More about pitch

[Scooter]

I think I have decided to go with a Prince prop for my 701. Don Parham said to go with a wooden prop. He also said he had sold over 250 direct drives and has never heard of a crank shaft problem with a Subaru. I've been dealing with Don a long time and really trust him.

Prince recommends a 68/47 P-tip prop. I e-mailed them my specs and what I wanted to do (or think I want to do).

I've always thought of pitch in terms of degrees. I don't know how to relate the 47 to degrees, or should I? I think the 47 (correct me if I am wrong, wouldn't be the first time), the 47 means that at max rpm it would move 47 feet of air in one revolution, or you would move forward 47 feet.

I'm really concerned about this prop selection. This prop is $994.16. I don't want a wall clock. So any constructive input would be appreciated.

[GyroRon]

I have a few short words of constructive input for you.

Reduction unit and Warp Drive.

[gyromike]

Larry,

I'm sure that those numbers are in inches.

The prop would theoretically move forward 47 inches per complete revolution, assuming 100% efficiency.

[Scooter]

Mike, thanks, I should have known that, just an oversite, I wasn't thinking.

Ron, thanks for you constructive advise. I'll pass, I've had both(twice), and don't want to repeat.

Before, I just put on whatever prop came with the kit. I didn't have to know why. Now I need to know why, so the best way to learn is to ask dumb ass questions. I'll try and not ask the same dumb ass question twice.

[GyroRon]

I can't wait to hear how it flys. ;D ;D ;D :P

[Scooter]

I must be missing something.

If I use the same engine, same prop. One with an Re-Drive and one with a direct drive. If I achieve the same prop rpm, say 2700 then the only difference I see is the re-drive weighs 35 pounds more and the engine is running 2000 rpm faster than the designer intended it to. The direct drive engine will have less wear, better fuel economy and less prone to engine failure. And, one less moving part to worry about, i.e., re-drive.
The only pro I can see in using a re-drive is that I can use a smaller, less weight engine to acheive the same horse power as the larger one. Which is a definate plus if that's what one wants.

Merry Christmas, guys

[gyromike]

Not quite, Larry.

Let's use the same engine, and the same prop.

Using a gearbox/belt redrive will allow the engine to spin at a higher rpm to turn a certain sized prop @ 2700 rpm. This is assuming we have the prop pitched to limit it's rpm to 2700, with the throttle fully open.

Remove the redrive, and the same engine will not turn the same prop @ 2700 RPM. The engine will not develop the same amount of power at 2700 RPM, that it would at 5000 RPM. Small displacement engines make their horsepower at higher RPM's, and big displacement Lycomings make it at half the RPM.

Direct drive will require a smaller prop with more pitch turning a higher RPM, which works better on streamlined, fast airplanes, rather than slow, draggy STOL aircraft.

[GyroRon]

The Re drive will allow the engine to be able to run at the ENGINES peak horsepower rating. Don may have a chart that shows a given rating at a given rpm, but on a high tech multi valve multi cam engine such as a EJ-25 that engine was designed to be run at and make highest power output at or near red line.


The Re drive will allow the prop to be turned slower than direct drive. Direct drive means the engines maximum rpm will be limited by the diameter of the prop to keep the prop under supersonic. With the size you quoted above, my un educated guess is you will have to keep the prop under 3000 rpm to stay in sub sonic prop speeds. At 3000 and below rpm the engine is not going to make anything close to it's 165 rated HP. With the re drive the engine can be allowed to spin at say 5000 rpm and the prop at 2500 with a 2 to 1 ratio. This allows you to pick a prop with a large diameter - which is always better than a shorter diameter - and use the props pitch to govern rpm by absorbing the engines horsepower. And another way to look at it is take this for example.....

Most gyros with direct drive EA-81 engines will produce static thrust in the area of 300 pounds. The same EA-81 engine using a re drive and larger prop typically produces between 400-450 pounds of thrust. The only difference is the higher RPM of the engine, the LOWER rpm of the prop, and the larger diameter of the prop. On a EJ-25 I would guess the difference between Direct drive and a Re drive would be much more dramatic.

