C

Camillo

Well Known Member
Hi. I am writing on behalf of a friend who is building an RV-4 with a Rotax. He is wondering why the VS is not angled to compensate for engine torque, as for the -9, for instance.

I am also building a -4, but still building the tail, so I did not look at the drawings.

Would be better for him to give a drift to his VS?

Thanks
Camillo
 
Rotax

Hmmm,

Which rotax.... you are probably going to get into some issues with this!!

1. Ok..... it will probably fly OK on 120 hp but it will not have anything like te resale value of a lyco 4.

2. The engine is offset to counteract torque in the set up of the engine mount, so whatever mount you build will need to be offset on accordance with the prop rotation for the engine you put in.

3 Weight and balance.... you may have to shove the engine forward quite a distance to compensate for the difference in weight between a lyco and a rotax.

4 You will need to do a stress analysis on the mount and pick up the same mounting points on the firewall as the standard mount, otherwise the airframe
would not be able to take the loads.

As someone who has gone through all this by putting a WAM engine in a 9 I can tell you its a lot of work!!
 
Good points. I would have never installed a different engine from one of the kit producer suggested. My friend will do. I hope he did all the necessary studies.

Nevertheless, he was wondering why VS is mounted aligned and it is not offset as per the -9 (and I guess for the -7).

Ciao.
Camillo
 
What Steve said . . .

The VS is not offset because the engine mount supplied from Vans is built with a small offset to the right.

In my case I have a Lyc O-320 w/ Dyna-focal mount. I really didn't notice the small offset in the mount until AFTER I installed the mount to the firewall & hung engine. Then while fitting the cowl, I realized the nose bowl area of the Van's supplied cowl (right behind the spinner) has a little more width on the left side vs. the right side. This is where the engine offset is most obvious; at the most forward distance from firewall.

When I installed the VS, I simply pulled two lengths of thread from either side of the VS aft spar, pulled them all the way forward to the centerline of the top of firewall. They provided a nice centerline measurement reference to allow me to center the nose of the VS between the two threads before drilling the first "pilot" holes in the VS front spar.

Best wishes on your project!
 
Answer

The answer is given at 2 in my first reply. No offset in the fin because the engine offset.

As answered above Lycos are offset to the right. I am not sure whether the Rotax rotates the same direction as the Lyco and this will have to be taken into account when designing an engine mount for the alternative engine.

He would be well advised to do some calculations for weight and balance to see how far forward he would have to push the Rotax to keep this acceptable, putting the battery in front of the firewall would help a bit, or maybe adding lead.
 
VS offset on the -4

I offset mine about 1/8" which is not easy to measure. Like others have posted, the engine mount is also offset, and you will realize that when fitting the cowling. I till ended up with a small trim wedge on my rudder, and wish I had gone 3/16-1/4" offset. Only slight rework was needed on the intersection fairing for my offset. Ultimately, I wish I had put electric rudder trim on like Pat Hatch did on his -6. I would be questioning the Rotax also(I dont know the weight), as the -4 is easily made tail heavy with PAX/BAGs and low fuel. I have a Lyco O320/wood prop, and wouldnt want the front any lighter at all.
 
Thank-you Less, Steve and Bill.
I read that engine mount is offset.
But my -9 has both engine mount and VS offset. So, why the -4 hasn't the latter offset? Bill felt necessary to give such VS offset. Maybe this would be considered an improvement to the project.
Regarding to Rotax weight and balance issues related, my friend built a new engine mount to give more distance from engine to firewall.
Thanks.
Camillo
 
Think again

Cammilo

1300 flying RV4s.......... I bet 1299 built to plans with no offset in the fin.

Not many people are complaining about torque effects given the offset in the engine mount, and why would you think you would know more about how this should be set up then Van??

Ok it could have a torque reaction when you test fly it, so you could remedy with a trim tab........ but by offsetting based on intuition.... or some vague feeling that it would be a good idea you could cause a problem.

