1.    Introduction

One thing that I have learned after 20 years of building and flying all kinds of UAVs and remote controlled vehicles is that a proper testing routine of all the aircraft systems prior to the maiden is essential to ensure the success of the test flight program.

You can decide to go straight ahead and maiden project fresh from the workshop. But this would mean that you’d actually test the aircraft components and systems in real flight conditions.

Or you can decide to follow a complete test routine and try to troubleshoot as many things as possible before the test phase, in conditions set as close to the flight environment as possible. The advantage of this approach being the following:

• You will be able to anticipate the problems in the workshop with plenty of time to correct them.
• You will be able to test the systems and get rid of the early failures or “infant mortality” as called in aeronautical jargon. The idea is to position your systems in the constant failure rate area ( or highest reliability zone ) of the bathtub curve before the maiden.

 

I have detailed the complete routine that I follow before any maiden below. This might look long and constraining but I believe that it is worth the effort considering some project investment are upwards several USD M and is required for certification of professional UAVs..

Of course and depending on your investment and confidence about your team's building capability and material reliability you could skip some of the following steps.

2.    Electrical system test

 

The electrical system test is quite important to my eyes for the following reasons:

• You will eliminate the possibility of an early component failure
• You will eliminate the possibility of a servo binding
• You will be able to characterize your system’s power capability, current consumption and electrical endurance and use it for characterizing the state of your electrical system when it gets older via our statistical log reader.
• You will discard the possibility of a regulator thermal runaway or receiver thermal lock ( Futaba FASST )

2.1. Individual component test

 

I check that the servos are operative at an early stage when fitting them in the plane. I just use a small servo tester like the Astro Flight servo tester and move the arm to each end of travel. I also use this device to center the servo hub before fitting the servo arm.

I then test again the servo when the control surface linkage and associated electrical loom is completely setup. I plug the very useful Hangar9 digital amp meter between the receiver and the loom. I test the servo from the radio in all the directions at a slow pace and make a note of the current reading. This will be useful for two reasons:

• To setup a basic consumption value at “age 0” that will be used to monitor the servo ageing.
• To check that there is no flight control binding or wiring problem ( bad cable, intermittent plug contact… )

An elevator servo readout at idle

 

Same elevator servo readout in motion

 

 

2.2. Torture test

 

When all the servos have been successfully tested and the electrical system is completed, I proceed with two electrical testing routines: the torture test and the endurance test.

The torture test is made with the flight controls loaded according to the maximum possible efforts found in flight ( 100% stick deflection at maximum speed ). To find the weights I’ll have to put on each flight control, I use the RC calculator spreadsheet.

This value is given in the “Fp max” cell that gives the load at the pushrod on the Control Calc software that is provided with all our kits. This weight is simply put on the flight control at the same distance from the hinging line than the horn height given in the “Yc” cell. I do this for each flight control except the rudder that is vertical by definition.

I then put the plane into the sun to simulate the maximum temperature at the field and activate the servo testing feature on the transmitter. I let the routine go for the equivalent period of two flights.

During the routine I check and make note of the receiver and /or regulators temperature as well as the maximum current consumption.

This routine helps me to check how the servos can cope with the maximum flight loads and the system peak consumption in a realistic environment. It also enables me to verify that there will be no regulator thermal runaway or receiver thermal lock for the Fubata FASST system.

The torture test in progress on an aviation Design Phoenix

 

2.3. Endurance test

 

The endurance test is very similar to the torture test except that it will be done with lower servo loads and for a longer period of time. It is mostly designed at eliminating the infant mortality failure points and providing a base reading for our statistical log. This reading will be used to evaluate the aging of your system season after season.

To do this I remove enough ballast from the previous test to simulate the load on the flight controls when using 25% of the stick travel at 75% of the estimated maximum speed. Once again I use the RC calculator to find out this value.

I then run the servo test with the weights on but continue until the battery system is empty.

I make a note of the endurance found that way.

I’ll repeat this routine 3 to 5 times. This is done in order to eliminate the possibility of an early component failure and to run in the battery system.