Also you can think of it like this, In a car or light truck with a stick shift tranny, Your going up a steady grade at a steady speed... Say 40 mph.... You could put the car in fourth gear and lug the engine down to a lower rpm and put the gas pedal nearly to the floor to keep the speed at 40. OR you could put it in second gear and turn much higher rpm but the gas pedal is now at only half way to the floor and your still going 40 mph. this is the difference between the Direct drive engines and the Re drive engines. It is like going from fourth to second gear. It is mechanical advantage. Will the engine recieve more wear and tear at the higher RPMS? Maybe but so does lugging a engine at or near full throttle. Will it burn more gas? Maybe a slight amount more but not a major amount more. Will you have more power? YES!!! In your case I think you will have roughly 400 pounds max static thrust. With the re drive I believe you can easily have close to 650-700 pounds with that engine and a good prop. That is a major increase in USEABLE power. This is the difference Larry in a 600 FPM climb to a 1500+ FPM climb, and this difference will likely cut the takeoff roll in less than half. Max crusie speed will likely be much higher to - probably be able to reach VNE in straight and level flight with the redrive version.


and it comes down to this..... as it is you will use a very large heavy engine and run it direct drive lugging the engien for horsepower. You save one moving part and a few pounds of weight and maybe save 1 gallon per hour fuel burn, that is fine. MY point is I personally think when it is said and done it will be underpowered and or overweight, and I think you could save your self a lot of hassle by designing in the Re drive now before you spend 900+ bucks on a prince prop, and before you finish the FWF and make it where to use the redrive later you have to make a new motor mount and new cowling etc...

This plane should fly just fine with the redrive equipped engine, You ought to be able to really throttle back and still run the engine in cruise flight at the lower RPM's you want to turn due to the better performance from the larger prop. BUT..... when you want to take off and or climb out you got the full 165 hp to do the job.

But look you do what you want, it is your plane. I just have a strong gut feeling that you will be disappointed with the performance of this plane built as your planning to do it. It all comes down to horsepower verses weight. I would take the higher horsepower.

[rehler]

Larry,

The same auto engine with a redrive will produce much more thrust than a direct drive. For example, a direct drive Subaru can be a real dog, bearly able to get off the ground. But addinga redrive can make it a good performing engine that allows the gyro to fly very well. There is a big difference!

1. The prop tip speed is limited which determines the prop diameter based on the prop rpm.

2. The engine hp is determined on the engine rpm.

3. The larger the prop the more thrust it can produce.

To get the best thrust you need the highest engine hp at the point that the prop tip is at the optimum speed. A reduction unit lets you have the best set up.

Plug in the numbers on one of the gyro calculation programs and see the difference.

[Scooter]

Guess I'll just try and see. Worst case, I'll have to build another cowl, but fiberglass is one thing I know. I already own a re-drive. I'll keep it for back up. I'm not saying I'm convinced, but I do believe in plan B. I'm almost a believer, because that engine I have turns 195hp at 6000, sure shouldn't lack for power.

As for as the Prince prop is concerned, anybody priced a 3 blade 68" HP prop from Warp Drive lately? I have, it comes to $855 with everything I would need before shipping. Not that much cheaper. Plus from my limited experience, it is bad noisy.

Don't think I don't listen to you guys, I do and thanks. That's why I ask.

[GyroRon]

The prince is one size and not easy to repair if dinged. The warp is cheaper and ADJUSTABLE!!!! So if you later put on the Redrive you can simply add pitch with a few tools and a little time, but have to buy a whole new prop if you go with the Prince.

[Aussie_Paul]

Ron, I think that Larry would definitely need more diameter or more blades with the redrive compared to direct drive.

Aussie Paul.

[GyroRon]

Typically yes your right, but with a warp he could later add the re drive and just add in a lot of pitch to take up the added power.

It beats shelling out 900plus bucks and not being able to use it after you do put on the re drive.

[Scooter]

If ya'll remember, I really wanted a ground adjustable prop in the first place. Warp Drive recommended the 3 blade 68" HP model. WPs sure seem noisy, but it may have been because the WPs I have experience with were on RAFs. It may appeared to be noisy because of the trust hitting the sides of the 4" keel that made it seem noiser than it would if it were in open air. I think I remember A. Paul said he could talk on a cel phone in his RAF with his wooden prop and he has the same type keel. So, that sorta says to me, it's the prop and not the location of the keel making the noise. I don't know, they just are noisy.

