Questions on a 4 stroke vs 2 stroke set ups.....

I was thinking today about Paul Walkers set up on his 4 stroke P-51. It occurred to me that it goes against the "grain" in what we've been striving for with our models. If I remember right, Paul uses 2 degrees of right thrust and a fair amount of right rudder in calm conditions and then he reduces rudder in windy conditions. I always beleived that we wanted zero thrust and either no rudder or very little rudder. These 2 set ups seem to be at odds of each other. Also wouldn't the right thrust and LO fight each other, let alone the right rudder? Can somebody explain why Paul can have so much right thrust and right rudder and still get away with it? Also then why can't this same type of set up be used on "regular" 2 stroke planes, either piped or non piped? Do the 4 strokes have so much power it can be "wasted" with right thrust? I thought we didn't want our planes with any yaw in them? Help me understand what is going on.
Thanks

Crist,

Go back and re-read the "Engine Offset, is it need and if so How Much" thread started by Doug Moon. I believe about 80% of the answers you are looking for are in that thread. The gist that I get from Paul's post are that he is trying a slightly different approach with the 4 strokes because their seems to be higher static line tension for a given lap time with a 4 stroke. Of course I think the extra tension is really supplied by the 4 stroke swinging a larger propeller, but that's just my take on why a 4 stroke offers increased tension. I don't have any actual experience to base that on, but hope to in the near future.

>Crist,
>
>Go back and re-read the "Engine
>Offset, is it need and
>if so How Much" thread
>started by Doug Moon.
>I believe about 80% of
>the answers you are looking
>for are in that thread.
> The gist that I
>get from Paul's post are
>that he is trying a
>slightly different approach with the
>4 strokes because their seems
>to be higher static line
>tension for a given lap
>time with a 4 stroke.
> Of course I think
>the extra tension is really
>supplied by the 4 stroke
>swinging a larger propeller, but
>that's just my take on
>why a 4 stroke offers
>increased tension.

I don't know why is *is*, but it's not simply diameter. Lots of people are running 13" 3-blades on PA 61-65s, but not necessarily getting tension on par with a 4-stroke.

It's not static thrust, either. A 40VF has twice the static thrust of a ST46, but the level flight tension is higher with the ST. The thrust in level flight, of course, is the same in either case.

It's not simply the lap speed, either. Two ST46's turning exactly the same prop at exactly the same lap time can have significantly different line tension. Same with PA61s, and presumably others, too.

The "why" of this question is probably the hardest thing to understand that I have come across. I've never even heard a plausible explanation, much less the right one. There's some simple physics that we have all missed.

Setting up the airplane with no engine offset and little or no rudder offset is just another theory. As long as there has been stunt, there has usually been some offset used in the engine from the good ole days of profile flying with a couple of washers under the front engine lugs.

All of my airplanes since 1968 have been built with 2 degrees of offset in the engine just as Paul is using. Only lately have I questioned that arrangement when applied to 4 stroke engines. Airplanes with 4 stroke engines seem to have substantially more line tension. The couple of 4 stroke tension, plus engine offset, can create an awful lot of line tension, particularly when the lap times get down to 5.2 or below.

You pays your money and takes your choice. The goal is to be able to keep the airplane on the end of the lines with enough tension to maneuver when the wind blows. The BBQB Bearcat is VERY heavy on the lines so I had a strong inclination to minimize rudder offset. However, the airplane's handling characteristics improve with increasingly large rudder offsets and movable rudder travel. It flies well in the wind, particulafrly in vertical and overhead maneuvers but my right arm chronically hurts and I use two hands on the handle after the pattern.

My new, unfinished Millennium F-51D is built around a RO-Jett 65 with my standard 2 degrees of engine offset. I just did a redesign of the Millennium nose for a Saito 72 installation for Richard Oliver. No engine offset but a movable, adjustable rudder is standard on my designs.

