More Prop RPM = more thrust?

[GyroRon]

Here is what I am getting at. I have a 60 inch prop on my gyro, a three blade warp drive prop. My engine, a Rotax 582, uses a gear reduction unit that has a 3 to 1 ratio. My max takeoff RPM is set for 6500 engine rpms. To set this RPM I have to adjust the pitch of the prop to restrict, or load the engine down to the 6500 RPM max I want it to turn. To do so with my prop and engine and gearbox ratio combo I have a total of 17 degrees of pitch measured at the tip of each blade on my prop.

I have not measured the static thrust on my gyro, but several others including my old Red one - that used the same prop, gearbox ratio and engine combo - have been tested and they all pulled about 325 pounds of static thrust.

My 3 to 1 ratio spins the prop at 2167 prop RPM at 6500 engine RPM, The lowest ratio I can get for my gearbox is a 2.62 to 1 gearset and it would spin the prop at 2481 Rpm at 6500 engine rpm - changing the gears out to the 2.62 to 1 ratio will spin my prop 314 RPMs faster at the same engine rpm.

I am counting on having to reduce the pitch in the prop if I change the gear ratio. To not do so, I believe the engine will be overloaded and won't reach 6500 Rpm where the power peak is.

So the big mystery to me is will the static thrust increase by Spinning the prop faster with less pitch than by spinning at it's current speed and pitch setting???

Part of why I think it might increase is I had heard - but now can't find anything to support it - that Warp Drive reccomends a mx pitch setting of under 10 degrees Pitch. I clearly have alot more than that. I figure I will be able to go from 17 degrees to maybe 15 degrees by spinning the prop faster.

Anyway what do you guys think?

[GyroRon]

Also want to mention something that makes me doubt it will make much if any difference.

At Bensen days 2003 Three of us 582 Dominator owners did a static thrust test pull on our machines, one right after the other. All of us had Rotax 582 engines. One of them had a smaller diameter 3 blade warp drive prop and he only pulled around 300 pounds of thrust. My machine - same set up as I currently have on my new gyro - pulled 324 pounds.

But here is the kicker... Tim Verroi's machine pulled 325 pounds, One pound more than me. Tim's machine uses a B type gearbox instead of the C type I use. The B box had a 2.58 to 1 ratio. With that ratio he was spinning his prop even faster than I could spin mine with the 2.62 to 1 gears.

The only part of the compairing my gyro to Tims for these purposes, is instead of the three blade Warp drive 60 inch prop I had, Tim had a 60 inch three blade Ivo prop.

[Harry_S.]

Ron:

The 1# diff. in static means nothing, really. The proof/pudding is the performance at AUW.

The general rule of thumb...1 degree of pitch=100 rpm.
Set the pitch for whatever ERPM you want and go fly.

[Chuck Irby]

Ron, of course you'll have to reduce the blade pitch. That's a given. But you should get more thrust with the same engine r's. What are you waiting on? Just do it and let us know the results. :D Remember, if you don't want to keep the gears, I want them.
However, I don't care whether I get more thrust or not.

[jos]

Hello,

Improving static thrust is a piece of cake.
Reduce the pitch and or increase the diameter.
Reducing the pitch from 17 degrees to a lower number improves the relative pitch. The relative pitch depends on the airspeed you want to fly.
I do not recal exact numbers but 17 degrees is Ok for over 100miles/hour(depending on aircraft drag).
At this speed the relative pitch is good but at take off the forward speed is very low and the relative pitch is not good.
The best relative pitch for static thrust is very small blade angles,like 5 degrees. This has a number off consequences, first you cannot fly fast anymore. At a certain(low ) speed the flow out of the prop has the same speed as your aircraft and the thrust is zero.
Second the power available is constant from the engine and the engine speed is limited. the given prop does nor absorb the power anymore and the engine overrevs.
Another limitation is the prop speed from 650 km/hour(blade tips) and higher the prop produces mostly noise and no extra thrust.
Increasing the diameter also increases the thrust surface and gives very good static thrust. Limitations are the space available and bigger diameter also increases the blade tip speed.
So if you change your redrive ratio and by consequence the pitch needs to be reduced the static thrust will increase.
The noise will also increase and the props performance will be less good.
If your prop is overpitched for your aicraft now, in other words at top speed the relative pitch is still to high, the pitch reduction will not affect your top speed.
So it is all a matter of compromises.
On the other hand you must consider that regardless the redrive 65 hp stays 65hp.
If you are looking for big differences in static thrust you need more hp.


