Torsional Resonance

[Scooter]

After additional research, I have to admit that I was wrong about my logic on "piston pounding". Some folks refer to it as "Spiking". I change my mind in a heart beat when I am convinced that my previous stance wasn't correct. The only thing I don't like about being wrong is that it's going to cost me more money to get it right. But, right always out weighs other options.

I saw an ad in the December issue of Experimenter about a new Light-Weight Prop Drive. The ad says it stops vibration and spiking. They have a pretty good web site, but not to informative, so I called them. They haven't put one on a direct drive but are very interested in doing so. It's a bi-directional drive which uses two reversable sprags (not sure about spelling) out of a tubro 400 automatic transmission. They are doing more research about my application and are supposed to get back to me. The price is $550 and it weighs about 6 lbs. Their site is www.aero-wing.com. Just thought I would pass it along.

[Scooter]

Steven, what are my other options? I've got too much time and money in the 2.5 to even think about another engine. Thanks, Larry

P.S. For some reason the profile lost my name.

[GyroRon]

Larry get a Prince P tip prop and leave the prop drive alone. The prince will do fine direct drive and will be the most quiet prop you can buy.

[Scooter]

I looked at the Prince and liked it Prince recommended a configuration in accordance to what I am building and what I think I want to do. However, if what I now think I want isn't what I end up wanting, then I'm stuck with it. I would like to have options. I want a ground adjustable prop. I may go with a DUC windspoon. NOt real sure, it's still debatable.

[GyroRon]

Yes a prince is not adjustable so you got to know what you need before you order it.

Personally I believe - and mark my words on this for future reference - that a direct drive Ej-25 on a 701 will be a mistake. To much weight in the nose to begin with, and a lot of noise and not much go from the direct drive application. I think it would be money WELL spent to put some type of reduction on this engine and be done with it. It will still be nose heavy and you will end up having to add a bunch of weight to the tail to get it to balance out, But i would also be concerned about the weight of the big heavy engine taking a big bite out of the planes useful load. Have you talked to Zenair about using a EJ-25?

[Scooter]

Yep, I talked to Zenith a few times and went there twice. They said it would handle it. According to Parham who did the cam work, my 2.5 will put out 135 hp at 3500 rpm. That should be plenty of go. With a RE-Drive it puts out 195hp at 6k, but that's about 40lbs+ on the nose. I'm having the engine mount built by Tech Weld with the engine 4 inches closer to the firewall than normal. I'll have to adjust the battery locations to get the GC within the proper range.

Noisy, no I don't see that, properly muffled at 25-27k, it should purr like a quite kitten.

Tech Weld is also building a light weight radiator and small header tank for me. The whole thing is only going to be about 80lbs heavier, so I don't take fat boys for a ride.

Plus I never really liked the idea of running an automotive engine at a continious 5k+ when it was designed by the manfacturer to run between 2 and 3k. I know people do it and do it very successfully but I'm not comfortable with it.

I think I have pretty much covered the bases, but I'm not closed minded. I take your suggestions as constructive. Worse case, I'll sell the 2.5 and go with a Jabiru 3300.

[GyroRon]

The Jabiru is nice. I had a 3300 on a Sonex and it is powerful and smooth.

i wish you luck with your 701, just would hate to see you put all that work into a alternative engine and hae it not work out good for you.

BTW the noise in direct drive Soobs is not the engine as much as it is the prop. General aviation planes like Cessnas don't turn much more than 24 to 26 hundred RPM on take off and throttle back to the low 20's in cruise. The subaru just won't be putting out much power at that RPM, it needs RPM to make power due to it's small displacement. I think you will cruise in the lower 3000 range and that is alot of prop speed and a noise maker.

But it sure beats spending 12 grand on a jabiru! Which btw turns a little over 3000 RPM on takeoff. But with a good muffler and the Prince prop it is the quitest engine prop combo you will ever hear.

[Scooter]

You could be right, but I beleive the 2.5 will have plenty of power to run the prop at any speed necessary. Just remember the engine is only at 1/2 throttle at 3000rpm. What I am saying is that I will have all the power necessary to run the prop at it's max. I just have to build some sort of control as to not over rev it.
The Jabiru 3300 is a nice engine, but bad over priced when you consider it's old technology, real old. It's $13k plus another 3k by the time you buy the install kit. It's 120 hp at 3k, which is maxed out) so you end up with less, probably around 105 or so at 25/27k. Plus I will take FI and water cooled over carbs and air cooled anyday. Not to mention I could buy 4 or 5 2.5s for the price of one 3300 Jabiru.
The Jab is 178lbs, the 2.5 is 259, only 81 pounds difference.
Boils down to prop, which I am still undecided.

