Question about twin engine design

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moparrob66

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Moparrob
Do multi engine airplanes offset the thrust angle from centerline more than a single? I understand some if not most singles have the crankshaft centerline offset slightly to counteract left turning tendencies. Since asymmetrical thrust is a bigger issue with a multi, would angling both engines away from the center of the fuselage help any or do they already do that? Seems like it could really help single engine behavior without hurting normal performance.

Im not multi rated but I have some right seat time in an Aztec, C-45 and Aerostar. Never pondered it before but now I gotta know...
 
moparrob66 said:
Since asymmetrical thrust is a bigger issue with a multi, would angling both engines away from the center of the fuselage help any or do they already do that?
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If talking about P-Factor, doubtful. With twins, you have other dynamics at play like direction of rotation of each engine. But with a loss of engine thrust on one side, which engine(s) are considered critical would have a bigger effect as it includes more dynamics than just P-Factor like torque.
 
Been a long time since I did any work in aircraft design (25+ years) and then it was only single engine aircraft.
But no one else has really answered so I will say what I can about it.

But I bet it is unlikely the engine are mounted with a 0 Thrust angle. It of course depends where they are mounted but they likely have some down thrust angle. I seem to recall the twin engine aircraft, especially with counter rotating props it is likely they have the engines mounted a few degrees outward to help maintain control with single engine operations. (reduces the yawing when on a single engine)

When I once consulted one of the engineers at the at the Ames Research wind tunnel about how to figure out the horizontal tail incidence for the airplane I was working on, he told me that they could spend many hours and a lot of money trying to figure it out in the wind tunnel and they would still get it wrong and we would end up adjusting it once we flew the airplane. So we just made our best guess and designed the tail mounts so we could adjust the tail incidence. I suspect airplane manufacture/designers do the same for thrust angles, Try a few and figure out what makes the airplane handle the best.

Brian
CFIIG/ASEL
 
Great! Thanks! I'm considering a twin experimental and looking at ways to design some safety into it. Its a long ways off though.
 
moparrob66 said:
I'm considering a twin experimental and looking at ways to design some safety into it.
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1) Design an interlock that prevents the pilot from turning off the wrong engine during OEI ops
2) Use counter rotating props that turn toward the fuselage
3) Use engines that can maintain failure altitude during OEI ops
 
Oh it will be way jankier than that. Turbines are far out of my budget. Picture the two of the cheapest homebuilts stuck together a-la P-82 twin mustang. Something like that. It'll make a Piper Geronimo seem like a G5 by comparison. Looking at the Affordaplane and Rotax two strokes. If it has an N number and it doesnt kill me, I'll build multi time on the cheap, and become proficient at single engine landings and changing rings...
 
Thats awesome! Ive never seen one but I'll be looking... Super cool!
 
Saw this twin while getting gas on Tuesday. Turns out it's a UAV and is being developed/flown by Purdue University students with their ground station (guessing that's what in the 2nd pic).
Maybe you can ask them if it's got enough #UL for you to throw a seat in it!
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Cri-Cri is awesome. If I could build up some multi turbine time in that bad boy, I'd have all kinds of job offers from the heavies!
 
moparrob66 said:
........... a multi, would angling both engines away from the center of the fuselage help any or do they already do that? Seems like it could really help single engine behavior without hurting normal performance.........
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Yes "They" do that! Turbine Grumman Goose.......
2021-07-28-15.45.39-1520x1140.jpg

 
Oh that turbine Goose is the stuff dreams are made of. Hard to tell if the spinners toe out or if its an optical illusion from the long, narrow nacelles. I guess it would be easy enough to try different angles with a protractor and some fender washers. I need a parachute too.
 
moparrob66 said:
Oh that turbine Goose is the stuff dreams are made of. Hard to tell if the spinners toe out or if its an optical illusion from the long, narrow nacelles. I guess it would be easy enough to try different angles with a protractor and some fender washers. I need a parachute too.
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Yep. Sure looks angled! I bet its got good rudder authority one engine. Great shot....thanks!
 
moparrob66 said:
Great! Thanks! I'm considering a twin experimental and looking at ways to design some safety into it. Its a long ways off though.
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Thinking about designing your own or buying something already built? Curious as to what you are thinking as there’s not a lot out there. Beside the Velocity twin and Aircam (and discounting the Cri-Cri) only aware of 2 twin RVs one of which crashed on a high speed taxi test that went South.
 
Im considering adapting two affordaplane fuselages together like a P-82 Twin Mustang
 
moparrob66 said:
Im considering adapting two affordaplane fuselages together like a P-82 Twin Mustang
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Affordaplane? That’s a unicorn. And two unicorns stuck together make a two-headed antelope.
 
Yeah its definitely out-of-the-box thinking, but if its not a complete death trap, it could be better than renting a proper certified twin for building time. I would definitely get my multi rating in something normal before I tried to fly it.
 
moparrob66 said:
Yeah its definitely out-of-the-box thinking, but if its not a complete death trap, it could be better than renting a proper certified twin for building time. I would definitely get my multi rating in something normal before I tried to fly it.
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It won’t be better. The effort required to accomplish what you propose is monumental. Doable, google Twin Jag, but it’s a major engineering effort and probably the hardest way into twin ownership. Just trying to manage expectations.
 
tsts4 said:
It won’t be better. The effort required to accomplish what you propose is monumental. Doable, google Twin Jag, but it’s a major engineering effort and probably the hardest way into twin ownership. Just trying to manage expectations.
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@moparrob66, in case you didn’t know, this guy built an incredible plane. He knows a bit about the subject.
 
Youre probably right. We are using our 172 for my son to get his private, then both of us want to get instrument rated in it. He wants to be an ag pilot, and he's been told that he needs at least 500 hours of tailwheel time. Spending $50k on luscombe rental isnt an option. We're thinking of building an Affordaplane, which appears to be about the simplest taildragger in existence. If he needs twin time, the simple design theoretically could be doubled and tweaked into a simple twin.
 
moparrob66 said:
Youre probably right. We are using our 172 for my son to get his private, then both of us want to get instrument rated in it. He wants to be an ag pilot, and he's been told that he needs at least 500 hours of tailwheel time. Spending $50k on luscombe rental isnt an option. We're thinking of building an Affordaplane, which appears to be about the simplest taildragger in existence. If he needs twin time, the simple design theoretically could be doubled and tweaked into a simple twin.
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Here's a cheap way to build tailwheel time and have fun doing it.

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Thats pretty cool. Just flying it back to Idaho would be 1/10 of the time he needs!
 
yak110.com

Yak 110

Official Home of the Yak110 Project. Yak 55 + Yak 55 + GE Jet Engine = Yak 110 the newest and most unique aircraft on the airshow circuit today
yak110.com yak110.com
 

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Rutan Boomerang may have some ideas you like.

aerocrafted.com

The Rutan Boomerang

Meet the Boomerang: a one-of-a-kind experimental aircraft designed by Burt Rutan and maintained and operated by the Aerocrafted team. Photo by Chad Slattery The number one question Aerocrafted gets asked when operating the Boomerang is, “Why does it look like that?” The Boomerang’s striking...
aerocrafted.com aerocrafted.com

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Another great idea! Im not at all discouraged! Thanks for all the input ...great crowd!
 
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