thrust expected from an 1834cc VW?

[Pops]

I just don't understand props. My 1834 cc, VW engine on the SSSC just turns 3200 rpm at WOT but climbs 1200 + fpm with my fat 235 lbs and still cruises at 80 mph at 2650/2700 rpm. The prop is a Culver 60"x 26" pitch.
32" pitch on it would be to much pitch and the WOT rpm's would be a lot lower. ?????????????

Do you think the pitch on my prop is really much more than the 26" stamped on the prop ?
Added--- Going to WOT at the start of the take-off roll, it has very good acceleration.

 

[Vigilant1]

I just don't understand props.

I don't understand them either. Some sort of black magic is needed to get them right.

My 1834 cc, VW engine on the SSSC just turns 3200 rpm at WOT but climbs 1200 + fpm with my fat 235 lbs and still cruises at 80 mph at 2650/2700 rpm. The prop is a Culver 60"x 26" pitch.
32" pitch on it would be to much pitch and the WOT rpm's would be a lot lower. ?????????????

Do you think the pitch on my prop is really much more than the 26" stamped on the prop ?

Observations:
- There's no ironclad rule about where and how prop pitch is measured. A general convention is to measure it at 75% out from the center and to use the mean chord line (rather than, say, the bottom of the airfoil), but not everybody does that. So, one guy's 26" pitch might be somebody else's 30".
- At the same pitch, a blade with higher camber will generate more thrust than one with lower camber.
- Do you keep the RPM constant at your higher speeds? As the plane picks up airspeed and the effective AoA of the blades decreases (the prop "unloads"), the engine will be able to produce more RPM (at the same MP). This will allow it to keep providing thrust at higher airspeeds.
- For a given prop and a fixed RPM, thrust falls off very fast above the prop's designed optimum speed, but operating below the prop's designed airspeed has a lower thrust penalty (provided the engine/prop can still make the same RPM and power at those lower airspeeds).

But, I'm certainly not an expert.

 

[TFF]

To add to it, more camber makes more thrust which is lift; it also makes more drag.

Thick blade Is like a Cub wing. Lots of lift at a slow speed but does not want to go fast. A Midget Mustang has a thin wing and unless it’s cutting through the air at a good clip, it’s not going to fly.

Translate this to a prop. In one family of props, there is some sort of consistency. Same formula of airfoil, material, craftsmanship, thought process. A different brand has a totally different formula. The physics are the same, execution is different.

I only got to see this once. A man held a prop carving demo at Oshkosh. He was making a very average prop, a Sensenitch wood prop type of prop. I still is the best thing I ever saw there. People started asking all these high tech questions like they were going to build their own MT constant speed prop. He kind of gave them a talk. “This is a basic propellor for a basic homebuilt that’s going to go maybe 150 mph at best. Hand carved. If you have a CNC and space age airfoil, knock yourself out, but don’t use this as a pattern.” He had a good number failures there as,dare I say, props. Mostly his issue was getting the blade too thin and it fluttering. If you were going high tech and thin, that is what you have to beat. I don’t think he was in it to make a better prop, to him it was about making your own prop.

 

[BrianW]

Running into the trees at the end of the runway while still in ground effect?

What has thrust/drag got to do with lift/weight? <g>

 

[BrianW]

/snip/

I only got to see this once. A man held a prop carving demo at Oshkosh. He was making a very average prop, a Sensenich wood prop type of prop. It still is the best thing I ever saw there. People started asking all these high tech questions like they were going to build their own MT constant speed prop. He kind of gave them a talk. “This is a basic propeller for a basic homebuilt that’s going to go maybe 150 mph at best. Hand carved. If you have a CNC and space age airfoil, knock yourself out, but don’t use this as a pattern.” He had a good number failures there as, dare I say, props. Mostly his issue was getting the blade too thin and it fluttering. /snip/

How did you find out that they fluttered?

 

[BrianW]

This thread started with this innocuous sounding query:
"Hi all,
What kind of thrust would you expect to get from an 1834cc VW?"

