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Retractable Gear

The standard Cozy Configuration utilizes a fixed main gear with a retractable nose gear. The retractable nose gear is implemented to overcome the stability of the design when the aircraft is parked since the empty CofG is at the same location as the main wheels. When parked the nose gear is retracted and the empty CofG moves ahead of the main wheels resulting in a stable condition for parking. The standard Cozy is thus a retractable design and extending this concept to include the main gear should not be thought of as a radical change in concept.

The Cozy designer, Nat Puffer, has made it abundantly clear that he does not see the Cozy as a fully retractable design. Moreover he actively dissuades builders from making such changes. The Cozy news letters have, on numerous occasions, addressed the pitfalls of retractable undercarriages however many of the adverse claims are unfounded and inaccurate. Without a doubt there are some adverse issues when incorporating such a modification as RG (Retractable Gear) and these are:

On the negative front:

  1. Increased complexity

  2. Increased weight

  3. Possibility of gear-up landing

  4. Increased cost

  5. Increased building time, and;

  6. Increased maintenance.

On the positive front the following benefits should be considered:

  1. Increased speed

  2. Increased range

  3. Increased payload

  4. Centre of Gravity

  5. More attractive aircraft albeit there is limited access to view the a/c in this condition.

The downside.

Increased Complexity

The complexity increase is somewhat obvious, there are a number of systems that all need to be integrated, hydraulics, electric/electronic and mechanical. Many builders are comfortable with these technologies, but there are also many builders who are not. This fear of complexity is a good reason to stick with the fixed gear since any nervousness may result in lack of maintenance which is clearly a NO-NO, but for those builders who are not frightened by technology, there is no reason to stick with the semi-fixed gear configuration.

Increased Weight

The increased weight issue is a little bit of a "Red Herring". The completed fixed gear "hoop" without wheels and axles attached weighs about 45 lbs. To this weight may be added the wheel pants, which with attachment hardware comes to about 5 lbs. i.e. a total of 50 lbs. Compare this with the Infinity Retractable Gear. This weighs in at 32 lbs. per gear leg and 10 lbs. for the hydraulic pump which totals 72 lbs. right? Wrong! There are two gear doors, a mass of mounting hardware, wheel well inserts, hydraulic lines, and reinforcement. This puts the whole package at about 90 lbs. Initially this looks like a 40 lb. penalty compared with the fixed gear configuration. However there are compensations to be made when considering Increased Payload .

Possibility of gear-up landing

There is an old adage that says, "there are only two types of RG aircraft, those that have landed gear up and those that are going to land with their gear up." This is an old engineering expression that has been applied to all forms of engineering. To strike a corollary, the sailplane pilots say, "There are only two types of powered aircraft, those who have had an engine failure and ......". The pilots of the certified spam cans say ," There are only two types of home built........." Well you get the picture.

Again this is all a question of proficiency and a desire for a specific style of proficiency.

Increased cost

When I conducted my trade-off between the two landing gear configurations the landing gear hoop was $550 (now $680) to which costs for wheel pants and mounting hardware had to be considered. At that time the overall cost was about $1000 (now $1200). The retractable gear configuration cost $4200 but no wheel pants were necessary, however there are a number of hydraulic fittings and hydraulic lines to consider and the overall cost comes in at about $5000. Thus it's going to hurt your wallet to the tune of approximately $4000 (now $3800) to install the retractable gear. I believe that it is worth it, in particular when you compare the cost with the money thrown at gizmo's such as electronic ignition, fancy avionics, satellite dish actuators used for nose gear retract etc. I selected the cozy design as an affordable cross country aeroplane with high cruise speed and long range. The RG enhances this requirement.

With typical flying of 200 hours per year, the typical pay back time would be approximately six years assuming the fuel savings provided the only cost impact. The jury is out on whether there will any increased insurance costs since this will depend on the type of insurance required. Definitely if no hull coverage is required, and this is most often the case since the owner is also the builder, then there should be no increase in insurance because there is no additional risk to the insurance company.

Conversely, if you dispense with flying altogether, the cost approaches zero. We all know that to fly is expensive and the least expensive is flying ultralights. Most of us have moved on from that domain and have sought an affordable high performance aeroplane. Adding retracts increases cost but the performance gains are worth it. I used to own a Piper Warrior II. The previous owner had installed a speed kit which consisted of a few pieces of plastic which sealed the gaps between the control surfaces. The cost was a few thousand dollars. The cost of the  Infinity Aerospace retractable gear seems excellent value in comparison.