After this routine you’ll have a fairly good idea of how many flights you can safely do with a freshly charged battery system.

Here is the electrical test report table that I fill for each airplane.

1.    Electrical system test
1.1. Individual servo test	center	Drain+	Drain-
Aileron L
Aileron R
Elevator L
Elevator R
Rudder L
Rudder R
Flap L
Flap R
Other
1.2. Torture test	Weight	Temp	Amp
Aileron L
Aileron R
Elevator L
Elevator R
Rudder L
Rudder R
Flap L
Flap R
Other
Receiver temperature
Regulator heat sink temperature
Peak current reading
1.3. Endurance test	Weight	Time	mAh/f
Aileron L
Aileron R
Elevator L
Elevator R
Rudder L
Rudder R
Flap L
Flap R
Other
Time to drain the battery system
Consumption per flight

 

 

 

 

 

3.    Pneumatic system test

 

The pneumatic system can be really tricky to troubleshoot on a completed plane. This is why I consider that an individual component system test can save you a lot of time and hassle.

 

3.1. Individual component test

 

I try to test each component outside of the plane as far as practical. I use a much higher pressure than the operational one for this. I most of the time use a value of 160 PSI. I’ll dip the component in a bucket of water if possible ( not for electronic valves ! ) and check for visual leaks.

The component is plugged to a piece of tube connected to a BVM fill valve and inflated to 160 PSI. It is then dipped into water and checked for leak.

An air tank leak test. Do not dip the gauge into water since it has a venting hole and you might introduce water into the spring mechanism, creating possible corrosion.

Carefully dry the component after the test to avoid corrosion.

If the component cannot be removed easily from the plane or is electronic, I test it by monitoring the leak rate.

I use a pump connected to a pressure gage with a T and very short pneumatic tubes the other end of the T is connected to a BVM fill valve. I then pump 160 PSI in and check how long the device can keep the pressure. A drop of 1 PSI every 5 minutes is a good value.

An air cylinder leak test. Each side of the piston is tested under pressure. You might also want to move the cylinder while under pressure to check for leak at half travel.

 

Note that most of the air tanks made in China have a very high leak rate from the cap seal. I usually fill the cap seal with Hysol E20-NS to get rid of this leak. Make sure that there is no pressure in the tank when you apply the glue.

Every item tested is then identified with a permanent marker to make things clearer.

3.2. In aircraft test

 

I always proceed with a complete test of each pneumatic system after the plane is completed. I check each system one after the other the following way:

I first check the leak rate when the radio is off by pumping the normal pressure in the system

I then do the same test with the system in each state ( gear up , leak check, then gear down ,replenish the air tank, leak check, for example ).

The leak shall not be more that 1 PSI per minute or a 10 PSI drop for 10 minutes.

Note for trouble shooting purposes  that if a leak is detected when the radio is off, it then means that the latter is located upstream of the valve. If it is detected with the radio operating, it is then located downstream of the valve.

I make a note of the leak rate in each system state as a reference value.

 

Here is the pneumatic test report table that I fill for each airplane:

2.    Pneumatic system test
2.1. Component test	160 PSI
Air tank 1
Air tank 2
Left Main retract
Left main door piston
Right main retract
Right main door piston
Nose retract
Nose door piston
Gear valve
Brake valve
Other
2.2. Aircraft test	PSI/10’
Radio OFF gear
Radio OFF brakes
Radio OFF other
Radio ON gear
Radio ON brakes
Radio ON other

4.    Fuel system test

A sound and reliable fuel system is essential to preserve your beloved model. Most of the crashes/hard landings are the consequence of an engine failure. A lot of the engine failures that I have seen were due to a fuel system problem ( air bubble, line clogging, fill line venting… )

Here again, the best routine that I have found is to do an individual component check at an early stage and to then do complete system tests as detailed below.

4.1. Component test

The first component that I test carefully is the fuel tank. I first rinse it with raw kerosene to remove any dust or layup residues. I then glue the required hardware on it. When the glue is set I proceed with the leak test. I close the tank with the required hardware and slightly inflate it by blowing air by mouth or hand pump into an open tube. All the other tubes shall be closed. I then dip the tank into water and look at any air bubble escaping.