JOS

[rehler]

Ron,

For what it's worth - a couple of years ago I had to replace my cog drive on my old Verner engine (got messed up and they no longer made the same one) which slowed down the prop RPM. Even with a change in prop pitch there was a significant reduction in thrust - not any scientific measurement, just "experience/feeling" while in flight.

In other words, my experience indicated that a faster spinning prop does significantly increase the prop thrust.

[GyroRon]

I guess my goal in this discussion is to see if I would get more thrust - or some other favorable side effect - by changing these gears out and having the prop spin a few hundred rpm faster.

I know that with the prop spinning faster, the pitch will have to be reduced. So my question is will there be much difference between the prop turning 2167 with 17 degrees of pitch, or 2481 with say 14 degrees of pitch?

Remember were talking about a gyroplane and not a Glasair, My speed range is mainly from 30-75 mph.

And what about Chucks idea??? not to twist his words, but what I believe he is saying is with my 3 to 1 gears, with the gyro in cruise flight at say 60 mph, due to the speed of the prop with that ratio I will need a given amount of power or engine rpm to fly at that speed. By changing to the faster gears, under the same conditions I could either fly faster at the same engine rpm or fly the same airspeed at a lower engine rpm - mainly cause the prop is spinning faster with the 2.62 gears over the 3.00 gears. I would agree if I DO get more thrust by spinning faster with less pitch as opposed to spinning slower with more pitch... But if the speed verses pitch between the two choices turns out to be a wash, then I don't think it will make much difference on what Chuck is saying. That is if I am understanding what Chucks point is correctly.

Ken, do you have any more details about your experience? What was the ratios? How much pitch did you take out?

[GyroRon]

And I guess the core reason for why I started this thread is I am thinking 17 degrees of pitch is excessive for my little 60 inch Warp Drive, especially at the speeds I am flying at.

Since there is no option of going to a larger prop - which I KNOW would make a positive difference - I want to make my current prop as best as can be. I am not hurting for more power, but if there is power there for the grabbing just by changing out a gear set, then I want to do it.

[GyroRon]

And lastly...... Why don't I just go ahead and stick these gears in and just try it?????? Well :) The big gear that is attached to the prop shaft in the gearbox can only be removed with a special Rotax puller just made for the big gear in a Rotax C type gearbox. It is a tool no one locally has and no one would probably need, except for me for this project. Are you sitting???? The cost for this tool... 130$ plus shipping

No other puller can be used, there is no room for a regular puller to grab the gear.

I don't want to spend 130-150$ just to find out I wasted my time. I am hoping for either more thrust in the speed range I am flying at, or some other positive side effect that may come by changing the ratio.

Last thing that I am unclear of, is will there be a harmonic or some other ill effect with a 2.62 to 1 ratio on my set up. I know that certain ratios with certain props can result in a nasty harmonic that will wear out things much faster. The 3 to 1 ratio works really good and it smooth.

[Chuck Irby]

Ron, you make a good point. In addition, you will need a 200 pound torque wrench and a mushroom thingamawatchi that cost another $25.

I appreciate you bringing this up again as you have about convinced me to keep my 3 to 1 gears. They look like new and work fine, and I don't need any more thrust. Like I said before, I have a tractor to pull stumps with. :D

[GyroRon]

If I had the tool I would at least try these gears. But with no tool I figured It can't hurt to ask the rest of my pals here what they think. I know most folks here have more brain power than I do! lol

[Chuck Irby]

Ron, how quickly do you need to do something? Keep me informed, please. Seriously though, you do have me strongly considering just staying with the 3:1 ratio.

