1. Introduction:

This is an article written from my 20 year experience in flight testing airliners and industrial/ military aircraft.

This article is in no way trying to explain how to flight test the above vehicles, but rather is an adaptation of the core philosophy to the hobby/ UAV world.

This is a rather slow and resource intensive method but it is quite essential to the designer who puts a new airframe, never flown before in the air. The procedure below will also be useful to the professional modeler who wants to flight test a very complex scale model already on the market.

The flight phase must be started after the pre-flight tests have been completed. All the snags discovered during the previous phase must be cleared before the flight test phase.

 

2. Core philosophy

The core philosophy of a flight test program is to progressively open up the flight envelope and test all systems for functionality and reliability at the same time with support of a good telemetry system.

It is essential that no flight stability augmentation system is fitted on the airframe, as those tend to mask natural aerodynamic behavior. Test flight programs are designed to exhibit the natural defaults of the airframe so that the engineers can correct them before the stabilization systems are fitted in. This consideration is, of course, only valid for naturally stable aerodynamic setups. However, even for unstable airframes, I recommend to start the test phase from a stable configuration and tune, then move the CG to the unstable domain with artificial stabilization system compensation. This will still be helpful to characterize the natural stability modes before going unstable.

It starts from low speed, flaps extended, gear extended to progressively retracting the drag elements and increase the speed. The test program is completed when top speed has been validated as well as all the high G maneuvers intended for the plane. CG position in all configuration must also be validated. For this purpose, a digital balancing equipment is a vital tool.

Teamwork is essential during this phase as it involves many operators. Briefings, telemetry log analysis, videos review, debriefings, corrective actions elaboration, engineering design involvement are key in this process.

 

3. Resources required

A certain number of people are required to attend a test flight program, extract meaningful data from the telemetry logs and offer the maximum feedback to the design bureau.

Together with the pilot, 2 spotters are recommended. These are as essential to the test program as the pilot. They should be fully trained on their function and fully briefed for the program of the day.

3.1. Program spotter.

He will announce the next program maneuvers announce the displayed stall speeds, max speed, configuration changes, compute 1.3 Vs, and activate remote control switches on request of the pilot. This spotter should not look at the aircraft but solely focus on the flight program sequence and the remote control. 

3.2. Flight spotter

He will check the airspace/ VHF and possible conflicts/ ATC requirement and give feedback on the airplane behavior ( nose attitude, CG behavior etc ). The flight spotter is mostly required if the test flights are conducted on a shared runway ( aviation of hobby field ).

 

3.3. Camera operator and hardware

A camera operator is also recommended to capture every flight, as well as on-board cameras.

The camera man should be trained at filming flying objects, which requires some specific skills and a good knowledge of the equipment used. Active stabilization and a gimbal can be of a great help, as well as an external screen for full sunlight filming conditions.

For on-board cameras, we use a combination of GoPros setup to record the main flight controls deviations during the flight and 360 fixed camera usually placed on the fin of the airplane ( size permitting ).

Video files are essential for de-briefing and post flight analysis. On-board videos MUST be fully checked on all the flights as they can spot a defective/ binding/ sloppy flight control/ flutter condition that is not visible from the ground. 

 

3.4. Telemetry and log

A full telemetry system with ground return and flighht log recording is absolutely essential to get a proper data analysis of the many parameters required to tune the airframe behavior as well as system performance. This includes:

 

. Control stick and switches position during the flight

. Flight pitot speed

. GPS position, speed and altitude

. Antennas frame received as well as RF link quality

. Servo torque recording

. Regulators intensity

. Batteries voltages

. Engine running parameters

. Sticks position on the remote control

 

Some of these parameters must be available to the program spotter in flight, but most importantly, all of these most be recorded on a flight log, both on the receiver and transmitter.

Proper flight log analysis and extraction of meaningful data for the next flight is essential to the development and fine tuning of the airframe during the test flight program. 

