What Prop Size / Speed?

magilla

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What gives? Is it the nature ofthe material (wood versus fiberglass) or the design which gives the difference?

Culver Prop Calculator says .75 - .80 mach for max thrust
Warp Drive Calculator says .88 - .92 mach for max thrust.

I am trying to calculate which PSRU reduction and which size prop to get, and with this much margin of error, I am now confused.

Engine hp: 120
Max Engine RPM: 4200
Cruise RPM: 3400
PSRU reduction: 1.6 to 1
Max Prop size: 80 inch

using the above calculators, my best option is to go with a 72 inch prop (4200 eng rpm gets me 2600 prop rpm, about .75 mach.)

I could go with a 76 inch prop, but eng rpm would only be 4035, about 110 hp.

An 80 inch prop puts me at 3800 eng rpm, about 105 hp

So, what is more important - size of prop, or matching prop size to reach .75 mach at peak hp?

Can you see my confusion? will 80" at 105 hp give me as much thrust as 72" at 120 hp??

HELP!!!
 

Ga6riel

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.92 mach seems too dam fast to me
very high tip speeds that approach mach1 lose efficiency and gain noise
a lot of noise. effective tip speeds should be within 500ft/sec to 800ft/sec

when you exceed a tip speed of 880ft/sec (600mph) compressibility effects cause a rapid increase in the power required. 880ft/sec requires a section of 6% of prop chord, and where it is known wood props need to be thicker. so tip speeds for wood, carbon fibre or metal props differ, and that could be the cause of the differing speeds on the data shown

edit to add 'tip speed'
 
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Screw

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Sounds like your trying to fing a prop for a Great Plains Liquid cooled VW engine. Culver does put those numbers out there for wood props, and of couse same as Warp drive.

I don't know what machine you planning on using this combo on, but if you have room for an 80inch or better prop, that would be the way to go. However, your not gonna be able to swing it well with the 1.6:1 gear. I suggest going to the 2.0:1 and pitch out the prop.

As it is my understanding, the VW are very torquie at midrange-high RPM (3400-4000).

Here are some problems your gonna have. It you get the Warp Drive 3 blade at 72 inches, you may be OK with the 1.6:1 redrive and all may be well. I remember GyroRon went to go fly someones VW tractor with a 72 inch Warp and it seemed underpowered and really overheated the engine. The advantage of using a three blade Warp is the ability to Change the pitch.


However, to maximize thrust, you try and stuff as large a prop as possible, swing it as slow as possible with as much pitch as possible as neccessary to get your engine RPM up to 4000RPM. Now you may find flying it that your max only going 50-60mph, but yu damn sure got there quick.:whoo:

The problem with Big Props is finding the right one. You are running a standard Rotax pattern, therefore, some of the prop makers like culver and Prince may have to make yours special because you don't have the meat at the hub to swing an 80+ inch prop. I had Prince make me an 84x50 and I relied on the prop makers experiance to find the correct pitch. Plus, he made it so, I would have nothing to lose.

Prince made the offer to me when I ordered it that if the pitch wasn't right, he would fix it at no cost to me. I don't know how well it would have worked because I sold the project before completion, but that was the route I was going.

Screw-Out
 

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magilla

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Screw - I need to ask you some questions!!

Screw - I need to ask you some questions!!

Screw -

I bought my tractor gyro from Joe Terminella. Basically Ron Herron's LW on steroids (frame is 1.25" od tubing). Kind of like a 20" wide dune buggy with a rotor system. I at 270 can climb all over it!!

The motor mounts were set up for a VW - bolts right to a Diehl accessory case.

Joe ran a 2100 revmaster with 65hp, with a home-made 1.6:1 redrive and a 72" Warp Drive 3-blade prop. Prop was pitched at 10 degrees, or about 37".

Engine RPM maxed out at 3400, he was only .59 mach tip speed at 37" of pitch. He could have dialed some more in, methinks. Pure conjecture from the second-guessing new owner. (The original owner had to be wrong about something, right?) :D

As it was, the aircraft flew around the pattern a couple of times, but engine was maxed out, according to Ron Awad. Oil came out of the breather and onto windshield because it was over-serviced, and the flight ended rather shortly thereafter. Ron had to leave, and further testing could not be done.


