Quantcast

motor mount offset for "P" factor

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
8,509
Location
USA.
Piper made a lot of their airplanes with the engine angled both to the right and down. Angling it to the right counters both P-factor and the spiralling slipstream strikng the fin, both of which pull the nose to the left. Angling it down reduces the amount of P-factor. The Ercoupe had considerable down-angle on the engine, too.

The higher-HP Pipers had a startling amount of angle. Stand in front of a Comanche 250 someday.

I never saw anything but engine aligned with the aircraft centerline on any Cessna single. A Cessna pilot is a real pilot and knows how to use the rudder.:beer:




Very nice Coupe. I still remember the first Ercoupe I saw just after WW-2. A lot to be liked in that airplane. I have owned 2. Going by the N number I would guess that this Ercoupe has a serial number around 3500-4000. I like the looks of the flat windshield better, and was told that the flat windshield has less drag. Don't know for sure.
 
Last edited:

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,546
Very good discussion. One factor, as Tralika explained, is due to angle of attack: the blade going down generates more thrust than the blade that goes up. There is one more factor, though: the propeller "twists" the slipstream, and consequently whatever is behind the upper part of the propeller, for most propellers (except VW direct drive engines, which turn in the opposite direction), is aerodynamically pushed to the right and whatever is behind the lower part of the propeller is pushed to the left. This second factor depends very much on the geometry of the vertical tail.
As for 2 inches being excessive, my suggestion is that you verify the offset angle. The higher the power/prop diameter, the higher this offset needs to be . I know a few aircraft that have 3 degrees to the right and 1.5 degrees down. And I've one flown an aircraft (a long time ago) which had 1 degree to the right and it was clearly not enough.
And there are two more factors yet. Four altogether. Propeller torque reaction puts more weight on the main wheel on the side away from the downgoing blade, adding rolling friction drag on that side, which is the left side in airplanes powered by North American engines. Very powerful aircraft had a real problem with that; the later Spitfires, for instance, with the 2100 HP Griffon engine, would sometimes blow the right main (left-turning engine) on takeoff due to the huge extra load on it. They ended up having to take off at about half power. Some other fighters couldn't use full power, either, on takeoff, because there wasn't enough aileron to counter the torque roll until the airspeed came up in the climb.

The last factor applies to taildraggers. Propeller gyroscopic precession will pull the nose left as the tail is raised. Or to the right with a CCW engine.

All four forces pull to the same side.
 

pwood66889

Well-Known Member
Joined
Feb 10, 2007
Messages
1,712
Location
Sopchoppy, Florida, USA
Very good discussion. One factor, as Tralika explained, is due to angle of attack: the blade going down generates more thrust than the blade that goes up. There is one more factor, though: the propeller "twists" the slipstream, and consequently whatever is behind the upper part of the propeller, for most propellers (except VW direct drive engines, which turn in the opposite direction), is aerodynamically pushed to the right and whatever is behind the lower part of the propeller is pushed to the left. This second factor depends very much on the geometry of the vertical tail.
Quite correct, Heliano. That is why my ride-of-choice (see avatar) has the verticles mounted outside of the prob stream.
There is a lot going on that has to be balanced for a single-engined aircraft to fly straight.
Also, Dan, the fuselage goes through more angle from start to controlled on take off than the trike gear. Means you really gotta be on the ball - I've heard tail wheeled aircraft have "Take Off Gear" since landing is so tricky.
But there's nothing one can't get used to - `cept maybe COVID... :)
Percy (inside and loving it)
 
Last edited:

Riggerrob

Well-Known Member
Joined
Sep 9, 2014
Messages
1,584
Location
Canada
Let's stick with common terminology by describing propeller direction of rotation as seen from the pilot's seat (or tail). Most Continentals and Lycomings rotate clockwise (right blade descending), while direct drive VWs roatate counter-clockwise (left blade descending).
 

Heliano

Well-Known Member
Joined
Dec 24, 2015
Messages
111
Location
Campinas, SP, Brazil
To be frank, I have no idea where that article came from. I've downloaded it quite a couple of years ago.
As for direction of rotation, here goes another comment: the fact that most engines turn clockwise (based on Riggerrob's terminology) results, when it comes to two-engine aircraft, in the loss of left engine being more critical than losing the right engine (that's why the left engine is called the "critical engine"), because in addition to the "P" factor there is the thrust asymmetry due to one engine out. Some manufacturers developed aircraft that have the left engine turning clockwise and the right engine turning counterclockwise. The Piper PA-31 Navajo is a good example. It resulted in much improved handling qualities in case of engine failure, but created a parts inventory nightmare.
 

Rockiedog2

Well-Known Member
Joined
Dec 11, 2012
Messages
2,377
My RV8 factory mount had a noticeable amount of R offset when standing in the seat and looking down. Seemed excessive. And yeah, it turned out so...took very little R rudder in the climb and a lot when reducing power for the descent.
On my planes that I built the mount for, I set it up zero zero and the vert fin leading edge offset left about a 1/2". From there it's usually just a small rudder tab to get it like you want it for the type planes I build.
Like somebody said, we can always shim the mount within the limits of the cowling setup. I did that on my 8 and got it better. I always thought Van got it offset too much.
Just cause it's factory doesn't mean it's perfect. Job one is the bottom line.
 

floridapilot1

Member
Joined
Nov 14, 2013
Messages
19
Location
Cologne Region Europe
Maybe a dumb question: Instead of keeping (more or less) the spinner tip on the symmetry line of the AC and tilting the engine for some degrees, why not shift the engine some inches to the left without horizontal tilt?
With alternative / asymmetric engines that might sometimes simply things.
Yes if you are able to look at both sides of the cowling at the same time, you will detect an asymmetry...


I have never found a discussion on that point.