 

On that matter, I recommend the use of a Jeti remote control system or Embention Veronte system as these brands are leader in the hobby/ UAV Worlds respectively. We have been using both for years now and I cannot stress enough how important it is to record, analyse and store all flight data. There is no point investing several 100 k USD in a new airframe design and fail in the test flight phase due to a lack of high quality telemetry equipment at a fraction of the cost of the project.

 

4. Test flight program

 The typical flight test program is described below:

 

4.1. Initial ground and taxi tests:

Configuration: fixed, gear down takeoff flaps.

1. Ground program:

Test all the flight controls in all flight modes for direction of deflection, neutral and max deviations.

Verify all telemetry live data readout, including pitot test into the wind ( should read wind speed ).

Verify all telemetry recording are valid on the flight log ( requires to shut down the model and connect the transmitter to a PC for log readout )

Do a range test.

2. Taxi program: 

Low speed taxi, validate the steering effectiveness.

High speed taxi, validate the steering effectiveness

Rejected takeoff medium speed: validate the brakes effectiveness

Rejected takeoff high speed: validate elevator effectiveness by pulling near Vr. The nose should start to lift. DO NOT TAKE THE PLANE AIRBORNE at this stage.

3. Post flight verifications:

Steering servo, brakes wear, tire wear, gear components, turbine parameters, battery consumption, fuel consumption, air trap air quantity ( should be full ).

4. Analysis:

This phase is essential to test the landing gear components, fuel system behavior and air trap effectiveness/ vacuum condition, and get a first feel of the Vr  speed, CG position and elevator authority.

 

4.2. Second taxi tests:

A long runway with good runout areas must be available for this.

Configuration: fixed, gear down takeoff flaps.

1. Rejected takeoff high speed: validate elevator effectiveness by pulling near Vr. The nose should start to lift. Let the plane get airborn, then cut the thrust to bring the plane back on the runway in straight line.

Land.

 

2. Post flight verifications:

Steering servo, brakes wear, tire wear, gear pins, wheel bearings, turbine parameters, battery consumption, fuel consumption, air trap air quantity ( should be full ).

3. Analysis:

This phase is essential to test the landing gear components, fuel system behavior and air trap effectiveness/ vacuum condition.

The hop test will give many valuable information:

Vr readout will approximate stall speed for the current flap setting ( Vs=Vr/1.3 )

This will allow to establish a first protection and approach speed approximation of 1.3 x Vs initially.

Longitudinal Vee and stabilizer angle position. This in turns give a good idea of the position of the CG and current airframe stability mode.

Elevator trim position.

Elevator authority at low speed.

Brakes performance at high speed.

Make a snag list and clear it at the workshop. Review all the flight parameters and data. Briefing with the spotters. Modify the dual rate/ control throws if required. Modify the CG if required.

 

4.3. Flight 1, configuration stalls and approach speed validation: 

Configuration: semi fixed, gear down takeoff flaps then landing flaps.

Verify all dual rate offer several orders of magnitude of control on each axis and that the program spotter is well briefed on operating the D/R switches.

1. Program: 

Takeoff,

One to two patterns for trimming with takeoff flaps. Check CG behavior in turns ( how much up elevator ? ). Check the controls throw and adjust if required ( via dual rate ).

Extend landing flaps.

One to two patterns  for trimming with landing flaps. Check CG behavior in turns  how much up elevator ? ). Check the controls throw and adjust if required ( via dual rate ).

Climb and stall at comfortable altitude. Record stall speed, implement 1.3 vs that is the approach speed ( spotter's job ).

Come back for landing. Approach at 1.3 Vs.

Landing and full stop.

This flight takes typically 4 to 5 minutes.

2. Post flight verifications:

Steering servo, brakes wear, tire wear, gear components, turbine parameters, battery consumption, fuel consumption, air trap air quantity ( should be full ) internal components fastening, servos fastening, servo arm, pushrod, control horn, fuel system, flight controls slop, gear slop, bearing and gear pin slop.

