SAITO 56 or 72 for Impact

Which engine is a better choice for Impact: SAITO 56 or 72.
The model with SAITO 56 is 60 o.z. heavy, and I intend to use 15% nitro fuel. My flying site is about 1000 m above sea level.

Well, everybody will have an opinion. Paul Walker flies his Miss America (certainly an Impact derivative) with a 72 and it is AWESOME. I fly my 75oz, 68 inch wing span stunter with a 56 and it's a great flying plane. The 72 has more grunt, but the 56 has plenty and is pretty easy to deal with. What's overkill?

If retro-fitting an existing 2 cycle powered engine, the .56 makes sense. However, if you are building a new model, the .72 would be the way to go. Then again, pilot size & muscle comes into play here, and just how much do you like a LOT of line tension? The .72 is not for everybody.
Sammy

>If retro-fitting an existing 2 cycle powered engine, the .56
>makes sense. However, if you are building a new model, the
>.72 would be the way to go. Then again, pilot size & muscle
>comes into play here, and just how much do you like a LOT of
>line tension? The .72 is not for everybody.
> Sammy

Exactly.

Marek: At that weight, I'd lean towards the 56. It can easily pull that weight around, and weighs 3 oz. less than the 72. I like both these motors, but prefer the 56. The 72 feels like it is going to pull my arm off sometimes. It does make quite a bit of difference having 3 oz. less weight on the nose of the plane.

Sidenote: In the recent issue of RC AirWorld magazine in Japan was an article about the new OS 70 FS Ultimate. This motor is 4 oz. lighter than the current OS 70 FS Surpass, is noticeably smaller and has a built in crankcase fuel pump and regulated carburetor. A very interesting looking motor. It appears the carb can be removed and replaced with a manifold/venturi setup; unlike the YS 63. Something to look forward to checking out.

>>If retro-fitting an existing 2 cycle powered engine, the .56
>>makes sense. However, if you are building a new model, the
>>.72 would be the way to go. Then again, pilot size & muscle
>>comes into play here, and just how much do you like a LOT of
>>line tension? The .72 is not for everybody.
>> Sammy

With the new Eather props the line tension on the Saito 72 is very nice. Not really hard at all.

The Saito 72 is the engine if you like having a little "overhead" as far as power is concerned. It is very nearly required in the summer months here in Dallas. The Saito 56 is very adequate for models up to 70 oz if you never see any real hot weather with real thin air. It is really better suited to models in the 55 oz range IMHO.

The Saito 72 offers a bit of "breathing room". That room always seems to lend itself to a lot of consistency.

I think an Impact could be flown with both. Maybe that is what you should do, get one of each! They bolt into the same holes. Then you will be the expert, and you can tell all of us which one is better. That is part of the fun!

I like the 56 for all the reasons Steve mentioned. My Impact is going to have a 56 when I get around to building it.

>I like the 56 for all the reasons Steve mentioned. My Impact
>is going to have a 56 when I get around to building it.

....and when I get around to buying you a new 56.... I promise I havent forgotten...

Thanks for all comments. I had some reservations about my engine choice - but now I think this is going to be a good combination: Impact + SAITO 56.

Thanks again for your help.

The Saito 56 in the Impact has been done successfully by several NW fliers. Paul Walker's initial 4 stroke tests were done with one of his retrofited Impacts. Dan Rutherford also retrofited his Impact with a Saito 56 to create the "Powered by Pond Scum" special. See attached picture of same.

I have flown a Saito .56 powered Impact. It flew very well. I thought it was easier to fly accurately than my two-stroke OS .40 VF powered Impact.

Howard,

Such an admission!! Does this mean we might see a thumper in your future?

My Impact with a SAITO 56 is flying. It has lots of power and looks like it is little to fast: my lap time: 4.5-4.7 sec. with the ground RPM about 9000. I am using 12X6 APC prop, 15% nitro 20% all synthetic fuel, and the 63 oz Impact is attached to 66’ lines.

