Panel Drilling - Part 6

This blog covers the drilling for the instruments for the instrument panel

Overview
The single task that can ruin a panel is the most basic - drilling instrument mounting holes. Having tried with the lower panel to mark out holes for the 1 1/4 instruments only to find the need to fine tune each hole I decided to use the instrument as a jig.

Marking out
Guide lines were marked out on the panel using the base of the panel, square and felt tip pen.

Adding vertical references at various locations

Locating the instrument's average center
See notes below

Ready to mark final two holes

With the a single instrument in an opening it was moved vertically and horizontally and its average position marked referencing the layout marks. Two instruments were placed into a pair of the openings with a ruler across one side to square them up about the horizontal average line's. Next the transfer punch was placed into a mounting hole and the panel marked. 

Transfer punch used for marking

Note: A hex 1.5 mm drill was used as it was short and stiffer than a standard twist drill.

The hole was drilled using a hex 1.5 mm drill then enlarged to 9/64'' to suit a 6-32 screw, next the instrument was fitted back to the panel with a screw, tightened around the average horizontal and appearance checked from the front, then repeated at the diagonal mounting point.

Refitted with two screws, tightened and the final two holes marked and drilled.

This process is repeated for all the instruments and when the trial assembly conducted, all but one instrument was square and this was only a few degrees. Best of all the 1 1/4'' gauges were square when fitted as these have the least tolerance for any errors.

Comment
To avoid all this work and achieve a perfect job draw the panel, produce a DXF file and have it CNC machined.

Canopy Skirt - Part 2

This blog covers the manufacture of the canopy skirt

Manufacture
After completing the cowl re-work the canopy was closed and re-taped off using brown packing tape with all the skirt lines re-established to allow glassing.

Canopy re-taped

The cloth was cut into panels and laid across the canopy, taped down to prevent unwanted movement. Two separate sheets were requires as the canopy is over 1200 mm over its perimeter at its center.

Laying up two sheets cloth

The epoxy was mixed in small batches in disposable cups then brushed onto the fiberglass cloth starting at the top, allowed to run down the cloth while being squeegeed through the cloth using a soft spatula to create a dry finish with the minimum weight of epoxy. The epoxy was spread to go past the black guidelines on the packaging tape to allow for final hand profiling.

Port/Starboard view of cloth after epoxy has been applied [brown area]

Note: Scrap pieces of 5 mm foam core proved to be ideal squeegees - soft but firm - disposable.

Once the first coat was completed the second layer was applied and the process repeated and then the same for the third.

The completed layup was now allowed to cure for 24 hours.

Skirt Removal
The skirt was now removed by applying a steady pull / lift at one corner allowing the skirt to peel away from the packing tape and was repeated from all for corners until the skirt released. The pliability of the skirt at this point was critical as if it was ridged it may have been impossible to removed.

A Dremal motor tool fitted with a plastic cutting disc was used to trim the waste leaving enough waste for final profiling by hand. Cutting was done on the aircraft very carefully,

The skirt is refitted and tapped back onto the canopy to fully cure over a week. 

Skirt rough trimmed ready to be refitted to canopy
The skirt is floppy but is is anticipated that it will 
stiffen with a application of macro.

Skirt refitted ready to be taped and fully cure
Insert show the profile at the front of canopy

Comment

The finished item is a bit more flexible than was expected but it did allow removal as this was one of the major concerns. I have used this technique before but not on this scale. The skirt may stiffen up when fully cured and macro is applied - standby.

The view from the hangar at Rylstone Airpark
Time for a beer..!

Panel Openings - Part 5

This blog covers the finishing of the panel openings.

Construction
With the panel drilled for all the circular holes time had come to mark out all the square holes. Using the drawing in combination with a square and ruler all the holes were marked onto the panel surface. 

Experience has shown that by marking out the edges of the holes with masking tape located on the edge of the marked line, yield's a more accurate result.

Marking out the holes for Ray Allen trim indicators

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Cutting holes with a scroll saw

Once all the holes were rough cut with a scroll saw, then opens were dressed carefully using a combination of flat file's, a vernier to achieve a accurate profile. Final edges were created using a fine flat file with a safety edge. [side teeth ground off]

Trial fit upper panel

Cowl Skin - Part 2

This blog covers the installation of the cowl skin.

Preperation
Installation of the cowl skin required a decision on the type of riveting and it was decided that flush would offer the best appearance. The skin was dimpled using the existing dimpler die with a new riveting gun - GESIPA Flipper.

