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gear ratio and prop sizes?


23 posts in this topic

Posted

Looking for a better understanding of gearing and prop size... If a guy wanted maximum stol performance would he choose the biggest prop he could clear, run the lowest gear ratio ie- 4:1 and pitch it accordingly? I'v read in several places that larger dia props turned slower provide better takeoff and climb performance. So where are avids/kitfox's limited, is it ground clearance for the prop? I think 3:1 is the lowest gearset i'v read of people using on these planes, does anyone have experience with the lower gear ratios?

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Posted

Call Darial, warp drive props!

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Posted (edited)

Agree - call Daryl at Warp Drive - He has a good knowledge of what works with your engine/redrive.

Other than ground clearance, you have to consider tip speed and not let it go supersonic.

Another thing to consider - If your prop is too large, then your blades have to be flattened enough for you to make your static engine rpms, and you may start out good, but your terminal speed may be too slow.

Sometimes a shorter prop will give you the best takeoff, climb and cruise speeds.

I would also get some input from guys who have experience flying your engine/redrive and see what prop they use.

Some use a 2-blade and others use a 3 blade prop.

I think a couple or three on here have found the ultimate prop for non-sport pilots - 3-blade IFA IVO!

EDMO

If 3:1 is lowest ratio that Rotax makes - Are you planning to build your own experimental "unproven" redrive?

Edited by Ed In Missouri

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Posted

At the risk of a thread jack ,Ed's comment on tip speed got me thinking.

Is there an ideal tip speed for various purposes ?

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Posted

Hard to go wrong with a 70" Warp and 2.62:1 whatever you do make sure and get a ground adjustable one. One or two degrees can have significant effects on performance and EGT's. Yes prop pitch plays a major role in tuning a 2-stroke.

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Posted (edited)

Hard to go wrong with a 70" Warp and 2.62:1 whatever you do make sure and get a ground adjustable one. One or two degrees can have significant effects on performance and EGT's. Yes prop pitch plays a major role in tuning a 2-stroke.

3 blade 70" ? That seems to be the most common prop used for our planes whether we have a 582, 912 or Subaru -

Daryl at Warp Drive told me that 2" more or less would not make much difference -

One flyer said he got the same takeoff and better cruise with a 68" -

Donno what ratio he had, but cruise probably wont vary more than about 5 mph.

Sorry Bandit - I left out the scarce 503!

EDMO

Edited by Ed In Missouri

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Posted

Sorry i let this drop for a bit.

I believe there is a 3.47 and 4:1 ratio available for the C box, also I called and left a message for Daryl hopefully he gets back to me. I too think IFA is the way to go just wondering if a bigger prop turning slower might have some increased benefit.

I have considered going belt psru but i would like to have a clutch and im not sure if i want to tackle it.

Either way i wont be doing much experimenting until i have some seat time.

Looking to have an Avid Mk IV real soon.

 

Thanks to all who responded.

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Posted

I'm coming in a little late on this thread but... I have a 2.59:1 gear box on my Hirth and I swing a 2-blade 72" Power Fin prop currently pitched at 11.25 degrees for the higher summer temperatures. I usually pull about 6200 - 6300 RPM on climb out and set cruise at 5800 RPM. I typically get between 85 and 90 MPH in cruise. If I hold back on the elevator I can climb at 1000 FPM at around 55 - 60 MPH. I can maintain a 500 FPM climb at 70 - 75 MPH.

MarkD

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Posted

Thanks for sharing MarkD, how do you like the Hirth? I always thought one of there 3 cylinder liquids or that flat four they have would be a hotrod in an Avid/Fox.

 

Daryl from warp drive called and I'm a bit overloaded with info, but i think for what i want to do with a 582 the setup would be 3.47 gear ratio with a 72" three blade IFA. Max Stol and still be able to dial more pitch in for cruise.

 

However i'v never heard of someone using a 3.47 ratio on a kitfox/avid, although a few people who have ran both 3.00 and 2.62 seem to like the 3.00 better.. maybe 3.47 is better yet.

 

Anyone know how big a prop you can hang on a C box? or Avid for that matter?

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Posted

I'm jumping in a bit late but just came across this thread. I've been having real power issues ever since we finished building almost 60 hrs ago. having no prior experience with these birds I didn't realize how bad it was at first as i was just excited to be flying again. but after flying for a while and now being able to take people I realize my performance sucks. I've played with rigging / weight/ carb jetting prop changes etc. but am starting to think my problem is just that I have the wrong prop/gearbox combo. I would like to see some of you with the 582 put up your empty weights gear box and ratio as well as what prop, make, size, 2 vs 3 blade info. and what performance you are getting out of it.. .climb rate and cruise.. I would like to go to a C box and preferably keep my current prop if it will work but want to know before I keeping sinking money into it with no results..

 

-Specs-

Avid C with IV upgrades

Heavy Hauler extended speedwing (yes you read that right)

Empty weight 526 lbs

Rotax 582

Rotax B gear box w/ 2.58:1 ratio  (spinning too fast?)

