Thorp Air Command - T18.net

So I have a current 68x68 Demuth prop on my o290 (fresh top end). Derek from Olney had a Demuth prop in the hangar he wasn't using and it was a 68x74. I was going to try it out since I was thinking about getting a prop with a little more pitch. That 74 pitch seems pretty aggressive for this engine and I don't know how to calculate if it will be ok.

I have 3400 ft runway and currently use about 800-1000 feet for take off with my 68x68. Are there any other things I should check when using it like the range for my static RPM test? I know with more prop it can effect my manifold pressure although I don't have a gauge to monitor it.

Any info on testing before putting this on and doing a test flight would be appreciated.

I am running a 68x74 on my O-320 so it may be a little much for the O-290 but it doesn't hurt to try. As for static RPM, that is usually found on the aircraft or propeller Type Certificate. I would guess the O-290 should turn between 2100 and 2450 RPM based on what I have seen over the years on the "certified" airplanes.

Manifold pressure can be useful for selecting a prop. It would let you know how hard the engine is working to turn the prop.

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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

That prop is designed for an O320. It should be too much pitch for an O290.
Cubes

I bought a Sterba 68x74 for my O-320 T18, it currently has a nearly new Sensenich 70x77 on it, and it's way overpitched for my engine. Ed tells me he can reduce pitch on his props easy and for free except shipping both ways if I want to go to 73, 72 etc.

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Base: KALX
N8786
SN 929

I figured it was a little too much pitch. My main concern is how much runway it will take and if it will cause any damage to the engine.

As for static RPM since it's experimental there is no type certificate or supplement for the plane giving info on static rpm requirements unfortunately.

I figured a 68x71 would be ok but it ended up being a 74 pitch which I think is probably going to be too much. I might not even put it on to test it if there is a chance it will put too much stress on the engine.

IMHO static rpm is just that, static. So regardless of the airframe the engine should make xxxx static rpm with the same prop length and pitch on a comparable airframe. Problem is, as you state there is no cert airframe with a sterba 68x74 on it to my knowledge. Maybe some T18ers will chime in with what their O290s do with various props. My O320 with the aforementioned 70x77 barely does 1900 static. That prop is on ebay soon.

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Base: KALX
N8786
SN 929

A Demuth 66x68 on 818TR -- works well

The effect on the engine could be the same as running to high of a gear on any land vehicle. Essentially bogging the engine down, not allowing it to develop full power and forcing it to run hot. This is where the manifold pressure gauge comes in handy. If the manifold pressure 25 inches and the prop is turning 2300 rpm then the engine could be working too hard. The term is "over-squared" and most people will tell you not to run that way because it can make the engine more prone to detonation. HOWEVER, if you read the engine operation manual and some POH, they tell a slightly different story. I use to experiment a little when I had the PA-20 with a constant speed prop. I never hurt anything but I never saw any advantage to running over-squared either. I was able to pull the engine down to the point where I lost airspeed and the CHT started to warm up but I could get a little more speed by running slightly over-squared.

It would make takeoff roll longer but I don't know how to determine by how much.

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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 O290-G with D2 cylinders, 140 HP, my previous prop was a Great American 68 x 68 wood. It was well matched to my aircraft with decent climb and cruise. That said, I now have a Whirlwind ground adjustable composite prop, it is more efficient than the wood prop, I have a faster cruise for the same engine rpm with equivalent climb rates that I was getting with the wood prop. I have the prop set for 2200 static rpm.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

[quote="blueangel59"]A Demuth 66x64 on 818TR -- works well

Jeff, I’ve read some testing on over square and under square. I think the biggest evidence that it doesn’t cause harm is the fact that quite a bit of the time fixed pitch engines will be over square t some durations of flight.

Bruce, I like my 68x68. flat out at about 1000msl. Climb is decent until I’m high altitude. If I can justify the expense it sounds like a newer design could be the way to get a little more out of this small engine.

I think all fixed pitch prop engines always operate over square until the power is reduced for cruise. Mine pulls 2000rpm/29" static and 2300/27" reducing with alt in the climb. Low comp engines just don't care. It's worse to run high rpm and low manifold pressure as you can get ring flutter.

Tony Shischka
T18C
ZK-VMS

I have heard of this in race cars but not airplanes. I am reasonably sure most people who fly behind piston engine with a constant speed prop were taught the following:
1. reduce power (manifold pressure) before pushing the prop up to land.
2. increase rpm before manifold pressure to climb.
3. descend by reducing manifold pressure alone (cruise descent).

If ring flutter was a concern, these would not be the standard procedures. The only warning I recall reading concerning low manifold pressure (idle) and high rpm was to not allow the CHT to drop below 300°F during a long descent. That was in an IO-520 manual. I may have read others but this is the only one that has stuck.

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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

Dave Hardin has a 68x70 sterba that he is going to let me borrow to try. I was planning on replacing with a sterba for cost so this would be a good test and just a slight amount of pitch which is what I wanted.

I have a Hendrickson 68x68 on my O-290 powered T-18. It seems perfect.
As a cheaper 5 lamination wood prop, I intend to upgrade to a Whirlwind ground adjustable when I see age related delamination or whatever...
My wife has a Sensenich ground adjustable on her O-320 powered RV-4, Bruce and Gary both have Whirlwind ground adjustable props on T-18 (O-290 and O-320). Flying locally and cross country (Seattle WA- Sedona AZ) these light and efficient props seem like winners.
A free chance to try another prop is always fun. 2" more pitch and a different 'brand' is interesting. Tell us how it works out?

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Scott Emery
EAA Chapter 326
T-18 N18TE