We have an expression in England..... dont fix it is its not broken!!
 
Steve, I will not offset my VS. I am not an engineer and certainly I do not feel myself better than Van's, of course.

My friend is using a Rotax and gave me a couple of photos to show on the forum. He would like to have his airplane under "advanced ultralight" category, which here in Italy is some around Kg. 500 weight. That's why he mounted Rotax, since it is lighter than Lyco.

His engine mount is slightly right oriented (Rotax works as Lycoming), but he thinks this will not be enough since the arm is very long. So he thinks it will be better to slightly offset the VS, also. He doesn't know how much to offset it, though, since he is not an engineer.
I asked him to give me more informations (Rotax engine type, arm lenght, etc...) and then I will write them here.

2434y9c.jpg


20s9f2x.jpg
 
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Just figure that the offset mount for the engine (to the right for clockwise prop rotation from the cockpit view) is going to counteract torque (roll), while the main job of the vertical stab & rudder with either an offset or rudder tab is to counteract yaw. The "6" uses a straight vertical stab also. You can count on using a rudder tab for the six to maintain straight flight in cruise. I have yet to see to good statistics to whether there is any real efficiency advantage between using an offset vertical stab, or trim tab on the rudder. The only advantage I've seen,....... is to pronounce that your plane fly's hands off on it's maiden flight, without the requirement for a rudder tab. Personally, I think tabs look kind of cool.....:)

L.Adamson -- RV6A
 
Offsetting

Well as he has built the mount presumably without any thought to offsetting he may be able to shim the engine mount.

The photos do show another of my concerns...... the extended mount probably means the view over the nose is lost for taxiing.

I wonder what effect it will have on the moment of inertia round the C of G having the engine stuck way out there???
 
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Nope

L.Adamson said:
......Just figure that the offset mount for the engine (to the right for clockwise prop rotation from the cockpit view) is going to counteract torque (roll),
L.Adamson -- RV6A
Click to expand...

Negative, Larry...ailerons control roll...the engine is offset in our airplanes to minimize yaw....and pointed downhill to minimize P-factor.

Best,
 
Would like to see more discussion on this subject.

pierre smith said:
Negative, Larry...ailerons control roll...the engine is offset in our airplanes to minimize yaw....and pointed downhill to minimize P-factor.

Best,
Click to expand...

Pierre,

This is an interesting subject, and I'm open to discussion, as I mentioned in a different thread. I've spent many hours in the last month researching this phenomenon, as well as looking at many aircraft rigging specs, and talking with many pilots about the need, or perceived need to correct for torque roll, while in flight..........with the exception of low air speed & high engine power.

I've been doing this research for the purposes of flight simulation design. From what I've seen, the early P-51 didn't have an offset engine, and we know it's strong with yaw. Same for some aerobatic designs that use torque as an advantage.

It appears that some early designs used different wing incidence to counter torque roll. This later went to the offset engine design, because the differing incidence or aileron trim tabs will cause the opposite effect, when power is pulled back. I've seen explanations that the offset engine is moving more air across the left wing to produce more lift, which counters roll. And of course, this varies with throttle, which is a nice benefit. Is this exactly correct? I don't know.

I've reviewed hundreds of in-flight photos of my RV6A in climb, cruise, and descent. Once I got that heavy right wing problem solved after the initial flights (left trailing edge radius larger than the right).......you can see that I'm not using aileron to correct for roll during the climb or cruise. I had aileron trim to correct for minor imbalance, as I prefer perfection in trimmed flight. But, I was not correcting for a constant roll while on the climb or cruise.

Most of us know, what will happen with low airspeed & high power. The RV will immediately yaw hard to the left, and perhaps want to roll to the left also. A P-51 Mustang will simply want to roll over on it's back, as has happened to some unfortunate pilots.

