Simpler Times and / Briggs and Stratton propellers

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blane.c

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In the 1970's and 1980's when I received the majority of my basic aviation knowledge it was a simpler time.

Bernoulli's theorem was not questioned at least not in my world and wings and propellers worked by "suck".

Super Cubs like Bic lighters were cheap and expendable IE, a hunter would pay more for a animal than a Super Cub was worth and therefor a Super Cub was considered a write off. They were used thusly.

I could set up a target in my back 40 and shoot to my hearts content. I no longer enjoy a back 40.

My dog died this past week. It was a poor death ill befitting him. I now take exception with God. And I want to kick his *. Further I look forward to the opportunity.

I wonder about redrives on Briggs an Stratton engines, are they really worth it? I mean why not smaller propeller diameter? A 48" (1.219 M) diameter propeller is good to around 4000 RPM and a 42" (1.067 M) propeller is good to around 4600 RPM so what is the need of a redrive it's complexity and weight and how does it justify a longer propeller?
Thank you.
 

TiPi

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Hi blane.c,
One of the drivers for prop diameter is the aircraft speed (or speed range). Large dia props (turning slow) have more thrust at low speed. Smaller dia props turning faster perform better at higher speeds. Putting a small and high-revving prop/engine on a slow plane will sacrifice performance. Small prop dia are also not that suitable for large fuselage cross sections.
 

Dana

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Larger propellers are more efficient. It's all about how much air you move.
 

rv7charlie

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Action-reaction; for low speed operation, you have to throw a given mass of air aft in a given period of time to make a given mass of plane go forward, and it's more efficient to accelerate a big disc of air at relatively a low speed difference, than a small disc of air at a big speed difference (through the prop). For the extreme in low speed efficiency, think about a helicopter.

When the a/c is moving fast, the quantity of air is effectively moving through the prop faster (because the plane is moving forward at a high rate), so the acceleration through the prop can stay relatively low, with the mass of air remaining similar.

Was that confusing enough?

Charlie

edit: google 'propeller mass flow'
 

Tiger Tim

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How big a Briggs? IIRC there was a V-twin that put out 40-ish horsepower at 3600-ish RPM which to me reads as a plausible swap for a small direct drive VW and may drag around a traditional but efficient single seat plane like a Jodel D.9 or something.
 

blane.c

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So a McCauley Borer propeller was originally for 125hp but now most common on 150hp. It is 82 inches in dia. and came in the past from 39 inch pitch and now advertised 41, 42, 43 and 44 inch pitch. It has an area of (41 x 41) x pie = 5281 square inches or 36 2/3 square feet or 3.4 square meters. It can turn up to 2600 rpm continuous or 2800 rpm for five minutes on a O-290 D of 125hp or a tip speed about 680mph and aluminum propeller (the older 39 inch pitch).

Take a B&S, size dependent largely on what you choose to believe but say between 30hp and 45hp and 3400 rpm to 4100 rpm. Figure a wooden propeller so tip speed below 580mph (51000 feet per minute or 15,545 meters per minute) or more conservatively 550mph (48000 feet per minute or 14,630 meters per minute).
A 54 inch dia. propeller turned around 3600rpm (580mph) or 3400rpm (550mph) 15.9sq ft or 1.48sq m.
A 48 inch dia. propeller turned around 3900rpm " " or 3700rpm " " 12.5sq ft or 1.17sq m.
A 45 inch dia. propeller turned around 4300rpm " " or 4075rpm " " . 11sq ft or 1.02sq m.

When you divide the hp of the 125hp engine by the hp of the B&S engines you will depending get a number(s) around 3 to 4 and when you divide the disc area of the Borer prop by 3 or 4 you will get a disc area similar to those shown above for the B&S engines. Conversely if you multiply the disc area's of the B&S's by 3 or 4 you will get parity with the 125hp/Borer prop combination.

While not the whole story it gives me pause because longer propellers will be thinner due to the hp available to drive them and at some point there must be diminishing returns?