I built the Millennium II last winter which locks in the 2 stroke engine choice. Since I started flying the 4 stroke Bearcat, I have been very pleased with the way the Saito runs as a stunt engine. If I wern't committed to a 2 stroke in my new Millennium II, I might very well have built it like Richard's. Comparing them next year will be interesting, a 4 stroke with no offset versus a 2 stroke with 2 degrees.

Crist: I have been flying a Berenger GeeBee Sportster since we got back from the Nats with a Saito 56. At first, we had it set up with no offset. It had very poor line tension. I added about 3 degrees of offset and it instantly flew better. Granted, this did not solve all of it's trimming needs, but it was a BIG first step. This plane simply flies better with a 4S motor with offset.

>Crist: I have been flying a Berenger
>GeeBee Sportster since we got back
>from the Nats with a Saito 56. At
>first, we had it set up with no offset.
>It had very poor line tension. I added
>about 3 degrees of offset and it
>instantly flew better. Granted, this
>did not solve all of it's trimming
>needs, but it was a BIG first step.
>This plane simply flies better with
>a 4S motor with offset.
>
>
>Later, Steve

I have a question concerning 4-strokes that these posts bring up. I have heard a lot about the immediate tension gain of a 4-stroke engine, but now you are saying that without offset there wasn't much line tension. Paul, Al, and others are using offset to get line tension. Is this, then, needed for the 4-stroke, or is this merely another trimming tool--you set it your way and I set it my way?

And this is not intended as a knock, merely a question. When Matt first flew his pipe set up, it actually fell out of the hourglass--twice. It took about 250 flights for him to get it trimmed to where he was truly comfortable with it. This is not a knock on the pipe set up, either, just a comment. But after a lot of fiddling, props, pipes, head shims, fuel, venturi, chart reading, not to mention a number of telephone conversations with Randy, it finally came together. But it came together without engine offset, there is now considerable pull at the end of the lines. He is using a 13.5 inch 3-blade prop by the way.

So, back to my question, is offset needed for the 4-stroke to get this good line tension that you are reporting, or is it simply a trim technique that you are using and tension can be had by using other methods?

Len,

The Gee Bee is a massive plane and weighs only 54oz with the Saito in it. Without the offset it seemed too light for the size of the wing. I dont have the numbers but that is what it seemed like. Offset it and, BAM! Good even line tension.

Brad Walkers Impact based model with a 72 has no offset and also weighs in about 13 more ounces. I flew this plane and it was pulling me all over the circle.

The airplanes in question on the first thread were very light so the offset may be needed to keep them out there because they dont have the additional overall weight that causes line tension along with the motor.

I thought it would always be really light needed offset due to actual flying weight not cuasing line tension. If the plane was of normal or heavy weight then the straight forward approach would work just as well due to the fact that the weight of plane is going to add to the line tension.

Paul proves me wrong there. His plane is 66oz not a light weight compared to the Chinese but not a heavy weight either. Maybe with a 72 in there and all the horses it pumps out he might get more line tension than he wants and straighten it out some. Wait a minute he flies a B-17 right? I dont think there is such a thing as to much line tension after that.

It seems to me if you are making enough power then no offset or rudder is needed. But then again the weight factor is there. Also style of flight. I absolutley hate heavy line tension. It feels awful. I like it smooth even and light. It feels very easy to give it a precise turn.

Doug M said:

> I absolutley hate heavy
>line tension. It feels
>awful. I like it
>smooth even and light.
>It feels very easy to
>give it a precise turn.
>
>
>DMoon

Bingo........That was my sentiment and plan for many years with the tuned pipe. I believe I have proven that that approach will work in the US. Now, I am approaching this differently. I am trying to develop an approach that will work in air typically found in Europe (much the same as was found in Sebnitz this year) AND appeal to European judges as well. This involves line tension to help cut through rough bouncy air. In my efforts this year, I found that additional rudder offset helped the plane turn and track better than with less rudder offset. The line tension did not increase much, but the drive out of the corners was improved.