PS, You have more than your share of the brain power, you AH. :D

[GyroRon]

Well the nice fellow that sent me the 2.62 to 1 gears HAS the special puller and I thought I might ask him if he would send it to me to use. But before I bother him I want to look into further, or will send the gears back to him or to you if you want them. I am in no big hurry.

[Dean_Dolph]

Ron, I don't know if it would help you but Phil Lockwood, of Lockwood Aviation in Sebring Fl., seems to be the accepted Rotax expert in the U.S. I say that because every aviation mag I have read lately has a blurb about a seminar he is giving some where.

There is a 1-800 number on their web site http://www.lockwood-aviation.com/las.php You may want to give him a call since he also sells props.

[Chuck Irby]

Ron, I was primarily wondering if you had a time limit imposed by the guy you got the gears from. He sounds like a super nice, and trusting guy. Is he a rotor head, or a UL FW type?

[GyroRon]

He is a Farmer that uses his T-Bird ultralights - he has two - for spraying his crops. No pressing time limit, but don't want to still be messing with this next spring either.

[Chuck Irby]

Gotcha, & thanx. I can't help but wonder why there are so few of the 2.62:1 gears being used. It might be interesting to know.

[GyroRon]

Most flying machines can swing a larger prop. With a larger prop using the 3 to 1 gears is good, even better is the 3.47 to 1 up to the 4 to 1 ratio now starting to be used. But these ratios are commonly used with 68'' or 72'' or larger props, not a little 60 incher.

I had a X-air ultralight trainer for a month or two last summer. It had a Blue head 582 engine on it with the 3.47 to 1 ratio. The builder instead of buying the best size prop for that plane, found a sweet deal on a 62 or 64 inch GSC prop, so he used it. Long story short, the X-air had the same problems. Slow turning prop with alot of pitch and although I felt it had plenty of power or Thrust, Everyone who had flown in other X-airs and then mine told me I was not making the power or thrust that 582 should be making. When asked what should I do, everyone said I needed to either go to a longer blade prop or go to a faster turning prop by changing the gearbox ratio.

I don't have any firsthand experience with this but I have heard that some of the fixed wing ultralights and trikes and so on that can turn these 72 inch props are putting out in some cases over 100 pounds more thrust - Out of a 503 Rotax! - than we can get with our bigger engines and little 60 inch props. On any flying machine 100 pounds of thrust is a sizeable increase in performance, but on our little gyros, 100 extra pounds of thrust would be a Major increase in performance. Anyone that has watched Carl Schieder fly his homegrown gyros with the 72 inch props can see what I mean, it looks as if he has 300 horsepower the way his machines fly, but he doesn't... just a big old prop back there!

[Chuck Irby]

Yeah, I agree Ron. I didn't make myself clear though. What I meant to say was: I can't help but wonder why there are so few of the 2.62:1 gears being used on gyros with 60" props. It makes no sense to me.

Incidentally, it seems that Carl is being pushed with a 110 HP Hirth F-30. That in itself would make a tremendous fdifference.

[GyroRon]

Chuck I think that one reason we see mainly 3 to 1 gears in the bulk of the gyros with 60 inch props is because when we order the engine we just go with the " normal " or Standard gear set. I believe 3 to 1 is the ratio the gearbox would normally come with unless you specified something else at purchase.

I am also still wondering if there is another reason we normally see the 3 to 1 ratio, such as the harmonics I mentioned in a earlier post in this thread.

And yes Carl has a powerful engine on his machine now, but if you remember just a few years ago, He had a 65 horsepower Hirth 2706 powering his two place and it would fly two people with still great performance with that little engine and solo it was not far behind in performance is enjoys today with the larger engine. I remember my first cross country at Bensen Days, when we flew to Sebring for breakfast one morning. Carl was flying with us with Toni in the front seat and even with that little 2706 hirth, he was flat out smoking us! He was flying along at close to if not over 100 mph, while the rest of us are putt putting along at 70...