Ivo 68" 3 blade ultralight prop with standard pitch

set to 6300 RPM's static as read from MGL digital tach. 

Take off at 60 mph 600 fpm solo and 400 dual

Cruise 70 mph indicated solo and 68 dual (flying at 6300-6500 rpms) below 6000 and I'm going down. 

Other - I'm 150 lbs wife is 130 lbs / no baggage / usually have 10-28 gal fuel on board 

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Posted

How many RPM are you actually turning on take off, what are your EGT's?  Have you verified the Tach readings with any other source?  Tiny tach or optical hobby tach?

 

How much washout did you build into the wings?  Where is your CG at?  I can fly mine around at 4900 RPM on wheels and 5300 on floats and stay in the air.  I normally cruise at 5800 to 6000 depending on head wind and how fast (ummm for an avid) that I want to get to my destination.  6300 RPM cruise is not cool. 

 

Did you fair the massive gear legs any?  That is a lot of tube hanging out there creating drag.  I would get some real thin aluminum if nothing else and make a down and dirty fairing with it and a few pop rivets down the trailing edge just as a test to see what difference that makes!

 

:BC:

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Posted

I've played with the pitch settings and tried it several times even at 6500 on T/O performance sucks. I had better performance with the 66" GSC but it was self adjusting in flight so it had to go. the lift struts are faired but not the gear legs yet. I have not checked the tach with another I will have to just to see. I'm running the MGL E3 engine monitor seems accurate for water and egt's. I'm right at 1050-1100 on the egt's after rejetting the carbs.  the wings are built with Speed ribs on HH spars and set at 1.5" washout. 

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Posted

Thats a lot of washout 1/2" would have been much better.  But at that weight you should be getting MUCH better performance.  I would look at the gear leg fairings as a quick, cheap test.  I would also get a cheap 20.00 hobby optical tac and use it to verify the engine RPM.  an optical tac should show around 2442 RPM on the prop at 6300 RPM on the MGL tac.  I am sure you set it up right, but you do have it set to the right mode for the 582 that fires twice per revolution??  Just trying to get my head wrapped around why performance sucks.  I have seen most guys that said they had poor performance (and they were using the supplied analog tac) find that when they used the tiny tac they were turning around 5300 when the analog tac said 6300. 

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Posted

I built the wings to the book specs before reading that I should have done them less... oh well lessons learned i guess. spent yesterday going over everything, cleaned the plugs, new thermostat, added a tiny tach to confirm the MGL. took it up early this morning to check it. still no good. climb out was 200-300 fpm at 60 mph, 6300 RPMs confirmed exact by the second tach. EGS's 1050-1060 water is a bit of concern though best was 111* I was hoping the new thermostat would get it up higher but it didn't. not sure where to go with that. I'm still wondering if i'm spinning this prop to fast. still hoping to see what gear ratios/prop combinations some of you are using. I've been fighting this for a while and I'm running out of ideas.

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Posted

A silly question: Where are your flapperons rigged?

If they are reflexed very much (nose up trim) his will kill much of the wings performance.

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Posted (edited)

I feel that it is fairly nose up even in cruise but when checked with a level the flapperons are just as the book says they should be, level with the bottom of the wing just outside of the lift strut attach point. Pulling in one notch of flaps gets me off the ground much faster and at about 38 mph. vs 50 and climb out is much flatter attitude but performance is still not good. is there something I'm missing here? maybe I should try putting some weight towards the rear to pull the CG back? I will have to double check the W&B but I know it was towards the front of the envelope as most of these birds are. 

 

EDIT: I believe the empty w&b was -13.66

Edited by High Country

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Posted (edited)

Robert,

      Thinking that your cg needs to be farther back - but could be in prop/ratio? ....  What is your altitude and temps?

EDMO

Leni,  Is 6300 static not showing enough prop pitch for a 68" prop??? 

I thought it should be slower - but I wouldn't know....My Soob is supposed to static at 4200 and unload at 5000 with a 70 or 72 prop with 1:1.84 redrive - Randy's Stratus runs a little faster, but he has a higher ratio redrive at 1:2.2.

Edited by Ed In Missouri

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Posted

I played around with 2.62 and 3:1 on mine and I settled on the 2.62 and the 72" IVO 3 blade.  It gave me the best performance on floats. 

 

6300 static should let you turn 6450 to 6500 on take off.  Max HP and TQ is in the 64-6500 range on a 582 anything more than that and your just burning gas.  Your EGTS tell me that your jetted right for your prop pitch as mine runs ~1100 on take off at 64-6500 RPM.  If it feels nose up, try pulling in just a touch of flaps.  I know when my flaperons are flying "level" as I play with the flap handle you can feel the roll forces on the stick get more or less.  I set it for the sweet spot in flight with the least amount of stick force needed for roll.  Then I trim with the elevator.

 

I would try strapping a divers weight on the tail spring, 2 to 4 pounds at a time and see if that does anything for you.  I like my CG around 18" as it seems to perform the best there.