This was purchased as a replacement for the traditional hand riveter because it offered a mechanical ratchet, one handed operation making installation 4 mm rivets practical in small places. 

A trial was conducted using the Flipper with the dimple die and it was found to produced a high quality dimple. 

This was a result of the one hand operation allowing the other hand to center the die using the ratchet to lock the assembly. 

The first click tightens up just enough to lock the die while allowing final centering with the last click bottoming the die locking the assembly. A push forward on the handle unlocks the tool releasing the dimple die. 

The  only change was that the nails used with the die require changing about 20 pulls to maintain consistency. 

Highly recommend are the Australian agent ToolFix at Kings Park Blacktown. I managed to kit the new gun because of PRM [Please read manual]. They reassembled the gun checked it out at no charge - truly excellent.

Next the firewall and new tabs were dimpled using the same procedure. Finally the channel spreader was countersunk using a micro stop and 120 degree tool. The skin was fitted, fully clecoed, doubler's fitted, sealed and riveted onto the airframe. With all checks completed the clecos were removed one at a time and riveted using 3.2 x 8 mm countersunk pop rivets.

All faces dimpled ready to rivet

Skin fitted and dressing edges straight to accept covers

Note: cover is hand made and this is not required with the
factory panels which are CNC punched.

Silicon applied to weather proof cover.

Note: Spread silicon very thin to allow doubler to be flush after riveting
Too thick as shown

Packer was installed under channel
before final riveting

Skin and doublers riveted to airframe

Aluminium 0.020'' added to cover gaps created by the the profile adjustment

Engine Cooling Ideas

This blog covers the background behind a Rotax cooling system design

Overview
This builder would like see what the effect of closing up the front door would have on the Tucano. To do this requires a solid foundation as currently the gear opening is the primary means for ejecting the waste heat from the radiator, oil cooler, exhaust and other minor sources. It is hoped that this would remove a perceived source of drag as well as some cooling drag - maybe.

All the reading says that air outlet's have to be twice the size of the radiator area based on heating the air. This did not have a convincing ring and seemed to be a easy way of solving the problem.

John Thorpe stated in one of his many articles on his design philosophy for light aircraft  that the largest drag left in T18 was cooling drag but required careful design due to the danger of overheating. 

Minds much more qualified have discussed this issue in great detail but the short article below outlines the basis of the my current opinion.

Research
In roving I can across a article on the Merdith Effect used as the basis for the design on the P51 cooling system. The most interesting comment was that at full power at altitude the outlet was squeezed down to 5'' square - See article: Merdith Effect fact or fiction.

Explanation of the P51 duct operation

Position A: High speed air with ram pressure enters the diffuser
Position B: Air is slowing, pressure rising as the diffuser cross-section area increases.
Position C: Air is at minimum speed and highest pressure on entering the radiator. 
Position D: Air has been heated on passage through the radiator. Pressure falls a little due to viscous losses in the radiator. 
Position E: Thermodynamic expansion is taking place in the nozzle. Temperature of the air is falling as the heat energy content is converted to kinetic energy. Pressure is falling as the air speeds up down toward the exit.
Position F: Adjustable exit scoop is set for high speed flight. Temperature of the air is much reduced, but not all of the heat can be turned into kinetic energy. Pressure is close to ambient, while the exit velocity is close to matching the airspeed of the airplane. The momentum change going from radiator rear face to the exit is seen as thrust, cancelling the drag of the radiator.
Position G: The scoop is wide open for low speed operation, where the cooling needs must be met by low velocity of the air at position A. No thrust generated due to low pressure rise in the diffuser.

Beign forced to dabble in Bernoulli a long time ago it did have a ring of truth. More roving lead to a discussion at of all places a car blog that was saying the same things.  

The first diagram below shows what I would believe is happening with a lot of radiators installations. Air is like any fluids and dose not like work and will seek the path of least resistance so if nothing else a radiator hung on front of a engine with a big hole in front as its only protection it will be inefficient by the air looking for a easy way out - aka a big hole in the back of the cowl.

Extract:  "Then what you want is a Kuchemann & Weber trumpet shaped diffuser. The name comes from a couple of scientists, Kuchemann & Weber performed a lot of R & D on this subject during World War II and published their findings in Aerodynamics of Propulsion sometime in 1953. These diffusers are highly utilized in aviation as well as in automotive racing. 

