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Q-talk 144 - Corvair Weight and Balance

Update 06/24/2009

Oh shit. The airplane is nose heavy!

Last week I invited an EAA tech advisor to conduct a mock DAR inspection. He recommended that I do one more weight and balance, and , if all was well, schedule the DAR inspection. On Wednesday I had an A&P, IA do a weight and balance. He utilized digital, calibrated scales. Here are the A&P's results and recommendation to add 30 lbs. to the tail.

Here the airplane has been leveled according to WL15. The scales were then placed under each wheel. Each scale is connected to the black CPU which provided a digital readout of the weights and the total weight. This data was inserted into a weight and balance spreadsheet.

I had done weight and balance prior to Wednesday using some old platform, balance beam scales borrowed from my EAA chapter. I never had confidence in these scales because I could not match the balance weights with the three scales. Also, I made a calculation error. I divided the total moment by the total weight to get a CG. But I tried to use the CG on the QAC weight & balance envelope. This happened because, for years, I have been calculating Cessna W&B by dividing total moment by total weight, then locating the CG on the envelope. I now know to use the total weight and total moment to find the CG location on the QAC envelope chart.

I will be transporting the AC back to the home garage Monday-Tuesday. More updates when I get there.

Update 07/18/2009

The aircraft has been back at the garage since July 3. Since that time I have been attempting to determine accurate arms for the occupants, header fuel, main fuel, and baggage. Also, I needed to check the original arms for the main wheels and the tailwheel. During this time, I also had to complete a flight instructor renewal course.

Here is the aircraft on the trailer in a level flight attitude. Measurements were made to determine the arms for the main wheels, arm=35.375 in.; and for the tailwheel, arm= 207.5 in.

I am using identical digital bathroom scales for the main wheels. They have a capacity of 425 lb.. I used a board to spread the weight over the surface of the scale and a 1/4 in. aluminum plate with wheel stops to position the tire. The scales have a "zero" function which eliminates having to subtract a tare value.

Here is the tail scale. It is also digital with a zero function. Situation: In the empty weight configuration, the tailwheel scale would not register on the scale. I discussed this with Chuck Widowski who suggested adding a bucket of something over the tailwheel. So, I used a small bucket of spackle, 8 lb..; read the value, then subtracted the 8 lb.. Worked like a charm.

Using this setup, I was able to acquire a set of weight measurements (L.wheel, R. wheel, Tail wheel) for the following configurations: empty (full oil), pilot only, occupant only , full main fuel tank only, full main and header tank, baggage only. Problem, but no surprise; my empty weights were different than those from the calibrated digital scales. I used my weight values to determine the arms. Many thanks to Paul Fisher for his explanation of this process and a spreadsheet to speed up the calculations. The data shows that I am still nose heavy, but now I have the means to measure my progress. Further, I used the arms to project another weight and balance analysis using the empty weight values from the calibrate scales. It shows that if I limit the header to 6 gallons (thanks Earnest) and carry a passenger and baggage, I can get the airplane within the forward limit of the envelope. I can't achieve this with a single occupant without adding ballast in the passenger seat.

So, on to the weight reduction and shifting. Here is a list of possible changes.

  1. Move alternator to the back of the engine
  2. Move the starter to the back of the engine
  3. Create carbon fiber cooling plenums; remove the baffeling.
  4. Move the engine toward the firewall and partially recess it.
  5. Modify the engine mount to accommodate #4.
  6. Create a new cowl using carbon fiber.
  7. Move ring gear to back of motor.
  8. Eliminate alternator pulley; use harmonic balancer belt groove.
  9. Fabricate brackets for alternator and starter.
  10. Consider lighter starter if available.
  11. Fabricate lighter intake runners to accommodate engine position change.
  12. Modify oil filter to accommodate changes.
  13. Either limit the header capacity or modify the header tank and move the header tank further back (about 4 in.)
  14. Consider backup battery alongside the current battery (FS117).

I ran across some photos which were sent to me by Larry Koutz prior to his unfortunate death. Larry was modifying his Q200 by installing a Corvair

This is the back of Larry's Corvair engine. The alternator is positioned over the stock oil cooler and the starter is position over the oil gallery Delcotron and next to the distributor. The ring gear is bolted onto the harmonic balancer with a spacer. The alternator is driven by a belt from the balancer to an oversized alternator pulley. It appears Larry was planning to use the Corvair's stock exhaust logs for headers.