The next step will be to reduce the sprinkler type venturi from 4.5 mm to 3.9 mm, which should give me 12.0 mm2 effective area, which corresponds with .285 w/ST NVA - I am aiming for 4.8-5.1 lap time.

mkozera,

If you wish to stick with APC props, try the following: 13-4W, 14-5N, 14-4W stock and cut to 13.25 and 13.50 diameters. I had to add a moveable rudder when going to monster props. Ugh. Lots of junk hanging out in the air. But it does work...

I am fiddling with all of the above props, .281-bore venturi, YS 20/20 fuel per Godzilla's (good) tip, 4-ounce plastic tank plumbed per another of Godzilla's tips. Stock Impact, although it was converted from ST51 power about a year ago. In addition to Heavy Metal there is a bunch of lead--about 1 3/4 ounces--in the nose. Balance point is currently at 7.125 from hinge line. Elevator throw has been increased, although I cannot give you a specific ratio.

While the 56 and an Impact are a nice combination and the motor is quite tolerant as to the way it is handled, I'm not completely sold on the four-stroke program.

There is potential, it just hasn't been fully tapped, in my opinion.

>There is potential, it just hasn't been fully tapped, in my
>opinion.
>
>Dan Rutherford

Somebody please tap it so it can live up to its potential. The non-Ztron 4 stroke has been on the scene for two whole years now. It certainly should overtake 50 some odd years of two stroke technology by now. GEEZZZ!!!!

Underacheivers.... get a move on!!!!

Marek,
If it's going too fast try an APC 14x5n YS 20/20 fuel . This is what i use with a punched out Venturi,.It should be ok.
John

John, have you tried reducing tip pitch to 3.5 or so? It would seem like a good way to reduce load on the engine and also reduce windup. Dan's looked pretty good Saturday with the cutdown 14 x 4W APC... pulled like crazy as I waited impatiently for Dirt to wave it away. Acceleration was almost instant, WOW! It looks like a good combo. I don't understand the CG shift forward, tho. A heavier engine plus lead???? The mind boggles... Steve

Steve,
I tried a series ofprops since last July 11.5x4through 14x4 Zinger & (nearly)every thing in between excluding any 3 blades.The thing seems to work the best with a 5 to 6 pitch.
John

Godzilla,

I deserved that. And while there is simply no way the Impact would be converted back to an ST 51, I was hoping for the Giant Killer setup. You know, the instant-gratification sort where one simply snaps his fingers, changes fuel and dominates the weenies.

Part of my problem has been that as the balance point got pushed further forward, an inherent linkage problem (you don't want to know...and I'm NOT going to tell you) caused forces at the handle to go way up. As in uncomfortable, let alone accurate pointing of the model.

Whines, nothing but whines. However, my buddy Bruce "Heavy Stunt" Hunt put it to me in both Classic and PA last contest and so good-enough just ain't.

By the way, this is my first Impact we're talking about. It's led a hard life. So it was not a big deal to cut into the cockpit/turtle deck in order to fix the problem I'm not going to discuss.

The idea is to get this model sorted out. No matter what it takes. And then the 72-powered Impact--which still needs to be covered--will be an easier, more productive proposition.

I'm still with ya on these things, continue to follow your advice, even if I am going my own way on props, the APC numbers quoted above showing good promise.

LAST EDITED ON Jul-15-03 AT 09:19 PM (CDT)
 
Dan,

Picking a prop for a Saito isn't an easy chore. Here are a couple of suggestions and some lore which may be of some use to you.

First, I treat a venturi equipped Saito like a throttle with a fixed throttle command. Changing the needle valve just doesn't have much effect on RPM. The needle just richens and leans the mixture without affecting RPM. Since you can't change RPM with the needle valve it is necessary to adjust lap time with prop pitch. This can be fairly straight forward as long as you don't try to change the diameter, blade area or shape at the same time.