Increased building time

When the Infinity Aerospace retractable gear is installed during initial construction, compared with a retrofit, there should be no increase in building time. This statement is only true when, as usual, there is no departure from the installation plans. Typically builders who install RG are already, by definition, "departures from the plans" so it is likely that each installation will be different to some degree or other and with this premise the construction time will increase over the stock builder installing fixed gear strictly in accordance with the plans.

I am no different to most of the RG installers in that I have made modifications to the design. In particular I utilized the hydraulic system to provide a hydraulic nose extension/retraction system that will raise the nose with full weight pilot and co-pilot. (Note: the hydraulic nose lift is about the same weight as the crank system and significantly lighter than the Satellite dish actuator). I have also included a hydraulic landing brake actuator. With all of these modifications to the system I have extended the build time significantly.

Increased maintenance

Whenever new systems are added to an airframe the complexity increases. There is no way around this, and increased maintenance is necessary. Many builders actually enjoy the maintenance cycle however many do not. This is a personal thing and I have found that the benefits outweigh the downside.

Benefits

The above paragraphs list the downside of installing Retractable Undercarriage, but against that come some significant gains. The gains are varied and depend on the scenario through which the comparison is made. I have seen many very biased comparisons made such as increased weight means less payload, or the wheel wells eat into the fuel tank capacity so the range is reduced. It is this type of glib remark that makes me write this web page.

Increased speed

Data from a small sample of flying canard pushers, incorporating Infinity RG, claim increases in maximum speed in excess of 14 mph, and typically 16 to 18 mph. How does this differ to the Velocities, Wheeler Expresses (or whatever they are called these days)? These other birds are claiming much lower figures, why?

Of the aircraft that I have seen, all of the other models do not completely close up the wheel door. They do this for simplicity. The gear door is configured to be attached to the gear leg and, to allow for ground clearance the bottom part of the portion that covers the wheel is removed leaving an open half moon segment. This half moon segment results in significant drag thus overcoming the benefits of the RG. It is important to completely close up this area of the gear door to reap the full benefit of the construction.

Nat Puffer claimed a maximum speed of 216 mph from his aeroplane. With RG an expected 230 mph should be attained and this should be achieved with no increase to the fuel consumed. 

Increased range

It has been stated that the wheel wells eat into the fuel tanks thereby limiting range. If, again, the "same speed" scenario is adopted then the reduced drag increases the range by 17% assuming the same fuel is on board. Alternatively the same range is achieved using 8.5 gallons less fuel. The wheel wells occupy approximately 5 gallons each however most builders incorporate a sump tank in the location reserved for the fixed gear installation, (fair trade eh!) this space This returns about 5 gallons. The result is the RG system is deficient to the tune of five gallons but consumes 8.5 gallons less to do the same job. Five gallons gives about 5% more range ( 117% x 45 gals / 50 gals = 105% ). The five gallons in the sump is behind the CofG at an average FS114. This five gallons represents the mandatory 40 minutes of reserve fuel at landing so, providing flight is made in strict accordance with the regulations, no change in CofG will be experienced due to consumption of fuel. If the flight is made outside the regulations, i.e. emergency conditions a steadily increasing forward CofG may be encountered. 

A further 5 gallons may be recovered during initial build by not installing the foam block in the fuel strake put there for CofG reasons. This is possible to accomplish without disturbing the CofG because the fuel that is displaced by the wheel wells is also behind the CofG. Using approximate numbers: the wheel wells occupy ten gallons of fuel ten inches behind the mean CofG, the foam block recovers some five gallons of fuel approximately 15 inches behind the mean CofG. Thus the overall CofG for the fuel is moved forwards a small amount. The CofG for the fuel is at a fuselage station of FS103 which is 3 inches behind the mean flight CofG. Moving the fuel forward by any number, to a maximum of 3 inches, improves the balance since the change in CofG during the flight is reduced under these conditions, this is a desirable feature.

Increased payload

Again flying the "same speed" scenario, the 17% reduction in cruise fuel consumption results in a saving of 8.5 gallons on full tanks. This weighs 51lbs. yet the installation added only 40 lbs. this results in a payload increase of 11 lbs. The thought that the RG adds weight is ill founded when considering the mission profile. Only in the empty, static condition does it appear worse.

Centre of Gravity

The standard MK 1V cozy has a CofG range from 97.5 inches to 102 inches with a mean value at 99.75 inches. The fuel is located at FS103 thus resulting in a CofG that shifts as fuel is consumed. Incorporation of the Infinity Aerospace RG results in fuel being displaced from a location behind the CofG.
 
 

Fuel CofG with wheel wells installed.
  Weight (lbs) Arm (inches) Moment (in lbs)
Fuel
300
103
30900
Subtract Fuel in Wheel Wells
-60
109.5
-6570
Totals
240
101.375
24330

The table above indicates that the CofG is moved forwards to a position of 101.375 which is close to the mean CofG of the aircraft. The 240 lb change in weight over the course of a long flight will shift the centre of gravity far less than in the plans location of 103 inches. Thus an improved flight profile is attained.
 