[rehler]

Ron, in answer to your question:

I changed from a 1.85 to 1 ratio to a 1.94 to 1, which at 5,000 engine rpm changed the tip speed of my 65" prop from 767 fpm to 731 fpm. I don't recall the amount of prop pitch change I made, but I think it was around 2 degrees.

The faster tip speed was noticably better than the slower turning prop.

[GyroRon]

Okay thanks Ken.

[Chuck Irby]

Ron, re harmonics; Have you noticed a problem with your 582 and 3:1 at a certain rpm range?

[GyroRon]

not that I can remember, other than the below 2200 rpm chatter that all rotaxs have - now fixed with clutch

[Chuck Irby]

I asked because I do notice a situation that feels like it may be harmonics. It occurs between 4500 and 4800 rpm. It may be just with my engine or maybe with all 618's.

I ran an ad this morning on Barnstormers for 2.62 to 1 gears. If I can find some at the right price, or a trade, I think I will go ahead and do it, due to having to run so much pitch (18.5 degrees) with the little prop, and not being able to get the right prop hub for a 4 blade prop.

[GyroRon]

Chuck I can't fly and maintain altitude at that RPM so I wouldn't know. That rpm may be a rpm where the Rave valves are moving???

[Chuck Irby]

Ron, I can't maintain altitude in that rpm range either, but there are times when I would fly in that rpm range, in a real slow descent, if it wasn't for that problem.

[Aussie_Paul]

..........apparantly any gearbox with an exact number ratio will have harmonics. I did not know this when I got my 618 with the 3 to 1. I ordered the 3 to 1 to reduce the noise level which it did with the 68" props I was using on my early side by side Air Command trainer, the HTML one that I did over 1500 hrs in!!!!! and did not know any better, but that is another story!!!

The lighter more efficient (at that time) ivo props that I was running had run approx 500 hours each and when I put them on the 3 to 1 redrive they bothe developed cracks within 10 hours and were wrecked. Enquiries revealed that the harmonics from the exact redrive ratio would be the trouble. The "hunting tooth" in a non exact ratio is what solves the problem I am told.The tougher W/Drive props could handle it. That resonance is damaging something.

Aussie Paul. :)

[Chuck Irby]

Thank you for your reply and for sharing your experience with us, Aussie Paul.

Based on your experience with the 3:1 ratio, and Ken Rehler's experience with different re-drive ratios, Ron and I should both benefit by going to the taller gear ratio. Thank you too, Ken.

[automan1223]

Ron it sounds like to me like you got way too much gear for a prop diameter that small. at 17 degrees your prop is acting more like a paddle with a blade that thin/wide.

Why cant you go with a larger prop ? 60" is not all that much most prop charts recommend a 2.5 ratio for something that small.

I dont think that rotax makes a truly even gear ratio. I thought the 3 to one was 3.1 to 1. count the teeth and divide um up and see what it is. Truly even numbers will give you a case of the wha hwhwhwaaasass. resonance in sound is most annoying. I thought it was between the exhaust and prop noise that created the resonanace.

No one here has talked about rotational inertia rating for a given gear box. This is most important for the life of the gears and the prop not to develop cracks. Rotax publishes an inertia rating and I am sure warp drive and ivo list the numbers too. If you put too heavy a prop on a drive you will blow it up sooner or later.

Rotational inertia is most important.

Jonathan

[mceagle]

2.62 to 1 is the correct ratio for 60" or shorter props. My experience concurrs with Paul about the harmonics of the 3 to 1 ratio. The typical harmonic sound sometimes heard is usually caused when ratios are very close to an even number. In any case, the exhaust should never point directly at the prop.
3 to 1 plus ratios are often used to minimise noise pollution and allow the use of longer props.
There is a lot of free thrust to be gained by using the longest prop that is practically possible. Changing from 72" to 76" on an EA 81 Subaru is worth 30 lbs of thrust with no other mods. That is only 2" extra prop around the outside edge.

