Engine positioning

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Mark Roberts

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Apr 19, 2021
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I am not sure where the engine should be mounted in relation to the cg and center of drag. I thought the center of thrust should be inline with the cg to not induce any moments but if it is inline with the center of drag it would cancel out the moment created from the drag. Does the center of thrust need to be at a specific point or does it not matter as long as it is in a general area?
 

Riggerrob

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Thrust-line can be above or below the center of drag, but if it is too high or too low, it can create pitch problems when you change throttle settings.
Thrust-lines are rarely much below C. of D. because prop blades would gouge the runway ... and we all know how expensive runways are.
Hah!
Hah!
Boeing 737 have engines mounted so low that they have thrust-lines below their C. of D. When you advance the throttle, the nose pitches up and when you reduce throttle, the nose pitches down, just like you want it to.

OTOH if the thrust-line is significantly above the C. of D. it can exacerbate trim problems. Lake Amphibian is among the worst with its single prop mounted high above the fuselage on a pylon. When Lake pilots advance throttle, their planes try to dive. The opposite occurs when they reduce throttle just before landing. Trim problems are so bad that Lake split the elevator in half so that half act as trim tabs and the other half are controlled by the yoke.
 

BJC

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Boeing 737 have engines mounted so low that they have thrust-lines below their C. of D. When you advance the throttle, the nose pitches up and when you reduce throttle, the nose pitches down, just like you want it to.
Until there is too much pitch up, a la the 737 Max and its MCAS.


BJC
 

TFF

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It matters plenty, but reality always sets in that keeps perfect possible. You and stuff have to fit in the airframe. Engines are usually only off the shelf. How big an airplane are you designing to fit it all in? Where is the wing, tail, and you in it all?
 

TFF

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The site has been helping with that plane for a while. It’s too small to be ideal and fit a person and be strong enough with the constraints of Pt 103. Go up in weight and speed allowance and something like a Cassutt can more ideal.
 

Mark Roberts

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Apr 19, 2021
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I am part of the group working on that plane. Our fuselage will fit a person and according to all of our research and simulations will be plenty strong. We are also under the weight requirements for an ultralight. We are determining the center of drag for the engine location. We assume the center of drag will be higher than we can mount the engine so we were curious about how far below the center of drag the center of thrust can be without creating a large moment.
 
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Old Koreelah

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One aspect often overlooked is the need to offset the engine slightly to one side to counter thrust torque caused by the prop’s direction of rotation.
One way is to allow at least 10mm adjustment on the engine mounts.
 

BJC

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One aspect often overlooked is the need to offset the engine slightly to one side to counter thrust torque caused by the prop’s direction of rotation.
One way is to allow at least 10mm adjustment on the engine mounts.
There is an old thread here discussing engine angle.


BJC
 

bmcj

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One thing to keep in mind is that an engine usually makes up a noticeable percentage of the total weight of an aircraft (especially in ultralights. That means that unless the occupants are quite a bit higher or lower than the engine, the vertical CG position will probably closely follow the engine placement.
 
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