Consensus between Joe T and Ron A was that the 65hp revmaster 2100cc just didn't have enough guts for this open frame gyro, and everything was not dialed in perfectly, as it was the maiden voyage. Aircraft flew to 300 feet, but was sluggish. Forgive me Ron or Joe - that is what I kind of merged between your two reports. All up weight was close to 800 with pilot, and he had only 65 hp - a little more than 9lbs per hp. (not ideal). i think everyone is in agreement that a 100hp or 120hp monster VW will do the trick.

Screw - I have read all your previous posts about prop size, engine selection, etc. - interesting stuff

Here's the biq question for all the aerodynamicists:

Big prop, big pitch at lower rpm, or smaller prop at faster rpm? Seems to me the key is getting close to .75 mach at tip speed, then figuring out redrive and engine rpms to max out the prop.

If the GPAS watercooled VW makes 120hp at 4200, then wouldn't swinging a 76" prop closer to .75 mach provide more static thrust than swinging an 80" prop at .59 mach??
redrive engine rpm
Prop size .75 mach rpm 2.0 1.6 1.3
80 2396 4792 3833 3114
76 2522 5044 4035 3278
72 2660 5320 4256 3458

If the GPAS engine makes 120hp at 4200, the chart shows that I should make 498 lbs with a 72" prop (Jukka Tervmaki's program). At 4000 rpm, hp is at 115, and a 76" prop makes 499 lbs. At 3800 rpm, engine power is down to 110 hp, and swinging an 80" prop yields 503 lbs thrust. Basically, all of the values come up the same... But this is with a 1.6 redrive.

Why would I want to go down to a 2:1 redrive with an 80" prop, and make it swing at 2100 rpm (.69 mach)??? Arguably, if the engine could get to 4200 rpm, I could produce 534 lbs of static thrust.

hmmmmm.

I am sold on VW. Just not which one yet.
MANUF HP NOTES PRICE
Great Plains: [email protected], Watercool $9360 test run
Valley Eng LLC: [email protected] rpm $9500 test flown w/prop
VW Eng Centre [email protected] rpm Fuel inj $unknown (no response)

Great Plains and Valley Eng both use the Type I VW blocks, VW engine Centre uses Type IV block exclusively. VW Eng centre is Fuel injected. if i get a carbureted VW, I will buy the Aerovee Aerocarb...

Anyone having knowledge of these engines please give me a shout
 

WHY

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Hi Joe

Speaking as a shade tree builder, I have found that if you give most good prop makers the max rpm , the desired cruise rpm and the torque curve and hp curve they will give you the best possible combination that their prop can produce. My observations on props is that when you are trying to find the "perfect" prop, you can spend a fortune on fixed pitch props. Prince P Tip will probably come the closest to mating a prop to you engine and make you happy, remember a "P" tip will give you the equivelant of a prop 2 inches longer. You will probably have to take a valium when you price on though. Another great prop is the Aeroplast adjustable (French I think") but you may have to take 2 valium when you price it, The standard of course is the Warp Drive. Also remember with a re-drive the lighter the prop the better and with direct drive you want the flywheel effect of a heavier prop.

Tony
 

Screw

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Everyone must take their own path. I chose the path I went based on some of the same information you just posted and here is why:

According to your own chart, to be at the disired .75 mach your only off by .06 with the 80 inch prop. Big deal. Besides that .75 is based on who's prop? Prince, Warp, Culver, Powerfin will all have different tip speed limits, so ask whoever is building your prop. As you have noted, an 80 inch prop spinning at 2100RPM has close to the same "Tip Speed" as a 72 spinning 2600?

The error in your chart "Assumes No Slip." Welcome to the world of experimental. Do the best guess you can.

I recommend Great Plains engines. I've had 2 of their engines, without a moments of trouble from iether one. Steve can build the sh*t out of a VW. I reccomend the 2276cc 120hp liquid cooled period. Call Lonnie Prince. The 84x50 prop he built for me was specific for that engine to produce 3800RPM static on ground. This will give it some room to get up to about 4000-4100 in the air.

