Hi Bob,
I typed in the address into my address line and got nothing. Went to HorizonHobby forum typed in 4-stroke and gave up after I don't know how many pages. So, I am going ask you a stunt specific 4-stroke question.

One of the comments I keep hearing about 4-strokes is:
A 4-stroke doesn't have a 2-4 break so you have to fly it fast 5.0-5.1 lap time in order to have enough power to fly the pattern but this is just the opposite of my experience.  Many years I flew a Saito 45, using a front-pickup chicken hopper tank, on a Nobler size plane with an oversized tail and because the 4-strokes have better torque I ran bigger 13 1/2" -14" props that had been thinned,and narrowed to reduce the load they induced on the engine but still delivered increased thrust. The big tail was to let the plane turn with the bigger props.
This plane on 63' lines center handle to center of plane flew5.9 laps in moderate wind and once flew at 6.2 sec. per lap in dead calm.
When I pulled the plane up into a maneuver I felt an increase in power but not rpm that pulled the plane through.  I named it a "4-stroke break" .
Perhaps, it was that front-pickup chicken hopper that I made that created this effect, and not the nature of the 4-stroke engine itself? I don't know.

The bottom line is with increased thrust I did NOT have to fly FAST in order to fly stunt with a 4-stroke engine.
Bob, you are the most experienced 4-stroke stunt guy I know,so my question is:
Is this idea that you have to fly 4-strokes fast one of the myths you talked about?

                                                        Pat Robinson

Hi Bob R,
I did my 4-stroke experiments a few years before I saw the first artice on someone using a 4-stroke in a stunt plane so I didn't have anything to unlearn.
My modified props used an 8" pitch.   I put a stock 11/8 wood prop
on the Saito 45 and adjusted for maximum tached RPM. I then modified my 13.5"- 14" props till they equalled the same max RPM on the engine so I didn't create an excessive prop load on the engine. These props looked a lot like a narrowed APC electric prop but they put out good thrust.

It seems that in my case, if you produce enough THRUST you don't really need the SPEED, unless the airframe needs the speed to produce lift.
I read on SSW where I think it was Steve Moon, was flying 5.8 lap times using a PA-75 and a 15" or 16" multiple blade prop with a pitch under 4" on max length lines so for him thrust worked instead of speed.  We live in the era of wonderful flexible, high thrust stunt power so I share Steve's wonder about why the increase of stunt speed that has occured over recent years. 

I stopped flying this setup because I was maxed out on performance and it couldn't handle more difficult wind and turbulence. There were no viable engines I could upgrade to on the market at that time so the experiment ended.

It might be interesting to try this setup with say a Saito 62, Chicken hopper tank and bigger modified props put in a 610+ sq" plane. Humm, that's an idea.

Lap speed is a big deal with me because I just don't enjoy flying fast so anything faster than 5.3 second lap time is a waste of my time just flying out the tank.

Now obviously, if you have enough thrust and power and line tension you can vary line length and change the lap speed to a more desirable number.
So I guess, my question is does the fast lap times I see talked about with 4-strokes apply even to models on max length lines?

I guess I will just ponder if my tank design and modified props made that big a diference in my 4-stroke experience or has 4-stroke design and run qualities changed that much over the years. Say if it has moved away from torque and more toward rpm. I don't know but ,this is interesting stuff!

I liked using a 4-stroke engine so I may have to give it another try.

Bob Z. this thread was a good idea, thanks.
Bob R. I defer to your superior computer skill about locating the website.

                                                                     Pat Robinson

Hi Bob R,
I don't want to hijack Bob Z's thread about wrong myths about 4 strokes
but so far we just tried to provide as factual a data, as experiences will allow, so I hope Bob Z. will indulde us a little longer.

Well, I went online and checked out the Score ARF  and the Saito-62 and I have a few questions for you:
1. Given that the Score seems almost intended for the ST-51 which would weigh out around 13+ oz with an after market muffler and the Saito-62 weighs out at 16.4 oz.. So, did you need to shorten the nose to adjust for the increased weight of the Saito?  I might be inclined to shorten the nose anyway
in order to to reduce the fulcrum arm so that the force of gyroscopic inertia which is created by using bigger diameter props won't degrade the planes ability to turn as much as it would with a longer nose moment.

2. 4-strokes tend to be very frugal in fuel use so how much does your 
Saito-56 use for a pattern?  If I were going to fabricate a tank I'd need to know the required capacity. The Score has a 6 oz plastic tank. Is that correct?