 

The only other thing I can think of is the drag created by your extra wide long gear legs.  I picked up 250 FPM in climb when I put the streamline material on my lift struts and a 100 or so FPM when I faired my gear legs.  Those tubes are A LOT of drag on an already "draggy" airplane.

 

What is your density altitude there?  On a 90 degree day at sea level here on floats (at gross) I can get off the water in about 500' but it takes a LONG time to climb out, she just doesnt want to get out of ground effect.  Anything less than about 75 MPH and the tail wants to drop down and it just feels sluggish... At times I have been lucky to climb out at 100 FPM.

 

On wheels, I can be off the ground in 100-150' and climb out at a minumum of 500 FPM on a hot day with the 30" airstreaks on the plane.  With the 24" that I have on it now I should be doing better than that but I have not flown it on a hot day yet with the new gear and wheels on it.

 

:BC:

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Posted

I built the wings to the book specs before reading that I should have done them less... oh well lessons learned i guess. spent yesterday going over everything, cleaned the plugs, new thermostat, added a tiny tach to confirm the MGL. took it up early this morning to check it. still no good. climb out was 200-300 fpm at 60 mph, 6300 RPMs confirmed exact by the second tach. EGS's 1050-1060 water is a bit of concern though best was 111* I was hoping the new thermostat would get it up higher but it didn't. not sure where to go with that. I'm still wondering if i'm spinning this prop to fast. still hoping to see what gear ratios/prop combinations some of you are using. I've been fighting this for a while and I'm running out of ideas.

Just curious what is the air temp and what elevation are you at? I kinda wonder about the water temp, I don't have any 582 experience but it seems most people run higher than that by a bit. That would indicate to me that you either have a really good cooling system or your not working your motor very hard. Another direction would be to compare speed with climb or even glide ratio (in similar conditions) with other avids regardless of rpm, if your way off then I would think plane setup. If your not too far off there then I would think your not getting full potential out of the engine, which your cool water temps would back up unless your overcooled in some manner. I would try pitching it to get 6500 rpm on climbout and see how much that changes things.. The other thing I would do is a conditional checkup on the engine, compression test, I'm not sure if the timing is adjustable if it is i would check it.. Seems like your down on HP or prop effeciency or both.. If it isn't a drag issue.. But like I said I don't have any experience with 582's just general two stroke principles.

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Posted (edited)

Robert,

     I could give you some figures if you want me to get my calculator going;  but just listening, I am hearing that Leni pulled a 72" prop with 2.62 ratio for 2405 prop rpms at 6300 engine, and you are pulling a 68" with 2.58, for 2442 prop rpms at same 6300 - Both are far below speed of sound, so you are not overspeeding prop, and would not be overspeeding with a 72" and your 2.58 ratio.

     I am also wondering if your engine is not putting enough power out to where you can pitch your prop enough - maybe you should compare your pitch degrees with someone who is getting good performance on the same type of prop and engine?  

     Doesn't the IVO convert from 3 to 2 blade?  Could you try that?

     Still haven't heard you say what your DA is....or just altitude?

Now, to add more confusion to the prop thing - When I talked to Daryl, he said that with the 72" Warp Drive prop on my Soob that it had to be flattened too much to make static rpms, and suggested I go to a 68" and put more pitch in and get same climb and more cruise( :huh:)  I think either Bandit or Jim Chuk said they experienced that with a shorter prop.

     I now have a 70" square-tip 3-blade Warp Drive that should go 2610 rpms at 4800 max cruise, or 2280 at 4200 static, but untried.  I wonder if I should get the tapered tips?  But I will try this one first.

EDMO

Edited by Ed In Missouri

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Posted

High country

Sad to hear your performance issues,keep going you will get there ,

Regarding engine water temp, we have a similar Low temp on our kf4 582 blue top.

A new thermostat made no difference ,we have soldered the vent tube on the head and drilled a smaller hole,

We believe there is enough flow to bypass the thermostat giving a slightly low water temp.

We have yet to run her up but will post when we do.

If your engine inverted it may be a completely different setup

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Posted (edited)

klares,

      We have sort of gotten away from your "theories" while trying to help High Country with his immediate problem.

In reference to your "big prop and high ratio":  Leni Akflyer, Joey C5E, and lots of others have plenty of experience flying the 582 engines and props - I think I would pay close attention to what they are saying.

  Anything more than a 3.0 ratio may not help, and could hurt performance.

     I think someone said that IVO would not sell them a prop with a certain ratio redrive?

      The largest GA or IFA that I know of is 75", and most are max 72", but turning them too slow is not the ultimate answer.  You can "overload" even the old "aircraft" engines with too much prop pitch.

I believe I read that max performance will be less than or about 75% to 80% of Mach 1 tip speed.

     As Leni said, the EGT will tell you about how you are loading a 2-smoke....I wouldn't know, and just listen to them.

EdMO

Edited by Ed In Missouri

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Posted

ED, good point I'm getting a bit off topic from the OP so I think I will start a new thread. Think we're getting somewhere though :) 

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