We were shooting for 500lbs of thrust and according to your chart your right there. Even your chart shows you that the bigger the prop, the slower, the more pitch is giving you more static thrust. I know it's not by much, but it's still there. Picture in your mind a tornado. Turn it sideways. This is you how the air is comming out of your prop.

On most props the tornado starts about two inches below the tips. On the prince "P" tip, it starts at the tip. For me, running an 84 inch Prince Prop was like running an 86 "Anybody Elses" prop. The bigger the tornado, the larger volume of air is moved.

Lonnie can pitch that prop out to to over speed your engine (4400) or under speed your engine (3400). 3800 on the ground is about right for that engine. The 1.6:1 redrive is perfect for a 72 inch prop. It will also spin an 80 inch prop, however, it will limit your engine rpm because the engine is working too hard to spin it. The 2.0:1 redrive is better leverage and allows you to use the torque where you need it on the 80+ prop.

My 2 cents.

Screw-Out
 

WHY

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Hi Spencer (sorry I missed type Joe in last time)

Also don"t forget to think about some extra ground clearence for those less than perfect landing flares.

Tony
 

PW_Plack

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Tony said,

...with direct drive you want the flywheel effect of a heavier prop.

Some direct-drive conversions specify limiting prop mass to save the bearings from excessive gyroscopic forces.
 

magilla

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Ahhhhhhh, the old "assumes no slip" problem

Ahhhhhhh, the old "assumes no slip" problem

Thanks guys for making this the greatest forum ever. I am starting to get the picture.

Here are some ground rules for props as I understand them (Please correct me if I am wrong)

1) Different manufacturers have different recommended tip speeds. PPonk and WD calcs both say max thrust comes at tip speeds at .88-.92 mach, Culver and others say max should be about .75 mach. The manufacturer in most cases knows about his prop's characteristics more so than the buyer. Therefore, it would be a pretty good idea to follow their recommendations, i.e., not spinning a Culver to .92 mach, as a starting point

2) If an engine/redrive manufacturer says X brand props are the best, and don't use brand Y, then you probably oughta listen to them... Valley/Great Plains say Culver / Prince the best, and NOT to use WD blades with their redrive, as they are too heavy

3) Prop thrust will change as you start to accelerate, and as the AoA changes. Max thrust on ground will limit cruise airspeed, and vice versa. Sometimes, max static thrust on ground at 0 airspeed is not the best choice, as thrust will change dramatically with increased forward speed.

4) Redrive and pitch the prop on ground (static thrust) so that the engine reaches max hp/tq about 250-350 rpm before prop tip speeds go supersonic. Make it so engine cannot reach redline on ground. This means that the prop and engine will spin up faster as you gain forward speed and more prop efficiency, somewhere about 250-350 rpm. Engine will still remain below redline if pitched correctly.

5) Different applications and different manufacturer's blades means different engine / redrive rpm combinations. Ambient conditions (temperature and pressure altitude) can affect these greatly. Therefore, an adjustable pitch angle gives more versatility for ambient conditions, and for fine tuning the thrust for your application (climb or cruise).

This puts me in a quandary :suspicious:

GP and Valley Engineering say that wooden props are the best with their engines and redrives, and not to use Warp Drive, as they are too heavy. Yet WD gives more flexibility because it is adjustable.

What's a guy to do?
 

magilla

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What size prop based on clearance?

What size prop based on clearance?

Again, thanks for the help

here's the situation:

With acft tail on ground, center of prop hub will be 49".

With tail horizontal, center of prop hub at 48".

With tail 10 degrees high (whoa boy), center of prop at 47"

What would be max prop size?

(I ask because I have heard two different rationales so far - 9" prop clearance from ground min per FAA; Second was that 6" clearance is all you need. I have NO CLUE as to the veracity or logic of these statements

based on clearance above I should be able to go with an 80" prop, right?

Or would 72-76" be more reasonable?

Another quandary - the shorter I go, the more I need a three blade prop to maximize thrust due to limited size... :wacko:

I need a cure-all like the ZCAV prop to give me 11lbs of thrust per HP. :flame:

Hmmmm. WD out, looks like IVO or Kiev now if I have to stay at 72" -

ugggghhhh - Culver and GP say that their 2 blade props outpull IVO....I don't know what to believe anymore........ :noidea:

I think I'll stick with a wood prop with the manufacturer's recommendation for size and pitch, and then if it doesn't work, I will go the adjustable pitch multi-prop route.

