deljon

My Feedback: (4)

Just wondering if there is a easy way for choosing a prop size when you add a reduction drive.

I'm sure there is some mathmatical formula for figureing this out, but math not being one
of my and others strong subjects is there a good "Rule of Thumb" for determining this.

Say you have a engine with a 14/6 and you add a 2:1 reduction, what size prop now??
How about if you have a 26/8 then add a 2:1 reduction, how do you determine how big to go??
Or maybe you go to a 1.75:1 reduction??

How about some practical suggestions.

Del Johnson

av8tor1977

My Feedback: (6)

What we do with ultralight airplanes with reduction drives is put the biggest prop on that will fit the airframe, (within reason), and then set the pitch to keep the engine at max rated rpms at full throttle. This optimizes the airplane for take off and climb. You can dial in a bit more pitch to bias things towards cruise and/or higher speeds, keeping in mind that with the inherent drag of an ultralight, you're not going to get a lot of speed.

With models, you would need to decide what you want to optimize, speed or climb/static thrust. Smaller diameter/more pitch for a fast plane, larger diameter/less pitch for more static thrust and climb. (3D)

As far as actual numbers, I bet PE Reivers could get you pointed in the right direction... You might also peruse Tony Clark's website.

AV8TOR

deljon

My Feedback: (4)

My primary interest is WW1 1/4 and 1/3 fighters, so high speed would not be a concern.
A good strong climeout would be good and swinging a more scale size prop and at a lower rpm
for a real good effect.

Del

Pelle Gris

A good start would be to multiply the pitch with the reduction ratio. If you run a 22x10 direct drive you would need a 20in pitch prop to provide the same speed when used on a 2:1 drive. For figuring out the diameter try playing around with Pé´s spreadsheet available here. www.mvvs.nl , under propellers I believe.
Set your rpm with the standard 22x10 as one example, and reduce the rpm by 2, and set 20in pitch in the other. Then you can play around with diameters until you find one that absorbs roughly the same horsepower.

I ran a Mick Reeves 1:1,75 drive on a G-62 some years ago. The only prop I had available at the moment was a 3W 32x12. It would spin at around 3800-4000 rpm depending on muffler, but it could only just get above stall speed. Even a steep decend for landing would be on full power, right untill I was 1 meter above the runway. Cutting the power would then have the aircraft standing still just 5-10 paces further ahead.

av8tor1977

My Feedback: (6)

That's a very good point and I should have included it in my post. You have to use enough pitch to fly the airplane at the speed you want, and with a reduction drive the lower prop rpm will necesitate more pitch. That's a good idea to jungle the numbers with Pe's prop chart. That chart is really handy and informative.

AV8TOR

pe reivers

My calculator ( www.mvvs.nl propellers, top of sheet ) allows you to calculate any prop, if you know the power and rpm of your engine. Play with reductions and prop sizes before you plan your building. Up to 5 configurations can be input at the same time
There also is a graph in which you can see the speed you will get. (only for first two configurations) For any prop, typical pitch figures should be within half diameter and full diameter. Smaller pitches than 1/2D are very inefficient at plane flight speed. Pitches larger than 1/1D are for speed planes only.

Pelle Gris

I believe that I have read somewhere, that in general, the most efficient prop is one where diameter and pitch at about 1:1.

pe reivers

efficiency depends on flight speed. The wright brothers prop also was 83% efficient. Low pitch or high pitch, it just shifts the speed at which those 83% are obtained from low (3D) to high (pylon) So with efficiency ratings, you also must state the type of application for which the prop must have best efficiency.
At stall speed (static) efficiency is zero. At 120% of pitch speed the efficiency also has been reduced to almost zero.
First case, forward speed is zero, second case, thrust is zero.
We do not have much use for prop efficiency, because we do not carry payloads. We want thrust, and juggle props so the thrust happens in the flight envellope that we want to have it.

deljon

My Feedback: (4)

All my Fokkers seem to like an 8 pitch the best. My 1/4 DVII with a 20/8 prop on a Saito 36 gasser
flies very will.
My 1/3 Dr.1 with a GT80 uses a 26/8 and flies extremely well.

I am getting ready to start on another 1/3 Dr.1 and will be using the GT80, but thinking of gearing
it to swing a bigger prop. 1/3 scale prop,(depending on full scale mfg.) would probably be in
the 34 to 36 inch range.
So, if I'm reading right I need to go to a 16 pitch then figure out what geared ratio I need to
to swing a prop in that size range.

I tried to get that calculater sheet, but kept getting a sheet about oil??
Thanks

Del Johnson

pe reivers

The 8" props of course provide the low landing speed you need without the super low rpm. The GT80 has about 7hp starting at 8000+ rpm, so you would like to have your prop use that amount available.
For a 36" prop, with 18" pitch you would need about 3200 rpm full bore. The GT80 likes to rev up to at least 8000 rpm, where it just starts to deliver, so your reduction should be 1:2.5 to 1:2.8.
Prop idle at 1500 engine rpm would then be 545 rpm. Low enough to land comfortably with that 18" pitch prop.
Top speed would be about 90kmh (56 mph), a very good value for a dr1!

BTW,
when you want to download my calculator that I just now used, go to the top of my page, (webshop), section propellers! First paragraph.

deljon

My Feedback: (4)

I checked Horizon for the specs on the GT80. They are rating it at 6hp with a APC 24/10 (largest they recomend) at 7500rpms
Since the Dr.1 is not a high performance plane I figured I could get away with a 26/8 with no issues, I'm getting 5600rpms (engine
fairly new) full throttle and get about 1300 idle.

Del

pe reivers

Here is the copy from the 80cc manual. 7.5hp at 10,000 rpm
Like all Zenoah engines, this one needs to rev up.

deljon

My Feedback: (4)

Looks like the manual differs slightly from their online specs, but close.

Thank you very much for the info, very helpful.
I was finely able to get the calculator page.
Thanks again


Del

pe reivers

A reduction drive lets you have the best of all worlds. Engine happy and plane happy.
A direct drive will only bring disappointment on a WW1 bird.