Throttle Body Injection

This blog covers the design criteria used to select the Rotec Throttle Body Injector for installation onto the Rotax 912 used on this aircraft.

Overview
The Bing Carbs along with the inlet manifold used on the Rotax 912 are a practical compromise but far from ideal, there are many aftermarket EFI systems but all share a flaw - single channel.

The Rotax 915 features dual-channel electrics/electronics or in plain language a redundant system with two means of fuel/ignition, while ideal, its complex and adds weight with the 915 tips the scales at 84 kg plus extras. 

Aftermarket EFI systems have no reported history of failure, most pilots will never in their flying careers use any of the dual redundancy incorporated into a typical certified aircraft engine but this only means there is a very small statistical chance of a failure but when it is required its 100% significant to that pilot at that point in space and time.

Regardless of which EFI is used, they share one common requirement, that's for the Rotax's regulator to keep working and it's the one item incorporated into a Rotax engine that is considered a weak link. The electronics operating an EFI system use's 5 - 8 amps, leaving only 10 amps available from the Rotax alternator to maintain the battery and panel electric's so a larger alternator may be required, more weight/cost but at least there is a backup available. 

The need to have an operating electrical system also means you have to have a larger [heaver] battery and the 1 kg battery that is installed will not provide the necessary flight endurance to an alternative airfield.

Note: In the event of total electrical failure there is a high drain on any battery and at 11V a battery is considered dead flat. 

Dog Aviation review on the RV12 and the Standard Ducati Regulator

 Photos are taken from Dog Aviation images investigating a series of 
RV12 regulator failures

SDS EFI  "Aircraft engines are essentially constant rpm load devices, spending the majority of their lives at fixed power settings for long periods of time. Once a power setting is established, the injector on-time and ignition timing remain the same until power is changed. 

For this reason, we feel that 3D mapping offers no useful benefits, it merely complicates understanding and operating the system for most people"

An alternative was not available until Joe Newman from Marwen discussed with Pual from Rotec that the TBI should work with a supercharger as the new Mk2 version incorporated an internal pressure reference, at this point both agreed that they could not see a reason why it could now not operate under positive pressure, it was unproven but now worth a look, Joe had with one in his bag on his return to Rylstone. 

Note: The Mk 2 Rotec TBI now an upgraded version of the proven Ellison TBI

Installation of the single Rotec TBI onto the Tucano engine was the only option as the dual TBI setup would not fit.

Service kit - it's that simple

A suitable TBI manifold would require flat runner exits, equal length runners and large volume. 

At this time the FAA approved plastic Ultem 9085 was selected to manufacture all the necessary parts using FDM printing.  A lot of time was put into to optimising the design with FEA to obtain a feel for how the plastic would perform. FEA can create an unreliable results with plastics so the design parameters used were at 140 degrees C to analyse a single state, while not a 100% accurate it's better than a guess. 

A Rotax 914 engine cannot be operated at greater than 88 degrees C and therefore 140 degrees C was selected as the structural design point with 100 C as a maximum operating temperature. A Rotax cam has little overlap so pressure waves from the opening of the inlet valve have been ignored as a design load.

Ultem 9085 plastic was chosen for its superior Izod number and heat deflection temperature [HDT] and strength at elevated temperature.

Material Specification

A 3D model estimated an assembled weight of 1.8 kg for the throttle, body/manifold/primer which is 1.2 kg lighter than the Rotax assembly it would replace.

Proposed Manifold/TBI/Inter-cooler
3.5 kg as shown [design weight 4 kg]

Elevation

Plan

Note: A Rotax engine MUST NOT exceed 88 degrees C, so a temperature probe is to be installed at the inter-cooler outlet allowing the temperature to be monitored.

The biggest issue at this time is this may not work, why, the Mk 1 Rotec will not operate under positive pressure but the Mk2 may because of a major design change Joe from Marwen noted at Oshkosh.  

To prove the design a 3D printer will be purchased early next year to manufacture a working prototype and it up to both of us to prove it works as a system. If it does work it will be 2/3 the price of an injection system and lighter - worth the look.

If it does not work back to the drawing board and on go the Bings.

Meshing basic manifold

Stress at peak [red] about 4 Mpa with 20 Mpa on Z axis allowable at 140C
Simple manifold used as this is within the limits of the package available
The model has a 3 mm thick wall, a wall thickness of 2.5 would be acceptable 

Comment

What if, what if?