3. My setup is based around the idea that with a constant speed engine the goal is to increase prop diameter in order to provide the thrust needed for good vertical performance and then modify the prop to greatly reduce drag loads in the inner 60% of the prop and then insure the pitch is set to produce
the speed you want and the outer 5% at the tip is used as a fine tune of prop speed.  Often you move to the next step of narrowing the prop planview shape in order to reduce drag even more to acheive the the desired rpm and vertical performance but not create a prop-load to overwhelm your engine.
Well, thats how it worked the last time I tried it.

I notice in specs for the Saito-62 it can swing a 13/7 prop so I will probably be looking at a 15-"16" prop to modify.

One thing I didn't see listed was the rpm where the 62 produced it's best torque output and its best HP that defines a "sweet spot" where the engine is
happy and is able turn props well.  Bob do you have those numbers?

4. Finally, How big a tail does the Score have? I you run bigger props you will increase  gyroscopic inertia and will need more force to turn the plane. You will need a bigger area or a longer tail Moment or both to acheive the same turn you had with a smaller prop.   I guess to be more precise I should use the term tail volume coefficient  but I didn't compute that with the other airplane so I have no frame of reference to compare. I just knew the tail was around 30% of the wing.  So what are the tail moment & tail area for the Score?

Bob,I'm sorry, I have run on so long. Your suggestion got my mind going
with questions and possibilities. If you have a collection of model mags look up everything you can find on props and prop modification written by Hal DeBolt.  Hal's articles were the bluebrint for all my prop modifications.
His articles opened my eyes to what is possible with prop modification.

Bob, thanks for listening and I will go away now. 

                                                              Pat Robinson




 

Hi Bob,
Several years ago I went to local college and talked to a physics professor.
I asked him about gyroscopic inertia. One question I asked if you take a force like gyroscopic inertia and place it at the end of a fulcrum that's attached to a body can you increase or decrease the effect of the force on that body by increasing or decreasing the fulcrum length. He answered yes but in order to quantify it I would have to wrap my mind some equations
which I had never heard of before.
 
Ever since then, when I am looking at increasing prop size substantially I also look at shortening the nose moment if possible to reduce the effects of gyroscopic inertia.  I have flown planes that were already a tiny bit nose heavy with a normal nose moment and when you put a bigger prop on them they wouldn't turn at all.

In short, I am less worried about the increased engine weight than the combined effect of the weight+ inertia.

Bob, I agree with you that four strokes are torque machines which is why I asked about "sweet spot" between max torque and max HP. I don't want to load the engine below it's max torque rpm and I don't want to strain the engine trying to run a bigger prop at the engines max HP rpm.

I read on the Saito website about issues on running the engines inverted
and reccomended starting out running the engine on its side and it also simplifies setting up tank height alignment to the engine. I addition, vertical CG is not effected as much by a heavier engine as it would be if was inverted.

A 4 ounce tank means you don't have a heavy fuel load to deal with.
Computing the volume for the complex angled shapes of my front pickup chicken hopper tank is a pain in the rear but I still have some of them around and the bucks I shaped them on so it may not be so bad.

Replacing the landing gear is a given with bigger props.

I think I should try to make the tail removeable so if I need to go bigger then it is just a bolt in process.

I think a  Tom Morris 4" bellcrank is called for given the prospect of greater control loads on the system.

Bob, the Rev-up is a nice light fairly low drag prop but I have even improved their performance by doing prop mods. Any engine power wasted on prop drag is not available to fly the plane so even simple prop "cleanups" are worth doing. A plane with a cleaned up prop feels like you added nitro to the fuel because the engine performs better.  On a K&B 40 I would cut down 12" -13" Rev-up props to 11 5/8 / 5 and clean them up so the engine could turn these wider props at the same rpm as a narrow prop which yielded a net gain in thrust.

I did some experiments on some larger props with a OS 61 gear reduced
engine. A 16/10 prop could only turn at 4,000 rpm but a after progressive prop mods this same engine turned it 8,100 rpm. I was amazed by what can be accomplished with prop modification. Hal DeBolt was right.

There are limits, don't weaken the inner section to the point you risk hub failure.  Dont thin the prop so it becomes too flexible. On a wood prop 10% - 12% of chord is the minimum blade thickness (Carbon can go thinner and maintain stiffness).