I am ashamed to say I responded with an assumed airspeed, for a static thrust suggestion.
I am even more ashamed with some of the other answers, so now I will give the strictly accurate answer. Here it is.

Max Thrust may be measured statically, if the airscrew is not stalled. If it IS stalled, then thrust rises until the prop unstalls at some speed, then thrust begins to decline with increasing speed through takeoff until it reaches a minimum at max cruise speed.

 

[Pops]

Back when I was 12 years old I got my first controlline model airplane. Props didn't last long as I had never seen a controlline model fly before and it took me 17 tries to made the first circle with crashing. Neighbor man had a wood shop and carved a prop for me since I didn't have any and it worked great. I had him so busy making props one day he said, let me show you how to whittle out a prop. Darn, had to start making my own. That was when I realized I didn't know what I was doing, and still don't. Like Vigiliant 1 said, " its a black art" for sure.

 

[Vigilant1]

Max Thrust may be measured statically, if the airscrew is not stalled. If it IS stalled, then thrust rises until the prop unstalls at some speed, then thrust begins to decline ....

How does that work? If the prop blades behave like other airfoils, then when they are stalled they are past their critical AoA. At these high AoAs, the airfoil is beyond it's Cl max and drag is very high.

I would think that during the takeoff roll prop drag would remain high until airspeed increases to a point where the blade is no longer stalled. At that point, the airfoil's Cl will be highest and thrust will be considerably better. Speeding up more (and therefore decreasing AoA) will reduce the Cl of the blades, but if our engine's RPM increases a lot (due to reduction of drag) we could still see in increase in thrust.
A lot depends on the slopes of the various lines:

- Prop efficiency vs Airspeed
- Torque curve of the engine
- Normal direct inverse relationship between prop thrust and airspeed.

I definitely am not an expert.

 

[plncraze]

Look in the EAA's Sport Aviation archives for Neal Willford's article on propellers. Then find Lu Sunderland's interview with Mr. Rose who designed props for Sesenich (spelling?). Then read this Wood Props - Flying Flea There are a number of variables and some of them are a guess. But these will help you get a grip on what the "real" prop designers look at when they start to play. Happy reading!!!

 

[Marc W]

If you really want to understand propellers read this man's book: Jack Norris

You can purchase it here: Propellers Explained & Logic of Flight

Jack is a character and he is a treat to talk to. I met him at Camarillo, CA, KCMA. He is a member of EAA1373.

Propellers are complicated. There are a lot of variables. Everybody talks about diameter and pitch but, as mentioned above, the airfoil rarely gets mentioned. Also pitch distribution and blade area are a big part of prop design.

Yeah! Read Jack and when you understand it all, please explain it to me.

 

[plncraze]

That is quite a book! I read it and have appreciated editors ever since.
Seriously, that is one that thoroughly covers the topic.

 

[Pops]

If you really want to understand propellers read this man's book: Jack Norris

You can purchase it here: Propellers Explained & Logic of Flight

Jack is a character and he is a treat to talk to. I met him at Camarillo, CA, KCMA. He is a member of EAA1373.

Propellers are complicated. There are a lot of variables. Everybody talks about diameter and pitch but, as mentioned above, the airfoil rarely gets mentioned. Also pitch distribution and blade area are a big part of prop design.

Yeah! Read Jack and when you understand it all, please explain it to me.

Do you want to make a bet on me understanding it all ? Just thinking about it makes my head hurt.

 

[Marc W]

I feel your pain, Pops!

I do like more diameter. Like you I have a 60" prop on my 2180 VW. I don't know why so many people use 54" and even smaller props on VW 's. I suspect it started because people were looking for ground clearance on low slung airplane's like KR's. I really like pulling the power back to 2600 rpm and cruising around at 2.5 gph and 95-100mph.

 

[Pops]

I feel your pain, Pops!

I do like more diameter. Like you I have a 60" prop on my 2180 VW. I don't know why so many people use 54" and even smaller props on VW 's. I suspect it started because people were looking for ground clearance on low slung airplane's like KR's. I really like pulling the power back to 2600 rpm and cruising around at 2.5 gph and 95-100mph.