So in answer to Len's question: I use it as a trim device, not a line tension maker.

Sorry for the divergence---

Steve,

Chris Cox and Al Resinger both built The Berringer GeeBee last winter. Al seems happy with his, but Chris tells me that the planes are very hard to trim for stable, level flight. Chris' wanders all over and no attempt (so far) has been able to get the plane to groove.

What do you do to get yours to groove in level flight?

Chris' wanders all
>over and no attempt (so
>far) has been able to
>get the plane to groove.
>
>
>What do you do to get
>yours to groove in level
>flight?
>

In talking to Alan R lately, I understand that Chris had his CG too far back, and has since moved it forward, with better results. I am hoping to see both of them fly this weekend in Salem. We'll know more then.

>So in answer to Len's question:
> I use it as
>a trim device, not a
>line tension maker.
>
>P. Walker

Line Tension and the ¡§Godzilla Wall¡¨

Since we are discussing the effects of line tension, and how they affect the 4-stroke airplane in particular, I thought I might share some of my observations. In the process of developing the ¡§Dallas setup¡¨ for the Saito 72, I have used several airplanes. These have included the Buccaneer 746, The Zone (based on the Time Machine 60), the Unassuming (Time Machine 60), the Saturn, and Shear Panic (Impact). All of these airplanes required SLIGHTLY different tweaks to the setup (propwise) and all created very different results. One of the main differences was perceivable line tension. All of these airplanes had different levels of line tension, and I believe that line tension had a lot to due with the ultimate performance of the airplanes.

Starting from lightest to the heaviest:

1. Buccaneer 746 (650 sq. in)- this is Ultra Hobby Products kit box airplane. It is a ridiculously light 56-oz with a Saito 72 installed. This airplane flew very well, and was a good wind plane. Problem was, since the airplane was so light the possibility of hitting the ¡§Netzband Wall¡¨ was very real. I had to run the engine exceptionally hard to get any line tension (read that again-I had to run the engine HARDER). Because I had to run the engine very hard it would have a tendency to drive hard through the corner, any attempt to SLAM the corner resulted in a bobble. So the pattern was a little soft if the airplane was flown in control. This airplane is a winner with the Saito 56, I bet. We need to try that.
2. Saturn (650 sq. in)- this was Doug Moon¡¦s repaired PA 61 pipe airplane. With the big Saito 72 it weighed in at 62 oz. We all chuckled at the thought of installing the big Saito in the little guy, but we did it anyway. As, a result, it was probably one of my favorite airplanes-ever. This airplane was VERY good in the wind and VERY good in good air. The Saito only had to put out reasonable power for the airplane to stay on the end of the lines, and lap times could very greatly with good results. Maneuver speeds were very consistent, and the controls were firm but nimble, as a result very small patterns were possible. Windup in the wind was evident but not uncontrollable. Trim was not that important, frankly. Laugh if you want to¡K
3. Unassuming (660 sq. in)- test bed Time Machine. Weighed in at 68 oz with the Saito 72. This airplane flew OK. Started to notice the effects of the extra weight on the Saito. The Saito liked it to a certain extent. Instead of fighting windup in the wind, now we start to see some SLOWDOWN in the calm. Interesting.
4. The Zone- (660 sq. in) ¡Valso weighed in at 68 oz. Designed specifically for the Saito 72 (although it was mainly a Time Machine). The line tension was really quite manageable although very heavy. As a result, this airplane was really quite good in the wind, but required very heavy control throws and a strong back. The only real limitation of this design was the short tail moment. The heavy weight and a 14¡¨ prop equaled a loss of tail power in a hard maneuver. As a result the corner was very flat and predictable but never very impressive. This airplane also exhibited tendencies toward slowdown in the maneuver (especially when trying to fly SMALL), and only marginal windup problems. I liked this plane a lot. It was a good plane until it folded in two, 3 weeks before the Nationals.
5. Shear Panic- (700 sq. inches)- UHP laser cut Impact with a 9.5¡¨ nose. This airplane weighs 72 oz (hey, it was built in 14 days and painted in 5 hours). This airplane has line tension for days. The longer tail moment really helped the turn and the weight has never really been a factor if you don¡¦t try to fly UNDER 45 degree patterns. This airplane actually hits the ¡§Godzilla Wall¡¨(which is the opposite of the Netzband Wall). In high winds the line tension gets so great that the controls don¡¦t move. Even moving the CG back doesn¡¦t help the situation much. Especially in the outside square. This is also the first airplane that the weight is just too high to be super duper impressive. The Saito 72 starts panting a little bit on the second maneuvers with this plane, especially in the heat. With the Saturn, you could do consecutive hourglass maneuvers for the whole tank and the engine never got hot or slowed down. I don¡¦t think that this kind of flexibility of power is possible with the Shear Panic, and it is simply due to the weight, that¡¦s all. By the way, don¡¦t get me wrong, I love this plane. My new plane for next season will be based on this design; it is far superior to any of the previous airplanes and more closely matches the needs of the Saito 72. Built at a correct weight it will be a super plane.