I think you can start with a 2 blade setup on the DUC windspoon and later add a 3rd, of course you have to have a different hub. I can't get the USA DUC dealers to answer my e-mails, I guess they got more business than they need. The UK guys answer, they recommend 2 blade 68".

I sure would like for someone to join in that has experience with the DUC windspoon.

The 2 blade Windspoon is $717.00 USD, with spinner and adjustment tool it is $970.00 USD.

[Aussie_Paul]

Ron, you said, "Typically yes your right, but with a Warp he could later add the re drive and just add in a lot of pitch to take up the added power."

The amount of increased pitch that Larry would have to add would have the prop still stalled at 100 mph.

Example, My first Raf 2.1 redrive and 68" 3 blade W/D, approx 14 degrees at the tip. I changed to the 2.37 redrive and had to increase the pitch to over 17 degrees and still could not keep the engine rpm down. The prop had very little get up and go until I got to 70 mph.

Now that is only a decrease in prop rpm/tip speed of 12.9%

Borrowed a 68" W/D 4 blade and at 14 tip degrees performed nicely with the correct engine rpm.

This 13% made a reasonable amount of difference to the noise level. I could use my cell with a bit of trouble. A WE/D on a standard Raf 2.1 redrive is very noisy!!!!!!

Then went to the 2 blade wooden props, the first was the Prince P-tip. Enjoyed that except for the resonance at around 3900 engine rpm. Lonnie was going to work out some airspeed and engine rpm combinations for me to test for him to give him some feed back, BUT I smashed it!!!!! I was so annoyed, BUT that made me look in Oz (because of the monetary exchange rate), and I found the maker of the props that I use now.

These props are quieter than the Prince, and have no resonance problems. Performance wise I really can't say which was the better the prince or the Oz. Also, the composite Prince would be a little tougher than the Oz wooden with the composite leading edge.

I do know that the W/D gives a tiny bit more acceleration when you add full power after the nose comes up than the 2 blade props.

Anyway just a little prop talk from Down Under.

Aussie Paul

[mceagle]

"The direct drive engine will have less wear, better fuel economy and less prone to engine failure."
Larry, this is one misconseption held by many people.
In my first EA 81 installation, the direct drive turned 4000rpm with a 48" prop, giving 80mph @ 23 lts/hr, and cruised at 3600rpm for 65 mph @ 18 lts/hr.
With the re-drive fitted, it turned 5000rpm with a 72" warp, giving 90mph @ 24 lts/hr, and I could still cruise at 3600rpm for 65 mph but the fuel consumption was 12 lts/hr.
A faster cruise of 4200rpm yielded 80 mph @ 16 lts/hr - still less than the direct drive.
Once the re-drive was fitted, the vaccum secondry on the carby came in at 4300 rpm which meant that you could do all your flying, if you wished, on the small throat on the carby.
With the direct drive, the secondary throat was opened nearly all the time.
The engine worked far easier with less throttle setting than did the direct drive.

One of my customers flys in regularly and he also calls me on his cell phone when he is 20 miles out. We can converse as though he is in a car. He uses a 72" 3 blade warp, but we turn the prop a lot slower than most, which probably accounts for the quietness. Any interference close to a prop will create noise.

If you still wish to run direct drive, then you would be using only a fraction of your 195hp, especially if the prop is 68". You will be limited to something like 2700rpm, at which rpm's the engine would only produce approx 89 hp

[GyroRon]

89 horsepower in a plane normally powered by a 100 horsepower Rotax. Also this engine will greatly outweight the normal engine in this plane. So less power, More weight and a cut into the planes useful load - limiting the size of passengers to kids and boney women....... Sounds like a bad plan to me. He has the re drive now at his disposal and hasn't yet bought a prop - which is a sizable investment and one I personally wouldn't want to have to make twice because the Direct drive was a dog - and has the easy option of using it now.

He is expecting to get the same level of power out of a 2.5 liter engine direct drive that I get out of my Lycoming which is twice the displacement. I know modern tech makes more power, but what really makes power at low revs a direct drive turn is cubic inches. the Re drive is the only way to go in my book.