Quantcast

Engine offset

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

slociviccoupe

Well-Known Member
Joined
Apr 1, 2011
Messages
351
Location
Palm Bay Fl.
When doing auto conversion with a grlearbox that reverses rotation (spg-5 on subaru) does the offset of the engine change to the opposite side as say a lycoming, continental or other would with a standard rotation prop?
Two other subaru installations are using marcote gearbox and with its internal helical gear setup prop rotates in standard direction.

Does reverse rotation because of gearbox cause any other issues you normally wouldnt think of?
 

Rockiedog2

Well-Known Member
HBA Supporter
Joined
Dec 11, 2012
Messages
2,498
I always set mine on zero xero whether geared or not and offset the rudder
 

Victor Bravo

Well-Known Member
HBA Supporter
Joined
Jul 30, 2014
Messages
8,281
Location
KWHP, Los Angeles CA, USA
The direction of the propeller is the only thing that matters. The gears, crank, and ratio will have nothing to do with it. On a tractor, start with one or two degrees towards the side of the descending blade. Leave room to adjust and fine tune.
 

slociviccoupe

Well-Known Member
Joined
Apr 1, 2011
Messages
351
Location
Palm Bay Fl.
I was just thinking say lycoming is offset to the right and up. Would i be offset to the left by same amount just the other direction.
I just dont want a plane that requires rudder trim to fly straight.
 

slociviccoupe

Well-Known Member
Joined
Apr 1, 2011
Messages
351
Location
Palm Bay Fl.
The direction of the propeller is the only thing that matters. The gears, crank, and ratio will have nothing to do with it. On a tractor, start with one or two degrees towards the side of the descending blade. Leave room to adjust and fine tune.
How do you fine tune? Washers or spacers? And seems dificult when making an engine mount then using the spinner to make your cowling and have it line up. Was just wondering if plans say rngine left of thrust line 1" and 1* up if i would switch to 1" right of thrust line and keep the 1*up
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,606
I was just thinking say lycoming is offset to the right and up. Would i be offset to the left by same amount just the other direction.
I just dont want a plane that requires rudder trim to fly straight.
If there's any offset on a Lycoming or Continental it's always to the right and down.

If it's any help, Cessna didn't offset the engines in their singles. Piper did, in their low-wing airplanes.
Champs and Citabrias have no offset but the vertical fin is angled a bit.

Whatever you do, it will cause some drag. Can't help that.

You'll need left rudder on takeoff. That, more than anything, takes a little getting used to.
 

pwood66889

Well-Known Member
Joined
Feb 10, 2007
Messages
1,772
Location
Sopchoppy, Florida, USA
"Piper did, in their low-wing airplanes."
Influence of Fred Weick, Ercoupe design lead.

"Whatever you do, it will cause some drag. Can't help that. "
`S `bout right, Dan. Compromizes ad infinitum is aircraft design.
 

Dana

Super Moderator
Staff member
Joined
Apr 3, 2007
Messages
9,530
Location
CT, USA
Right thrust for a RH prop, left thrust for a LH prop. If the engine is straight, you'll need rudder trim for cruise, then you'll need to hold more rudder for climb, and opposite rudder for descent. With offset thrust, the amount of rudder correction needed will be less. I presume my Hatz's engine is straight, it's trimmed to cruise straight, I need right rudder for climb and significant left rudder during glide.

I've owned two airplanes with LH props, needing left rudder for climb. No big deal, easy to get used to.
 

rv7charlie

Well-Known Member
Joined
Nov 17, 2014
Messages
1,149
Location
Jackson
Some RVs I know that are using rotary engines are set up with no offset. Supposedly there's little effect in cruise (a good thing); you just need more rudder in low speed climb. One originally had left offset for a left turning prop. Later changed the gearbox to a wider ratio which allowed both more HP and a larger dia prop, but reversed rotation to right rotation. Between the extra thrust and the 'backward' thrust angle, it was very interesting on his 1st takeoff.

I haven't flown yet, but I built my mount with no offset.

FWIW...
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
8,695
Location
USA.
Right thrust for a RH prop, left thrust for a LH prop. If the engine is straight, you'll need rudder trim for cruise, then you'll need to hold more rudder for climb, and opposite rudder for descent. With offset thrust, the amount of rudder correction needed will be less. I presume my Hatz's engine is straight, it's trimmed to cruise straight, I need right rudder for climb and significant left rudder during glide.

I've owned two airplanes with LH props, needing left rudder for climb. No big deal, easy to get used to.
When I flew a VW powered airplane for the first time I kept telling myself on take-off, "Left Rudder, Left Rudder". After a few flights I didn't have to think about it.
 

Air Trikes

Well-Known Member
Joined
Dec 21, 2015
Messages
60
Location
Montreal Canada
I was just thinking say lycoming is offset to the right and up. Would i be offset to the left by same amount just the other direction.
I just dont want a plane that requires rudder trim to fly straight.
1 degree left +/- 0.5 for a tractor, then you can adjust if necessary. About up or down, depends on the wing position (up, down, middle).
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,606
As I mentioned, whatever one does will result in some drag. The offset helps control the yaw in the takeoff and climb, but since the P-factor and slipstream effects are the largest at low speeds, that offset thrust has to be countered somehow in cruise. Rudder trim is the usual means. An offset rudder generates drag, to say nothing of the increased prop slipstream blasting against one side of the fuselage and the very small loss of thrust. (Cosine of the angle. I think.) With an offset engine you end up with a rudder offset the other way to make the airplane fly straight in cruise. I think Piper used offset to make the airplane easier to fly.