Considering clearance issues, weight, and complexity I wonder is there a real advantage of longer propellers and redrives or at what point the diminishing returns kick in?

image_2021-02-24_105407.png
All Pusher, twin boom. And still iterating, but definitely 3 engines.
 

Vigilant1

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While not the whole story it gives me pause because longer propellers will be thinner due to the hp available to drive them and at some point there must be diminishing returns?

Considering clearance issues, weight, and complexity I wonder is there a real advantage of longer propellers and redrives or at what point the diminishing returns kick in?
Another factor, esp with wood props, is the impracticality of props with very short chords and very thin blades. These small HP engines only have a limited amount of torque, especially at the typical 3600 RPM or so where you'd want to run them in direct drive. With engines of about 30 HP in direct drive mode, as you make the blades longer you'll typically reach impractically thin and "wispy" wooden blades before you encounter tip speed limitations. It's not worth it to reach for an extra 1% improvement in prop efficiency by going with a 2" longer prop if it increases the risk of a cracked/thrown blade. I don't want that kind of excitement.
With a PSRU the torque to the prop is increased, so beefier blades are possible. This, together with reduced prop RPMs (thus tip speeds), enables longer blades if a PSRU.
As others mentioned, the biggest advantage of a large diameter prop is at low airspeeds. So, for these small engines, PSRUs are popular with ultralights, trikes, and paramotors. Cleaner aircraft (Luciolles, SD-1s, etc) go with direct drive because it saves money, weight, and complexity and gives acceptable performance in TO/climb and in cruise. As far as numbers: When I looked at it last, IIRC, with a 28hp engine and an airspeed of about 60 mph (best angle climb speed for the Tri-mower?) , there was only minor improvement with props longer than about 42 inches. At higher airspeeds a longer prop would be even less significant.
More here: "Micromaster"-- Centerline twin using small industrial engines
 
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n45bm

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In the 1970's and 1980's when I received the majority of my basic aviation knowledge it was a simpler time.

Bernoulli's theorem was not questioned at least not in my world and wings and propellers worked by "suck".

Super Cubs like Bic lighters were cheap and expendable IE, a hunter would pay more for a animal than a Super Cub was worth and therefor a Super Cub was considered a write off. They were used thusly.

I could set up a target in my back 40 and shoot to my hearts content. I no longer enjoy a back 40.

My dog died this past week. It was a poor death ill befitting him. I now take exception with God. And I want to kick his *. Further I look forward to the opportunity.

I wonder about redrives on Briggs an Stratton engines, are they really worth it? I mean why not smaller propeller diameter? A 48" (1.219 M) diameter propeller is good to around 4000 RPM and a 42" (1.067 M) propeller is good to around 4600 RPM so what is the need of a redrive it's complexity and weight and how does it justify a longer propeller?
Thank you.
I liked all of your comments until you blamed God and wanted to "kick is a*" for your poor dog's death. Life isn't fair - for most of us.
 

n45bm

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So a McCauley Borer propeller was originally for 125hp but now most common on 150hp. It is 82 inches in dia. and came in the past from 39 inch pitch and now advertised 41, 42, 43 and 44 inch pitch. It has an area of (41 x 41) x pie = 5281 square inches or 36 2/3 square feet or 3.4 square meters. It can turn up to 2600 rpm continuous or 2800 rpm for five minutes on a O-290 D of 125hp or a tip speed about 680mph and aluminum propeller (the older 39 inch pitch).

Take a B&S, size dependent largely on what you choose to believe but say between 30hp and 45hp and 3400 rpm to 4100 rpm. Figure a wooden propeller so tip speed below 580mph (51000 feet per minute or 15,545 meters per minute) or more conservatively 550mph (48000 feet per minute or 14,630 meters per minute).
A 54 inch dia. propeller turned around 3600rpm (580mph) or 3400rpm (550mph) 15.9sq ft or 1.48sq m.
A 48 inch dia. propeller turned around 3900rpm " " or 3700rpm " " 12.5sq ft or 1.17sq m.
A 45 inch dia. propeller turned around 4300rpm " " or 4075rpm " " . 11sq ft or 1.02sq m.

