another possibility is that the aircraft will not fly as intended.  However that possibility is significantly reduced simply because I have clearly stated what I want.  I will consider everything I know about each parameter to help ensure I reach my goal successfully.

Note that the specification did not mention the size of anything except the engine.  Ultimately the specification must be met as closely as possible using the engine as a base point.

This is a conventional aircraft with no particularly radical features.  The model will be similar to pod and boom sailplanes but having a lower aspect ratio wing that can handle higher flight loads.

The aircraft will be aesthetically pleasing due to its aerodynamically clean appearance and simplicity.  All I care about is what the air sees.  The design will not be compromised just for looks.

The model will be primarily of wood construction, fully skinned, fiberglassed and painted.  The airframe can be built to its target weight with proper engineering, material selection and construction practices.

Target Weight

The vertical performance requirement dictates a target weight (dry, ready to fly) of no more than 4 lbs.

Wing Loading

I can build to any wing loading I want.  What I don't want is the model bouncing around in the air with a low wing loading or the sluggish roll rate of a high wing loading (assuming at least some of the weight is in the wing).

Therefore the wing loading will be in the range of 16 to 18 oz/ft².  I could choose a higher or lower wing loading and build to it.  This wing loading is a good compromise to achieve the most desired flight characteristics.

I never go over my target weight and often beat it by up to 20% because I stay focused on the purpose of the model and don't let myself get off track doing the "what if's".  For example, I don't start thinking about maybe putting a .46 engine in the model or building it to survive something it isn't intended to do.

If the model isn't intended to be yanked out of terminal velocity power dives, then don't build the wing to survive it.  You're giving away weight.  Of course it goes without saying that if you don't design the model to do something then you probably shouldn't turn around and attempt whatever that is with the model.

Remember that if you build the model too light you can always add ballast.

Design Specifics

The Airfoil

There is no question that a symmetrical airfoil is the best choice for this model.  The only question is how thick it should be.  I've had good success with the NACA 00XX airfoils and will probably select one in the 11% to 14% range.  Depending on how much I choose to taper the wing I may use a thicker tip airfoil than at the root to help prevent tip stalls.

I really don't think tip stalls will be a problem though so I will probably use one airfoil for the entire panel.  I'll look at the charts before making a final selection but my first thought is to use a NACA 0013.

The Wing

The wing will taper and have an aspect ratio in the range of 7 to 8:1.  It will be all wood construction with plenty of ribs, sitka spruce main spars, shear webs and a full skin.  A second pair of spars may be necessary for the aft portion of the wing.  The wing panels will slide onto tubes permanently glued into the pod fuselage.

Knowing the weight and wing area allows me to determine the wing area which in turn allows me to calculate the wing span based on the aspect ratio (A/R).  Figures in the table below are rounded.  The chords given below are the average chords.  Again, the target weight is 4 lbs (64 ounces).

The wing will be thinner than a typical aerobatic sport design in order to achieve higher airspeeds.  Narrow strip ailerons will be shaped as part of the airfoil rather than flat plates.  The ailerons will be sealed to prevent efficiency losses, drag and reduce the possibility of flutter.  The ailerons will be driven by individual servos that are buried in the wing to prevent drag.  Another flutter reduction measure will be ending the ailerons short of the wing tips.

The wing will probably be too thin to house retracts that are reliable so I might build this plane to be a hand launch/belly lander.  Other options are wire gear mounted in the wings or a dural gear mounted to the pod.

Fuselage Specifics

The fuselage will be pod and boom construction.  Formers can be turned using a drill or drill press.  The pod will be planked with balsa and can be built using the carbon fiber tube as a jig.

The engine will be mounted upright and fully cowled similar to that of control line stunt ships.  The pod must hold an 8 oz. fuel tank, three servos (four if dual elevator servos), receiver and battery pack.

The model will be as symmetrical as possible about all axis.  The thrust line, wing centerline and horizontal stabilizer centerline will be located along the centerline of the pod and boom.  A small degree of right thrust will be incorporated for trim purposes.

The boom will be a lightweight carbon fiber tube.  In this case I will have to find a tube that isn't too heavy but also isn't too flexible.  Off the top of my head I would say a thin wall 1/2" diameter tube should be close to the right size.  Another consideration is that the tube isn't weakened too much by the exits for the control system.

The model will have a generous tail moment with small, thin airfoiled flying surfaces.  The fin can not extend too far below the boom or it will be damaged on landing.  I can make the fin longer in chord and shorter in span to get the area I want while putting some of it below the boom.  This will allow me to balance the areas better than if it had a longer span and shorter chord.  Sort of like an arrow.

Using the "that looks about right" method, I'll start by designing a horizontal stabilizer having approximately 16% of the wing area.  From there I'll adjust it in proportion and size until it looks right.  The model may have a fully flying horizontal stabilizer depending on whether it can be practically implemented.

More to come...

Back to Radio Control Aircraft Design
Airfield Models Home

Copyright © 2005 Paul K. Johnson