The Problem

When I design I always start with the problem. When you have fully understood the problem then the best solution will present itself.

In this case the brief was narrow and focused with little compromise.

Design an electric air racer that can take off and complete 5 laps of a 5km course in the fastest possible time and still have 30 percent battery in reserve at the end for the landing pattern and go-arounds.

The Solution

I saw this design exercise as foremost one of drag reduction. 

Also paying particular attention to the specific mission this aircraft would have to fly.

Over half of the course is spent with the aircraft turning (and pulling significant G), thereby generating a lot of induced drag.

To combat the increased induced drag I went for the highest aspect ratio wing possible (practically) within the rules (a minimum wing area was one of the rules).

Due to the high average speeds of the course the next most important drag to reduce was form drag. By using a highly laminar-flow wing and fuselage, with any external structures faired and reduced in size, the form drag is kept to a minimum.

The fuselage has extensive laminar flow due to the absence of the normal accelerated (and turbulent) prop flow that a tractor configuration suffers from. 

Features and Innovations

• Taking the ‘best in practise’ for drag reduction from the glider world

• Laminar flow fuselage of minimum frontal dimensions

• Twin counter rotating pusher props to eliminate torque and other prop effects

• Props airflow outside of airframe so does not contribute to accelerated form drag

• High aspect ratio wing with maximum laminar flow for minimum induced and form drag

• Full span flaperons for reduced take off and landing speeds and distances as well as reduced wing lift at high speed

• Fixed twin in line main gear with retractable ‘outrigger’ tail wheels to minimise drag

• Cooling drag minimised by utilising both air and liquid cooling

• Superior visibility and G tolerance for pilot

Design overview

The MA Maniac is of conventional configuration with a shoulder mounted high aspect ratio wing and T-tail type empennage.

The placement of the motors is unconventional, being out on pylons above the wings. Stacked in-runner motors would be used to make up the 75kW on each side while still keeping frontal area to a minimum.

The landing gear configuration is also unconventional with twin in line main gear (to meet another of the rule requirements) and 3 tail wheels. 1 fixed and 2 as retracting ‘outriggers’ which only serve to keep the wings level on take off and landing.

Design detail

Mass calculation:

Based on existing glider architecture (minus the reduced wing span and removing the airbrakes) it is estimated that the empty weight would be 300kg.

Basic Airframe weight: 218kg

Battery: 12 kWhr, 12 x 3.33 = 40kg

Motors & Props: 32kg

Motor Pylons: 4kg

2nd main wheel and retracting tail wheel outriggers: 6kg

Total empty airframe weight: 300kg

Adding the min pilot load of 80kg makes for a Take Off and Landing weight of 380kg.

The rules required a minimum of 307kg TOLW so I was 73kg OVER the minimum with my design.

Aerodynamic drag: 

I used existing CD figures from within XPlane for sailplanes to give the most realistic model of drag.

Drag Calculation:

Drag (D) = THP x 550 divided by VMax = 170.85x550 divided by 543.475 = 168.82

= 168.82 D (lbs of drag at Max Level Speed)

Coefficient of Drag (Cd) = D divided by Roh x Wing area x VMax squared = 168.82 divided by .00119 x 66 x 295,365 = 0.0072

= 0.0072 Cd (Coefficient of Drag)

Coefficient of Lift (Cl) = W divided by Roh x Wing area x VMax squared = 837.7 divided by .00119 x 66 x 295,365 = 0.036

= 0.036 Cl (Coefficient of lift)

Drag Coefficent with regards to aspect ratio of wing (Cd1) = Cl squared divided by Pi x 9 (AR) x 0.87 (Taper of wing) = 0.0013 divided by 3.14 x 9 x 0.87 = 0.000052

Parasite Drag Coefficient (Cdpar) = Cd - Cd1 = 0.0072 - 0.000052 = 0.0071

= 0.0071 Cdpar (Total parasite drag - estimate) 

This drag figure is just slightly more than half that of a standard Cassutt racer (with 0-200 powerplant) which explains why the MA Maniac is so fast.