When you divide the hp of the 125hp engine by the hp of the B&S engines you will depending get a number(s) around 3 to 4 and when you divide the disc area of the Borer prop by 3 or 4 you will get a disc area similar to those shown above for the B&S engines. Conversely if you multiply the disc area's of the B&S's by 3 or 4 you will get parity with the 125hp/Borer prop combination.

While not the whole story it gives me pause because longer propellers will be thinner due to the hp available to drive them and at some point there must be diminishing returns?

Considering clearance issues, weight, and complexity I wonder is there a real advantage of longer propellers and redrives or at what point the diminishing returns kick in?

View attachment 107882
All Pusher, twin boom. And still iterating, but definitely 3 engines.
Then there's jets ...
 

rv7charlie

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The Borer example assumes that 82" is optimum dia for that HP & the speed range of the a/c on which it's installed. That likely isn't a valid assumption. The dia was probably limited by ground clearance. The Valley Engineering VW powered demonstrator that flew back in the 1990s was a great illustration of what a reduction drive can do. It was a 2 seat, open cockpit a/c with a VW/reduction swinging, IIRC, a 96" prop. When flying off the ultralite strip at OSH, it consistently left the ground in distances where everything else was just getting rolling.

The cleaner, faster a/c mentioned above that use direct drive industrial engines spend most of their time going fast enough that the small diameter isn't as big a penalty, *and* they are restricted by ground clearance in the diameter they can swing.

Everything (in aviation) is a compromise. Using a relatively heavy industrial engine on a legal ultralite probably means a reduction drive can't be part of the weight budget, so you are forced to accept relatively poor static/low speed thrust for the HP. The faster stuff that's ground clearance-limited also accepts poorer than optimum static/low speed thrust, but isn't penalized as much because it cruises faster.

I can tell you that even small increments can make a significant difference in performance. I've flown fixed pitch props from 68" to 72" in diameter on an RV4; the 72" provides significantly better acceleration and climb rate, with zero penalty in cruise speed. And its speed envelope is *much* wider than anything that could fly with an industrial V-twin.

Charlie
 

blane.c

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Yes Helio Courier had 10 foot dia. propellers and geared Lyc's.

So you know with 30ish 40ish hp machines it just seems unlikely that STOL is a real objective?
 

rv7charlie

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All depends on your goals. Ever watch a Rotax 447 powered single seat Kolb on takeoff? Very STOLish. The mission might be simply landing on sandbars, while carrying a flyrod & a few flies. The Luciolles, SD-1s, etc have a different mission.

Your original question seemed to ask, 'Why the redrive; won't it work without one?' And the answer is, 'Yes, in specific circumstances, with compromises that should be understood.'
 

TFF

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Are you thinking you can equate it to a 1/2 VW? What prop does a 1/2 VW run, 52x 24? As a not quite rule of thumb if you add up pitch and length of a good combination, you can algebra your way to equal load on the engine. 52x24 will load the engine like a 50x26 or a 54x22. Not quite rule of thumb, but trading a inch or two between pitch and length is about 95% on the money. What the prop does will be different on how it pulls the airplane. Are you making the horsepower at a reasonable RPM?You either are or not. Is the weight in a reasonable range? More subjective in that you can trade a loss in performance per pound added until it just won’t take off. Where on that sliding scale is ok? With an unknown, you will have to make a guess and see if it works. That is always the trade. Can you afford to be wrong? If the answer is no, something off the shelf will save you money, just on that it works. If you jumping to get your hands dirty, you are a hero of a combination if it works.