[GyroRon]

Larry 81 pounds is a HUGE HUGE HUGE difference!

Also I wonder if you understand direct drive aero engines.... just because the engine is designed and can run at 6000 RPM, in a direct drive application at 3000 rpm you will not be at half throttle.

If you look at a EJ-25 with the re drive and without the difference between the two is what RPM they can acheive at a given throttle opening.

With the re-drive you can turn 6000 rpms at the engine, and depending on the ratio, the prop turns half that speed or lower. NOW HERE IS THE IMPORTANT PART.... The props pitch setting is selected to load the engine down to the point of limiting it's possible wide open rpm to the 6000 rpm.

In the direct drive application your limited not to the engines RPM limit, but the props rpm limit. Depending on the diameter of the prop, at some speed the props tips WILL become supersonic. At supersonic speeds the prop will lose thrust, make a extremely loud noise, and quite possibly shred itself apart. In a nutshell you do NOT want to go supersonic. So.... in a direct drive application the larger the diameter of the prop the slower you have to turn it to stay under supersonic. If you use the little 52 inch props on most direct drive subaru ea-81 gyros, your wide open rpm will be limited by keeping the prop speed under supersonic which is somewhere right around 4000 rpm. So on these gyros, the props pitch is selected to load the engine down to the point of only allowing 3600-3800 Rpm at wide open throttle settings. - note: if you were to increase power on this engine somehow, say a turbo charger, the engine would now have more power and could turn this given prop even faster than before, possibly going supersonic, so to absorb the additional power of the turbo charger, the props pitch setting will have to be increased.

So the whole point I am getting to is that your EJ-25 will be limited by the prop size and pitch to only turn 3-4 thousand rpm - the rpm wil first be decided by the props diameter then the loading of the engine will be decided by the pitch - SO........ at 3000 rpm your throttle setting will likely still be nearly wide open. Half throttle will be somewhere in the very low 2000 range.

So your not running the engine at the higher speeds a re drive equipped engine would turn. But your still running the engine under the same high loads, possibly even higher loads due to the lower rpm and less mechanical advantage.

[GyroRon]

I also wanted to mention that the Jabiru is ready to run all up weight at the 178 pounds.

the subaru at 259 pounds does not include anything except the bare engine. Add the assesories and it will be quite a bit heavier.

so the 81 pounds may turn into 100 or more pounds difference. And the Subaru does not make that much more power to compensate for the added weight. IOW's it will be a dog.

you really ought to try to put a re drive on it from the start.

[Aussie_Paul]

I can't believe this but I have to agree with Ron on this one. That was very well explained Ron. Who wrote it for you!!!!!!!!! LOL. Just joking.

Larry , it will be interesting to find out how it does really work, but it may be an expensive learning curve.

Is the ej-25 the quad cam engine?

Aussie Paul.

[Brent_Brown]

Larry If Don said it will make 135 @ 3500 did he say what prop would work best? I will be running a 68 warp drive on my Corvair direct drive. 2800 will be my max rpm. I am sure that Don told me before that a 58" prop would work at 3500 max that sounds about right to me. your SUB at this rpm will out pull any Jabiru. But the weight might be high. I asked them about putting the Corvair in a 701 they told me it was to much weight and this thing will fly at 225 pounds. Saying that I have seen just about anything put on this AC like EA81 with 45 pound PSRUs.
Good Luck

[GyroRon]

The subaru is 2.5 liters.

the Jabiru is 3.3 liters.

I don't care what that the Jabiru is built using old tech and the Subaru is built with new tech. The old tech was to use torque from displacement, to produce horsepower at low Rpms.

the new tech like Soob, is to use less displacement and more RPM's to make the power. It is also a lot heavier.

No doubt I would rather have the Subaru ALL things equal, but in the real world things AREN'T equal. the only downside to the Jabiru is the price tag.

But Larry, I can dig up a quote from you on the other forum, but it said something like " if you ain't got the money, you don't belong in aviation...."

I say spend the money and do it right from the start. Aside from the engine you will have a lot of time and money into building this plane. In the long run the plane's increase in value with the " real" aero engine, the Jabiru, will more than make up for the added cost of the engine.

The Subaru would be a whole better alternative engine if used in a airplane designed to use a engine of that basic weight and size. The Vans RV 4, 6, and 7, and 8 comes to mind. These planes use lycoming engines with 150-200 horsepower and about the same weight or slightly more as the big Soobs. Of course you would have to use a re drive to get the target horsepower.