The book is out of print and extremely difficult to find, except in college/university libraries. Anyway, their research showed that the following type of bell mouth intake is inefficient: 

 

...and that more of a trumpet shape prevents cavitation/turbulence and also traps more airflow, forcing it through the heat exchanger and preventing a lot of airflow from stacking up... 

 

Here are some CG images of a theoretically perfect Kuchemann & Weber diffuser: 

 

While you might not be able to achieve the exact shape that their calculations would generate, a rough approximation of using about 1/4 to 1/2 your radiator surface area (depending on your specific cooling needs, i.e. street or track) for the diffuser opening and then gently curving walls (like you're making a trumpet) that join to the edges of the radiator will get you a lot more efficiency than running without any duct work."

A trumpet is not practical but the diagrams would reinforce the idea that were outlined in the article on Meridths work. 

The final word belongs to Lee Attwood the designer of the P51 when he gave his last talk in England reported here in an article from Historic Racer.

Overview

What dose this this mean - with a cowl closed cooling air has to be ducted to achieve a acceptable level of cooling efficiency while minimizing losses and a large exit is not required in a well thought out and ducted system. Replica aircraft lose freedom in design so no great gains could be envisioned but if we dispose of the air in a controlled manner to cool the engine in climb it may be possible to close the door's on wheel well opening.

A water cooled motor has a singular advantage over a air cooled motor, you place the heat exchanger anywhere and with the Tucano that's is under the engine. This preclude's the use of a standard Rotax exhaust and requires two separate;exhaust and mufflers to free up the underside, such a system is installed on a Highlander located at Rylstone.

To concentrate the heat load a Silent Hektik water/oil heat exchanger would be installed so both these heat load's are at one location. Silent Hektik have stated that a single radiator will meet the requirements of a standard motor so a single radiator installation will have to be at 100% efficiency to cool a supercharged engine. 

As shown below the air is taken in, slowed to raise the pressure and extend the time for a volume of air to cross the radiator interface before being accelerated and dumped out of the two fake turbine outlets. 

Forget thrust as Merdith's work at the outlet side is only applicable to aircraft traveling in excess of 300 mph.

The outlets are fake turbine exhaust exiting though the underside of the fuselage.

The inlet and outlet are fiberglass moldings with the two 45 degree standard oil cooler ducts. Radiator is a stock Rotax with the connections 4'' scat hose. The outlets would have to be fabricated over two half CNC machined foam cores that should survive manufacture of two sets 

The duct assembly / radiator would have to be mounted independently to the engine using support rail's incorporated into the lower cowl supported by the firewall and rigid engine mount. The exhaust would be two independent systems to open up the base of the engine to allow the installation of the system.

Will it all fit - $64 question as there is a lack of 3D models so it will have to be checked by comparison to the installation on the aircraft. What I will have to do in the spare time when at home is a little more thinking with numbers. 

So the thinking is out there and I will await comments.

Cowl Modification - Part 1

This blog cover the modification to the cowl skin to align with the canopy frame. 

Overview

When fitting the canopy and top cowl as part of the preparation for the shroud a alignment issue was noted. It was decided to modify the cowl profile to match the canopy as this appeared to be the most direct route. 

I can see no error in how the work was undertaken to create this issue but I did email the factory for advice. Franco at Flying Legend advised that a new frame design was to be available in a few weeks but I elected to modify the cowl to avoid the lost time. 

A pair of metal horseshoe's that has begin incorporated into the new design will be fitted onto the canopy on their arrival from the factory.

Misalignment noted at both ends

The factory cowl sheet aligned perfectly with all holes in the fuselage allowing it to be used as a drilling jig. 

Manufacture

The mounting lugs on the front bow were removed and a duplicate manufactured but with longer leg on one side.

The existing sheet was laid on a new sheet of 2024-T3 sheet on a under sheet of foam insulation.  The foam allows the sheet to be drilled and clecoed with the foam providing bite and fixing the sheet. The side that attached to the fuselage side was extended by 25 mm and then all the factory holes duplicated except the side holes. 

Duplicating cover existing cover
The blue tape is to prevent drilling unwanted holes

Removed existing tabs 

Port side tabs installed

The new sheet was refitted to the aircraft and then a set of new tab's 75 x 20 x 0.020'' folded for re-installation. The canopy was shut and with a tab placed under the cowl a spring clamp secured the tab to the frame. Next the canopy was closed, fit checked and if OK, the holes marked, drilled and then clecoed back into the frame, this operation was repeated 14 times over a few hours until a new profile was created.