Just today, at an FBO open house, in which the Liberty Foundations "movie Memphis Bell" flew in, I asked the B-17 pilot about torque, as well as a friend who owns a single place Pitts, Cessna 180 taildragger, and a Cub clone; and some other flight instructors.

I did this because of a desktop flight simulator, that has always promoted itself as having very real "on the fly" flight dynamics. The "sim" problem is..........is that you're always correcting a roll with "ailerons" during the climb & cruise, and anytime you change throttle settings. The B-17 has four props that all turn the same direction. The pilot said that they do not have to correct for an inherent roll. Same response from my friend, and those flight instructors as well.

Knowing that my RV was built straight as an arrow, and had excellent trimming qualities that were often mentioned by other pilots, and the fact that the wing incidence is the same...........I'd have to say that ailerons are not correcting a tendency for torque induced roll........except for perhaps right after the wheels leave the ground.

In conclusion, I'll say that the flight simulator is wrong. I know that aileron rigging can be used to compensate for rigging factors. The same with trim tabs. I know that with your crop dusting business, that you have many more flight hours than do I, and I'd certainly like to see some more discussion on this subject. In the meantime, I'm not agreeing.

L.Adamson -- RV6A with no offset vertical stab. Used a hand adjustable rudder tab.
 
rudder trim

To add to this thread, I have been in the P-51, and rudder trim is something not to be overlooked...but it has 1500hp. In my -4, I have the "wedge" dialed in for 150-160kts, centered ball. During take-off and climb, still the usual right rudder, and during high speed down hill runs, slight left rudder is needed .
 
fixnflyguy said:
To add to this thread, I have been in the P-51, and rudder trim is something not to be overlooked...but it has 1500hp. In my -4, I have the "wedge" dialed in for 150-160kts, centered ball. During take-off and climb, still the usual right rudder, and during high speed down hill runs, slight left rudder is needed .
Click to expand...

Been in the P-51 too. Had a friend that owned one, but he's passed on now. Like your 4, I also used a taped on wedge for a while, then riveted on a permanent rudder tab. Took a lot of right foot pressure to center the ball, until I did so. Would just fly crooked sometimes. And as I've thought about it, this afternoon, the engine was already angled, while the yaw persisted.
 
Some other news: Rotax is 100HP, but when available the new 120 Hp will be installed.
My friend told me distance from firewall to engine ring is 25'', but I guess he mistook since 25'' is too low.
Empty weight should be 355/365 Kg.
Yes, cowl comes from another airplane and was adapted.
This -4 has a -8 tail and a slider canopy.
 
Nope.

My late boss, Jack Sliker, had both a Bearcat and a P-51, in which I had a lot of back seat time. It always amazed me how much aileron could be seen used right after takeoff and into the climb, to offset torque roll. When I started my spraying career, I was already Comm, CFI, IFR rated , multi-engine and Commercial glider.

When we got to altitude, the ailerons were level, with reduced power and the higher speed negating any aileron input.

My 700 H.p. Air Tractor even needs some aileron at 1600 ft. lbs of torque (direct-reading torque gauge) and a foot full of right rudder, on takeoff and initial climb. It has an extremely long nose because the engine only weighs around 330 lbs without the starter/generator, with the prop arc 14' ahead of my seat and 9' ahead of the leading edge of the wing.

Air Tractors are equipped from the factory, with locking tailwheel assemblies, once the stick is neutral, to help with directional control on takeoff. Pushing the stick forward unlocks the tailwheel (a cable runs down to the tailwheel lock from the elevator pushrod), and is how we all taxi.

P-factor accelerates more air over the inboard right wing, causing more lift than the left wing, adding to the left-rolling tendency.

The basics: Aileron for roll, rudder for yaw....agreed?

Best,
 
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pierre smith said:
My late boss, Jack Sliker, had both a Bearcat and a P-51, in which I had a lot of back seat time. It always amazed me how much aileron could be seen used right after takeoff and into the climb, to offset torque roll. When I started my spraying career, I was already Comm, CFI, IFR rated , multi-engine and Commercial glider.

