An O-360 in an experimental super cub: what would be the best prop?

[frequent_flyer]

Anything over 80 inches goes supersonic at just over 2700 rpm and looses all it efficiency.

According to my calculations:

An 80 inch prop turning 2,700 rpm has a tip speed of 942.5 ft/s.
An 86 inch prop turning 2,700 rpm has a tip speed of 1013.2 ft/s
Speed of sound at sea level standard day is 1,126 ft/s.

I am unable to reconcile my calculations with your statement. What did I miss?

[Gordon Misch]

FWIW I get the same tip-speed answers for both props.

Edit: same answers as Frequent did, for each.

[frequent_flyer]

I am unable to reconcile my calculations with your statement. What did I miss?

I did some browsing. Prop tip speed has to consider the aircraft forward speed not just the speed due to rotation. I also found references that indicate efficiency falls off rapidly as tip speed exceeds about 0.85 Mach.

I found a few interesting articles and will provide links for the curious. (No, I don't claim to understand all the details).

https://www.kitplanes.com/wind-tunnel-52/

https://www.nar-associates.com/techn...ncy_screen.pdf

https://www.warpdriveprops.com/propspd2.html

https://www.idc-online.com/technical...%20Factors.pdf

I'm happy to be able to spin my 80 inch prop at 2,700 any time I want but it spends most of it's time closer to 2,100.

[skywagon8a]

I did some comparison testing years ago with a 150 hp PA-18 and a 150 hp 7GCB, both on floats comparing the original 7456 with a 1A175GM8241, 8244 and an 8046. The 1A175s all seemed to pull out of the water and climb about the same. The only real difference was in cruise. The 82" seemed to be thrashing around not doing any better than the 80". It was as though the extra two inches was doing nothing. The 80" and 82" had about a 30% better TO and climb than the 7456. The 8046 cruised the same as the 7456.

I've also compared the original 2 blade 185 seaplane prop with a shorter 3 blade. The 3 blade seemed to pull better on the initial take off run. I don't remember the diameters 86" and 80"?

Historically a long 2 blade turning slowly will produce more low speed thrust. This is why the Helio uses a geared engine driving a long prop. As the rpms approach the high mach numbers the tip efficiency falls off.

[Gordon Misch]

Good point re forward speed - - So I ran some numbers just for grins and giggles, 30 mph (takeoff), 60 mph (climb), 100 mph (cruise). All at the same rpm. Machs are

80 in 0.84, 0.84, 0.85

86 in 0.90, 0.90, 0.91

For our Cubs, the forward speed is small compared to tip speed so doesn't have a huge effect.

[Paul Heinrich]

Other than top level STOL competitions, how often would anyone absolutely need 100% maximum performance out of an idealized engine/prop combination?

[stewartb]

The only STOL competition I have is with myself. I use everything my planes have every time I commit to taking off.

[flynlow]

I recently attached video cameras to a Cessna 180 Skywagon equipped with a large-diameter “seaplane prop” set to maximum rpm and recorded both its performance and decibel level. With the propeller knob full forward and wide-open throttle, the Skywagon’s takeoff roll was 620 feet and lasted 14 seconds. Once established in a 75-knot climb, it ascended at 1,100 feet per minute, took 1 minute 50 seconds to go from brake release to 1,000 feet agl—and the sound beside the runway was a screaming 98 decibels. Then, at pattern altitude, the pilot reduced engine/prop rpm to 2,500 as per the pilot’s operating handbook.

But what would happen if the pilot set 2,500 rpm on the ground and then went to full throttle for takeoff? Would performance suffer?

On the next two takeoffs, the pilot did just that—and his takeoff roll was 20 feet shorter and three seconds quicker, his rate of climb was 150 feet per minute higher, and time from brake release to 1,000 feet agl was reduced by more than 20 seconds.

Oh, yeah, and the noise near the runway dropped 10 decibels.

The mind-numbing, ear-splitting jackhammer noise of propeller tips hitting the speed of sound is really the mark of inefficiency, wasted effort, and a needless loss of aircraft performance.Klaus Savier, founder of Lightspeed Engineering and designer of multiple aircraft speed modifications, said all airfoils—including propellers—see a sharp increase in drag as they approach the speed of sound.