[GyroRon]

Hmmmm, well I got the gears, just need the tool!

[CLS447]

Tim, what is the max RPM you like to see with an EA-81? I am thinking about getting a Prince prop... Are you familiar with it? I am now running a 68" Warp at about 5100 rpm full power at 13 degrees of pitch. My redrive is 2.18-1 .

Will I get better climb If I reduce pitch a little to increase rpm?

Before I order my prop I will see if I can go a bit bigger in diameter.

[Chuck Irby]

Ron, is there a chance your gear guy has another set?

Tim, Thank you for the info.

[GyroRon]

Chuck there is no chance Sorry.

[mceagle]

Chris, pretty close to the mark. with all the variations of the EA 81, the max HP is developed at around 5000 RPM so there is not point in going above that (at full throttle at best climb speed).

[Sonnyj]

Hey Ron
If you could give me an idea what the puller looks likeI may have one that would work [no promise]I have a wide array of pullers,installers and home made pullers.

[GyroRon]

I got one coming so for now lets wait and see, thanks though

[bones]

It's not too hard to get them off, with a bit of bush teco, just get a flat plate bearing puller, one with threaded holes in the corners screw in two 12mm bolts a piece of angle iron between them, with another bolt to put tension on the shaft in the middle of the other two, then just put a sh*t load of tension on the shaft with that bolt, then with the oxy torch with a small welding tip just run it up and down two opposite sides of the cog for about 5 seconds making sure that you dont stand in front of it as it comes off in a hurry, if done properly the main crank shaft is still cool enough to hold your hand on it, a bit crude but it works very efficiently.

[Chuck Irby]

Ron, I know you've been busy with ROC this past week, and I trust that you all had enough fun for all of us who couldn't be there.

Since I couldn't make it to ROC, and the weather report was for rain all weekend here, I made the decision to send my gearbox to Lockwood for a gear replacement. I should have it back this Thursday. Then we have a fly-in next weekend.

I don't suppose you have changed yours out yet, right? So, if you want to wait another week, I should be able to report the difference it made in mine.

(I never got the first reply from the ad I ran in Barnstormers. So you got real lucky with yours.)

[GyroRon]

Got the tool in the mail while at ROC. Will do the change out in the next few days. Sucks you missed ROC. We wanted you to come

[Chuck Irby]

Thank you Ron. I am really happy for all of you that were at ROC, it sounds like everything was perfect, except for the ants. :D You won't believe the deal i'm getting at Lockwood. They're furnishing the labor and exchanging their 20 hour used 2.62:1 gears for mine for $100 plus freight.

[GyroRon]

Well just to update everyone, I got my gears swapped out in my gyros Gearbox. Went from a 3 to 1 ratio to a 2.62 to 1 ratio. The Prop spins faster now by about 400 PROP RPM at a engine speed of 6500 RPMS.

I did a static thrust test just before pulling my 3 to1 gears out with a friends thrust scale. It pulled 370 pounds. After the change to the new gears and repitching the prop it appeared to pull about the same or possibly 5 pounds more.

My prop was set with 17 degrees of pitch before with the 3 to 1 gears and with the 2.62 to 1 gears I went down to 12 degrees of pitch. Before with the 17 degree pitch and 3 to1 gears the top engine speed was 6640 on the ground static. Now with the new gears and new pitch setting I am only seeing 6450 RPM on the ground static. SO..... I could stand to take out about one more degree of pitch if I wanted to.

In flight the prop unloads slightly and I gain about 100 extra engine RPM. Even at 90 mph indicated the RPM with the new gears and pitch stays under 6600. Before with the old gears and old pitch setting the prop would unload more and there would be more of a unloading effect and more RPM gained the faster the airspeed.

As for performance, it certainly climbed out much better now. Of course the temps were nice and cool this weekend but it climbed noticeably better than before. Not a huge difference just a noticeable difference. I lost 5 mph on top end speed before I could get to around 97 mph wide open and now it is around 92 mph. As for cruise settings I haven't gotten to test that yet, but believe it will be about the same as before.

