Flight Mechanics Midterm Exam, Class VP$21$HK

Instructor: Assoc. Prof. Dr$.$ Ngo Khanh Hieu

The aircraft used for this test is a remote$-$controlled aircraft model with a $3$m wingspan using $02$ electric motor propulsion systems with APC Thin E $126$ propellers as shown in Lecture $5($see figure$).$

Technical specifications:

Drag polar: CD $=0\mathrm{.}035+0\mathrm{.}0398*$CL$^2$

Mass $($M$)=4\mathrm{.}5$ kg

Height $($H$)=100$ m

Ambient Temperature $=32$C

Velocity $($V$)=80$ km$/$h

Using Lipo $3$S battery, $5200$ mAh, maximum allowable battery usage is $80\%$ of rated battery capacity

Electric motor type: ECO $2826$C$-1080$Kv

The propeller characteristics and motor characteristics are given in the paper "Performance Estimation of Fixed$-$Wing UAV Propulsion Systems" by Mohamed Etewa et al$.,$ drones, MDPI, $2024.$

Questions:

$1.$ Determine the Reynolds number on the main wing at the given cruise speed?

$2.$ Determine the thrust characteristics of the aircraft at takeoff state, assuming the aircraft speed is approximately zero, RPM of each motor is pushed to $8000$ rpm$.$ Specifically:

o$$ Total thrust?

o$$ Thrust$/$Weight $($T$/$W$)$ ratio?

o$$ Propulsion system efficiency?

o$$ Electric motor power usage ratio in this case?

o$$ Battery current supplied to the propulsion system?

o$$ Maximum battery usage time for this case?

$3.$ Similar to question $(2)$ but considering the case when the aircraft is in steady level flight at an altitude of $100$ m with a speed of $80$ km$/$h$.$ Specifically:

o$$ RPM of each motor?

o$$ Total thrust?

o$$ Thrust$/$Weight $($T$/$W$)$ ratio?

o$$ Propulsion system efficiency?

o$$ Electric motor power usage ratio in this case?

o$$ Battery current supplied to the propulsion system?

o$$ Maximum battery usage time for this case?

$4.$ What is the maximum distance the aircraft can fly in the case of question $(3)?$

$5.$ In the case of question $(3),$ if the motor RPM is increased to $8000$ rpm$,$ what would be the steady level flight speed of the aircraft while maintaining an altitude of $100$ m$?$

$6.$ What is the maximum distance the aircraft can fly in the case of question $(5)?$

$7.$ What observations can be made about the maximum flying distance of the aircraft in questions $(4)$ and $(6)?$

$8.$ If an additional battery pack is installed to double the current battery capacity of the aircraft, how will the flight time in question $(3)$ be increased? From this, provide an assessment of solutions to increase the flight time of electric aircraft?