Last year I carved a number or props with 14" diameter and the same blade template with pitches varying from 4.5" to 6". I wound up the year using the 14-5.0" turning 8900 static and producing 5.5 lap times. This was using a Pat Johnston manifold, a .300 venturi and a large (.155") needle valve.

This year I'm using a new Saito and the Ultra-Hobby manifold assembly, a .285" venturi and the small OS needle valve (.135"). This set up gives me 10200 RPM with last year's 14-5" prop, a gain of 1300 RPM, I think due mostly to greater venturi area. this RPM gave me lap times of 5.2 which is much too fast for the BBQB. I tried to slow the engine and lap time with a larger prop, so I carved a 15-5.0". It slowed the static RPM to 9300 from 10200 but the larger diameter prop produced more thrust and lap time dropped to 5.0. A hand carved 15-4.5 turned 9500 but still had 5.3 lap times. A 15-4" turned 9800 and gave 5.5 lap times. I finally settled of a 14.5-4.0" which turns 9900 and gives a 5.5 lap time. I was trying to get the lap time down to 5.5 while at the same time working the RPM up to 10000 to get the engine up higher on the horsepower curve and to unload the engine for longevity. Above 10000, I'd be afraid or valve float. Also, I noted the larger venturi opening, while giving more power, also lacked the ultra stability of the smaller venturi. Last year, I would have lap times starting at 5.5 and 5.45 at the end of the pattern. With the larger venturi, I am picking up about .3 on my lap time. It starts at 5.5 and ends at about 5.2. I wonder about the fellows running .300 plus venturis with props which load the engine down into the 7000's. How does this affect lap times? I can't begin to guess whether this would give more or less thrust, but, that is way down on the horsepower curve and a terrific load on the engine. This could cause the engine to overheat. It might also be hard on the unbushed rod which seems to be the Saito's only noticable weakness.

Also the APC is a very heavy prop. It is an efficient prop and puts out lots of thrust, but it is also a very good gyroscope due to its weight. If you were to try a lighter wood prop, it would require less rudder travel on a movable rudder and perhaps eliminate the need for a movable rudder altogether. Hope some of this helps.

Al,
I've tried the wood props on my .56 but for the thrust I get it loadsthe engine way too much. The APC just lets the engine rev with a great deal of pull.
John

> Al,
> I've tried the wood props on my .56 but for the thrust I
>get it loadsthe engine way too much. The APC just lets the
>engine rev with a great deal of pull.
> John

A Saito .56 loads completely differently than a .72.

Try a Bolly 13-6 for the .56. That's a good place to start.

Brad -

I've heard Tom Dixon reccomend the Zinger 13x6 for wood "prop kits" for the 56, says they pitch closer to 5 than 6. Ever try them yourself?
EricV

>A Saito .56 loads completely differently than a .72.
>
>Try a Bolly 13-6 for the .56. That's a good place to start.

Wow. A person who still carves his own props. I'm really impressed. But seeing the airplanes Al builds, I shouldn't be surprised. How repeatable is the process? Is it possible to produce a half dozen that are identical?

So I have reduced the venturi size to 4.0 mm, which gave me 12.5 mm2 of effective area (which corresponds with .285 w/ST NVA) and I went flying yesterday. WOW – that’s all I can say. Ground rpm 8900-9000 and the lap time 4.8-4.85 sec. lots of line tension everywhere and for the first time in my life I was NOT uncomfortable going into outside square loops. This is great setup.
With the larger venturi my Saito was picking up noticeably more RPM after launch – which was making the runs fast – now looks like it is steady.

Gents, he gave some very important info that no one has acknowledged. He lives at 3,000 plus feet above msl. Not 1,000 feet, but meters. In my very limited experince, at sea level a .56 will do great, but above 3,000 feet msl, a .72 would do much better. Never too much power above 2,500 msl. If you fly at sea level and have never tried the same plane, fuel and prop at 5,000, try it, it will open your eyes. Guys in Denver area and SLC any coments??