But this my friend is a doomed project. I suggest going to Yahoo.com and looking for a 701 builder forum. I bet there is one for the 701 or possibly the whole Zenair line of aircraft, and ask the people there what they think of it.

I am not trying to pee on the mans party, I just have a big time gut feeling that if no one stops him now, he will have wasted a lot of time and money trying to get this installation to work out.

[gyromike]

Damn Ron,

You're getting pretty good at this aeronautical stuff! :o

A 912S would be killer on a plane like the 701. Big prop, low RPM is what's needed for a hi-drag lo-speed design. I'll bet it would be a blast to fly...as long as you're not in a hurry!

I like the 701. It's like a bulldog.
They're so ugly...they're cute!

The 801 with the Lom inline 6-cylinder engines look even better. Looks like a turbo-prop!

http://www.zenithair.com/stolch801/builder-pics/kb-02.jpg

Is this not mean-looking?!?

Of course, the above is just my opinion.

[GyroRon]

It looks pretty cool to me. But after all that crap Larry used to say about how dominators and other function over form machines were SO UGLY..... and how the low to the ground RAF was the better than the others simply because it was better looking, Well I just can't believe he choose this design! It is butt ugly with a capital B! ;) But yall don't get twisted, I am just having fun with Larry for some of his past comments..... ;D ;D ;D no harm intended.

Ah.... off to go flying. not above freezing yet, but I got a heck of a cabin heater in the Pacer!

[Scooter]

Yep and I still think they are ugly, not crap, just fact. So is the 701. But it is the only thing other than a copter I could find that would do what I want to do. Besides, when I get done it won't be so ugly, just a little ugly and mean looking.

I've been to the Zenith forum. It's not near as much fun as this one. As for the engine, I'm gonna do what I'm gonna do. If it doesn't work out, then I'll do something else.

I started to get an 801 but Zenith said I might have problems using my short field. I already had the engine and re-drive. Zenith said there was no reason why what I wanted to do with the 701 wouldn't work. The re-drive is still for sale. If Zenith said no way, then I wouldn't be doing it. Plus I'm using larger wings with re-tractable slats, (Pegastol). Which will give me more lift, lower stall and higher cruise.

Half throttle was only a figure of speech, I know that's not realistic. I've looked at the power curve charts. I'm very aware of the weigh penalty. The 259 included everything except water and radiator. No one's told me anything I didn't already know except about the spiking.

I'm certainly not a know it all, but I like doing things differently and my way. And, I always have a plan B.

[gyromike]

Larry,

If Zenith sez it can handle the weight, then I'm sure they would be the ones to know.

Keep us informed. Pictures too.

[Scooter]

In my continuing research on Torsional Resonance, I ran across this article,

http://www.prime-mover.org/engines/Torsional/contact1/contact1.html

I'm really confused especially with this statement, because I was thinking it was a problem with a direct drive and not re-drives. He's saying just the oposite.

DRIVE SYSTEM CONFIGURATIONS
Any time a propeller is connected to an engine in any way other than directly to the crank it would be wise to realize that torsional vibration can be a problem.

[mceagle]

Torsional vibration is absorbed by the propeller (flywheel) if it is bolted securely to the crankshaft.
If there is a gear train between the crankshaft and the flywheel, then all the destructive torsional pulses are fed directly into the gears.

[Aussie_Paul]

Tim, I guess that is why an 11 lb flywheel is better that a 3 lb flywheel when a gearbox is fitted to a 4 stroke. I have a new Sub4 geabox mated to a 120 hp ej-20 for the Firebird frame. I asked Sub4 for a heavier flywheel than last time to compare the differences.

Tim, a number of the Yanks (term of endearment) told me that I would not get 20 hours out of a Hirth gearbox fitted to my ej-22. I have, as well as youself with the Rotax gear boxes, have had tremendous success by running an 11 lb flywheel. My Hirth behind the 130 hp ej-22 has given trouble free operation for over 300 hours. I have not had to replace anything. These hours have been with a gross weight of 1100 lbs.

Aussie Paul.

[GyroRon]

Tim, a number of the Yanks (term of endearment) told me that I would not get 20 hours out of a Hirth gearbox fitted to my ej-22. I have, as well as youself with the Rotax gear boxes, have had tremendous success by running an 11 lb flywheel. My Hirth behind the 130 hp ej-22 has given trouble free operation for over 300 hours. I have not had to replace anything. These hours have been with a gross weight of 1100 lbs.




PAUL!!!! go better be knocking on wood when you say things like that!

[Aussie_Paul]

Yea, I know Ron, 'just could not help myself!!!!!!!!! LOL

[mceagle]

"Of course you would have to use a re drive to get the target horsepower."

