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Old 08-13-2019, 12:36 AM   #1
BW. Hicks
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Mounting Engine on Old Plane


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I have an old scratch built plane that I would like to mount an engine onto. The old mounts are gone and I would like to mount dubro antivibration mounts on it. Where in relation to the wing is a good place to start? Do I level the fuselage and pick the center line and center the engine on that? I know down/side thrust can be added. I'm just looking for a place to start then tweak it from there....
Hopefully what I have asked makes sense. Thanks!
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Old 08-13-2019, 08:00 AM   #2
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Re: Mounting Engine on Old Plane

Most installations will include side and down thrust angles to counter torque and power change effects.

With many kits and ARF's I've seen that is often built into the firewall installation and to keep the prop shaft centered the mount will be offset up and to the left side, (from the position of the pilot seat).

If your firewall is perpendicular to the centerline running down the aircraft you will need to add a tapered shim plate to align the mounts to the engine. If memory serves Sig or another used to sell these with different angles years ago.
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Old 08-13-2019, 08:51 AM   #3
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Re: Mounting Engine on Old Plane

Side and down thrust are only needed when the plane being powered is a high drag airframe.
If the model has a flat bottomed thick wing it will probably need off set, both down thrust and right thrust. If the model is a sleek, pattern style airframe and a fully symmetrical wing it can usually get away with a 0 - 0, no off set firewall.
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Old 08-13-2019, 10:05 AM   #4
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Re: Mounting Engine on Old Plane

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Originally Posted by Harvard 387 View Post
Side and down thrust are only needed when the plane being powered is a high drag airframe.
If the model has a flat bottomed thick wing it will probably need off set, both down thrust and right thrust. If the model is a sleek, pattern style airframe and a fully symmetrical wing it can usually get away with a 0 - 0, no off set firewall.
DOWN thrust is, if needed, usually on high wing planes.
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Old 08-18-2019, 11:52 PM   #5
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Here is the plane and wing shape. I want to put a YS 120 in it. It almost fits with the original motor mount that is in the plane. Should I just mount it and try it?
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Old 08-24-2019, 09:48 PM   #6
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Thanks for your comments on mounting the engine. How would I find the cg on this plane? Is there an easy way to balance it for flight once the engine is on? I'm not sure if this engine is heavier than the one that was originally on it. I think it probably is though. I'll have to increase the size of the fuel tank because the 12oz that is in it wont last long running the .120
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Old 08-25-2019, 08:33 AM   #7
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Re: Mounting Engine on Old Plane

wing position and air foil have a lot to do with engine position and thrust . a low wing plane tends to pitch down when the motor is above the wing and a high wing plane tends to pitch up when the motor is mounted below the wing . as a general rule I would start with zero thrust on a low wing plane and some down thrust on a high wing plane .

all wings regardless of there design create drag if you pull above or below the point of drag it causes pitch , you can use the design of the air frame or engine thrust or both to counter the forces ..

that air foil looks like it would have the balance quite far forward ..
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Old 08-25-2019, 11:44 AM   #8
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Re: Mounting Engine on Old Plane

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Originally Posted by BW. Hicks View Post
Thanks for your comments on mounting the engine. How would I find the cg on this plane? Is there an easy way to balance it for flight once the engine is on? I'm not sure if this engine is heavier than the one that was originally on it. I think it probably is though. I'll have to increase the size of the fuel tank because the 12oz that is in it wont last long running the .120
Hands on experience with the same model is better but here is a piece of free software allowing you to calculated center of gravity; start with "static margin" midpoint of 10%:

https://rcplanes.online/cg_calc.htm
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Old 08-25-2019, 12:08 PM   #9
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Thanks ramjet and michaely. Interesting info about the high wing and low wing. I'll do my best to figure out the cg calc from the link as well.
I just discovered this must have had an os fs 120 already in it because it says that on the engine mount. So whatever the original builder did to set up the cg makes me think an ys 120 shouldn't throw things too far off. Does this make sense?
See pics attached.
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Old 08-25-2019, 03:46 PM   #10
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Re: Mounting Engine on Old Plane

Without using the calc a quick and dirty CofG locator would be at the spar, generally the thickest point of the wing.

There are different versions of each, but due to some internal and external design differences the YS would likely be slightly heavier than the OS. Either may fit that Dave Brown mount, but moving it fore and aft even slightly would have a greater affect on balance as it is usually the single heaviest element.

Looking at the pics would doubt it was one of the 4 early style YS since there is likely no access to the regulator mounted below the crank:

https://www.google.com/search?q=ys+1...w=1267&bih=579

Depending on condition, a normally aspirated 120 4-stroke should run for approx 7 minutes on your 12 oz tank. However, needle settings can alter consumption up to 2X.

Having said that your throttle work holds ultimate sway. Full throttle the entire flight will drastically affect how long that will last.

I can present examples from my own experience: Saito 125 would require almost double the fuel until break in, then radically less. On a 16oz tank it will now end an average 9 min flight with half remaining. Not the case at first since I risked dead stick at the same duration.

The YS is a completely different animal. The 120 can put out the same torque as a normally aspirated 150 at full throttle. The negative is it also has the same fuel needs as the larger cube example, (no such thing as a free lunch).

One other positive is YS has a near instantaneous throttle response, the reason these were popular with pattern flyers.

On a final note, there are several factors that would normally control what engine gets mounted and a lot of times the original designer/manufacturer will list what is best, (or at least been tried). Finished weight, and type of flying will be the final arbiter with everything else following. If you want to drag shock waves through the air you would want the biggest engine and tank you could fit. I have a Saito 100 powered Super 'Bolt, (listed for a max 91 four-stroke) and it comes down with half what I believe is a 12oz tank. I recently watched a vid where someone stuffed a Saito 125 ahead of the firewall!

Depending on what type of performance doubt that would run fill tilt the entire time, but could be wrong. Most of my flying with that same 'Bolt uses moderate throttle, however at a show in front of a paying crowd last weekend kept it near full to get snappy response and to bring it back to show line quickest. Under those conditions used the same amount of fuel in half the time.

Although I have and worked on 4-strokes down below 50, most of what I run are in the 90-125 range, the exception the OS 160 twin. Bring that one up since stock would absolutely gob fuel and oil regardless of settings. Once modded the carb it will reliably idle down to close to 2K all day, sipping fuel the entire time. That one is getting mounted to a bipe with a 16oz tank and feel comfortable expecting the same average 9 minute flights with lots remaining.
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