Like I said, Cessna didn't use any offset in its singles, not even in the 300-hp airplanes. No offset fin either. These airplanes used a cockpit controllable rudder trim system instead. A spring bungee was adjusted by a jackscrew to move the rudder bars. The smaller singles used a ground adjustable tab, and the cockpit-adjustable trim was available in the 172 and up. It was commonly seen in the higher-hp airplanes.
 

fixnflyr

Well-Known Member
Joined
Mar 7, 2009
Messages
59
Location
Springfield, Ga. USA (2ga2) Swaids Field
I wouldn't really worry so much about which rudder to push on take-off or during the climb, that is usually a small portion of the flight as most of the time one is in cruise. Rudder input will change with flight profile, you can't set one trim that works perfect for all phases of flight. Just fly the plane and give it the control input it needs for coordinated flight. I see a lot of Super Cub types with a lot of down offset on the engine now days but what they are doing is trying to add Alpha to the wing for quicker T/O. I guess it really depends on the mission of the design. A fixed tab on the rudder is real easy to add and easy to trim once you get a flying airplane. An offset engine mount, not so much. (cowling alignment is effected when you shim the engine mount).
The Turbine Lancair 4P I used to fly it did not matter. You had to fly the rudder at all times. That airplane did not have enough vertical tail. Any power change or any pitch change resulted in the slip-skid ball "going for a walk". (BTW I sure miss that plane, 285 Knot normal cruise!)
I think it is more overall drag to offset the engine mount than trim it with the tail. I race the Formula 1 class at Reno and I won't offset the engine for this reason. Also IF1 is a very mission specific airplane as we are turning left as much as we are flying straight. Also we don't climb too much. ;)
 

Rockiedog2

Well-Known Member
HBA Supporter
Joined
Dec 11, 2012
Messages
2,498
I've mentioned this before here but will again.
My RV8 QB when standing in the front hole looking down at the engine mount(factory) the R offset is a bunch don't know the specs. When climbing out the R rudder required is very little but when starting down on power reduction it takes a buncha L rudder. I think most old school purists would rather have it the other way; this one would. I washered the mount and got it a little better but the already installed cowling limits that. I think they guessed wrong at the offset when drawing it up. I would have much preferred zero offset but the Cherokee folks probably wouldn't have liked that.
In contrast, my very light 100 horse Rotax 701 takes a heavy R foot on steep climbout. And then for the level off/power reduction it takes a buncha L rudder. I like that(it's relatively overpowered and works hard in climbout). I got no idea what the offset is but wouldn't be surprised if none in the factory mount. Anybody know?
I like zero zero, then it flies like the planes I grew up in.
Some of you engineer types or knowledgeable like Pops, explain this to me...for a given airplane and condition(let's use stable cruise for instance) it's gonna take X amount of trim drag no matter if it comes from the mount, tabs, vert offset or wherever. Is that right? I've always expected so(speaking in redneck engineering thought, meaning for practical purposes) it doesn't matter where it comes from so zero zero is good and tabs etc trim it out.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,974
Location
Memphis, TN
Offset of any kind is there to reduce pilot loads. Up or side and fin offset. Incidence is offset to. It’s going to suck to pilots want to be pilots. Not good for aerobatics because a pilot wants to have a neutral control. A plane that acts the same in any direction. It’s a little bit crazy on a simple plane. Planes for low time pilots and planes that have lots of pitching moment or crazy amounts of power it helps tame them at a cost.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,606
Some of you engineer types or knowledgeable like Pops, explain this to me...for a given airplane and condition(let's use stable cruise for instance) it's gonna take X amount of trim drag no matter if it comes from the mount, tabs, vert offset or wherever. Is that right? I've always expected so(speaking in redneck engineering thought, meaning for practical purposes) it doesn't matter where it comes from so zero zero is good and tabs etc trim it out.
The prop-induced yaw is minimal in cruise, so any rudder trim (and drag) is also minimal. Since most pilots spend more time in cruise than in climb, that airplane needs little or no offset of the engine. Most of the airplanes I flew had no offset and I learned to fly them just fine. I never noticed any need for left rudder in a descent.
The 180 and 185 needed a bootful of rudder on takeoff. A left crosswind made it all the more interesting. The ones that had rudder trim made it a lot easier, but you still need some strength to fly that airplane. If you're running light, with CG near forward limits, you need a lot of elevator trim in the approach; if you have to overshoot, you need a LOT of push on that wheel to keep the thing from nosing up dangerously until you can spin the trim back down. For that reason, the seat structures need to be inspected really closely at every annual. I've found way too many seats cracked or otherwise ready to fail. You're putting a lot of stress on the seat when you have to hold the elevator forward hard like that. The trimmable stabilizer has a lot of authority.

Pilots that want fingertip flying should buy Pipers.
 

Dana

Super Moderator
Staff member
Joined
Apr 3, 2007
Messages
9,530
Location
CT, USA
The prop-induced yaw is minimal in cruise, so any rudder trim (and drag) is also minimal. Since most pilots spend more time in cruise than in climb, that airplane needs little or no offset of the engine.
Prop induced yaw (P-factor) is only one of the turning tendencies. There's also the spiraling slipstream and torque (torque is a rolling tendency but correcting for it causes adverse yaw). Ideally you offset the engine enough in cruise to counteract that without rudder trim, then when power is reduced the plane still flies straight. That won't eliminate P-factor when at high AOA so you'll still need rudder in climb.

If the airplane needs opposite rudder in glide, then there's not enough engine offset so that rudder is required, and that rudder trim doesn't go away when you're gliding.

As I mentioned before my plane needs significant left rudder in glide, which just seems wrong when making left turns in the traffic pattern, since we've all been schooled against using rudder to make skidding turns in the pattern... even though in my case the rudder is needed to keep the ball centered.
 
Top