The Kevlar tank is SLIGHTLY pressurized ( not more than 10 PSI )

I then do the same thing with the air trap tank or UAT.

I finally make sure that the filling system is totally leak proof. On that respect I prefer to use a simple fuel dot system with its threaded sleeve.

The fuel dot is the simplest, yet most reliable fueling system…

4.2. Aircraft cold test

 

I then hook up the complete system in the plane to set the correct tubing length up to the fuel pump. The engine shall not be connected but the system should be in close loop. The pump output or safety valve output should be connected back to the last tank vent. This way the fuel loops backs into the system. I do a functional test by running the fuel pump up to the maximum voltage relevant to the used engine.

The fuel test on the F-84G. The fuel system is looped back to the main tank after the ball valve ( the fest and tygon tubes are joining above the engine FOD, which is not visible on this picture )

The pump is operated via the “test function” menu on the GSU or data terminal. While doing this test routine I make sure that the air trap tank or UAT remains  completely full. I then check carefully the pump outlet/valve outlet to make sure that no air bubble is found there.

The second step is to move the model while doing this test in all the expected  flight situations ( nose up, nose down, and inverted ). In each phase I check the pump outlet/ valve outlet carefully to make sure that no air bubble comes out.

If any air bubble is found at any time, a trouble shooting routine shall be done. Simply check the line upstream to see where the bubbles are generated from. Then re-plug the faulty equipment or change it.

After you have done a satisfactory cold fuel test, you can connect the pump output to the engine for the hot test.

4.4. Aircraft hot test

 

The next step is carried out during the engine test phase. Check the pump output with the engine running at full thrust to see if any air bubble is passing through.

 

Here is the fuel test report table that I fill for each airplane:

3.    Fuel system test	Tick
3.1. Component test
3.2. Aircraft cold test
3.3. Aircraft hot test

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5.    Engine test

Testing the engine is another important step. To make sure that I will not be tempted to do the maiden  flight straight after the test, I usually do as much as I can at the workshop or in the garden ( mind your grass ! ).

5.1. Component test

The first step in the engine test routine is to check the engine components. I use the “test function” menu when the engine is set in the airplane to confirm that every component is working as expected ( the first two points will be already tested during the cold test if you do it ):

• The pump runs correctly in the correct way ( I prime the fuel system at the same time )
• The fuel valve is opening correctly
• The start valve ( kero or gas ) is opening correctly ( I’d prime the gas line at the same time if required )
• The glow plug is working correctly ( adjust the tension at the same time so that the filament is bright red for a gas start engine)
• The EGT sensor is reading a correct value
• The starter motor is working correctly

5.2. Start routine test

 

I then fill up the fuel system and do a short start test. The idea here is to check that the starting sequence is working correctly. I usually do this in front of the workshop up to the end of the start sequence ( learn idle sequence finished ).

During this first start sequence, I make a note of the max EGT and max current drained from the battery ( requires a Hall Effect amp meter to avoid opening the battery line).

The starting sequence shown with a Hall Effect amp meter hooked in to the black ECU supply wire. 8 amperes are drew.

 

When the engine is stabilized at idle I check the heat buildup in the back end of the fuselage. I read the upper fuselage skin temperature with an infra red device. If the value gets above 65 degrees Celsius, I shut down the engine immediately and cool the fuselage down with a blower. This means that an additional heat protection method shall be used ( increase the  Heat Shield + coating or lay up some ceramic blanket or aluminum foil… ). In any case, this step is critical because heat buildup in the back fuselage can result in structural failure or melted servo wire/ damaged servo if not identified at an early stage. The idea is to fly a plane that remains cold or mild in this area even on the ground during taxi.

5.3. First runs

The next step is to do some more extensive engine run -p when the potential start or heat buildup problems are solved.

The engine run up is done primarily to test two things: the max EGT and the acceleration capacity.

