Light Engine CG question

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braol
Posts: 8
Joined: Sat Jan 29, 2022 12:32 pm

Light Engine CG question

Post by braol »

Hello!
When installing an engine considerably lighter than the 'stock' Model A engine (say, a small Rotax), what is the solution to the obvious weight/CG issue? Is it just to add weight at the firewall and extend the motor mounts? I ask for 2 reasons...the obvious is the basic weight/CG issue, but the other is aesthetics combined with engineering: If one were to simply extend the motor mount enough to 'make up' for the reduced engine weight of a Rotax 503 for example, not only would the nose be MUCH longer but the moment-arm of having the weight so far forward would probably lead to stability issues. I understand adding weight, but what I am looking to do is to benefit from the reduced weight a lighter engine would give. I'm not trying to go all Part 103 or anything (I could care less, I have my commercial license and would be flying LSA, not Private), simply trying to get the max benefit from the reduced weight...like decreased length of takeoff and some increased payload. I've tried searching the forums but have not seen much from an engineering standpoint on this topic. One thought I had was to add a fuselage fuel tank forward of the pilot, but once the tank starts to empty you'd lose the weight which doesn't solve the overall problem.
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Richard Roller
Posts: 240
Joined: Mon May 22, 2017 11:14 am
Location: Olathe, Ks.

Re: Light Engine CG question

Post by Richard Roller »

A problem associated with a very light engine and an extended engine mount is the aerodynamic balance of the a/c. As you extend the nose of the a/c you tend to negate the effective tail volume of the a/c. Pitch and yaw damping will be reduced.

N34KP has a Model A and an extended engine mount. When originally finished the a/c was very tail heavy with a 230lb pilot. The original builder didn't want to tilt the wing, so we moved the engine 7 inches forward. There have been pluses and minuses with the move. A minus is, with my limited experience in other Piets, is that 34KP is not quite as stable in pitch and yaw as other Piets. A plus has been in large cross wind landings the a/c is very neutral in handling, with no tendency to nose upwind or down wind.

Perhaps some of the builders with lighter Continentals will chime in.
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taildrags
Posts: 637
Joined: Tue Mar 07, 2017 10:39 pm

Re: Light Engine CG question

Post by taildrags »

Wish we weren't so far apart geographically, Richard. I'd gladly have you fly 41CC to see what you think of the handling vs. Four Kilo Papa. I'm curious to know how much difference there is between Ford A Piets, Continental Piets, and Corvair Piets. My plane weighs about 636 empty and has the wing canted back about 4".

The first Piet (well, GN-1 actually) that I ever saw, touched, and sat in was Ernie Moreno's N2431 with Franklin 4AC-176-B2 engine.
left01.jpg
Very long nose (26" long engine mount), 803 lbs empty but that includes a 40 lb lead weight up in the nose somewhere to make up for the light engine.
cowl02.jpg

The plane has two fuel tanks, one up in the wing centersection and the other up behind the firewall over the passenger's seat. That tank really needs to have fuel in it to keep the CG within range.

Oscar Zuniga
Medford, OR
Air Camper NX41CC, A75 power
danoliver
Posts: 89
Joined: Sat Feb 25, 2017 7:58 am
Location: Cincinnati

Re: Light Engine CG question

Post by danoliver »

I'm not sure why you haven't found many results searching the forums, this subject has been talked about a lot. A good starting point is to pick up William Wynne's weight and balance book available from this site.
I just looked up the specs for the 503 and it looks like your talking around 110 pounds total depending on accessories. WOW that is light. My O-200 powered ship (no starter, generator or vacuum pump) has a max pilot weight of 210. That is with the wing shifted back 3-1/2". I used the A65 engine mount from the plans. I wonder if there is a limit to how far back you can move the wing? Have you looked into the Scout maybe as a better match to a lighter engine?
braol
Posts: 8
Joined: Sat Jan 29, 2022 12:32 pm

Re: Light Engine CG question

Post by braol »

Thanks for the replies.

I guess my new question is: Since I have not built the fuselage yet, and I already know I need to position the wings further aft, can I place my cabane attachment points further aft on the fuselage...or do I need to place them in accordance to the plans and adjust the length of the cabanes to fit the new wing position? Everything I've seen about moving wings relates to already-completed planes with lengthened camabes/turnbuckles. I figure, looking at the plans, that the cabane lengths need to be changed because the landing gear and cabane attachment points are located at the same point on the fuse. I am also wondering, theoretically at this point, how far aft the wing would have to move for the type of engine I am considering. If it is around 4" for an A65, how much further could it be for a 65lb lighter engine?
kennyrayandersen
Posts: 4
Joined: Sun May 21, 2023 5:40 am

Re: Light Engine CG question

Post by kennyrayandersen »

I’m looking at doing that as well due to some ‘modifications’ that I’m considering to increase the overall payload. One thing to consider when moving the cabane struts is where they attach to the fuselage. You may have have to/want to move the bulkheads slightly so that the attachment of the cabane strut has a solid load path into the fuselage. Otherwise you would just angle the cabane struts to move the wing aft. I’m thinking if I’m going to do that, just stretch the cockpit and move the cabane struts with it.
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taildrags
Posts: 637
Joined: Tue Mar 07, 2017 10:39 pm

Re: Light Engine CG question

Post by taildrags »

Dan Oliver: there are at least four things that affect how much the Piet wing can be pivoted without a lot of new things happening. One: as the wing is pivoted aft, it gets lower and approaches the top of the pilot's head. Yes, you can stretch the cabanes to make them taller and yes, you can include a cutout or flop-over panel in the trailing edge of the centersection so the pilot can still get into the rear cockpit with the wing shifted farther aft, but those things add their own complexities so it's better to build the plane with the CG as close to the right spot from the start and just use the wing shift to make small corrections. Two: the forward and aft cabanes are not the same length, with the forward cabanes being a bit longer to give the wing the design angle of incidence. As the wing is pivoted aft, that geometry changes. Three: the aileron control cables leading up to the wing from the pilot's control stick cross in an X in front of the pilot. As the wing is shifted aft, those cables approach the pilot's windscreen and the inside face of the instrument panel and reduce the available panel space. At some point, things touch. Four: the wing lift strut lower attach fittings are already a bit tricky to get everything to fit and line up, but as the wing is shifted aft the attach fitting forks have to be be adjusted in length and angle in order to mate with their attach fittings on the fuselage. Some builders have used Heim-type ball joints on the ends to permit adjustment of the angle, but it only gets more accentuated as the wing shifts aft.

Moral of the story: work within the limits that years of experience have shown to be workable and useful, or else be prepared to redesign a lot of little things that are consequences of making changes that have not been proven over time.

Oscar Zuniga
Medford, OR
Air Camper NX41CC, A75 power
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