Engine Supercharger Upgrade

This blog covers preparing the engine for the supercharger and some of Marwen's work and discoveries about Rotax 912ULS cylinder heads

Overview
The selected engine had a total of 140 hours when it was removed from a wrecked Foxbat which as a result of the accident suffered a prop strike. The engine was inspected and signed back into service by an approved Rotax agent at the time of purchase but it was decided to verify.

To achieve this, the engine was stripped, cleaned, painted black and fitted with a full set of O rings etc to bring the engine back to nearly zero time. One issue noted was that this engine had seen a lot of service running LL Avgas and this required about two [2] hours of cleaning by Joe from Marwen.

Checking engine mechanicals

Engine assembly

All the gears and crank checked out to specification so Joe [ex-race bike mechanic] decided to check the final compression ratio as he always had done when racing. While a relatively simple operation it revealed a static compression ratio of 9.6: 1 which would create an effective compression ratio close to 13.6: 1 [worst case].  

Joe was concerned and contacted me, we looked at options like knock sensors but the real issue was the compression ratio. 

The starting ratio should be 8.5: 1 - where was the error?

Note: Worst case was defined as the smallest combustion chamber - they varied by about 3 cc and due to the small swept volume this had a large effect.

Joe checked his figures again & again and they came out the same, there was an error somewhere. Joe next calculated back to the factory compression ratio, it was 11.1: 1 with the accepted ratio for a 912ULS generally understood to be 10.5:1.  This was answered when Rotax released it's current heavy overhaul manual which clearly states the compression ratio is now 11.1: 1 for these engines.

Extract Rotax Heavy Overhaul manual - page 12

Modifications 

Marwen Spacer 1.5

We have decided to decompress the motor further with the original 1.2 mm spacer rings replaced with 1.5 mm rings and the heads CC to match the largest measured volume. 

This realized a compression ratio of 9:1 and gave a effective compression ratio of 12.7:1 with a 6 psi of boost.

The next issue was that the original 1.2 mm spacers left the hydraulic lifter with only had 0.8 mm of travel and now with a 1.5 mm spacer it would be 0.5 mm [0.020''], we were edging close to mechanical lock up. 

This issue is to be addressed by manufacturing custom lifters placing the hydraulic lifters back onto their mid position [+/- 2 mm]. 

CC combustion chambers as part
of setting the final compression ratio

Comment: 
The comment below was taken from the Pro Power site but generally, any search will yield the same comments at many sites - edited here for clarity with this case.

"For 9.5:1 EFI/TPI applications running without an intercooler, boost levels above 5 psi will require the use of ignition/timing retard on pump gas, and will produce horsepower gains of 35-45%.  Of course, lower compression motors will be able to run more boost, and higher compression motors should run less boost, everything else being equal. 

For carburetor motors, the rules are slightly different. 

Carburetors deliver the vast majority of fuel in a liquid state, and as this raw fuel atomizes from liquid to gas, a chemical state change actually occurs. Due to this endothermic reaction, which draws heat and cools the incoming air, a carburetor motor can safely handle more boost than a comparable EFI/TPI motor. For carburetor engines with compression ratios of 9:1 or less and boost levels in the 8-14 psi range, pump gasoline works very well. "

Joe cool and sidekick

Does the current arrangement work, well yes but with the measured increased compression of 0.6 points an intercooler should be considered. Also as we intend to improve fuel delivery so any protection created by over fueling may not be there.

This one is open to comment .!

For this aircraft, we decided to err on the side of less power to help stave off any chance of detonation on those very hot Australian summer days.

Methanol injection would be an option, Joe Cool loved racing on methanol.

Wing Tips Installation

This blog cover the installation of the wing tips

Overview
It was decided to try and obtain a better match between the faring and the wing tip by mounting attachment strips to the side of the rib instead of using the 0.5mm [0.020"] aluminium strips fitted to the elevator stab.

To do this four [4] strips were laser cut in 1.0 mm marine grade aluminum by Laser Wizard using a DXF file provided by the builder.

Installation
The profiles were first folded into a right angle to allow installation, next they were offered up to the end rib and trimmed to fit and then alodined.

Starting at the front a 1.0 mm angle was used as a spacer between the top side of the skin and angle. Supporting the spacer while using a finger to determine the correct location for the angle a hole was drilled and cleco fitted. This operation was repeated for the length of the angle.