 

3. Analysis:

Controls throws used

Trims set

CG position

Stall speeds

1.3 Vs approach and protection speed for given configurations

Flight current consumption for each control servos

Antennae performance and RF link performance

 

Make a snag list and clear it at the workshop. Wait one week before flight 2 and review all the flight parameters and data. Briefing with the spotters. Modify the dual rate/ control throws if required. Modify the CG if required.

 

4.4. Flight 2, CG validation/ fine tuning: 

Make this flight when the previous snag list is clear.

Configuration: semi fixed, gear down takeoff flaps then landing flaps.

1 Program: 

Takeoff,

One pattern to confirm trimming with takeoff flaps.

One pattern to confirm the new CG behavior further ( if changed ).

Extend landing flaps.

One pattern to confirm trimming with takeoff flaps.

One pattern to confirm the new CG behavior further ( if changed ).

Climb and stall at comfortable altitude. Record stall speed, implement 1.3 vs that is the approach speed ( spotter's job ) with the new CG position.

Come back for landing.

Landing and full stop.

This flight takes typically 4 to 5 minutes.

2. Post flight verifications:

Steering servo, brakes wear, tire wear, gear components, turbine parameters, battery consumption, fuel consumption, air trap air quantity ( should be full ) internal components fastening, servos fastening, servo arm, pushrod, control horn, fuel system, flight controls slop, gear slop, bearing and gear pin slop.

3. Analysis:

Controls throw used

Trims set

CG position

Stall speeds

1.25 Vs approach and protection speed for given configurations

Flight current consumption for each control servos

Antennae performance and RF link performance

 

Make a snag list and clear it at the workshop. Review all the flight parameters and data. Briefing with the spotters. Modify the dual rate/ control throws if required. Modify the CG if required.

 

4.5. Flight 3, gear retraction and stall speeds: 

Make this flight when the previous snag list is clear.

Configuration: variable, gear down then up, takeoff flaps then landing flaps.

Program: 

Takeoff,

One pattern to retract the gear in front of you

One pattern to confirm the new CG behavior with gear up.

Extend landing flaps.

One pattern to confirm the new CG behavior with gear up.

Climb and stall at comfortable altitude. Record stall speed, implement 1.3 vs that is the approach speed ( spotter's job ) with the gear up.

Come back for landing.

Extend the gear.

Landing and full stop.

This flight takes typically 4 to 5 minutes.

Post flight verifications: Steering servo, brakes wear, tire wear, gear components, turbine parameters, battery consumption, fuel consumption, air trap air quantity ( should be full ) internal components fastening, servos fastening, servo arm, pushrod, control horn, fuel system, flight controls slop, gear slop, bearing and gear pin slop.

Make a snag list and clear it at the workshop. Review all the flight parameters and data. Briefing with the spotters. Modify the dual rate/ control throws if required. Modify the CG if required.

 

4.6. Flight 4, flaps retraction and stall speed ( do not push the speed ): 

Configuration: variable, gear down then up, takeoff flaps then clean, then landing flaps.

Make this flight when the previous snag list is clear.

1. Program: 

Takeoff,

One pattern to retract the gear in front of you

One to two patterns to retract the flaps and trim.

One to two patterns to check CG behavior.

Climb and stall at comfortable altitude. Record stall speed, implement 1.3 vs that is the stall  speed protection ( spotter's job ) with flaps up.

Come back for landing.

Extend gear and landing flaps.

Landing and full stop.

This flight takes typically 5 to 6 minutes.

2. Post flight verifications: Steering servo, brakes wear, tire wear, gear components, turbine parameters, battery consumption, fuel consumption, air trap air quantity ( should be full ) internal components fastening, servos fastening, servo arm, pushrod, control horn, fuel system, flight controls slop, gear slop, bearing and gear pin slop.

