Thorp Air Command - T18.net

Sure thing! I’m a little excited to see how it works out. I like my ground roll but David and the guys with 320 cruise around 170 and I’m max 166 true. I can’t justify a firewall forward plus new cowl on this cheap flier of mine.

I would like the nice forums adjustable but looking at them before it was mid 2000 for sensinich and it would add some weight compared to my current prop. For that kind of money I’d rather spend it to upgrade my radio and get it ifr certified so I don’t keep getting stuck on long trips. That’s the biggest reason I take my Cherokee for any longer trips. So it’s a tough battle of wants verses draining my bank account for my toy.

One thing you need to consider is the propeller's design Horsepower.
I tried another 68x68 that was designed for a 150 HP O-320.
The blade area is greater and it was great enough to reduce the static RPM (about 2100) on my O-290 compared 2340 RPM for the Ted Hendrickson 68x68 that I run.
The ground roll was longer and the initial rate of climb was poor. The RPM did not noticeably increase until the airspeed approached 120 MPH, then the two propellers started to behave similarly.
Above 140 MPH they were about the same. I only flew that prop once. I wouldn't carry a passenger with it.
I'm concerned that you are adding 6" more pitch, and greater blade area possibly designed for a 160 HP O-320. I think your static RPM will be low, maybe 2000 RPM.
Just a quick napkin calculation:
125 HP divided by 2700 RPM = .0462963
.0462963 x 2000 RPM = 92.59 HP Some Napkin eh?
Not much for take off, even on a 3600 foot strip if there are any obstacles to clear. Also the propeller may be semi-stalled and not deliver as much thrust per HP until it starts to gain RPM with airspeed.
If you decide to try this prop, I suggest you establish a point along the runway that you can confidently abort the take-off and stop before you reach the end. Then make sure you are airborne by that mark. Make the aircraft as light as you reasonably can.
@ 2340 RPM, I'm taking off with 108.33 HP. Soon after liftoff, the RPM is climbing along with the airspeed. So testing the other 68x68 prop designed for 150 HP gave my 2100 RPM, about 97.22 HP that did not increase until after I had cleared the trees and accelerated toward 120 MPH.
I hope this gives some perspective to your prop trial and helps the outcome.

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Scott Emery
EAA Chapter 326
T-18 N18TE

That is some good info from Scott. I have always focused on engine efficiency and just trying to keep things in the power band with the knowledge from grade school science class that anything below the power band is lugging the engine and wasting fuel and anything above the power band burns more fuel than is usually justified for a slight performance gain. I had never calculated the actual end result and was surprised the test with the 68x68 for a 150 HP engine calculated out to less than 65% HP available for takeoff.

Assuming Scott was flying his airplane from a field elevation of ~500 feet (his EAA chapter is based at 537'), the prop cost him about 53 HP. Using formulas I found on backcountrypilot.org, normal HP loss from sea level to 500 feet should be around 2.2 HP. Take that same configuration to a runway at 3000 feet and normal HP should around 136.5 however, if you factor in the test showing ~65% power at 500 feet, takeoff power with the bad prop is only 88.7 HP (82 @ 5000"...).

None of this is really hard scientific evidence but I believe; if you can't make that prop turn a decent RPM in the chocks, it can't turn an acceptable (to me anyway) HP producing RPM for takeoff and climb. I also believe a prop that will not allow an engine to unload and turn at (or close to) it's rated RPM is robbing HP and wasting fuel in flight.

***I deleted the original post so I could edit and re-post an expanded version.***

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I have an O-290G updated to 135 HP with a Sensenich W68LM x 72 wood prop. I was told that Ed Burke worked with Sensenich engineers to re-form the tips and he reported 10 mph faster cruise afterwards at around 160 mph. I don't know what my static RPM is yet, but I expect it will be around 2000. I'll let you guys know. I have the Sensenich papers for the prop if anyone needs to see them.

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John Twigg N12055 SN.79 Pittsburgh, PA (KFWQ)

That's a decent increase.

I didn't try the 68x74. I did try a 68x70 and it didn't help. I thought I posted the results on a page here but maybe not.

I lost 200rpm in flat out at 2000ft elevation. Down to 162mph true at that alt. I had a weird left yaw as well. After changing prop the yaw went away and I was centered again. It was a sterba borrowed from Dave Hardin.

I guess your next effort could be drag reduction?
I have the old Ratray wheel pants with brakes and hoses hanging in the breeze. Bruce Finney has newer, possibly Rocket pressure recovery pants.
I bet there are at least 4 different types on T-18s and S-18s.
I land on grass, so no tail wheel pant for me.

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Scott Emery
EAA Chapter 326
T-18 N18TE

Ya, i have the original designed wheel pants open on the sides and everything hanging in the wind. I looked into cubes' wheel pants, wing roots and gear root fairings. The cost of all that was well over what the small mph I would gain. a few grand (plus the work to install) for a few mph isn't worth it. I'd rather spend the cash on more fuel to fly!

I think my demuth 68x68 seems to be optimal speed/climb combo for this particular plane. I'm just at 2650 rpm full throttle so I can get the benefit of no turbulence in the venturi (flat throttle plate) and not over rev the engine for cruise.

jtwigg, 68x72 sounds a bit much for this engine. The 68x70 I tried was too much. 2000 rpm static is quite low and seems like you would be very low on power for take off. The top rpm is rated at 2800 rpm so you would be 800 rpm shy of that and 600 rpm off your cruise hp. At sea level that would be sub 100 hp take off power. Test it with plenty of runway for the first time!

Props are kind of like shoes... what one manufacturer calls one size will fit differently than the same size from a different manufacturer. I.e “brand x” 68x70 may not work at all but “brand y” 68x70 could be perfect because they all have their own way of shaping the blade and taking measurements. I would love to experiment in this area some but I lack the funds.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Ya, it definitely shows that each manufacturer has different shapes. The sterba had a very thigh inbound area compared to my demuth. The demuth was thin all the way to the hub. The sterba was easily an inch thicker in the more inbound areas. Details like that can really change it the way it flies.

Binder - I will be flying it this month if all goes well and will let you and the forum what the actual numbers are.

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John Twigg N12055 SN.79 Pittsburgh, PA (KFWQ)

Awesome! Long time coming. Mine has a static of 2300 rpm. With a high DA and moderate load (under 1200lbs total weight) it will eat up some runway. Low DA it's off pretty fast. You'll know more once you get a good static test. Dropping 300 rpm off my static seems like it would eat quite a bit of runway on my thorp.

Binder, I sent you a PM.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Have a 68x73 Sencinich metal prop (m76) on 0-290g T-18 that’s been running 40 years+. Was repitched some years ago from a 68x68. N6CM gets up and goes and is a delight to fly. It’s pretty light at just a hair over 900 pounds. Static and take off RPM is 2250...not great, but she climbs over 1,000 FPM with one person. At full power does 2700 RPM and a little over 190 mph on a cool, smooth air day and gives our IO-360 T-18 a run for the money. Cruises 160 to 170mph at 2450rpm. Burns 8 gallons an hour at cruise. We love her. Read the newsletters. All the guys back in the day seemed to have landed on 68x73 as the best all around prop for the 0-290GPU. All the best.