Design > Stunt design

Charles Mackey - Monster FM Feb/Mar 1961 engine offset?

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Dennis Toth:
Anyone have a scanned copy of the . February/March 1961, page 17, Flying Models, “Monster” (offset engine/wing) by Charles Mackey? I am curious about his thinking with the big offset out thrust of the engine in this series of design he did. I think this is the extreme in engine offset (I think the BiSlob is close) and wondered if he gave any design insight about it's use and how it actually impacted performance.

Best,   DennisT 

Steve_Pollock:
Dennis, not the best copy, but readable (part 1)

Steve_Pollock:
(part 2)

Steve_Pollock:
(part 3)

Brett Buck:

--- Quote from: Dennis Toth on October 28, 2020, 09:41:14 AM ---Anyone have a scanned copy of the . February/March 1961, page 17, Flying Models, “Monster” (offset engine/wing) by Charles Mackey? I am curious about his thinking with the big offset out thrust of the engine in this series of design he did. I think this is the extreme in engine offset (I think the BiSlob is close) and wondered if he gave any design insight about it's use and how it actually impacted performance.

--- End quote ---

   Note that this large offset is still straight through the CG, rather than causing a huge thrust line offset. So to first approximation it creates no torque in yaw. Large amounts of offset in a conventional model also causes a huge amount of yaw torque (which I contend you don't want). This one just aims the engine out to get some more line tension, it (to first approximation) doesn't create a lot of yaw torque at the same time. The approximation being that there is still a fin/rudder on it.

      So, simplistically, all it does is put some of your maybe 1 lb of thrust pulling on the lines. That is feeble compared to the 6ish lbs you would get from centrifugal force, which is why it isn't very effective. The added line tension F=thrust*sin(offset angle), figure you have 1 lb of thrust in flight, the offset angle is about 30 degrees, eyeballing it, so 8 ounces of added line tension, and 14% loss of maneuvering thrust (f=thrust*cos(offset angle)).

     So, engine offset is a feeble source of extra line tension if it goes through the CG.

      Howard keeps telling me that this sort of reasoning is simplistic, which is it, but so far I have seen nothing about thrust offset that is not pretty predictable from looking at the thrust vector.

       Brett

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