As for loosing the dog, yep it sucks. Was there in the last couple of years twice.
 

rv7charlie

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Something else to chew on is that some things don't 'scale'. I don't have the engineering chops to explain it well, but your earlier example of area percentages vs HP percentages may not work out in the real world. One example that's stuck with me for a long time is the 'scale fighter' idea from back in the 1980s-1990s. Builders discovered that when you scale an existing design down by, say, 50%, wing area shrinks a lot faster than span and weight. They got airframes with handling that was a lot more...spirited... than expected. You might find that when you scale the prop down by a factor of 3 or 4, the actual thrust efficiency may decay even faster.

edit: When you look at all the early designs that flew on 15-40 HP, they were using engines that made their power at less than 2000 rpm, allowing really large dia props. The prop efficiency was compensating for grossly inadequate HP.
 

Vigilant1

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For the TriMower, I'd assume (maybe incorrectly) that a primary objective is safe flight (including moderate climb) with one engine out. That would be the starting point for determining tradeoffs for direct drive vs PSRU, engine size, choosing a prop pitch that provides the best cruise >while< allowing for that safe engine out climb performance.
I confess to liking the 810cc Vanguard engine a lot. Good HP to weight, less expensive than engines that are larger or smaller, and Tipi is doing a lot of development on it.
With the two-engine MicroMaster concept, it seemed likely that the plane could be made to climb safely with one 810cc engine out AND with props with a pitch that provides okay cruise up to about 110 kts at 6K ft, which works out to about 145 mph TAS. If that is achievable in real life, I don't think it would too shabby for a total of 56 HP on tap. But, it would be a light, relatively clean single seat aircraft.
The TriMower should be a significantly less demanding engine-out scenario (with 56 HP still available). It seems you could easily go with these same engines in direct drive, unless you are going for a pretty high takeoff weight, STOL, etc. For example, I haven't looked at it closely, but I doubt a practical 2 seater could be done with 3 x 28 HP engines with safe 1 engine out climb in direct drive unless we made some significantly optimistic assumptions, very long span, extremely light empty weight, etc.
 
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Dana

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The Borer props are used by STOL guys who want acceleration and climb performance to get out of very short strips... at the expense of cruise speed. I'd love to borrow one to try on my Hatz, though.

I remember reading about a Bleriot replica. The original had a 37HP Anzani engine that turned a giant prop at something under 1500 rpm and flew "OK". The replica had a modern 100HP engine turning a much smaller prop at something over 2500 rpm and had appalling performance.

FWIW, Mosler recommended a 54x24 prop for their 40HP half VW turning 3200 rpm. The general consensus in the half VW community was that a redrive turning a larger prop slower would significantly improve performance but the uneven firing of the half VW means it's very difficult to design a redrive that will hold up.

Aero engineers don't "question" Bernoulli's theorem and never have; rather, they know it describes certain aspects of the pressure/flow distribution around an airfoil but it doesn't "explain" lift, which is caused not by "suck" or "pressure", but by the pressure differential" between the upper and lower surfaces.
 

Pops

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For the first 35 hrs on the SSSC I used a 1200 cc, 40 hp. 4 cyl VW engine with a culver 54"x 24" prop. Turned 3600 rpm on WOT on climb out at about 550 fpm. Cruised at 65 mph at 3200 rpm. Sweet little engine burning 2.5 gph. I wanted more ROC so I built the 1835 cc, 60 hp engine for it. 1200'+ ROC with a 80 mph cruise at 2700 rpm.
 

blane.c

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The propeller dance includes cubic inches, torque, rpm, hp, speed and the viscosity of the fluid it is working in, in relation to the diameter and pitch of the propeller. It seems so simple.

In my Cubby I could when light easily take off in a few hundred feet of runway just easing the throttle to 1700rpm O-290 D2 and 42 pitch Borer, of course it could rev higher and did when loaded or short.

"Suck" is so simple.
 

Tiger Tim

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edit: When you look at all the early designs that flew on 15-40 HP, they were using engines that made their power at less than 2000 rpm, allowing really large dia props. The prop efficiency was compensating for grossly inadequate HP.
This right here. I used to have my circle of friends overlap with folks who were into WWI replicas and they said all other things being equal a ‘modern’ engine in a WWI plane needs to have about twice the power of the originals for the same performance, the reason being the originals swung gargantuan props at very low RPM compared to the aero engines we see now.
 
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