When we got to altitude, the ailerons were level, with reduced power and the higher speed negating any aileron input.

My 700 H.p. Air Tractor even needs some aileron at 1600 ft. lbs of torque (direct-reading torque gauge) and a foot full of right rudder, on takeoff and initial climb. It has an extremely long nose because the engine only weighs around 330 lbs without the starter/generator, with the prop arc 14' ahead of my seat and 9' ahead of the leading edge of the wing.

Air Tractors are equipped from the factory, with locking tailwheel assemblies, once the stick is neutral, to help with directional control on takeoff. Pushing the stick forward unlocks the tailwheel (a cable runs down to the tailwheel lock from the elevator pushrod), and is how we all taxi.

P-factor accelerates more air over the inboard right wing, causing more lift than the left wing, adding to the left-rolling tendency.

The basics: Aileron for roll, rudder for yaw....agreed?

Best,
Click to expand...

The basics: Aileron for roll, rudder for yaw....agreed?

Yes, I agree.

But where does that put the engine angle; as from what info I've picked up, the P-51 didn't have. And BTW, even my RV could use a bit of quick aileron when the wheels left the ground (180 HP & C/S prop). I'm still going with the angled engine mount...... as a built in design factor for torque roll. It's not my guess, as I always thought it had to do more with yaw. It's from the large amounts of info, I've gathered.

There is also some interesting info on the net, in regards to Reno Racers,
and the advantage of keeping a bit of that left roll for the all left turn racing course.

P.S...... thanks for the info on the Air Tractor takeoff & initial climb, I can use that information, as it relates to power/weight.
 
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Larry, sometimes I think that we tend to overthink these issues. If an airplane takes a bunch of right rudder on the takeoff roll, just put in what it takes. In the case of my Air Tractor, if there's a crosswind from the left, I'm cautious and only advance as much throttle as the rudder can handle, until the airspeed increases the rudder effectiveness, since there's a huge left-turning tendency under this circumstance.

The aileron input to compensate for torque induced roll is negligible and really, not even a consideration. The rudder is a huge concern, or the airplane is headed for the grass on the left side of the runway. The nose is so long on the Air Tractor turbines and the P-51, that the engine has little or no offset to the right...so it's all on the pilot.

Best,
 
pierre smith said:
Larry, sometimes I think that we tend to overthink these issues. If an airplane takes a bunch of right rudder on the takeoff roll, just put in what it takes. In the case of my Air Tractor, if there's a crosswind from the left, I'm cautious and only advance as much throttle as the rudder can handle, until the airspeed increases the rudder effectiveness, since there's a huge left-turning tendency under this circumstance.

The aileron input to compensate for torque induced roll is negligible and really, not even a consideration. The rudder is a huge concern, or the airplane is headed for the grass on the left side of the runway. The nose is so long on the Air Tractor turbines and the P-51, that the engine has little or no offset to the right...so it's all on the pilot.

Best,
Click to expand...

Pierre, Just as you said, just do what it takes. The reason I even got into this matter of "torque" is for flight simulation purposes. My days of real PIC are at an end, at least for now. I've tested various sim programs over the years. There is a particular one that uses a form of fluid dynamics to simulate the actions of the aircraft, as controlled by the pilot. The problem is...........it's always requiring pilot action to trim for torque, even while in cruise, or throttle changes. It tends to make some "armchair" pilots believe that we're always counteracting roll with the stick, or constant aileron trim, or the airplane will just roll over. In other words, the action from torque is just over done, and the ways in which an airplane deals with torque isn't modeled correctly. The need for rudder on the takeoff roll & climb is there. Then I'm the person, one of a few, who say's this "roll" action isn't correct, and then of course I have to find out exactly why, and what's built into the design to compensate for it. Your last comments regarding aileron.....help too. Thanks!

L.Adamson
 
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