 For what I do, you need major prop mods including narrowing the planform shape of the prop. Narrowing the prop does cost you thrust but from what I have read and experienced prop diameter is "king" in producing vertical performance.

A tuned pipe setup generates loads of thrust by running the engine at the rpm where it produces max power with right prop and uses the pipe as a governor to control that power.  Randy Smith told me you could hand launch a full size stunter vertically into a wingover it has so much thrust.
A four stroke on the other hand is usually a lower rpm one-speed machine without a 2-4 break , so I reasoned that increasing the prop diameter was the best way to increase thrust and it seemed to work.

Bob, thanks for your information and for listening while I think this through on the fly. I admit I am getting more interested in this idea.
This is interesting stuff!   

                                                            Pat Robinson

This has turned out to be a great discussion. Pat - your information is very perceptive - definitely not hijacking.

Of the many things I've learned about 4S is the importance of props, especially in that I see so many of the pilots underpropping them [yes, I see this a lot but, after being told to mind my own f****** business, I never offer unsolicited advice]

Anyhow, to emphasize what Bob Reeves stated, go for the pitch - believe it or not, increasing it will often SLOW the plane. After installing a Surpass .40 in my Euclid, I found that it was way too fast with an 11-5 Top Flite wide blade wood.
I then tried the same prop in an 11-4 and all that changed was fuel economy. Still too fast but I lost about one minute, putting me into an incomplete pattern.

Then, going the other way, to an 11-6, I found that the plane slowed a bit but picked up about a minute and a half in run time. Things were getting interesting!!!

The next prop to go on was an 11 1/2-6 /12 and it slowed down even more, picking up about 30 seconds. The run was not only perfect but the plane pulled everywhere in the circle. Next to be tried will be an 11-7, probably an APC.

Lap times? I haven't a clue. I have never timed any of my airplanes. I fly by what feels comfortable to me. If the turns are crisp and smooth and I can stay with the plane, I’m generally pleased with the speed.

To find the true "sweet spot", I suggest going to the field with a selection of props, a few different diameters, pitched from 5 to 7. See what works best for THE PLANE IN QUESTION. The props that I'm becoming more and more comfortable with are the APCs.

Tachs? I only use them on rare occasions. Case in point - when I was doing a safety article for pampa/stunt news, I wanted to compare two identical props, one with the tips painted iridescent (Day-Glo) orange.
The test engine was my non-surpass .40, which had been turning in some rather pathetic performances on my ARF Cardinal, swinging a 10-6 Top Flite wood.
I found some 11-7 Master Airscrew plastic props, painted the tips on one and set out to take some picture to examine the effects of the paint.
I tied the plane down and was very surprised at the amount of pull – definitely more than the 10-6. Then, after comparing the two, it turned out that the .40 was turning the 11-7 and 10-6 at the same speed!! The engine is, by nature of the cam characteristics, designed to make maximum torque at a give speed and reducing the load (apparently) will not allow higher speed.

Back to engine speed, I have had enough experience to be able to set my engines by sound and feel. A setting can normally be dialed in after a few flights - then, one may need to go in or out a click depending on weather.

A point was made about the 4S engines being more fuel-efficient than the 2S. If you wish to examine it from a physics and thermodynamics standpoint, find a Marks Engineering handbook, go to chapter 4 and study the section on thermal properties of substances. Pay particular attention to the PV (pressure-volume) curve for the Otto Cycle, on which our engines operate. During the isentropic expansion cycle, the pressure is decreasing, the piston is going down and the volume is increasing. This is the section of the curve where torque and therefore power is produced. BMEP (brake mean effective pressure), piston area and engine kinematics account for the output torque, piston speed accounts for crankshaft speed and the product of the two times a constant yields horsepower. The ideal situation maximizes length of the power stroke – controlled by cam timing in a 4S engine. In other words, extract every bit of energy from the burning gases before setting them free.

Looking at the same curve as applied to a 2S engine, it becomes clear that during the power stroke, the exhaust port opens well before BDC, causing a sudden drop in pressure, as the gases depart through the open port. Pressure lost equals power lost. Burning gases expanding while exiting the cylinder contribute nothing. They will, in fact, tend to pull out (extract) some of the incoming fuel/air charge.

Comments please!!!!

Bob Z.

Hi Bob,
I just saw your drawing of your hopper tank. Very interesting. I appreciate your posting it. Good stuff!
                                                           Pat Robinson

Bob, you should try the TF 13-8 Powerpoint.

