Thrust Line

[Chucker]

I have found nothing in the plans that talks about thrust line. As all four engine mount spools are 2-3/8", it appears that the engine should be square to the fuselage centerline in both the vertical and horizontal axis.

The reason I am bringing this up is that, due to p-factor and prop wash, everyone seems to fly around with a little bit of left boot on the rudder. Alternatively, many have fashioned trim tabs to deflect the rudder just the right amount at their favorite cruise speed.

I am getting ready to mount a brand new cowling on 136DE as part of her engine change. I obviously need to center the cowling around the prop flange. To do that, I need to decide where to put the prop flange; on centerline?, or slightly left of centerline to compensate for the p-factor and prop wash.

So I am asking the crowd... Has anyone experimented with canting the VW engine off centerline to compensate for the aerodynamic forces of the prop?

I know that I am thinking too hard : ....but just humor me.

All the best,
Chucker

 

[Mytonic]

I recently mounted my engine and cowling and had the exact same thoughts. After a little research it seems the consensus was to align the center line with the fuselage and make any corrections some place else. I ended up shaving a few thousands off a couple of the stand offs so the engine would be straight. Sonerai airframes are not perfectly straight and square, having been built in a home made jig and dealing with metal moving when heated, I have had to do a lot of straightening on my project. I think if you offset the engine then had to make corrections after flying it you may end up with the cowling not aligned. John

 

[cosmic]

Our Sonerai 1 is powered by a Jabiru 3300 and 1 degree of offset was built into the engine mount. It flies straight and the offset is not apparent.

 

[Bil4381]

I recall it being on the firewall drawing and that it worked. The engine C/L was at the center of the engine mass for J3300 engines. 125 hp. I placed 1/8" thick washers on one side depending which direction your engine rotates.
The good news is that I used a fixed trim tab say 4" long deflected about 5 degrees. It flies straight with that. Bill E

quote author=Chucker link=topic=4644.msg35319#msg35319 date=1548426070]
I have found nothing in the plans that talks about thrust line. As all four engine mount spools are 2-3/8", it appears that the engine should be square to the fuselage centerline in both the vertical and horizontal axis.

The reason I am bringing this up is that, due to p-factor and prop wash, everyone seems to fly around with a little bit of left boot on the rudder. Alternatively, many have fashioned trim tabs to deflect the rudder just the right amount at their favorite cruise speed.

I am getting ready to mount a brand new cowling on 136DE as part of her engine change. I obviously need to center the cowling around the prop flange. To do that, I need to decide where to put the prop flange; on centerline?, or slightly left of centerline to compensate for the p-factor and prop wash.

So I am asking the crowd... Has anyone experimented with canting the VW engine off centerline to compensate for the aerodynamic forces of the prop?

I know that I am thinking too hard : ....but just humor me.

All the best,
Chucker
[/quote]

 

[oahupilot]

P factor or asymmetric loading only happens at high angles of attack. It does not have anything to do with your thrust line, its a function of attack angle.

Slip stream cork screwing from the propeller may induce yaw turns by impinging on the ruder surface. How ever this would be specific to the airframe and not the engine. I order to account for this you have to do trail and error or full blown aerodynamic analysis, if you change engine cowl or porp you need to repeat the process. Sometime from a design perspective its just easier to put a little boot into it.

 

[Chucker]

We work so hard to make our machines smooth and light. Doesn’t flying around with the rudder deflected sound like a waste of energy?

 

[Speed20]

P factor or asymmetric loading only happens at high angles of attack. It does not have anything to do with your thrust line, its a function of attack angle.

Slip stream cork screwing from the propeller may induce yaw turns by impinging on the ruder surface. How ever this would be specific to the airframe and not the engine. I order to account for this you have to do trail and error or full blown aerodynamic analysis, if you change engine cowl or porp you need to repeat the process. Sometime from a design perspective its just easier to put a little boot into it.

Not necessarily true. I just maidened my 2L today and noticed I had to apply left rudder straight and level on downwind, no high angle of attack. Going to try a ground adjustable trim tab

 

[10kDA]

Offsetting thrust line is a common way to address differences in response when a higher-horsepower engine is installed in a given airframe without altering angles of incidence or adding trim tabs to the tail surfaces. I've seen it successsfully address greater propwash & P factor characteristics in Junior Ace, Cub/Super Cub (clone), and Globe Swift airframes. A friend of mine got the very first 337 for the installation of a 160 hp O-320 in a Swift, replacing the original C125. The engineering process resulted in changing the thrust line 1 degree over, one degree down, to minimize accelerated airloads from the prop on the tail surfaces resulting in more yaw and climb than original. He said he took an educated guesstimate based on available data and the change turned out to be just about right, very little difference. His major concern was to avoid changing the power-off stall break which was OK as it was but would likely have been unfavorably altered if the tailplane & fin incidence had been changed or trim tabs used as the only compensation.

 

[10kDA]

I should have added: He thought one over, one down was a good place to start whatever the application and yes, he was an aero engineer.

 

[user 116312]

A slight set in down thrust line helps to dampen trim changes with speed, and offset is used to counter gyro forces of propeller rotation. A propeller that turns in clockwise direction when viewed from cockpit has offset to right. (Needs right rudder on takeoff)
Counterclockwise offset to left. (Needs left rudder on takeoff)

 

[tls258]

Tail Wheel 101 DVD has a good illustration of P-Factor as gyroscopic force. This is from the Wiki:
"P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller,[1] where the propeller's center of thrust moves off-center when the aircraft is at a high angle of attack. This shift in the location of the center of thrust will exert a yawing moment on the aircraft, causing it to yawslightly to one side. A rudder input is required to counteract the yawing tendency."
https://en.wikipedia.org/wiki/P-factor

 

[Speed20]

So I've always loved the F8F Bearcat, but recently watched a presentation by the CAF SoCal wing a few years ago and learned some interesting facts.
I knew she could climb, but I DIDN'T know that she held the time-to-climb record for 10,000' all the way till the F-16 finally beat it.
Also he said the engine was actually installed at a 2 degree angle plus an additional 6 degrees in the rudder (many of you recall recent discussions here about that). Even though it was a tiny plane with 2300ish HP, he said it would go straight as an arrow on takeoff (with a slight roll tendency). The manual told pilots NOT to apply excessive rudder on take-off!

Didn't think much about it till I saw the F-8F-1 at the Arsenal of Democracy Flyover the past week (Tom Wood Aviation) where I snapped this photo.

 

[shocktrooper]

So I've always loved the F8F Bearcat, but recently watched a presentation by the CAF SoCal wing a few years ago and learned some interesting facts.
I knew she could climb, but I DIDN'T know that she held the time-to-climb record for 10,000' all the way till the F-16 finally beat it.
Also he said the engine was actually installed at a 2 degree angle plus an additional 6 degrees in the rudder (many of you recall recent discussions here about that). Even though it was a tiny plane with 2300ish HP, he said it would go straight as an arrow on takeoff (with a slight roll tendency). The manual told pilots NOT to apply excessive rudder on take-off!

Didn't think much about it till I saw the F-8F-1 at the Arsenal of Democracy Flyover the past week (Tom Wood Aviation) where I snapped this photo.

Yeah man! What an awesome warbird for sure! ..was always one of my faves to admire and wonder; 'what-if?' as well. Had it made it into full production at the hight of the Pacific air battles, it would have been Imperial era Japan's "2nd worst nightmare".