Engine angle

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Wagflier

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I need input from an engineer, if there are any who monitor this forum.
What are the ramifications of changing the installed angle of the engine relative to the aircraft centerline? If a particular design calls for, say 2 degrees up, and 2 degrees right and you install it with 0,0 can that be a dangerous thing, or will it just require additional control inputs to control the aircraft? All due respect to everyone on the forum, I don't want opinions, only facts
Thanks in advance
 

Meyersg

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Our KIS4 with a Mazda rotary was installed 0 degrees instead of the recommended 2 degrees and it flew fine but always required right rudder or trim to center the ball.
 

rv7charlie

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Might or might (probably) not be related to engine angle. RVs are designed with offset, but there are several flying straight with 0-0 on rotaries.
 

jedi

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Why do you want to go to the 0-0? There are other ways to get the desired output but why redesign. Do you have an opposite rotation engine?

Quick answer is you can go 0-0 but it will not fly as nice unless you correct for engine torque with other methods. Depends on how big the engine is. On a 65 hp Cub it's not a big deal. With 2,000 hp up front it is a big deal.

If you are going to switch engines CW to CCW 0 is better than the wrong way. The advantage of engine offset is an engine change is no big deal to redesign the engine mount for the opposite rotation. If the correction is built into the airframe, it is a big deal to change engine rotation.
 

Victor Bravo

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I hope you get the specific information from an engineer you requested. Since I don't qualify, I will only offer this one tidbit from an old washed up model airplane builder: you build the airframe straight to fly straight without any power, and then you adjust the engine (thrust line angle) such that it compensates for its own torque, P-factor, airflow swirl, etc.
 

Hot Wings

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I don't want opinions, only facts
Without detailed numbers from your situation you won't get any 'facts' - just generalities. That is just the nature of this game.
Dangerous? Probably not. Offset thrust is often used as a tuning device to trim a plane for a specific flight mode, but there are other reasons.
This article might help you ask a better question?
Wind Tunnel - Thrust
 

Voidhawk9

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In general, yes, you nailed it already, as have the previous replies. Without the offset, and assuming all else is equal, you will need control trim (with the associated drag that brings with it) to fly straight at the design point (probably cruise).
 

Dana

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I haven't measured it, but I suspect the thrust line in my Hatz is zero, or at least not as much right as it should be. It flies straight in cruise, takes the usual of right rudder in climb, but I have to hold left rudder to keep the ball centered in a glide.
 

Appowner

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As I understand it from a modelers perspective, the basic idea behind offset is to allow you to keep your control surfaces at or near zero during normal, level flight. The purpose being minimum drag for maximum flight efficiency. I would wager that your plane with the called for offset would be slightly faster on slightly less fuel than it would be without the offset.
 

TFF

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Not an engineer.
Lots of the aerobatic guys put the engine at 0/0 and live with inputs. They want inputs as linear right side up as up side down. Most people don’t experience P factor upside down. They want to drive the plane not the plane drive.
As a modeler, it’s one of the first concepts you learn when you build your first plane.
 

rv7charlie

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I haven't measured it, but I suspect the thrust line in my Hatz is zero, or at least not as much right as it should be. It flies straight in cruise, takes the usual of right rudder in climb, but I have to hold left rudder to keep the ball centered in a glide.
This.
Also not an engineer, here, but consider when you need the most rudder input; takeoff, correct? Slow speed, higher angle of attack, wide open throttle. And higher power/weight ratio a/c require more rudder. I've flown Swifts with 160hp, 200hp, and Continental 210hp (with a really big prop). The C210hp took a *lot* more rudder on takeoff. The faster you're moving, the less rudder input you need to correct for propeller effects. The rotary powered RV guys I know that set up their mounts at 0-0 did it for max cruise efficiency; accepting the extra rudder needed on takeoff.

The only thing that gives me pause in the original post is the '2 degrees up'. I've never heard of a tractor-configured a/c wanting 'up' in the thrust line; all I've ever heard was right only, or down/right, for USA-typical clockwise turning props, as seen from the cockpit. Does the airframe in question have a high-mounted pusher engine? If so, the 'up' part might become important.
 

wsimpso1

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First off, engineers do not have any monopoly on knowledge or understanding. Some know a lot about something other than what you are interested. Let's just say you want help from knowledgeable folks. We got those, and some are engineers, others experienced in airplane configurations, etc.

