Case Study 1
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.
- 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
- Controls and
linkages designed and built to prevent flutter within
- Ailerons controlled by individual
servos to reduce slop and allow
- 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
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
- Wing panels remove for transportation.
- Landing gear is optional. If used it should be retractable or
- A color/trim scheme will be used that clearly differentiates the top
from the bottom of the aircraft to prevent
* 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.
- Low speed flight
- Knife edge flight
- 3D aerobatics
- Crash resistance
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
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.
The vertical performance requirement dictates a target weight (dry, ready
to fly) of no more than 4 lbs.
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).
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
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
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 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
Span (Chord) w/7:1 A/R
Span (Chord) w/8:1 A/R
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
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.
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
More to come...