Didja ever notice....

bluerooster

bluerooster

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that almost all aircraft engines are not straight with the longitudinal axis of the aircraft?
Most are canted down, and to the right. The severity of the angle is commiserate with horsepower rating, and prop size.
 
bluerooster said:
that almost all aircraft engines are not straight with the longitudinal axis of the aircraft?
Most are canted down, and to the right. The severity of the angle is commiserate with horsepower rating, and prop size.
Click to expand...
FYI: it's to offset the "P-factor" of the prop thrust. Even helicopters have certain mechanical offsets to deal with the same issue.
 
Also, most vertical stabilizers have their leading edges slightly offset to the left, largely to correct for spiraling slipstream at cruise. Really obvious on a Citabria or similar.
 
bluerooster said:
that almost all aircraft engines are not straight with the longitudinal axis of the aircraft?
Most are canted down, and to the right. The severity of the angle is commiserate with horsepower rating, and prop size.
Click to expand...


unless it British, then it is canted left................................................
 
I remember when I first got my plane and had the cowling off for inspection and I thought the angle of the engine was off. I got all nervous, took a picture and sent it to my mechanic. He just laughed and said that it was normal. Not the first time he's said that to me and I've learned so much from him these past 6 years not only about my plane but the ones we've worked on together.
 
What got me thinking about that was the sagging 210 thread. Was wondering if it was actually sagging, or just appeared so.
 
That's why IO-520 Bonanzas have a different cowl from the IO-470 models. To offset the greater power, the engine was canted more than the original nose cap would permit. Compare the 1970 F33 (225 hp IO-470) with the F33A (285 hp IO-520).

5C253BD4-8651-4FB7-A0BD-38254481CDD8.jpeg

2CF05F81-AEC1-4E63-9E2E-1E3F4F2705E6.jpeg
 
One can also modify the aircraft "thrustline" to improve performance in certain areas. There was one alternation called "The Thrustline Mod" for Super Cubs that moved the engine/prop negative factory offset to a zero offset to improve takeoff performance. If I recall it added shims under the stock mount and provided cowling modifications. I believe there are other similar mods for different aircraft like racing applications, etc.
 
You can see the engine cant in this photo of a new engine going into my Warrior II (PA-28-161).
2011-11-01.Progress.Front.25.jpg
 
When I was installing the O-235 on my Zenith 601 XL I got freaked out cuz the engine was "crooked". I was sure the engine mount was screwed up and I was pretty ****ed. Then I Googled the problem....
 
Gee. All these years as a mechanic and pilot and I didn't notice that "most" airplanes have their engines and fins offset.

They don't.

The Cessna singles don't, for the most part. And they don't have offset fins, either. The 172, the most-produced airplane in history, doesn't, nor do the 150, 152, 170, 177, 180, 182, 185, and others. A 210's sagging mounts are just sagging.

The Champs don't. A few Citabrias do have an offset fin but not an offset engine.

My Auster, a British airplane which had the wrong-way-turning Gipsy Major, didn't have either engine or fin offset.

Piper and Beech used offset engines on many of their metal airplanes. Ercoupe did, too. But that hardly qualifies as "most" airplanes.

How easy it is to start aviation myths.
 
I was curious about the Cirrus.. from what I can find on line and based on the hundreds of photos I have, many with the cowl off and looking straight at the plane, I have not noticed any offset

**Many sailboats have fairly dramatic offsets.. there may be more reasons for this, but "prop walk" can be fairly insidious. On my father's C&C 36 it drove great in a straight line.. and if you were docking on the port side (as you're supposed to, hence 'port') you could look like a pro by popping it in reverse and letting the boat "pull" itself in
 
Dan Thomas said:
Gee. All these years as a mechanic and pilot and I didn't notice that "most" airplanes have their engines and fins offset.

They don't.
Click to expand...

That may be the case. I may have just been projecting from what was clear on Citabrias.

That said, the designers must do something to compensate for the yaw that the spiraling slipstream hitting the vertical stabilizer causes, otherwise in cruise we’d constantly need to hold a bit of right rudder.

Right?
 
Just checked, and the manual for my Sky Arrow with a “backwards” ROTAX 912 says “Deviation of the thrust axis 1° left in horizontal plane”.
 
When I was teaching ground school, our class would often head out to the maintenance hangar to see this first hand. Having a taildragger handy also helped teach about P factor.
 
Pilawt said:
That's why IO-520 Bonanzas have a different cowl from the IO-470 models. To offset the greater power, the engine was canted more than the original nose cap would permit. Compare the 1970 F33 (225 hp IO-470) with the F33A (285 hp IO-520).
Click to expand...

I never realized that was why the Bonanzas switched cowling design. Interesting!

If my memory serves me correct the early Bonanzas (at least all the E series powered ones) had the engines mounted straight. They had more pronounced left turning tendencies than the later planes do. Just another interesting Bonanza lineage thing.
 
FastEddieB said:
That may be the case. I may have just been projecting from what was clear on Citabrias.

That said, the designers must do something to compensate for the yaw that the spiraling slipstream hitting the vertical stabilizer causes, otherwise in cruise we’d constantly need to hold a bit of right rudder.