Ron, just a small point. A re-drive does not change the horsepower of the engine. Horsepower is a multiple of torque and rpm. If youhave two engines of 80 hp each and one is at 2500rpm and the other is at 5000rpm, then the one at 5000rpm must have half the torque of the one at 2500rpm to give the same 80 hp. It doesn't matter how you gear an engine, it remains the same hp at the same rpm's.

On re-reading your post, if you meant that the redrive allows you to achieve the higher rpm's for a particulal engine to reach its maximum horsepower, then you were quite correct.

[GyroRon]

On re-reading your post, if you meant that the redrive allows you to achieve the higher rpm's for a particulal engine to reach its maximum horsepower, then you were quite correct

Yes this is what I meant.

[GyroRon]

How about explaining why though, on some airplanes... let's use a Kitfox for example, Does a 912 cubic centimeter Rotax 80 hp 912ul engine outperform a 2200 cubic centimeter Jabiru 80 hp? Both are four cylinders and both are rated for 80 hp. Both are quite close to the same weight installed. The Rotax runs high engine rpm and a re drive and the Jabiru run low rpm and direct drive.

[GyroRon]

Not to sound like a smarty pants, I know the reason.... I just wanted to open it for a little debate.

Anything to get off the winding in the wind topics!

[PW_Plack]

Ron,

I think the Rotax 912 is 1200cc, not 912cc.

BTW, there's no guarantee fuel economy will be better on the direct drive, either. Unless it's operated at RPMs outside its "happy band," a modern four-stroke engine can be expected to burn a little under a half-pound of fuel per hour per horsepower, regardless of RPM.

[mceagle]

Yes - 912 is 1200cc and 912 uls is 1352cc.

80 hp turning prop at 3200rpm or 80 hp turning a prop at 2400rpm?
2400rpm allows for much larger diameter more efficient prop. (on low speed aircraft)

[Neil Hintz]

There seems to be a lot of misunderstanding surrounding reduction drives, it seems to me some people still think they are just additional weight and cost with little benefit, in this day and age I find it hard to believe anyone still believes this! How do we derive thrust to fly? With piston engines we rely on combustion of fuel air above a piston to force against a connecting rod to a rotating crankshaft, torque, hardly brain surgery. Torque is a function of cylinder pressure, cylinder pressure (measured as BMEP) is a function of basically how much uel air is admitted into the cylinder each cycle, this is torque, twisting effort, NOT HP. To rate this torque as HP we need a frequency (RPM) ie- number of bangs per minute. In simplistic terms, more 'Twisting efforts' per minute is more power. So the choices for making thrust are, limit the number of bangs per minute (prop diameter/ tip speed) and go for a large piston area/ long stroke/ lots of pistons or very high cylinder pressure via blower (big bangs!) or sensible size/ number of cylinders at a higher number of bangs per minute with a reduction drive to bring the output RPM into the optimum for a given size prop. Trade revs for torque, gear it down enough and you can run a prob big enough and slow enough to lift you straight off the ground (helicopter), try that with direct drive! It is so fundamental, did the P51 mustang have direct drive? Imagine the size of the engine it would have required. To get any amount of performance you must use a reduction drive!

[Aussie_Paul]

Exactly Neil.

Aussie Paul.

[Neil Hintz]

see what can happen when you get a window of opportunity on the computer Paul!

[Neil Hintz]

Here is a picture of an Airmaster prop fitted to the latest Dominator off my production line. 71" constant speed.
Can't wait to see how it goes, it's attached to an Autoflifght gear reduction, SUB4 130HP EFI, tuned induction engine.

Neil

[Neil Hintz]

Try this picture.

[Neil Hintz]

Steven, Me too! I'm keen to see this running, was hoping to have it running this weekend just gone. Possibly Wednesday or Thursday. I have limited the rpm to about 120 HP. Airmaster unofficially say it will be safe for 130HP but we need to approach this output slowly while keeping a good eye on the integrity of the prop, I have the full support of Airmaster. I'll post some more photos when it's outside running.

Neil

[Vance]

FSTexan,

Fifteen posts attacking Duc propellers at this moment.

You seem to be repeating yourself.

It would help us understand where you are coming from if you would fill out your profile correctly.

It seems unlikely that your real name is Luc.

It appears that you do not have an aircraft.

Most of your posts seem unrelated to the thread and many of the threads are from long ago.

Why do you feel so strongly about the Duc propeller?

What is it you are attempting to communicate?

Thank you, Vance

[i3 roberto]

Tis is one of the most briliant tread in the forum now