“The drag rise on propeller tips starts at about 0.84 Mach and goes up sharply from there,” he said. “The loss of efficiency is dramatic.”

Engine horsepower increases with rpm, but as propeller tips approach the speed of sound, the amount of thrust they produce drops off rapidly.

“It doesn’t do any good to add horsepower if you’re losing propeller efficiency as a result,” he said. “That’s a losing proposition.” Large-diameter, two-blade propellers on direct-drive engines are the most susceptible to high tip speeds. Think Cessna 180s and 185s, T–6s, and old Beechcraft Bonanzas.

Reducing rpm at high power settings can have negative consequences, even in normally aspirated engines. In extreme cases, high manifold pressure and low rpm can cause detonation or preignition that could damage or destroy engines. Check with your engine manufacturer to find out whether your airplane’s engine is capable of safely operating at less than full rpm


Sent from my iPhone using SuperCub.Org

[flynlow]

Above is article from Dave Hirshman, AOPA


Sent from my iPhone using SuperCub.Org

[stewartb]

It doesn’t make sense to this Skywagon owner. Here’s a post I made in 2012 regarding noise levels inside my old 160hp/Borer 82-42 equipped PA-12. Not long after I did the same test in my 275hp 180 with 86” 3-blade spinning 2700rpm. The 180 was 3dB quieter than the -12 in all reported operations.

FYI, I took my noise meter out in the plane tonight to identify my own noise levels. PA-12, 160hp w/8242 prop, and a Hot Rod muffler. 1200 rpm warm-up 85db. 2450 rpm cruise 98db. Full throttle 2700 rpm cruise 102db. Driving home in my Duramax pickup, 65db at neighborhood speeds. Turn on the amplified Alpine and turn it up to comfortably loud volume, 95db with peaks to 99db. My ANR Gallet helmet is supposed to have about 30db of total noise reduction. My stereo? no hearing protection there. I think I know why my ears ring.

[hotrod180]

According to my calculations:
An 80 inch prop turning 2,700 rpm has a tip speed of 942.5 ft/s.
An 86 inch prop turning 2,700 rpm has a tip speed of 1013.2 ft/s
Speed of sound at sea level standard day is 1,126 ft/s.
I am unable to reconcile my calculations with your statement. What did I miss?

Per pponk's tip speed calculator:
80" at 2700 = 643 mph (.8677 mach)
86" at 2700 = 691 mph (0.932 mach)
this is at 20' Celsius

ponk sez the sweet spot is .88-.92 mach

tip speed calculator here Props - P. Ponk (pponk.com)

[stewartb]

A stock 180 with an 88” seaplane prop spins 2600 rpm. Not a big noise threat. 185s are the noise makers and they do great at 2850 rpm with an 86” prop. I’ll add that those with IO-550s make less noise (2700 rpm) but I don’t see any takeoff length advantage from the 550s. Rate of climb? I hear that’s improved. I can’t observe that difference from my living room. I have a bird’s eye view for takeoff length.

You guys with 0-470s should try taking off with a 100 rpm reduction and see what you think. Better yet, do it on floats. Or draggy spring snow. Adding drag will make you want all your power. To that? I suspect the 185s with 550s benefit. More torque and less prop cavitation should favor the 550. Not like IO-520s are poor performers. They are not.

[stewartb]

For the record? Assertions like that in the article aren’t new. I’ve read them since the day I got my 180 and I’ve tried using different full throttle RPMs to see for myself. Regardless of DA, and I see very low DA at sea level and well below zero temps, I use all the RPM I have available. Full rated power is the best performer. Noisier? No doubt, but the notion that noise indicates declining aircraft performance is incorrect in my plane.

I get a kick from watching my 3 year old granddaughter when walking around the airport. She recognizes the noisy planes and puts her fingers in her ears before they go by. The offenders are the Skywagons, Maules, and 206s. She doesn’t react the same way to Cubs, but once in a while there’s a noisy one.

[frequent_flyer]

ponk sez the sweet spot is .88-.92 mach

That's higher than other references. E.g. "Prop Tip Speed in Mach. Maximum performance between 0.8-0.92 Mach." ref https://www.warpdriveprops.com/propspd2.html

Anyway, my only reason for computing tip speed was to confirm tip speed is not "supersonic" for 80 - 86 inch props at 2,700 rpm.