Side view of Larry's engine. Intake runners are fitted around the oil Delcotron and close to the back of the engine. The oil filter and manifold is positioned over the engine top cover. Larry was planning to use his Q200 cowl, I believe. The prop hub is unknown to me.

This photo shows the fitting of his MA3-SPA carburetor. Also visible is some of the lower structure of the engine mount.

Here is another photo of the upper portion of the engine mount. Larry was a fan of the swing-out engine mount. Tabs for the swing pivots can be seen on the vertical square tube.

This shows the engine mount positioned on Larry's firewall. The square tubes are flush on the firewall. The foreground shows the standard Corvair mounting tray. Apparently, Larry was modifying his header tank capacity and/or position. Of interest are the large round tubes to the right and left sides of the firewall. I believe Larry was planning to eject his exhaust through the fuselage just over the canard . He had done some testing and verified a low pressure area over the canard . The large tubes would surround the exhaust pipes and, using the exhaust pulses evacuate engine cooling air.

Update: 09-04-2009

It has been a while, and I have been working on it.

Here the engine has been removed from the firewall.

Stuff removed from the firewall and and tentative outline of cutout for the engine.

Back of the engine. Parts which will be behind the firewall include: steel braided oil hose (provides oil supply to 5th bearing at the front of the engine), oil filter, harmonic balancer, and about 1/3 of the distributor. I have decided to keep the starter and alternator on the front of the engine.

This measurement shows the distance from the firewall of a rear engine mount bolt. A new engine mount is being constructed with this bolt located 5.5 in from the firewall, moving the engine back 5 inches. The forward face of the harmonic balancer will be even with the firewall. The stock folded fin oil cooler is about 1/2 in. forward of the firewall.

Here is a photo of the new engine mount being constructed. Notice the firewall opening is much smaller than originally planned. Also, notice that the header tank has been removed. The inside of the fuselage has been prepped for reinstalling the header.

Here is the previous header tank. It will be reduced from 9 gallons to 4.

This photo shows the engine being positioned in the proposed location.

Another view.

This is a detail shot of a hinge for the swing-out motor mount. All of the welds are tack welds.

This is a detail of the lower hinge and the left lower firewall bolt fixture.

This shows the engine mount swung out and supported by the hinges. Also, notice the anti-torque tube at the lower right side of the mount.. The white tubes were cannibalized from the previous engine mount.

Another view. I have added a tube across the the top (not pictured here) to strengthen the mount during swing-out.

This is a photo of the engine mount bolted to a scrap steel frame to prevent creep of the attach points during finish welding. The mount is upside down. Later, this frame will be used to test the integrity of the engine mount to 5 G's.

Update 01-02-2010

It has been a while. I was out of town during the Thanksgiving holidays. My son was in town during the Christmas holidays. And I had to help the Ohio State Buckeyes win the Rose Bowl on New Years day. I am still working on the changes.

Initially, I was considering moving the alternator and starter to the back of the engine, it appears I can achieve my W&B goals without this change. Also, I was thinking about fabricating cooling plenums using fiberglass. This photo shows foam forms carved. I decided against this modification because I could not figure how to get the incoming air past the alternator, drive belt and brackets without leakage on the right bank. So, I am going to keep the baffeling for engine cooling.

Earlier, I showed a photo of the previous engine mount with the rear engine bolts at 10.5 In. from the firewall. The current location location of the rear engine bolts is now 5.25 in.


With the engine pushed back 5.25 in., the previous engine intake runners no longer work. Here is the MA3-SPA carb bolted to a new runner fitted with a bracket which will be bolted to the 3/8 in. bolt hole at the bottom center of the Delcotron. The barbed fitting is the vacuum pickup for the manifold pressure sensor.


This photo shows the carb and runner mounted to the engine.

This photo shows the intake runners continued to the right side of the engine. The left side runners look similar.

With the changes to the engine mount, the previous air filter and carb heat box did not fit. The foam above is the rough shape of a new air filter/carb heat box.

Here is the air inlet portion of the new box with a K&N filter in place.

Just as the previous intake runners did not fit the new engine position, neither did the exhaust headers. Here is the right headers being fitted prior to welding. The material is prebent SS tubing.

Here is the finished left side headers. The completed air filter/carb heat box is installed under the engine mount. I have not yet connected the carb heat control linkage.