The new settings result in a louder gyro, there is more prop noise now and from the pilots seat the engine and prop noises you hear are now totally different.

Anyway just a update.

[rehler]

Ron,

Thanks for taking the time and effort to do the teasting and write your report. This question has been asked often, but your actual test helps us to better understand.

[GyroRon]

Thanks Ken. wish I could be more scientific about my testing. I don't think that my results so far would be worth the efforts I have gone through to change the gears. Maybe there will be a difference in cross country flying, possibly being able to maintain cruise speed at a lower engine rpm... or maybe not.

One area of concern I have is that with the new gears, pre rotations seem to take more time to get the blades up to speed and seem to put a load on the clutch I put in my gearbox. I am guessing it is cause of the lower - or is it considered higher? - ratio may have lowered the torque I am able to transfer to the pre rotator. Less mechanical advantage I am thinking....

Overall though the engine prop combo has a happier "Feel" from the pilots seat when I took her up for the test flight yesterday.

The one thing I haven't done and should have and maybe still will do, is to call Warp Drive and talk to them about this whole matter. I have been assuming that 17 degrees is excessive as far as pitch settings go.

[brs]

I feel that I've tried to reinvent the wheel today. I just received and installed a new Ivo Prop on my Vortex. The prop is 68" long and my gear ratio is 3:1 on a 582. The tech at Ivo Prop told me to go with the two blade because of harmonics with my gear ratio. So I ordered the three blade but set it up using only two (for starters).

After trying it out this afternoon I found that my climb was so poor as to be unsafe. I need to install that third blade but searching the forum makes it clear that it is a bad idea considering my gear ratio. So I'm considering changing the gear ratio. Which led me to this thread which was very helpful though did not answer all my questions since I have a 68" prop and not a 60" as in GyroRon's case.

I have three choices: 1. Change the gears which is what I'm considering. 2. Sell or trade this new prop and get a Warp Drive or WoodComp. 3. Stop flying the gyro and sell it as is. Yes the gyro is for sale but I'd like to fly as long as I can.

So what do you think? If I do change the gears would it be better to go with 2.62 or 3.47?

[gyromike]

With the gyro tied down, what is your static RPM?

[brs]

With the gyro tied down, what is your static RPM?

Didn't do a static but cruising down the runway level at 10' I get 6300. I had it adjusted for 6500 but that still did not give me the climb I had with the Powerfin.

BTW - before someone asks why I'm putting a new blade on a machine I'm trying to sell. Last week I left a tool (dumb I know) on the engine which took out of one of the blades. Since Powerfin seems to be out of business and the hubs are suspect I chose an Ivo. Now back to the thread.

[gyromike]

You didn't give your airspeed when you were cruising down the runway at 10', but it sounds like you don't have enough pitch in the prop.

You should anchor the gyro down and do a full throttle runup (in the seat, belts and helmet on, brakes on, blah blah, obligatory safety speech) and see what your maximum RPM's are.

With that information someone else with a similar engine-gearbox-prop combo can help you get it setup correctly.

[Resasi]

Static testing our 3 blade Warp 62' on a rotax 503 DCDI B 2.58.1 we got 319 lbs at 6500 rpm with 6 degrees

Due to a fault on the tiny tach/shielding/ it began giving eroneous over reading, and before realising this, I had begun to pitch the prop up again. Discovered problem and pitched back to six but now at 6500rpm am getting only 290lbs.

Any suggestions as to why this might be the case?

[giro5]

A little info on a 503 didc on a trike. When I got it it had a 3 blade airplast 60 inch. C box with clutch 3:1 ratio. Before i got to fly it I tipped it over and broke 2 of the blades. ($600 each to replace). My instructor loaned me a 3 blade powerfin. We put it on and the instructor flew it and then I soloed. We had to put a bunch of pitch in it to keep the rpm down to 6500. It was a dog. Flew but just barely. So I got a 2.62 gear set installed and what a difference. Pitch was set about 8 degrees and the thing climbed and flew like it was supposed to .