I have two or three.

Bob, you should try the TF 13-8 Powerpoint.

I have two or three.

Hi Brad, Have pretty well decided to leave the Score as-is for the balance of the season. Have been playing with the engine and trim so much haven't been able to actually get many practice flights in. Going to try to make it to Dallas for the Labor Day contest, Memphis and of course ours. I would really like to come down for the Dallas Stunt Clinic this fall, the season would be over and we could play with all kinds of stuff..

One quirk I have noticed is if I drop the RPM below about 7400 it vibrates really bad. Not sure an 8 pitch prop would work unless I went to 70 foot lines and kept the RPM up. The vibration may be a resonance issue with the Score design, don't know. Really thought hard about moving to 7 pitch and increasing the RPM a bit so it wasn't running so close to the vibration RPM but didn't have time before Wichita.

I really need to get my Classic ship to some level I feel it can be competitive and Bill Willson gave me a few hints on how he makes a ST 51 work. I really need to play with that, the Score is so close, anything from here would really be getting into very fine details.

Pat, thanks for the kind remarks, heard many comments about how nice that Saito sounded and how well the Score flew. Great for ones ego 

Good Morning Pat,

Here is a link to a thread on my Saito from when I was playing with ideas for the adjustable venturi.

http://stunthanger.com/smf/index.php?topic=3511.0

The attached photo is the only one I could find this morning of the engine, this is after I bent the exhaust tube so the muffler wasn't hanging down below the fuselage. Somewhere on the forums is a thread on how I bent the exhaust tube, basically filled it with sand, heated and bent. Too bad the UHP forum went off line, it had a couple excellent threads on my two Saito set up's with almost step by step trial and error explanations with pictures of how I ended up with I have now.

Was told 4 strokes do best when side mounted so I did it. At the time I really didn't understand why just following blindly. Now I understand the why but not sure I know how to explain it.. It makes the needle less critical, with it side mounted you can actually richen up the needle a bit if you need to slow it down and don't have time to re-adjust the venturi. It will run just as solid as if you had the needle set properly. A side benefit is you need less tip weight because you have the head sticking out the outboard side. All my FS installations in the future will be side mounted.

On the Score I originally marked and drilled the holes with the pattern supplied in the instruction manual for an inverted mount. After I decided to go with a side mount I found one of the holes lined up that put the thrust line in the designed location, plugged and redrilled the other 3.

Huh, indeed!!! 

Well, Dennis - you convinced me - I just threw all my 4S engines in the trash. 

Around 40 of them, including my M-5 radial!!! 

Good riddance - I never really liked them - I was secretly on SAITO's payrol, to infiltrate and eliminate the use of 2S engines worldwide!!!!!   

Bob - no more 4s - Zambelli.

28 4S??

Tamed down??

Bob z.

Hi, Pat - yes, the information was exactly what I need. Thank you and thanks to the others who were so helpful.
For what it's worth, I am not on the market for an ST .51. Instead, I am evaluating a new stunt engine in that displacement range and I wanted some info for comparative analysis.

Actually, I lied about scrapping all my 4S engines. 

I kept one - it's a GHQ that I converted to a 4-stroke Diesel. It turns a 22/13 prop at 321 RPM, and does a wicked 4-8 break.
Runs for 72.2 minutes on 2 ounces of fuel, which is a mixture of ether, kerosene, ether, olive oil, ether, paradichlorbenzine and ether, with a touch of ether added. Since it will run in both directions, I can use counter rotating props. So, I can use a 22/13 and a 22/-13.

Problem is, I can't find a modern stunt plane with a tall enough landing gear.

Bob Z.

Hi Bob Z.,
You GHQ is obviously too much for a "small plane" like a "Sweeper" so the only appropriate place for your GHQ engine is on the nose of a ten foot span "Argus".  The paint bill might be a little steep but sacrafices must be made for such a Hurculean engine.   

Hi Bob R.,
I thought you might weigh in with some advice.  So, How is your classic
plane coming along on being sorted out?

I was also curious if either of you gentlemen were knowledgeable about the lightweight OS-56 that weighs around 14.8 oz
( don't know if with or without muffler).  It cost around $250.00. Most OS I have looked at were heavier than an equivelant Saito engine but this seems to be an exception. Just curious.

It's great talking with you gentlemen, again.

                                                                 Pat Robinson