[stewartb]

A prop is more efficient as DA goes up? How do you explain that? Props work best in dense air, as in cold.

[frequent_flyer]

A prop is more efficient as DA goes up? How do you explain that? Props work best in dense air, as in cold.

I had incorrectly applied the compensation for changing Mach number. Glad someone was awake.

I'll delete that part of my post.

[CenterHillAg]

I’ve done some anecdotal data gathering while working. My Ag Cat has a PT6 turning a 106” Hartzell 3 blade to 2200 on takeoff, I’m limited to 42.5 psi on the installation which I easily make year round. With maximum drag being made with the fertilizer spreader and a 2200# load of fertilizer, 2200 on takeoff gives great hole shot but doesn’t do much for building speed after breaking ground, 2050 is the sweet spot which lays down the power and builds/holds speed. If you leave the prop at 2050 and get another load, the takeoff is anemic and is slower to build speed once in the air. Flying an R985 seemed the same way with a 106” Hamilton Standard 6101, 2350 will get you moving but 2150 gets you on the step. 2150 on takeoff was nothing special.

[hotrod180]

..... 185s are the noise makers and they do great at 2850 rpm with an 86” prop......

If there's any truth to this:

"Over .92 mach the airflow begins to detach from the propeller which decreases efficiency and dramatically increases noise. To improve performance and public relations you should consider reducing RPM so as to fall within the .88 to .92 mach range. Your propeller will be producing maximum thrust which is good for you, and less noise which is good for all of us."

I would think that would apply here--
the tip speed calc sez a 86" prop at 2850 is .983 mach.
My ears are bleeding just thinking about it.

[Dave Calkins]

“…the notion that noise indicates declining aircraft performance is incorrect in my plane…”

I pulled that quote from Stewarts post above.

noise IS wasted effort. noise DOES indicate less efficiency. BUT, But……….

When I need outta the hole, stuck in snow, gotta get on step before the river corner kindof power, loss of efficiency is not on my mind. Max Thrust is, efficiency be damned!!!

I cant argue with CenterHill Ag about his Cat.

I can argue it over the 985 Beaver, Garrett Otter, SuperCub, 180/185, 206, and PT6 Caravan Amphib.

And my static pull testing has proved it scientifically.

edit: tjat sounded kindof snarky in my “thrust”. Thrust was intended, snarky was not. thanks. d

[frequent_flyer]

the tip speed calc sez a 86" prop at 2850 is .983 mach.

What temperature did you assume? That seems to be way colder than standard.

Still 0.95 on a standard day though.

[Dog]

After everyone has read the last page it might make sense to most of s why a shorter prop at higher RPM ( power) is still in

the sweet spot for performance over a long blade.

This is why Mr Cato stated it will out climb the longer Blade. I found the same thing. It may be foolish to not believe Mr Catto , his props hold lots of records.

At our RPM envelope anything over 80 inches in a fixed pitch only works to getting moving and fights you for everything else.

The 82 inch Whirlwind and Suns Neck have the swept tips to reduce tip drag and sound. It works well for that but the last bit does almost nothing for thrust.

If I was experimental I would have Catto build me a 82 inch prop or go with the Suns Neck. I have one on order for my Cub

[stewartb]

For guys who enjoy some science, here’s a good read. http://www.supercoolprops.com/articles/transonic.php

I look at my Whirl Wind prop and marvel at the shape. It is 80” with swept tips but the body of the prop has more chord than any prop I’ve seen, and more twist near the hub. It produces lots of thrust but requires lots of horsepower. From my perspective as the dummy with his hand on the loud lever? I like it.

In the Skywagon example? When my tips are in full noise mode the body of the prop must be working well since thrust is higher than it is at slower prop speeds. I’m sure there’s a limit to that curve but given that 185s use the same prop at 2850 rpm successfully I don’t worry about it.

[skywagon8a]

A couple of articles about Paul Lipps' propeller design of which the Whirlwind is one.
http://rexresearch.com/lippsprop/lipps.htm
https://canardzone.groups.io/g/canar.../message/25670