Y'all are giving me a headache. :help:
 

magilla

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Yeah, but....

KenSandyEggo had problems with his, and from what I've heard, they don't pull the static thrust numbers because they are lighter weight and flex more...

However, what the heck do I know?? Other than I don't have the $ to try 10 different props...

Thanks, Thom. I replied to your PM on the AR chapter.
 

mcbirdman

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Spencer - I am seeing all these theoretical prop sizes and rpms etc.

What I am not seeing is what the thrust #'s were on the setup he had. I see all the theoretical prop sizes hp of engines but do you know what he was able to achieve in thrust numbers to fly at the 800lbs weight?

Joe T called me as he was getting ready to do one more test using a high lift valve mod I think and told me the numbers he was getting. I did write it down I belive but think it in all my paperwork down in Indiana where we are rebuilding my weak engine.

I didn't realize he was using a redrive but what sticks out to me here is that it seemed to me that he was only getting thrust in the 220's, same as mine. Then we find out mine was built poorly and now it is 320's as a direct drive.

What I learned from the book C.Beaty sent was that the engine is just starting to breath at 3000rpm and can handle a bit more on a continuous basis.

Although I wouldn't call it a LW clone because it isn't a duplicate of a LW, it simply is a different open frame tractor design. However aside from it being uncovered, the main similarity is that it is a tractor but weighing about 50lbs more than a more streamlined covered LW. Joe had a neat prerotator setup too.

What I am trying to say is that I think he had similiar thrust numbers as mine before we rebuilt. What I wonder is that if mine would barely fly closed frame and 50lbs lighter - but the same thrust - would his have flown just fine if he had the 100 lbs of thrust we now have? Static thrust with 62" prop at 3200 providing 320lbs. If you can find out what the thrust numbers were you can at least use them as a baseline. Hope that helps you somehow....

Hoping things go well for you. jtm
 
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spoke

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Hi All. Would like some info from anyone flying an EA 81 direct drive as to length and pitch and rpm , I am running a 56x26 3200rpm static , 3500 plus at takeoff and climb,, Spoke
 

Ron Iaconis

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Sounds pretty good

Sounds pretty good

From my understanding,,,that sounds pretty good,,,,,some feel that closer to 3800 is more " better" but yours sounds that it's in the ball park,,,,,,,,,if I may use that term?????hummmmm????
 

Doug Riley

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Spencer, do you have solid evidence that your proposed Franken-Vagen can reliably produce a continuous 100 or 120 hp? I have very grave doubts.

The VW crank, crankcase and valve train were all designed for an engine that produced an "automotive" 40 hp. The automotive duty cycle involves a few seconds of redline operation at a time.

VW themselves hotrodded the engine up to 1600 cc and then went to an overall larger engine, the Type IV. Redesign is expensive; you can bet they wouldn't have done it if they could get more by just boring or stroking the (formerly) 40 hp. box.

Yes, water cooling allows you to burn more fuel in a given size engine per unit of time -- because converting water to steam soaks up an incredible amount of heat. Still, cooling isn't the only problem when you not only triple the redline power output of an engine but ALSO put it into a duty cycle where redline power is used for minutes at a time and "cruise" output is 70% or more.

The 2200 cc (92 mm x 82 mm) aircooled VWs of past years didn't overheat so much as disintegrate. They threw rods, dropped valves, cracked cases, broke cranks, fried bearings and even seized rings like a 2-stroke. (Yup, I flew one.)

I would suggest lowering your expectations regarding power output. If you want a water-cooled four-banger that will stay in one piece during full-throttle climbouts to pattern, look for a used Rotax 912. It'll cost less than those over-hotrodded VWs and last much longer. It weighs less, too. This "miracle" is accomplished by winding the thing like a 2-stroke (5800 RPM), but, very unlike the VW, it's designed for that.

About 10 degrees pitch gives the prop roughly the right angle of attack at gyro airspeeds.
 
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