When the aft rib was reached the limitation of the arrangement was encountered as this builder managed to hit a number of factory holes or just no metal. To compensate for this a sheet cover was fabricated and riveted to the face of the rib. 

While not perfect, it is a solution.

Angle strips ready for installation

One the starboard a couple of the holes were slotted using a 3 mm OD file to adjust the position while the port side went smoother - practice is of some advantage here.

Next the factory tip was offered up to check the fit and once satisfied the location for the fixings was marked from the rivet line using the same techniques used for the rudder / elevator stab.

With the top side fixed using 3/32'' clecos, time to look at the underside. Here there was a distinct gap due to the rib and skin fit. There was a blog on the "Cat in the Hat" , this was for the efforts to align the faring at the top of the rudder fin, here the builder tried to sand the tip into alignment and ended up with that hat from that famous cat.

It was decided this time to add material so the tip was remove and a strip of fiberglass added. This was manufactured using two layers of 8 oz cloth and epoxy. 

It was placed between two boards covered with packing tape then brick added to the top board. this produces a ultra thin strip of fiberglass tape. This was cut into a strip 25 mm wide and fixed to the inside face of the tip using 5 mm epoxy.


The edge on the tip was chamfered along the fiberglass tape using a Dremel and sanding drum, then Plasti-Bond was applied and allowed to harden. 

The tip was offered up again and the profile obtained with multiple sands / refit / sand - get the picture it's called time..!

The dark edge is the filler and the text - the instructions of what to do
when removed many times

Once satisfied with the profile the mounting holes were drilled as per the top side.

All holes were drilled 3/16'' and fitted with M3 rivet nuts pressed into the hole to engage the spline. All holes in the fiberglass were enlarged to 4 mm od using a tapered grinding stone fitted to the Dremel motor tool. 

Finally the tip was refitted and secured with M3 x 12 S/S flanged button head screws.

Next it was noted that the aileron fouled the tip with the solution begin to notch the edge to align with the aileron edge.

Foul with aileron

Comments
It is critical that the skins are in a line not staggered, the lower skin on the underside had to be removed and trimmed to rectify this issue. 

The angles worked well BUT if doing again it would be developed over a accurate drawing of the end ribs to ensure ease of assembly and accurate layout.

Overall not hard once you get a good assembly procedure. 

Always another job

Aileron Installation

This blog covers the final installation details for the ailerons.

Overview
The ailerons had been assembled previously leaving the final details to the final installation.

Installation

Note two [2] washers

With these issues sorted and a lot of lost time, the only practical way to assemble the brackets then connect the aileron was to fit the brackets to the aircraft first.

Both bearing mount require an AN4 aluminium washer to be inserted on both sides between the face of the bracket to avoid fouling but on final installation the inboard mount required two [2] on one side to clear the rivets. 

The next task was to fit the rod end bearing, to locate the bearing, a pair of 3 mm spacers were machined from 4130 tube to fix the rod end location. Additionally two [2] seal were added to protect the working face from the environment.

Stainless steel and aluminium have some known corrosion issues so all mounting face were painted in SLS Etch primer before installation. 

Primer on mounting faces

With both hinges fitted the aerlion was mounted with AN3 bolts / washers using the captive nut fitted to the spar. This task is not easy and dose require the use of a set of fine bent long nose pliers, assorted tools and floor gymnastics on the trolley.

Leveling Aileron

The next task was to adjust the shaft connection between the aileron / bell crank located in the wing. This required locking the aileron into a their neutral position, this was achieved  using a piece of aluminium angle and clamps.

Next the push pull tube was placed in it neutral using the center rocker arm, this was placed perpendicular to the spar face. The rod end at the bell crank was unbolted and adjusted in conjunction with the one attached to the aileron bracket, when correct the lock nuts were tightened and wax marked.

Note: It is critical this be undertaken while access is available from the underside of the wing  as there is no access to the rod end bearing on the bellcrank with the skin on.

Bell Crank

A particular issue that appeared was that the end rib was not square and this required a small amount of Plasti-Bond to be applied and sanded to achieve a flat face, once sanded the mass balance was reinstalled using three [3] M5 x 16 mm Titanium Button head screws.

Note: Screws were used over rivets at installation as it was suspected that something was not 100%.

This was noted when the tip was installed and will be covered in that blog at a later date.

Mass Balance and filler [upper right hand corner}

Comment
Generally straightforward once the best way to achieve assembly was understood.