One more interesting note about yawing and line tension. I started working more seriously with the Bolly 14.25-6 prop on the Saito. This prop always showed superior line tension over my wood props, but I have really started noticing some traits in regards to yaw that I really do not like. The Bolly weighs 1.5 oz as compared to the 1-oz wood props that I use, and I spin it almost 600 RPM faster. There are several interesting effects.

„h In the wind the ¡§Godzilla Wall¡¨ appears to almost double. The CG feels way far forward.
„h The shutoffs at the end of the pattern also became very erratic. If anyone watched me fly at the Nationals you might have noticed the quick shutoffs that my setup is capable of. Well, with the Bolly prop, the inevitable beeping at the end of the pattern started to appear.
„h The tip is starting to dip in a hard corner. The previous settings of the Rabe rudder are now not as effective. The airplane refuses to ¡§sit¡¨, especially when it is windy.

The only thing that has changed is the prop. All of these factors point to excess yaw. Moving the leadouts forward improves the situation somewhat, but¡K I will keep working on it.

I still do not hesitate to recommend the Bolly prop, by the way. If the Shear Panic were not so close to the ¡§Godzilla Wall¡¨ already, I think the Bolly would work much better. I literally think I have hit a wall of too much line tension. The Bolly, since it muscles the airplane to the outside of the circle naturally (I think this is precession), it exacerbates the line tension level even more. Of course, this line tension I think I love could just be the extra weight of the prop, which is, in reality, a detractor.

So in summation, I really feel that ¡§line tension¡¨ is, first, a function of the weight of the airplane (we need assume reasonably decent trim). I think I have flown enough of a variety of airplanes with the exact same engine and prop to be capable of making that assessment. To this end, I plan to build my next airplanes with a goal weight of 63 oz-no lighter, no heavier. There are two reasons for this weight, Paul Walker won the Worlds with an Impact at that weight, and that was the weight of the Saturn, which had the perfect feel. The second most influential factor for ¡§line tension¡¨ would be the engine and prop. Don¡¦t get me wrong, the Saito 72 is capable of spine snapping line tension, but all things being equal the weight of the airplane has even more effect.

The big double whammy for line tension is the heavy airplane coupled with the 4-cycle. That dude will really stay out there¡K. Is that ultimately what I want? No, it isn¡¦t.

Brad,

Did any of the test aircraft use any engine offset? If so, how much? When I get my Saito .72 sometime this fall or winter, I plan on putting it into a 686 Sq In. Legacy. The Legacy has a very large Stab/Elevator, in the neighborhood of 28% of the wing area and about an 18" moment arm. It sounds like this type design might be a good fit for the .72. A couple of other question, then i'll shut up and listen. What size clunk tank are you using, and about how much fuel does the .72 use for a pattern? Oh yes, before I forget, could you post a photo of how you are holding the clunk tanks onto the plane itself. Thanks millions for all you 4S efforts.