3. Analysis:

Controls throw used

Trims set

CG position

Stall speeds

1.25 Vs approach and protection speed for given configurations

Flight current consumption for each control servos

Antennae performance and RF link performance

Make a snag list and clear it at the workshop. Review all the flight parameters and data. Briefing with the spotters. Modify the dual rate/ control throws if required. Modify the CG if required.

 

4.7. Flight 5, flap up CG validation with inverted flight ( do not push the speed ): 

Configuration: variable, gear down then up, takeoff flaps then clean, then landing flaps.

Make this flight when the previous snag list is clear.

1. Program: 

Takeoff,

One pattern to retract the gear in front of you

One to two patterns to retract the flaps and trim.

One to two patterns to check CG behavior:

Climb at 45 degrees and invert the plane. Once the plane is climbing stable on its back, release the elevator pushing action. Let the plane fly alone. Note the behavior. If the nose is dropping heavily: too nose heavy. If the plane is dropping the nose slightly: good CG. If the plane is climbing more ( nose going up ): too aft CG.

Come back for landing.

Extend gear and landing flaps.

Landing and full stop.

This flight takes typically 5 to 6 minutes.

2. Post flight verifications: Steering servo, brakes wear, tire wear, gear components, turbine parameters, battery consumption, fuel consumption, air trap air quantity ( should be full ) internal components fastening, servos fastening, servo arm, pushrod, control horn, fuel system, flight controls slop, gear slop, bearing and gear pin slop.

3. Analysis:

Controls throw used

Trims set

CG position

Stall speeds

1.25 Vs approach and protection speed for given configurations

Flight current consumption for each control servos

Antennae performance and RF link performance

Make a snag list and clear it at the workshop. Wait one week before flight 6 and review all the flight parameters and data. Briefing with the spotters. Modify the dual rate/ control throws if required. Modify the CG if required.

 

4.8. Flight 6, flap up CG validation with inverted flight ( do not push the speed ): 

This is a repeat of the previous flight in case CG is pulled back.

Configuration: variable, gear down then up, takeoff flaps then clean, then landing flaps.

Make this flight when the previous snag list is clear.

1. Program: 

Takeoff,

One pattern to retract the gear in front of you

One to two patterns to retract the flaps and trim.

Two patterns to check the new CG behavior:

Climb at 45 degrees and invert the plane. Once the plane is climbing stable on its back, release the elevator pushing action. Let the plane fly alone. Note the behavior. 

If the CG is good, come back for an inverted flight in front of you ( if the airframe permits it ). Note how much elevator stick pushing is required. 

Come back for landing.

Extend gear and landing flaps.

Landing and full stop.

This flight takes typically 5 to 6 minutes.

2. Post flight verifications: Steering servo, brakes wear, tire wear, gear components, turbine parameters, battery consumption, fuel consumption, air trap air quantity ( should be full ) internal components fastening, servos fastening, servo arm, pushrod, control horn, fuel system, flight controls slop, gear slop, bearing and gear pin slop.

Make a snag list and clear it at the workshop. Wait one week before flight 7 and review all the flight parameters and data. Briefing with the spotters. Modify the dual rate/ control throws if required. Modify the CG if required: if too much pushing is required when inverted, get the CG aft. If to light on the stick push, get the CG forward.

 

Repeat flight 7 program till the CG is set to your preference.

 

Subsequent flights:

Gradually push the speed to the maximum allowed speed and check the airframe.

Start introducing aerobatic maneuvers if the plane is capable of it. Start with low G's maneuvers ( rolls ) then high g's maneuvers ( loop based maneuvers ).

The last maneuver to perform will be the snap stall if the airframe is capable of it.

Verify: no signs of flutter, no cracks on the spars, skin, control horns...

Once the test program is fully completed, the plane can be flow with confidence in meetings and competitions. Note that the airframe will gradually age and once the early failure rate is eliminated, will slowly get more reliable.