Standard advice is to stick to the plans unless there is a known issue and a known good fix for the issue, then stick to the plans for the known good fix. Deviating from the plans Is entering the world of airplane design, experimental fabrication, and test pilot work in new airplanes. Most builders, even the ones on this forum are simply not prepared for this… Build to plans and you know what you will get. Start altering the plans and who knows what demons you will excite. If you just gotta do it, well, do it with your eyes open.

Now that I have the warning out of the way, let’s talk about the small angles on the engine and what going zero-zero does.

Small offset angles have been experimented with and are widely used because they produce less drag in cruise and less rudder input over the operating range than a 0-0 alignment.

“Oh, but doesn’t that adversely affect prop efficiency.” Maybe… None of us should worry over efficiency of a single item, but instead worry over efficiency of the entire airplane. When you optimize the prop but make the airplane less efficient, you have made a poorer airplane. Ideally you want the prop perpendicular to the flow through it. A 2 degree angle to the prop is so small that you will have a hard time measuring its effect on thrust as compared to 0-0, so the losses are just not big enough to worry over. Basically the efficiency scales with the cosine of the angle. Cosine of zero degrees is 1. Cosine of 2 degrees is 0.9993, or really close to 1. The energy that might be recovered with a zero angle engine is miniscule. The wing causes a gentle up wash in the air ahead of the wing, so a small tilt down is usually a good thing for prop efficiency. Then the prop causes air to spiral coming out of the prop disc. This is the energy wasted in a prop, and we want to minimize it’s total effect on the airplane. While it might seem to make the prop more efficient to have its yaw axis align with the long axis of the bird, the 0-0 prop requires rudder input to make the airplane go straight at cruise and usually more rudder input over the entire operating range. Rudder angle produces drag too, and at cruise, rudder is sometimes thought to cause more losses than the tiny loss from the angled prop...

So, fool with it if you must, but you will probably have a better airplane if you build with the angles of the plans.

Billski
 

Victor Bravo

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Sorry, I have to add one thing which I know is a fact, so perhaps the OP will forgive my lack of a degree in engineering. Having no offsets in the airframe, and offsetting the engine to compensate for itself - creates a proportional remedy that is more likely to make the airplane fly straight at any speed or power setting.

Setting the engine at zero and making offsets in the airframe will force you to pick one speed/power setting at which the aircraft will fly straight, and it will require control inputs or trim at all other speed/power combinations.
 

OhAnElBirds

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Also not an engineer...

Based on several plans that I possess or have studied where the same exact airframe (Heath, Pietenpol, Expiramental Cub) had optional engine installations that had differing degrees of engine offset...

This says it better than I could...
Having no offsets in the airframe, and offsetting the engine to compensate for itself - creates a proportional remedy that is more likely to make the airplane fly straight at any speed or power setting.

Setting the engine at zero and making offsets in the airframe will force you to pick one speed/power setting at which the aircraft will fly straight, and it will require control inputs or trim at all other speed/power combinations.
 

Dan Thomas

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We had an extensive thread on this a while ago.

About that prop: Its blades will normally have an angle of attack of two to four degrees in cruise, sometimes less than two. If we cant the engine two degrees to the right, that puts the AoA of the blade going over the top at four degrees lower than the blade going under the bottom. Doesn't sound too good to me, and sure doesn't make for a vibration-free ride. Angling the engine down not only means down-thrust in level flight, but puts the upgoing blade at a high AoA than the downgoing blade.

In all the Cessna singles I worked on I never saw any offset in any direction. Piper did it a lot on their metal singles. The Piper was easier to fly, the Cessna made a real pilot out of you.
 

Voidhawk9

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In all the Cessna singles I worked on I never saw any offset in any direction. Piper did it a lot on their metal singles. The Piper was easier to fly, the Cessna made a real pilot out of you.
Curious, I have always found the opposite to be true! The Cessna trainers are almost too easy.
 

OhAnElBirds

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IDK, flown lotsa Js, and PA-11, 14, 17, 18. Easy peasy... Same, same with 140s, 150s and 170s... JMO :)
 

Victor Bravo

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Dan Thomas said:
the Cessna made a real pilot out of you.

“I don’t care who you are; that there is funny!”
I can see a Pitts driver having a good laugh over that.

But... do we have any Luscombe Phantom pilots here, anybody with ME-109 time? Any former Schneider Cup racers with Macchi M-39 or MC-72 time? Any HBA participants who happen to be women pilots with experience flying antique helicopters inside a building in Germany? Does Fitz Fulton happen to be with us today?

Perhaps one of those people might like to chime in :) :) :)
 
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