UST008

Dossier

Insitu N20 UAV 50 cc two-stroke single

the tank once a given pressure has

been obtained. This system has the

effect of warming the fuel, which helps

overcome the problem of waxing when

using kerosene-based fuel at very low

temperatures in flight.

Fuel delivery pressure is a function of

the regulated pressure created by the

delivery pump and the fuel regulator,

which can be anything up to 6.5 bar,

which is referenced (or related to) the

pressure of the compressed air supply,

which is normally in the region of 4 bar.

The compressed air is supplied by

a crankshaft-driven air compressor,

which uses a piston pump. Matching the

reciprocation of the combustion piston, the

air compressor piston supplies a given

amount of air at any given engine speed.

The increase in air supply with

increasing rpm is not a linear

relationship, given the design of the

porting that transfers air from the

crankcase into the piston pump working

chamber. Moreover, the air supply system

has an accumulator effect, such that the

compressed air supply is at constant

pressure rather than pulsed.

On the ignition side, while the N20 has

provision for a second spark plug, that is

not the initial offering – it is in recognition

of the fact that in some instances it will

be a requirement of certification. The

long-life iridium spark plug is fired by

a high-energy, long-duration inductive

coil. The coil is directly fired by the ECU,

which determines spark timing and dwell,

thereby controlling ignition energy.

In theory the coil – or pair of coils in

the case of a twin-plug arrangement –

could be directly mounted on the plug,

but in practice it is remotely located for

reason of overall craft streamlining.

N20 mechanicals

The N20 features a single-piece cantilever-

mounted crankshaft in a one-piece

crankcase (in other words, it is supported

only on the drive side of the crankpin). The

crankcase has a rear panel for access,

which houses the reed valve.

The propeller is directly driven by

the front of the longitudinally oriented

crankshaft (with main bearings between

it and the crankpin). The terms ‘front’ and

‘rear’ refer to use in a puller configuration;

as noted, in the ScanEagle the N20 is

swapped around to work as a pusher.

A generator fits onto the front of the

crankshaft, just behind the propeller,

which is bolted to its rotor. Integral with

the crankcase is the housing for the air

compressor, which is located between

the generator and the power cylinder. An

eccentric lobe on the crankshaft drives

the piston used for air compression.

This is sandwiched by the two main

bearings (a roller and a ball bearing).

The big-end bearing is of the roller type,

and Cathcart recognises that the fact

the crankpin is cantilevered “avoids the

need for a split crankcase that can be the

source of a leak”.

The cylinder head is integral with the

barrel, which makes plating and honing

the bore more challenging. However, the

base of the cylinder can then be formed

as a flange with four short bolts attaching

it to the top of the crankcase.

The combined head and barrel thus

saves the weight of head bolts as well

as of a head sealing system, and where

it attaches via its flange to the crankcase

is a low-pressure seal, so an O-ring

suffices. The only gasket in the engine

is that of the exhaust, which is pressed

metal stainless steel with a two-bolt fixing.

The single crankshaft web has a heavy

metal plug for reason of counterbalance;

the balance factor is undisclosed.

Cathcart notes that the air compressor

lobe incorporated into the crankshaft

and the generator’s rotor contribute to

engine balance. “The air compressor

and the rotor help minimise the vibration

couple,” he says.

The con rod is of tapered section since

that type is considered to minimise weight

while maximising stiffness and durability.

The rod is hardened steel, forged and

machined, and the big-end bearing

runs directly in it, as does a needle roller

bearing in the small end. Enhanced

lubrication for the big or small end is not

considered necessary, as both are served

by general oil mist in the crankcase.

The piston pin is hardened steel,

machined from solid and through-

drilled with tapered ends to the drilling.

Uncoated, the pin runs directly in the

small-end bearing and in the piston. The

pin is retained by steel wire C-clips, which

are a sufficiently tight fit in the respective

pin bore groove to stop rotation of them,

although the pin itself rotates.

The piston is described as “die cast

from a high silicon-aluminium alloy”, the

actual percentage of silicon being

June/July 2016 |

Unmanned Systems Technology

ScanEagle launch – field testing the N20