Richard
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
13,777
Location
Memphis, TN
People hate asymmetrical things unless looking at art. Even then it’s got to be good.
It’s all about compromise. Where do you want it? Certain people want it one and certain want it another. Most are designed for the biggest group. Essentially this fight is I don’t want the engine off or the vertical off, why can’t I have it both ways; fight about asymmetry. No one seems to care if the plane flies straight.
 

fly2kads

Well-Known Member
HBA Supporter
Joined
Jan 2, 2010
Messages
1,724
Location
Justin, TX
Maybe a dumb question: Instead of keeping (more or less) the spinner tip on the symmetry line of the AC and tilting the engine for some degrees, why not shift the engine some inches to the left without horizontal tilt?
Shifting the engine off to the side won't be as effective. Picture a top (plan) view of an aircraft, and draw a vector line through the prop centerline. Shifting the engine over a few inches will keep the vector very close to the aircraft centerline, but parallel to it. Angle it a bit, even a couple of degree, and the vector now swings away from the aircraft by a larger margin.
 

floridapilot1

Member
Joined
Nov 14, 2013
Messages
19
Location
Cologne Region Europe
Shifting the engine off to the side won't be as effective. Picture a top (plan) view of an aircraft, and draw a vector line through the prop centerline. Shifting the engine over a few inches will keep the vector very close to the aircraft centerline, but parallel to it. Angle it a bit, even a couple of degree, and the vector now swings away from the aircraft by a larger margin.
At least when the airstream reaches the empennage the lever arm is real great; that may be an essential point.
Another contribution will result from the fact that a lot of airflow originating from the right half of the prop circle will be shielded by the cowling, reducing the p-factor – but inevitable the prop effectiveness too.
Maybe these facts make further discussion unnecessary.

Richard
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,546
If we just made the blade on the left side a little bigger than the one on the right... then...
Wait, never mind. 😕
Thanks. Now some goof will try to build a prop that has a blade that gets longer or wider as it comes up on the left side. Or that has variable pitch with pitch links like a helicopter to increase the pitch in the left side of prop disc, with the swash plate actuated by airspeed and AoA and heading and pitch inputs. All this so that the pilot doesn't need to hold some right rudder on the takeoff roll and climb.

A few years ago some fellow here asked if there wasn't some way he could build a taildragger that had self-steering so it wouldn't groundloop on him. He wanted to fly taildragger but didn't want to learn how to control the thing.

What else can we expect of such a lazy and fearful society?
 

skydawg

Well-Known Member
Joined
Jul 26, 2016
Messages
63
Location
Denver, Colorado
For what's it worth, I put a 450HP V8 on a Cessna 172, the engine was derated to 220HP by redundant EFI system. Centerline offset was a concern of course and used different models to figure it out. In a nutshell, try to calculate required rudder deflection at Vc, as this is where you want zero trim or rudder input to maintain coordinated flight to lower drag. Then, do some testing at just above stall speeds and at climb speeds with Max power, using some method to cause rudder depletion needed to center inclinometer ball. Graph the result.

You will obviously notice less rudder input with airspeed. If rudder deflection at lower critical speeds, such as Vx, seems excessive and costing performance, be sure there is enough rudder travel left to maintain yaw control at speeds slightly below stall.

You can begin to reduce power which will also lower yawing force and watch climb rate on VSI. If your VSI increases with reducing thrust (allowing less rudder), you probably need too much rudder deflection which causes more drag than reducing thrust and needing less deflection.
In this case, consider more engine offset to reduce the need for excessing rudder deflection at lower speeds..... This will decrease control surface deflection drag and allow yaw control at lower speeds.

This is mostly concern on aircraft with small rudder, or twin engine aircraft when calculating best single engine speed that's fast enough to keep yaw control with it's critical engine failed without excessive control surface deflection and increased drag.

Biggest thing to remember when designs require excessive rudder at low speed and high power, is to be sure there is sufficient travel left if you find yourself near or below min speeds so you can get yourself back in control before deepening your issue with excessive drag with greater surface deflection into the on coming airflow.. Small rudders look cool but have a ominous history.
 
Last edited:

lelievre12

Well-Known Member
HBA Supporter
Joined
Jul 15, 2020
Messages
49
What about pushers like Velocity/Cozy IV? Does anyone know the engine offset setup for these?
 

Heliano

Well-Known Member
Joined
Dec 24, 2015
Messages
111
Location
Campinas, SP, Brazil
Two things:
1 - Shifting the engine to the left: does not address the problem 100%: there are two propeller effects: one is due to angle of attack, which causes the right side of the propeller disk to produce more thrust than the left side; the other is the twisting effect the propeller has on the airflow, which creates a component blowing from left to right on the vertical tail, which also yaws the aircraft to the left. Shifting the engine to the left would NOT address this second effect.
2 - Pusher aircraft with the propeller behind the wing are less affected by these things, because: the wing downwash reduces the propeller angle of attack; and usually (such as with the Vary/Long Eze) the vertical tail is NOT behind the propeller.
Now the yaw effect on high power aircraft is in fact a serious problem. The A-29 Super Tucano (and as far as I remember the higher power Piper Cheyennes too) uses engine bleed to create a rudder bias to mitigate this. I remember many years ago when I was flying the EMB-120 Brasilia (a relatively small regional turboprop with powerful engines) that people used to joke that the aircraft had four primary flight controls: elevator, ailerons, rudder and rudder trim.
 

Dana

Super Moderator
Staff member
Joined
Apr 3, 2007
Messages
9,431
Location
CT, USA
I don't know how much, if any, right thrust my Hatz has, but it's clearly not enough. I have a trim tab to add right rudder adjusted for cruise, it takes a small amount of right rudder in climb, and a significant amount of left rudder during a glide.
 
Top