Engines, Wind, and Nitro......

[Crist Rigotti]

I went flying today and here's what I learned about engines, wind, and nitro.
The plane:
Harbinger. 625 sqs about 53 oz. T&L ST .51 swinging a Bolly 13 X 4.25 prop release RPM 10,600 to 10,700 Lap times about 5.4 to 5.5 seconds.

I usually fly using 15% nitro because the airplane doesn't slow down in the square eight. The wind was pretty much straight line at about 10 to 12 mph.

Using the 15% nitro Sig fuel the airplane would show signs of winding up and would struggle a bit in the overhead eight.

I brought along some 25% Sig fuel and filled it up. It started and without touching the needle to ran about 11,100 rpms. I richened the needle to about 10,600 to 10,700 and released the plane.

The engine ran in a very deep but powerful 4 stroke. Line tension was very good and lap times were about 5.4 to 5.5 seconds. I tried some loops and the tendancy to wind up was all but gone! The engine didn't even break at the tops of the loops. Tried some square loops and the same thing. I was delighted! I tried some overhead eights and the engine pulled the plane very well in the inside loops with a little bit of a break at the top of the circle, but in the outside portion, it broke hard and just powered it's way throught the wind! Next up was a vertical eight. Nice break at the outside portion with great line tension everywhere. I do have my tank positioned to give a little leaner run inverted than upright. Not much but just a little.

Tomorrow I'm goning to try it again to see if I can duplicate the runs. As far as today goes, I learned that the extra nitro gave the power boost yet almost acting as a limiter to wind up in the wind.

Has anybody else experienced this and could somebody explain what went on.

Thanks.

[the original Steve Smith]

Crist,

What were the conditions at the field?  Was it very hot and/or humid?  I have read conflicting observations about higher nitro percentages in hot or cold conditions and now I don't know which is which anymore.

I hope someone has an understandable answer.

[Crist Rigotti]

Steve,
It was warm but not hot and humid.

[Ron King]

Crist,

Just a guess, but it sounds like the four stroking keeps the plane from accellerating through the back side of the loops and controls the wind up. A thirteen inch fan on the front will act like an air brake sometimes. We use props to slow the full size birds down, so it makes sense to me.

You never mentioned it, but I'm assuming the engine broke into a two stroke during most maneuvers with the 15 percent fuel.

I have a similar situation with the 76 engine versus the 65 engine in my Alouette. Using the same fuel, the 65 comes on the pipe sooner and breaks harder. There is some wind up in a stiff breeze.

I have my 76 set up with a long pipe and it just loafs through the whole pattern in a steady four stroke. It breaks way overhead, but not very hard. Zero wind up and it ignores the wind. Coolest flying plane I've ever owned. 

My two cents,

Ron

[Phil Coopy]

I have been using 25% nitro Omega with oil content boosted with castor to 21-28% lube depending on the engine needs. I used a lot of 25% when I was flying RC pattern and IMAC because it seemed to boost the low rpm torque a lot. So having a lot left over I tried it with a FOX35, LA40-S, and even a Cox Black Widow. I am turning a 12-4 Zinger with the LA40-S. Had some recent problems with runaway but I don't think they were nitro related.  Amazingly, all the engines seem to run cooler with tons a power. I think there is enough difference to continue the practice unless I see some engine damage. Fuel consumption seems to be about the same.
Phil

[Crist Rigotti]

Ron,
Yeah, it would break a lot more often with the 15%.  Your assessment of the 25% fuel is what I'm thinking is happening.  In yesterday's calmer air it hardly broke at all.  Only on the outside loops of the H8.  I have noticed that the 25% fuel seems to "pound" the plane a bit more.  What's interesting is that the plane is flying a bit slower than normal yet the line tension has gone up.  I'm going to have to play with it some more today and see if I can optimize the set up.