Fuel CofG with foam block removed & wheel wells installed.
 
Weight (lbs)
Arm (inches)
Moment (in lbs)
Fuel
300
103
30900
Subtract Fuel in Wheel Wells
-60
109.5
-6570
Add Fuel replacing foam blocks
30
115
3450
Totals
270
102.9
27780

For those who are more fuel conscious and need more fuel capacity the removal of the foam blocks moves the CofG to a position 102.9 inches which is marginally better than the plans condition but not as ideal as the previous condition. I elected to use the latter scenario because of my goal for longer range. Clearly there is a CofG benefit when installing the Infinity Aerospace RG.

More attractive aircraft

Clearly a retractable gear aircraft is more aesthetically pleasing than the fixed gear machine even though, for most of the flight time, the retracted gear is not visible. Many builder are content with the knowledge that the aircraft is more appealing, even though to a limited few, who might be in formation flying or being viewed in a close pass to the ground.

Construction photographs.

The image alongside is the package that is received from Infinity Aerospace. The package included both gear legs, hydraulic pump, pre-fabricated wheel wells, emergency blow-down apparatus, control valves, necessary electronic control module, and full installation manual. The quality of the components I found to be excellent. There was one AD that was issued and this was accompanied by all the appropriate hardware and necessary drills to accomplish this task. The installation manual was exceptional and was far from the usual scrappy pieces of paper that usually accommodate the experimental aircraft equipments. Full schematic and physical layouts for the hydraulic circuits were included. The necessary modification to the spar was very clear and concise and I had no difficulty following the plans.

 

 

The construction photograph illustrated alongside indicates the port side strake prior to installation of the upper skin. Conspicuous by its absence is the foam block usually incorporated for CofG reasons but is not necessary when incorporating the wheel wells associated with the Infinity Aerospace RG. The brake line is clearly visible along the front bulkhead of the strake. This front bulkhead is installed as a single unit that was jigged to be an accurate straight piece prior to assembly. The leading edge is latter shaped using the pattern from the plans. The rear gull wing door that I have incorporated is also visible in this image.

 

 

The aluminium rectangle shown in the image alongside is the mounting plate for the capacitive fuel sender. This plate is latter covered with PVC foam sanded to a thickness to match the bulkhead. The resulting foam is then covered with glass in a wet lay-up. This prevents fuel from leaking through the screw threads and passed the cork gasket.
 
 

 

 

This image illustrates the retracting mechanism. Not shown in this view and incorporated after installation of the upper strake skin, are the three hydraulic lines which run within the thickness of the upper skin. The three hydraulic lines are enclosed in a small amount of pour foam so that expansion and contraction of the lines will not cause deformation of the wing skin. To accomplish this process the foam of the upper skin is routed using an electric router. The resulting groove is then painted with wet epoxy and allowed to cure to ensure fuel tight integrity. After cure the three lines are then installed with small foam spacers to separate the lines. Pour foam is then added and the excess is sanded off after cure. Once these processes are complete the top glass skin is installed in accordance with the plans.

 

This image shows the gear in its retracted position. The final glassing of this lower skin is not yet in place. A slight recess of this skin is visible around the extremities of the wheel well. This is to allow for the thickness of the gear door when it is fully closed.
 
  
 

 

 

The image alongside indicates a 2.25" diameter tube penetrating the leading edge bulkhead. This penetration provides for removal of the trunion whilst maintaining the smallest possible aperture for the gear door. The final leading edge of the strake is to be equipped with a small cover plate to cover this aperture. Consideration has been given to incorporation of a small 2" diameter quarts halogen observation light at this location.
 

 

 

The curving of the bottom skin is accomplished through scribe lines parallel to the leading edge. It is possible, and I have heard accounts, that fuel leakage may occur through the inner skin, into these scribe line. The fuel then passes along these lines to one end or the other of the strake where final leakage occurs. In order to prevent this eventuality. Two 1" wide grooves were routed across these scribe lines. These grooves were then filled with PVC foam without scribe lines. Wet micro was used to bond these pieces in place. The location of these fuel blocks was selected to be directly in line with the internal bulkheads.

 

 

This final view of the landing gear illustrates the arrangement when the gear is extended. Notice the use of carbon fibre to brace the structure in the region in which the strake bulkheads were modified. The strength of these components exceeds the original strength of the strake.
 
 

 

 

 

 

 

 


 



  Last Updated:   

Thursday August 31, 2006