Trying for good engine runs at 5000 ft. altitude

I've been trying to get good engine runs at my location altitude of 5000ft.for two years.
So far have had marginal results after trying hints from Stunt
News
I basically get irratic runs (runaways,no break,weak breaks lacking power at the tops of manuvers).These occure with my
Brodak Fox 35,K&B40.and Dixon FP40
Am using Sig 10% nitro - 25 to 29% all castor.
Thunderbolt,Sig,Glowdevil,and Fox plugs all r/c Long.
Tried ST and Stock needlevalves.
Varied venturi size on the K&B and FP.
All engines run great on the test stand or plane when on the ground, 4 cycle and 2 cycle tach good, but when they get in the
air results are usually varied.
Props used are Zingers, BYO, 10-5'S to 11-6'. Had the best results with Master Air 10-6's.
My thoughts are,it has to do with the altitude.
Would appreciate any general advice on flying at high altitudes

LAST EDITED ON Sep-09-00 AT 11:46 PM (EST)

Have you tried varying the nitro levels? Most likely 15% instead of 10% will help the engine runs. Second thing is that the plane itself needs to be on the order of 10% lighte than is true in the thick air most of us can fly in.

Someone asked recently how much an Oriental should weigh to fly nicely with a Fox 35. It's easy to keep that one around 40 ounces, but it will fly almost as well in thick air at 44. But at your 5000, you need to keep that one at 39 ounces. You need more area and less weight to come close to lower altitude performance.

K.

P.S. You can still get the "high oil content" fuels that the C-L folks generally prefer in these higher Nitro concentrations, but just because a couple of us did mention trying 15%, doesn't mean to lower any oil percentage to accomodate the extra Nitro Methane. Instead, the Methanol percentage is reduced. I will never suggest anything less than 20% oil by volume for sport or stunt C/L flying, and for many engines used in C-L, half or more of that oil should be Castor. For instance, my message example mentions a Fox 35 and an Oriental.

That is an iron piston engine, unless you get a replacement set of ABC guts. It wants at the very least 25% oil and while a new Fox can get by with half synthetic oil, an older one should have 100% Castor, and probably 28-29% oil by volume.

More nitro won't hurt. Sounds like you may have a fuel system problem because the engines are running well on the bench.

Kiwi and Jim, Thanks for the suggestions,more nitro will be my first fix,the weight will happen on my next plane, Accentor.

I have had the engines in a Banshee,Oriental,Tangerer and Fancy
Pants,one thing in common is the brand of uniflow tanks,
Perhaps a change there might be in order.

I have heard some of the flyers in Denver use 15% Nitro.

Again thanks, Jerry Bohn

>I have heard some of the
>flyers in Denver use 15%
>Nitro.

I agree with nirto as a good first guess. I have had to deal with the effective altitude problem every year, and he most reliable fix is more nitro.

15% is a good start, but don't hesitate to use more. Ted and I have pretty much zeroed in on 15% even at Muncie, which is about 2500-3000 feet effective altitude once you figure in the altitude, humidity, and temperature.

I think an engine can be made to run smoothly, but it will not turn the same propeller at the same rpm as at low altitude. If there is an attempt to compare to sea-level performance, there will be perpetual disappointment. My suspicion is that 5000 feet altitude will allow a conventionally aspirated engine to develop somewhere around 80 per cent of its sea level performance. That's why not too many Cessna 172's are used in the Rockies... There is another element in play here, too, the air density, or lack of it, causes a given wing to behave very differently at altitude. At the same nominal groundspeed, at 5000 feet the 'true' airspeed is substantially less. Mate that with a different-performing engine, and a attempt to emulate sea-level flying parameters, and confusion is the product.

Actually, I don't think you'll find much of a loss of rpm over sea level, at least the guys didn't at the 1970 FAI FF Team Finals at Albuquerque, which is over 5k', if I remember right,
maybe over 6k'. As that was with 80% alky, 20% castor, the only
idea is that the thinner air unloads the prop just about the same as the power lost, resulting in only a small loss of r's. As I
recall, the dry air also required a reduction of compression, which I don't quite understand, but I did mess with G.15's and
Rossi's enough to find that "straight" fuel is actually touchier
than our 5% or 10% nitro, too much compression is likely, and venturi size and shape is much more important. Perhaps due to the
thinner air you are working with, you'll find some improvements
in venturi design and don't forget spraybar alignment and sealing. Let us know what you find out! Steve

Here in Salt lake city, 4300 feet above sea leavel, We often run the following engine mods to cope with thinner air. None.

What works best with your engine is simply going up in nitro to 10-15%. You can play with venturii openings, and compression, but you'll get the most and quickest results with more nitro.

The biggest improvements will come from the aircraft itself. Wingloading is the key. High altitude and thinner air do not take well to heavy loadings. We often equate the best wingloading to a 38 oz Nobler. What you want is more than 14 square inches of area per ounce of weight.

There are two ways to get this type of loading. First is to build light aircraft. The other is to add area to the wing by adding another rib bay. Be sure to add some stab and elevator area if you go this route.