Does the name "Shear Panic" imply that the designer is structurally overconservative?

How can too much line tension limit control movement? Have you hit your wrist hinge moment limit?

>How can too much line tension
>limit control movement? Have
>you hit your wrist hinge
>moment limit?

This comment probably is a result of our (Howard and myself) flying session last night. I have been working towards more line tension in the P-51 since the NAT's, and have accomplished that. Last night, I tried an experiment that I have wanted to do all year, but never found the time to do it before. I opened up the throw on the handle to get a quicker corner. Well....That part worked, I did get a quicker corner, but by the time the flight was over, I was about ready to call for help from Howard. The moment applied to my wrist as a result of that change was SIGNIFICANT. My wrist really hurt, and I considered calling it a night then. Remember, I have flown my B-17 which weighs considerablly more than the P-51, but have never experienced anything like this with that plane. I then put the handle back the way it was all year, and I was able to fly the pattern just fine! No pain!

Sooooo....Maybe I am getting close to the point of having "enough" line tension. "Enough" is defined as how much my body will take! Then, one last experiment. I removed a turn of rudder offset. I was AMAZED at how much the line tension changed with that adjustment. It backed down to NAT's type levels and flew a very smooth and graceful flight at that point. BUT...With that extra line tension, I was able to "bang" the corners much harder, but the flight looked a little more abrupt.

Bottom line is that I left that rudder offset out for the upcoming contest this weekend. Now with a good motor in it again, it should do well. (Thats another story).

> Well....That part worked,
>I did get a quicker
>corner, but by the time
>the flight was over, I
>was about ready to call
>for help from Howard.
>The moment applied to my
>wrist as a result of
>that change was SIGNIFICANT.
>My wrist really hurt, and
>I considered calling it a
>night then.

Funny you should mention that. I was fiddling with props before the NATS, and ended up getting a few sub-5-second flights by accident. The pull was obviously quite stout but not outrageous. The control loads also went up alarmingly, and after a couple of flights I had a sore elbow/forearm. Now, granted, I'm a big sissy, and heading rapidly towards "old fart" status, but I think these ariplanes may be getting close to the reasonable limit on physical effort.

BTW, the "old fart" factor is even more obvious when I note that this was all at the beginning of July, and, if anything my arm has gotten worse, despite relative inactivity and several cortisone shots.

>>>I am trying to develop an approach that will work in air typically found in Europe (much the same as was found in Sebnitz this year)<<<

Paul, do not believe the Sebnitz is a typical European place. We visited that place a month before WC and I had surprisingly move leadouts almost 1” back to get optimal trim. So it ended up more that 2” behind the CG (engine as well as ruder are at 0 offset). But as I went back home (flat windy place at much lower altitude) I had to move it back to its previous place. After return to WC I already knew what to do – it significantly helped again.

… but your rudder offset looks like a good idea, I think I will retrim my model autumn again – with more rudder offset…

>I'm a big sissy, and
>heading rapidly towards "old fart"
>status,

True...

but I think these
>ariplanes may be getting close
>to the reasonable limit on
>physical effort.

Also true.

> BTW, the
>"old fart" factor is even
>more obvious when I note
>that this was all at
>the beginning of July, and,
>if anything my arm has
>gotten worse, despite relative inactivity
>and several cortisone shots.

Seriously, THIS IS A REAL THING! I have had flareups of like, tendonitous. It was the worst right before I left for the Nats. It really hurt to pronate my wrist.

My airplanes are too heavy. Bob Baron was on this years ago.

BTW, just kidding a bit Brett. Love ya!;)

Remember, I have
>flown my B-17 which weighs
>considerablly more than the P-51,
>but have never experienced anything
>like this with that plane.

Huh. Weird. Must be the combination of the weight AND the 4 cycle then.