Aerodynamics:

Wing - The wing is of a high aspect ratio to reduce the induced drag to a minimum. The aerofoil sections used are highly laminar to provide the least drag at high speed. The tip design is highly tapered and swept with Winglets to reduce the induced and vortex drag to minimums. Full span flaperons are used to enable reasonably slow take off and landing speeds whilst not compromising on the wing profile at high speeds.

Horz Stab - Trim drag can be reduced to the absolute minimum value as we have a fixed pilot weight and carefully controlled C of G. Therefore the horz stab can be reduced in size to a minimum.

Vert Stab - Standard size and rudder throw. We must allow a safety margin in the event of the loss of one of the propulsors to be able to still achieve adequate yaw control, therefore the rudder and fin are of a conservative area.

Fuse - The fuselage is of minimum frontal area and has an unusually low drag due to the lack of turbulent flow from a typical tractor configuration (which typically adds an additional 7% to the total drag). Therefore laminar flow back to just forward of the wing junction is possible. The other benefit of this layout is the superior visibility afforded to the pilot over a typical central seating position in most racing aircraft. The reclined seating position also offers superior G tolerance over a conventional more upright seating position. The fuselage also features 2 crash structures for rollover protection. The first behind the pilots head and the second at the front of the cockpit, above the instrument panel.

Flight envelope:

A relatively short take off is desirable and we already have an excellent power to weight ratio so acceleration is fairly brisk. A short take off is achievable with the full span flaperons providing a large increase in lift. Climb is not important for this mission as we only need to achieve level flight between 9 and 75 metres (another rule requirement of the race). The transition from take off to high speed flight is a relatively smooth one with no change in pitch from the outrigger gear retraction and only a small change in pitch for each step of flaperon retraction. A little forward trimming is required after that as the speed builds. The aircraft is very stable once up to speed. Landing is straightforward with the flaperons providing a low stalling speed and the fixed pitch props serve well as ‘airbrakes’ at idle (stopped) to control the decent.

Loadings:

+10 to -4 G from VA to VNE.

Strength: 

All carbon composite construction, utilising the best in practise for high speed structures to maximise strength whilst minimising weight. 

Stability: 

The MA Maniac exhibits very stable flight characteristics throughout it’s flight envelope and is straightforward to fly. It retains a positive static margin at low speeds.

Control: 

Some technique is required for the initial take off run, with the stick held hard back to keep the tail on the ground as the power is increased. There is a slight forward pitching moment to account for with the props set up high on the pylons but as the elevator is partially in the accelerated flow behind the props then control is maintained throughout. Once airborne and the aircraft is configured for high speed flight it becomes very stable and is quite straightforward to fly the course. Landing is straight forward with good speed control from the flaperons, a touchdown on the mains is preferred before allowing the tail to lower after the speed has dropped.

Performance: 

Vh: 321 knots

Vne: 340 knots

Va: 240 knots

Vfe: 110 knots

Vlo: 100 knots

Vso: 55 knots

Best 5 lap course time - 2mins 46.036 seconds

Competition controversy

The judges rejected my submission (and several others) because they did not find my document to be up to their expected standard.

My time was 1.6 secs faster than the official 'winner' Iontrepid.

and 12.8 secs faster than 2nd place 'winner' Sparrowhawk.

and 21 secs faster than 3rd place 'winner' AFormX.

The rules were not explicit in what was required with the supporting document.

The competition was sold as a speed contest and not a document contest. "The winner will be the design with the shortest time around a course in a simulator."

This is very sad that this has happened, but my design is the fastest (despite being 54kg heavier than the ‘winner’) so my design decisions have been justified as the right ones.

It will be interesting to see what actual designs are winning this real world series in a few years time.

UPDATE

To prove the design is viable I made a scale RC model and test flew it several times successfully.

It is fast!

Surprisingly easy to fly, although it still has a high landing speed despite full span flaps. This wouldn’t be a problem with a better pilot as I am not that experienced with flying RC aircraft.

Video of 3rd flight here: https://youtu.be/lwzF4bfJfio?si=iSfLo0DDU4NRGxzf