This is the initial fabrication of the closeout box for the firewall between the engine compartment and the cockpit. I am using 1/8 in. plywood.

This is the first iteration. After positioning it on the cockpit side of the firewall, I found interference with the rudder/brake pedals.

This is the final configuration of the firewall closeout box. Both sides will be reinforced with BID and epoxy. The firewall side will then be covered with fiberfrax and Stainless steel sheet. I will make the box removable in the event I need to access inside the firewall at some time in the future.

This photo shows the cockpit side of the firewall box. Rudder pedals and toe brakes now have clearance. I will follow Jerry Marstall's heads up about preventing the brakes fluid lines from rubbing against each other. Notice that, unlike to Q2 plans, the motor mount attach bolts are at the perimeter of the firewall. 5 BID pads at the four corners extend from the firewall back onto the interior of the fuselage to transfer the thrust loads to the fuselage. It is not necessary to reinforce the middle of the firewall per Q2/Q200 plans.

With the box in place, the motor mount/engine was bolted to the firewall to check clearances of the oil filter, oil line to the 5th bearing, harmonic balancer, oil pump, and distributor. Of interest to Corvaircrafters is the Mark Langford style crankcase breather at the top center. I still need to attach an oil vent line overboard although Mark says he has never seen a drop of oil from his.

This is the engine primer solenoid which was moved from the firewall and attached to the motor mount. Below it is the voltage regulator. It too was removed from the previous firewall location and attached to the motor mount. I will be reversing the positions of the regulator and primer solenoid because of the possibility of fuel leaking from the solenoid onto the regulator.

Again, due to the changes in the motor mount, it was necessary to change the fuel line path to the carb. This photo shows the line coming from the bulkhead fitting at the top left. Next is the inline filter (Earl's). Then the fuel flow sensor which is bolted to the motor mount. Followed by a 180 deg. tubing bend to the fuel pump which is also bolted to the motor mount.

So, now I can remove the four engine mount bolts and detach the fuel line at the bulkhead fitting, and swing out the engine etc. to gain access to the back of the engine.

Update 11-01-2010

It has been a while....I have finished the changes to the aircraft, calculated a final W&B, , and returned the aircraft to the airport.

This is the final shape of the header tank. At the top left is the 3/8" inlet fuel line. On the right is the 1/4" air vent line. At bottom center is the 5/8" return fuel fitting (connected to the overflow tube inside the tan) to the seat tank. Not seen is the sump screen and fitting lead to the engine.

Here the header tank has been installed in the fuselage wedged into position while the epoxy cures. Notice that I have decided to remove the stainless sheet due to weight and multiple holes acquired during various changes.

Instead, I have painted the firewall with three coats of a fire blocking paint from Aircraft Spruce after repairing all the unnecessary holes

Here is a photo of the stuff. I lit the acetylene torch and tried it out with brief exposure to the torch flame; it charred, bubbled, thickened, but did not fall away. However, I think I will add a Halon injector through the firewall as suggested by Jerry Marstall.

This is a photo of the firewall closeout box. It has a layer of fiber frax on the engine side and is coated with the fire barrier paint inside and out.

The engine mount failed the load testing twice before I got it right. It was failing at the bed attach points.

I solved this issue by welding metal gussets or straps to the locations where the bed was breaking away form the firewall frame.

Here is the final engine mount test where it sustained 10G's without distortion; 2400 lb.. down force and 320 lb.. torque simultaneously.

It was necessary to reroute all the wiring, firewall forward. Next, was reconstructing the cowl.

I cut the top cowl about 1.5 in. from the firewall; Then cut out 5.25 " in front of the first cut. The same was done to the bottom cowl.

The two cowls were pieced together, gaps filled and contoured.

Taking shape!

It was necessary to reconfigure the bottom and cowl flaps to accommodate the air box changes

Upon completion of the changes, a new weight and balance was calculated resulting in N240JS conforming to the Q200 weight & balance envelope. Here is a copy of the calculations.

Here is a spreadsheet of the various flight scenarios showing that the aircraft conforms to the CG envelope during projected scenarios.

I am truly grateful for all the encouragement, advise, and help provided by Quickie Builders Association and EAA Chapter 118. Without your support I would not have been able to complete all these changes.

Joseph Snow
Q2xx/ N240JS