Right?
Click to expand...
A fixed trim tab on a lot of rudders does that. Other, more expensive airplanes, have adjustable rudder tabs or adjustable rudder trim bungee assemblies that put some spring force on the rudder bars. Besides that, the propeller slipstream spiral is minimal in cruise, nearly undetectable, and most pronounced at the start of the takeoff roll or in slow flight. Most pilots are just using rudder. Real pilots, at least:confused:

The 180/185 and some others have the rudder system rigged for a little bit of offset to the right. If the airplane doesn't have rudder trim it really doesn't achieve much, since there will be no centering function in the system. It just keeps the pedals aligned in cruise.

Next time you walk past a row of airplanes, look for fixed rudder tabs. They're bendable so that any turning tendency in cruise can be eliminated. I have sometimes found them straight or bent the wrong way, a sign that the wing or flap rigging is off. A flight test will confirm that. The airplane will want to fly in a nose-left slip to counter a heavy right wing.

proxy-image
 
I used to fly a PA-12 with a 135-hp engine mod...nobody could figure out why at the time, but with full nose-up trim, it still required back pressure on the stick all the time.

I later found out that it probably didn’t get the engine mount changed properly with the mod. :rolleyes:
 
Dan Thomas said:
A fixed trim tab on a lot of rudders does that. Other, more expensive airplanes, have adjustable rudder tabs or adjustable rudder trim bungee assemblies that put some spring force on the rudder bars. Besides that, the propeller slipstream spiral is minimal in cruise, nearly undetectable, and most pronounced at the start of the takeoff roll or in slow flight. Most pilots are just using rudder. Real pilots, at least:confused:

The 180/185 and some others have the rudder system rigged for a little bit of offset to the right. If the airplane doesn't have rudder trim it really doesn't achieve much, since there will be no centering function in the system. It just keeps the pedals aligned in cruise.

Next time you walk past a row of airplanes, look for fixed rudder tabs. They're bendable so that any turning tendency in cruise can be eliminated. I have sometimes found them straight or bent the wrong way, a sign that the wing or flap rigging is off. A flight test will confirm that. The airplane will want to fly in a nose-left slip to counter a heavy right wing.

proxy-image
Click to expand...
I used to see a Maule with a bendable tab riveted onto the ground-adjustable tab provided by the factory...apparently the owner and/or his mechanic didn’t know the factory one was adjustable.
 
When the first IO-520 powered Bonanza, the S35, was introduced in 1964, Beech ad writers touted "Stabi-Line Power" -- the engine canted 2-1/2 degrees to the right and 2 degrees down.
 
Dan Thomas said:
Gee. All these years as a mechanic and pilot and I didn't notice that "most" airplanes have their engines and fins offset.

They don't.

The Cessna singles don't, for the most part. And they don't have offset fins, either. The 172, the most-produced airplane in history, doesn't, nor do the 150, 152, 170, 177, 180, 182, 185, and others. A 210's sagging mounts are just sagging.

The Champs don't. A few Citabrias do have an offset fin but not an offset engine.

My Auster, a British airplane which had the wrong-way-turning Gipsy Major, didn't have either engine or fin offset.

Piper and Beech used offset engines on many of their metal airplanes. Ercoupe did, too. But that hardly qualifies as "most" airplanes.

How easy it is to start aviation myths.
Click to expand...


The champ does have an offset. I have a bare fuse in the jig fight now and you can clearly see it in how the front tube is welded to the fuse.
 
In the Pilatus, it’s 2 degrees down and 2 degrees to the right. It also helps that it has rudder trim and a yaw damper. :)
 
bluerooster said:
that almost all aircraft engines are not straight with the longitudinal axis of the aircraft?
Most are canted down, and to the right. The severity of the angle is commiserate with horsepower rating, and prop size.
Click to expand...
I noticed that 55 years ago, when I built my first rubber-powered model aeroplanes.
 
unsafervguy said:
The champ does have an offset. I have a bare fuse in the jig fight now and you can clearly see it in how the front tube is welded to the fuse.
Click to expand...
A 7AC?

Edit: I found a specification that says the 7AC has a 3/8" offset to the fin. You were right. Higher-powered Citabrias have a lot more than that.
 
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I shot this this morning - the vertical stabilizer of a friend’s Citabria.

49805964408_fdab8b30e2_z.jpg


Not as obvious as I recall on mine, but you can see the offset if you look carefully.
 
FastEddieB said:
I shot this this morning - the vertical stabilizer of a friend’s Citabria.

49805964408_fdab8b30e2_z.jpg


Not as obvious as I recall on mine, but you can see the offset if you look carefully.
Click to expand...
In the higher-powered Citabrias there is more offset.
 
FastEddieB said:
That’s a 7GCBC. At 150 hp, isn’t that among the most powerful Citabrias?

Maybe you meant Decathalons, which go up to 180hp?
Click to expand...
We had a 7GCBC that I recovered with Poly-Fiber. It had more offset than that. Perhaps it was increased or decreased at some point. Or maybe my memory is bad. It was around ten years ago I did that job.
 
You really have to exaggerate the right thrust and down thrust with rc models. Looks wonky, but makes them fly right.
 
SoCal RV Flyer said:
You really have to exaggerate the right thrust and down thrust with rc models. Looks wonky, but makes them fly right.
Click to expand...
That is what really surprized me. When I had the cowling off the Cherokee, and told my BIL that the engine looks a bit "wonky". He agreed, and suggested that I get it looked into. I then told him that it's supposed to be that way. Just like the R/C models that he builds and flys.
 
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