Static Thrust Number?

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Vigilant1

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However, while the climb was reduced by a third, when you leveled the plane, the top speed shot to 75mph (again, verified by gps, boxing the compass), which works out to a staggering 98% of theoretical pitch.
Thanks for the report, real measurements are always great.
FWIW, many prop mfgrs report the pitch as measured from the bottom of the blade. And, even at true zero AoA , there are normally a few degrees of pitch to go before we get to zero lift. Between these two things, it can be hard to know where the true zero lift pitch is on a particular prop.
 

Pops

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The prop has to be a match for the engine and the airframe. One size doesn't fit all. The shorter props with the higher pitch and higher rpm for the low drag fast airframes. Just he opposite for the slow high drag airframes. The SSSC and the SKyraider are slower high drag airframes.
I talked to "Pops" at Culver props a couple months before he died for a long time about the correct prop to use on the SSSC. In the end he recommended the 60"x 26" prop for the right prop for the engine and airframe. He got it exactly correct. Cruise at 2650/2700 rpm at 80 mph burning 3 gph with a ROC of 1200+ and 3200 rpm and a top speed of about 95 mph. After several hundred hours, I wouldn't change a thing.
 

TFF

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The usual correct way to measure pitch is 50% of blade length on the back of the blade, usually called the face. Every once in a while, you will see a line painted on there. What each brand does though is twist the props different and along with different airfoils;measuring at 50% much less anywhere is not 1 to 1 relation. Some brands are closer than others. Some may measure different just to hide “secrets”.

Depending on what you want the plane to do, you want the static thrust to be under best static. You want the engine to lug a set amount so it can accelerate to max thrust in the air. You don’t want the prop unloaded on the ground, you want it unloaded in the air at the speed you want to travel at.
 

Pops

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The Skyraider and the SSSC is very much alike in many ways. The construction is the biggest difference.
There is a picture of the Skyraider that a local friend built and my SSSC together. Both had the same performance. 1835 cc VW on the SSSC and the 60 HP HKS on the Skyraider. Same 120 sq' of wing area, same EW.



DSCF0009 (4).JPG
 

Vigilant1

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The Skyraider and the SSSC is very much alike in many ways. The construction is the biggest difference.
There is a picture of the Skyraider that a local friend built and my SSSC together. Both had the same performance. 1835 cc VW on the SSSC and the 60 HP HKS on the Skyraider. Same 120 sq' of wing area, same EW.



View attachment 113314
Two good looking airplanes.
If the performance numbers are the same, the SSSC airframe might have had some advantages, because that 60 HP geared engine would be expected to provide more thrust at those airspeeds than a 60HP direct drive VW.
 

tomz28

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The Skyraider and the SSSC is very much alike in many ways. The construction is the biggest difference.
There is a picture of the Skyraider that a local friend built and my SSSC together. Both had the same performance. 1835 cc VW on the SSSC and the 60 HP HKS on the Skyraider. Same 120 sq' of wing area, same EW.



View attachment 113314
do both planes have a high camber wing pops?
 

Pops

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do both planes have a high camber wing pops?
I don't know what airfoil that was used on the Skyraider. I was at my friends shop when he was building it several times but didn't pay attention to the airfoil and never ask.
On the SSSC, I used the airfoil used on the Fisher Koala 202 and Super Koala, Dakota Hawk, but I don't know what airfoil they used. I do know that its a good airfoil for the airplane. The prototype Fisher Super Koala is in the hanger next door and after flying it is when I decided to built the SSSC using the same airfoil. Great flying airplane.
 

Dan Thomas

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The most common reference I've seen is at 75% of blade length. 50% would normally be considerably higher pitch.
If the prop has an evenly distributed pitch along its span, it wouldn't much matter where you measured it. The distance the blade section at 50% travels is only 2/3rds the distance the blade section at 75% travels, and the lower pitch at the 75% point will bear that out. Pitch is measured in inches of advance, no slip. The pitch inboard has to be higher to advance the same distance in a short travel. It's simple triangulation.

I normally measure a prop at 70% span.
 

Vigilant1

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If the prop has an evenly distributed pitch along its span, it wouldn't much matter where you measured it. The distance the blade section at 50% travels is only 2/3rds the distance the blade section at 75% travels, and the lower pitch at the 75% point will bear that out. Pitch is measured in inches of advance, no slip. The pitch inboard has to be higher to advance the same distance in a short travel. It's simple triangulation.

I normally measure a prop at 70% span.
Yep, I wrote "pitch" when I should have said "blade angle, " they are different things.
Here's a Sensenich document on measuring blade angle and computing pitch.
https://www.sensenich.com/wp-content/uploads/2018/01/Miscellaneous_Documents_Calculating_Pitch_1297966148.pdf
 

Pops

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I'm not very smart, but I know enough to not attempt to design and make a prop. Way over my head. Would be a good waste of time and effort for me for sure.
 

Dan Thomas

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I'm not very smart, but I know enough to not attempt to design and make a prop. Way over my head. Would be a good waste of time and effort for me for sure.
I have a book here that has often tempted me to do it. It explains the layout, pitch distribution, airfoil and everything. The bigger hassles include finding enough nice clear straight-grained hardwood, and cutting the laminated blank carefully so you don't remove wood you didn't want removed. By the time you find and pay for the wood, glue, the correct huge wood bit to drill the pilot hole and the hard urethane to armor the leading edge, you could almost buy a prop.
 

Pops

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I have a book here that has often tempted me to do it. It explains the layout, pitch distribution, airfoil and everything. The bigger hassles include finding enough nice clear straight-grained hardwood, and cutting the laminated blank carefully so you don't remove wood you didn't want removed. By the time you find and pay for the wood, glue, the correct huge wood bit to drill the pilot hole and the hard urethane to armor the leading edge, you could almost buy a prop.
And add 10 cents an hour for your time. The cost of a prop is a good buy. I have bought 4 Culver wood props and have been happy with each one.
 

challenger_II

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Thank ya, sir!

As for "zero lift" angle, let's take the venerable Clark Y as an example:
As used as a wing, or a propeller, when you have "zero angle" based on the flat of the airfoil, you still have 3 degrees angle of attack, due to the chord line of the airfoil. And, to confuse things more, when the chord line is at "zero" angle of attack, you still have "lift", as the airfoil is still acting to the relative wind.
Now, to REALLY throw the monkey wrench into the works: when you have a sufficient negative angle to achieve "zero lift", you still have "lift". However, it is diminished, as it is being diverted as "translational lift", i.e: the lift force is now directed in the direction of the relative wind. Ask any good Gyro Driver, or Wind Turbine engineer.

Aspirin, any one? :)

Thanks for the report, real measurements are always great.
FWIW, many prop mfgrs report the pitch as measured from the bottom of the blade. And, even at true zero AoA , there are normally a few degrees of pitch to go before we get to zero lift. Between these two things, it can be hard to know where the true zero lift pitch is on a particular prop.
 

challenger_II

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Two items: The Tennessee statement was a recommendation, not the whole story. If you were to mount the 1835 on a Rand KR1, then the 52x28 was your huckleberry. The 52x28 would let the engine turn up to 3600, in the air. Such a prop would hugely destroy the performance on a slow, draggy bird like Pop's SSSC.

i see Tennessee props says to run a 52x28 prop
[/QUOTE]
 
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