Fuel system capacity

[JQCF1]

Hi all, Can somebody steer me to guidance on fuel tank capacity requirements when doing an engine upgrade? For example take out an A65/65Hp and put in an 0-235/118Hp. The original 13 gal tank is not quite adequate for decent ops. Is there some rule for Part 23 aircraft? Thanks for your comments.

John Q.

 

[GBor]

Here are two explicit statements about capacity:

(d) The total usable capacity of the fuel tanks must be enough for at least one-half hour of operation at maximum continuous power.

(e) Each fuel quantity indicator must be adjusted, as specified in §23.1337(b), to account for the unusable fuel supply determined under §23.959(a).

They are from FAR23.963. There is a whole section on Fuel Tanks starting in 23.951.


Garland E. Borowski, PE
Star Aviation

 

[der8110]

Fuel tanks might be the least of your problems when almost doubling the horsepower. First is maintaining balance, which usually means putting the engine further forward. The the engine mount then is a nice stress problem as it attaches to the firewall. This leads to problem two, which is the tail loads are now increased, and in the wrong direction. Problem three is performance - you will be at Vne vitually the entire time, transitioning to and from cruise becomes quite a task and slowing down without shock cooling the engine is a consideration.

Someone once told me an appropriate analogy: "you go and get the double-D boob job, then wonder why your back is sore".

 

[dgapilot]

Gross weight will be another problem. The A65 is 170 lbs dry, and the O-235 is 246 lbs dry and carries 2 quarts more oil. Right off you loose 66 lbs of usefull load before even thinking of adding fuel. Then if you add an electrical system and radios, you may not even have a single place airplane unless you are very small. You will have a single place airplane. I've seen a Champ (7AC) with an O-235 and I think it had a usefull load of less than 300 lbs and it had the stock 13 gallon tank.

 

[dgapilot]

Assuming you are refering to something like a Champ, CAR 4a is the certification basis. 4a.605 states capacity has to be at least (.15 * max continious horspower) gallons. The 118 take off is actually a 115 hp engine, so .15 * 115=17.25 gallons minimum for your application.

In short, when making alterations, it is best to start with the original certification basis and work from there. Many times FAA will require upgrading to the regulations in effect at the time of application for the STC, but not always. If you were to try to modify a Champ to FAR 23 fuel system standards it would be impossible, so CAR 4a will be the prevailing document.

I don't think upgrading a 7AC to an O-235 is a viable project based on gross weight alone. Stick with the A-65. There is an STC for the O-235-C (115hp) on the 7AC, STC SA3-372, but doing a search, I can't come up with the owner of that STC. I don't know about your FSDO, but I know none of the ones I work with would give a field approval for this alteration. You might also want to check out Order 8900.1 chapter on major alterations to see how it would fit in if you need to develop engineering data to apply for an new STC.

 

[der8110]

CAR 3.440 requires one gallon fuel for every seven max continuous horsepower, if it is a model with such a cert basis. Changed product rule wouldn't force you to latest & greatest (14CFR23), but, you should think STC. Advisory Circular 21-40A gives a nice overview of the STC process.

How about a Rotax 912? 121 lbs which gives a little wiggle room for radiators, etc., and electrical system for the glass cockpit and three-axis autopilot that surely follows. 80 Hp which gives the "kick" you are looking for, but not too extreme. "Just because you have a spare O-235 laying around" is not enough economics to pull this off. Then again, you can do whatever you want under experimental.

 

[GBor]

Just try not to kill yourself "experimenting"...

 

[JQCF1]

My thanks to all contributors to this string....Just what I needed. Your comments are all on the point. In actuality it is a vintage 11BC Chief, now with an STC'd 0-200 and we are going for twin 13 gal wing tanks and no fuselage tankage. Identical wing as Champ that has these tanks as option. So for 100Hp we need min of 15 gal and with 26 we are OK but will do a very thorough Wt & Balance exercise.
Thanks again for all the prompt comments, John Q.

 

[rb1957]

fitting this powerful an engine (compared to the original one) will also need a detailed flight test, no?

on W&B, what are you going to compare your results (calc or weighed) to ? i'd expect that the flight envelope would be quite different ... you're obviously after a faster plane, how are you planning on expanding the envelope (at least in the V direction?

Is there an up-engined version around for comparison ?