Mr. Rabe,

I appreciate the input, am currently appreciating your moveable rudder concept a lot more than, ahem, I would have predicted at any time in the past.

However, I am too stupid to carve props by hand, even if some (Zingers) are sorta 50% there to begin with. Thus it is that I have, for now at least, decided to work with APC props.

Yep, got CF pieces, Tom Lay selling me one each for the 56 currently in use, the 72 to follow. And they are beautiful pieces of work. But I can have a great many APC props for about the same amount of money, they are efficient props assuming I can match something to plane and motor. Yes, they are heavy; in the sizes John Leidle and I are currently using we're talking about (gasp!) 55-gram props...

Ya know, Helmick is not the first to comment on how hard my Impact pulls on the ground. I'm not saying it is anything special, but maybe I ought to act the stooge just to see what it's like. I mean, Steve, I can slip the handle-to-wrist tether on a bit quicker if you would like.

Refer to second paragraph. Note that I have already admitted to stupidity. And that I have thoroughly read your dissertation on props, pitches, diameters, etc. So. My comment: From (too-)limited testing it appears as if I can have 14-4W APC props in full-blade and cut-down numbers, each .250 reduced in diameter down to 13.0. The slowest of these props is full-blade, getting a little faster with each reduction in diameter. No other serious evaluations have been made as the weather wasn't cooperative and aforementioned linkage problems I refuse to discuss and/or divulge.

Does it make sense to think I can have a big handful of APC props, all alike except for diameter, and simply choose how fast I want to fly depending upon which prop is used? No pitch changes, just spins against the disc sander. And, yes, I have learned to not attempt using the needle valve to control speed.

LAST EDITED ON Jul-18-03 AT 03:15 PM (CDT)
 
Dan,

What is this Mr Rabe stuff? If your airplane has the gear length to swing a 14" prop, try a 14-5 Zinger. I think they make them. My wood recarved 14" props weigh 28 grams. My profile Beatcat was flown initially on APC props but didn't like them at all because of the weight of the prop and the prop's gyroscopic characteristics. the airplane twisted and hinged in corners until I switched to a 12-5 Zinger. The prop was much lighter and the airplane's corners improved dramatically. I still had to use nose weight in the form of a heavy prop nut but it was nose weight, not prop weight.

>Gents, he gave some very important info that no one has
>acknowledged. He lives at 3,000 plus feet above msl. Not
>1,000 feet, but meters. In my very limited experince, at
>sea level a .56 will do great, but above 3,000 feet msl, a
>.72 would do much better. Never too much power above 2,500
>msl. If you fly at sea level and have never tried the same
>plane, fuel and prop at 5,000, try it, it will open your
>eyes. Guys in Denver area and SLC any coments??

Good catch, Tyrone! I am wondering where Merek is located, but probably not USA, where the Metric system is still a major mystery. His physical size is also a factor, as is the price of Nitro in his locale. I could see 4 strokes NEVER being popular in many countries, as long as they like Nitro as much as they do, and as much fuel as we (should) burn every year, during practise.
Sammy

Sammy,

I live in Canada and to be exact in Calgary, Alberta. Here we use both metric (official system in Canada) and imperial system - prices in grocery stores are looking better when quoted per pound instead per kilogram. As far as the fuel I buy from Greathobbies (greathobbies.com) at the beginning of the season and for 15% nitro 20% synthetic XP fuel (this fuel is made by SIG in USA) I pay about 20 CAD (15USD) so this is not bad at all.

Al,

I didn't want to leave this subject hanging. But I have now completely reversed field on the issue of props and for the time being have set aside a bunch of the APC numbers.

Yes, they are quite heavy. But the real problem for my Impact was the 4" pitch. At PW's insistence I bolted on an Eather 12.5 x 5 three-blade prop at the Arlington Affair last weekend. Dramatic improvement. The power steering came back, with further changes the turn came back.

John Leidle does report success with an APC 14-5N and I will try it again. But for now I'm trying to not stray too far off the res.