That is hard to beat . What pitch and what airplane ? 60"x 30" ?

 

[TFF]

On something like a KR, you trade diameter for pitch. Speed is the reason. A 60x30 can be equated into a 54x 36 give or take. The load on the engine is going to be about the same. A speed plane that gives no flip about slow, like a KR, you want pitch; you want to see if you can go 200. Cubby like, well your not going 200. It’s going to hit a wall at some point, why pitch it so it could never break through. Low pitch big diameter. A plane with drag in between slick or draggy will have a pitch in between. I’m assuming this is with the same engine.

You start getting into the black art with such a big change difference in diameters that the blades widths and thickness can be altered to take advantage of the materials. You might be able to add another inch or two of pitch on a small Diameter prop because maybe more available strength is at that diameter. A trade of excess strength by thinning the blade for better aero and then using the better aero advantage to add two more inches of pitch, so you might be able to get a 54x38 out of some juggling.

You have to have made a lot of props to know how to take advantage of the numbers and shapes.

 

[flitzerpilot]

Tip speed staying (just) below Mach 1 when testing the Z-1 in a TV dive, the airscrew acting as a rotary airbrake, provides a clue to what propeller diameter and pitch works. Turning the largest diameter propeller was always my goal, one reason for the good propeller ground clearance on the Flitzer series in a level attitude. Maximum IAS on a typical 1834cc VW is around 90 mph, which is reasonable for a biplane whose draggy undercarriage makes up about 25% of the total. Much of what was discovered was based on original guesstimates but took a couple of propellers to optimise. The best Clark Y prop tested was 61" x 32" but the undercambered blades were best.

 

[Pops]

My draggy SSSC hits the drag wall at about 85-90 mph. Cost about 3 mph to fly with the windows off and the door down and fastened for the summer. Going from 5:00 x 5" wheels and tires to 850x 6" cost about 3 mph.

 

[Marc W]

That is hard to beat . What pitch and what airplane ? 60"x 30" ?

Thatcher CX4, 2180 cc and a Sensenich 60"x44". I climb at 80mph and see 2900 rpm there. My home airport is at 5,200' and I often climb to 11,500' or so to clear the local mountains. I don't have a VSI but I typically climb from takeoff at 5,200' to 10,200' in 7-1/2 minutes which averages out to 666 fpm. Wot in level flight at 8000' or so trues out to 137 mph or so at 3200 rpm.

The interesting thing is that last year I borrowed a Sensenich 54"x44" that came off a Sonex. Speeds and climb rates were virtually identical. The difference is that the 54"x44" took 400 rpm more to reach the same numbers.

 

[Vigilant1]

Marc, do you have a manifold pressure gauge on your panel? Were you able to compare fuel burn between the two props at similar airspeeds?

 

[rv7charlie]

Thatcher CX4, 2180 cc and a Sensenich 60"x44". I climb at 80mph and see 2900 rpm there. My home airport is at 5,200' and I often climb to 11,500' or so to clear the local mountains. I don't have a VSI but I typically climb from takeoff at 5,200' to 10,200' in 7-1/2 minutes which averages out to 666 fpm. Wot in level flight at 8000' or so trues out to 137 mph or so at 3200 rpm.

The interesting thing is that last year I borrowed a Sensenich 54"x44" that came off a Sonex. Speeds and climb rates were virtually identical. The difference is that the 54"x44" took 400 rpm more to reach the same numbers.

If all tests were at the same density altitudes and at wide open throttle, leaned to best power, then the 54" prop took a lot more power to do the same job. Did you compare fuel flows?

Smarter people than me talk about 'mass flow' and 'minimum acceleration' of the air.

I'm not up on VW rpms vs prop dia vs mach numbers, but in RV-x size planes, the typical 2700 rpm Lyc with up to a 74" dia prop won't start having problems with mach on the top end until you get well north of 200 MPH. But every inch of dia increase buys a *lot* of climb rate. (That mass flow thing.) The common 68" dia props give up a lot of climb, with no advantage on the top end.

General rule of thumb: bigger is better. ;-)

Charlie