The openings for the inspection panel were cut out for installation of the doublers then all edges were dressed using files to final profile.

The new cover was treated with Henkel Alodine 1200S Brush On alodine in accordance with the directions supplied and set aside to air dry.

When dry the panel was clecoed into position and all the lower edges were now matched drilled and the panel removed and the edge trimmed and dressed. 

When complete the panel was fully clecoed into position.

A pair of factory supplied doublers were now cleoced into position with no issues.

Ready for final riveting
The fold through the lower rivet line at 5+ degree [approx] was added 
to create a tighter fit when riveted

Antenna Access - Part 2

This blog covers the installation of a inspection hole on the starboard side for the antenna access.

Overview
Servicing of the antenna would be impossible due to no access for removal and connection

Manufacture
A aluminium 0.020'' 2024-T3 doubler was folded with 10 mm verticals on two sides. The assembly was riveted on a PCD around the 90 mm opening with 8 [eight] 3.2 mm pop rivets securing the doubler to the skin. 

A series of holes were matched drill from the cover 3.2 mm through the skin and doubler, then the holes in the fuselage were fitted with M3 rivet nuts. 

The center of the PCD was drilled 6 mm through and then the access hole cut with a 90 mm hole saw through the skin and doubler.

Access opening 90 mm

Canopy Skirt - Part 1

Shorts T1 Tucano
The Shorts Tucano has a unique shroud over the once piece canopy though the prototype lacked this feature

Shorts Tucano circa 1991

Model of Shorts Tucano canopy

The white lines represent the explosive charges 

fitted to the prototypes canopy

In service the Tucano gained this feature it was decided to add the feature to the canopy.

Preparation
The canopy frame had a scallop that was noted on the starboard side that need filling before commencing on the shroud. A mix of West System Epoxy and Micro Balloons was mixed and applied. Packaging tap was applied acting as a release system once cured. During the curing phase a piece of garbage bag was applied as it's very thin and flexible and useful to re-shape the fill countering the natural slump that occurs before setup.

Note: All micro balloons must be sieved to remove any lumps to create a smooth paste.

Once cured it was sanded to profile using a 1'' dowel and sanding blocks with aluminium open cut 125 grit.

Shroud Construction
Trill weave glass 285 g.s.m. cloth was chosen because of its ability to confirm to curved shapes. 

West Systems 105 resin and slow cure hardener  was selected as the builder is confident in it's performance and application.

The existing cover that was peeled and taped back in part 1 was now re-positioned and tapped to the side of the canopy frame. Aluminium duct tape 100 wide was applied to approximate the shroud and then covered with duct tape. Finally additional packaging tape was applied to allow for epoxy spread past the shroud boundary during manufacture.

Note: Packaging tape is the easy to use mold release for this type of work.

Laying out shroud profile
When top was finished, packaging tap was applied
over the blue making tape

With the tape positioned, an accurate outline was drawn onto the tape with a heavy felt tip pen to allow it to be transferred to the fiberglass shroud later. Additionally it allowed the builder to step back and confirm the profile was suitable.

Flap Controller - Part 2

This blog covers the installation of the flap controller

Overview
The flap controller supplied by Aircraft Extras FPS Plus NT allow's automatic 3 position with auto retraction of the flap by a simple tap of the flap switch. To tidy up the wiring the cores all were run in loom tube and it was decided to install the electronics into a larger box to allow the final termination to be hidden.

Manufacture

Ready to install

A plastic jiffy box was selected and the original cover used to match drill mounting holes for four [4] 6-32 screws in the base. Once drilled the surplus area behind the PCB was cut from the case to lighten it a little.

Two 0.020'' 2024-T3 aluminium lugs were manufactured then drilled to take four [4] 5/32'' pop rivets each. These were placed on the box, drilled and after painting [front only] pop riveted to the plastic case with the 5/8'' lighting hole's added.

The PCB was fitted using the original standoffs & 6-32 screws as required - all screws were treated with small drop Loctite thread lock at assembly.

Once completed it was pop riveted to the longeron using 5/32 x 7 mm pop rivets. 

Finally all the loom tubes were installed into 9.5 mm holes drilled at assembly completed the necessary preparation for termination.

Flap Controller
The cable entering from the side is the flap motor
cable supplied with the package with the other
begin the required cores back to the panels