If the engine gets a very high EGT or spit flames or hiccups during acceleration, I reduce the acceleration parameter ( low RPM acceleration, high RPM acceleration or acceleration delay parameters )

I also check during this phase that the venturi effect at the engine cone is working correctly. The engine casing temperature taken with an infra red thermometer shall remain moderate ( around 80 degrees Celsius )

I also check the idle RPM and stability during this test. If the idle RPM has a tendency to drop below the minimum value during deceleration, I increase the minimum RPM parameter by a few thousands.

Finally this is the moment to check that no air bubble is entering the engine as explained above.

 

Here is the engine test report table that I fill for each airplane:

4.    Engine test	Tick
4.1. Component test
Fuel valve test
Fuel pump test
Gas valve test
Starter motor test
Glow plug test
EGT sensor reading
4.2. Start routine test	Temp	Amp
Max start EGT
Max start current drain
Fuselage temperature
4.3. First runs	Temp	x/x	RPM
Max EGT
Acceleration parameter
Min RPM
Venturi effect

 

 

 

6.    Aircraft balancing

I use a modified Great Planes CG machine to check the position of the Center of Gravity before the maiden.

http://www.greatplanes.com/reviews/gpmr2400-man.html

The modification enables me to balance planes up to 30 kgs/ 66 lbs. It consists in adding two 7 mm carbon tubes to the steel arms to triangulate them. These tubes are linked to the steel arms with Kevlar rows and CA.

The modified CG machine

The horizontal support rods are extended to 70 cm and the graduated rulers extended to 50 cm. Balancing my giant 1/7^th scale F-18F from FEJ becomes an easy game with this device.

Remember when you balance your plane that the air trap tank or UAT shall be full. The landing gear shall be extended unless otherwise specified by the manufacturer of the kit. Finally the main tanks shall be empty.

Do not forget to fit the all batteries in the plane and remove all the protective coveres before balancing…

5.    Aircraft balancing	mm
CG position

 

7.    Radio and range test

7.1. Radio programming

 

Make sure that you note all the travels on a document while programming the radio ( have a look at the document at the end of the article ).

I consider very important to program 3 rates for each control axis for the maiden. I usually go with 0% exponential / 100% travel on the first position, then 15% exponential/75% travel, and 25% exponential/ 55% travel. I am not a big fan of the exponential function, but of course this is a matter of taste and habits.

I also tend to keep the programming as simple as possible initially.

7.2. Programming test or flight simulation

 

The final test that I do in the workshop is a flight simulation after having programmed the radio. I set the plane on its cradle with the pneumatic system refilled and simulate all the flight phases of the maiden as per the pre-established program ( I’ll come back to this point in a future article ).

This includes selection of the gear up, retraction of the flaps, trimming all the channels in both directions, selecting all the possible dual rate combinations, using al the programmed flight modes and any other specific function like wheel brakes,  speed brakes, bomb drops…

The idea here is to check that no programming bug was introduced.

7.3. Range test

 

I finally do a preliminary range test in front of the house to verify roughly that the system is working as expected. For a detailed range test routine, refer to the Weatronic article that I wrote in the previous issue.

Here is the programming and range test report table:

6.    Radio and range test
6.1. Control throws	mm	degrees
Ailerons up
Ailerons down
Elevators up
Elevators down
Rudder right
Rudder left
Flaps 1
Flaps 2
Steering right
Steering left
6.2. Programming test or flight simulation	Tick
Taxi
Brake
Takeoff mode
Gear up
Flaps retraction
Flight mode
Dual rates test
Approach mode
Flaps down
Gear down
Braking
6.3. Range test	Meters
Range distance

 

Conclusion

 

All the aspects of the pre-flight tests are time consuming. However they might trap some  errors and even save your model. They will definitely make you become familiar with your aircraft systems before the maiden. This will contribute to greatly reduce your level of stress for this decisive event and will increase the chances of a successful first flight.

 

Note that all the complete report table can be found here. I fill this type of table for every plane and it is a great tool to check again all the plane system at the beginning of the flight season and to monitor the ageing of the airframe and components.

The associated file is downloadable on my ftp:

http://www.geohei.lu/olin/data/modelisme/Weatronic/Pre-maiden-procedures.doc