Tuning Engines for Weather

In other threads there are references to adjustments for high / low humidity and high / low temperatures. Let's make a huge (no data) leap here, and assume that the only input variable we change for weather is the fuel. That may not be true, but I don't want to change gaskets or plugs because of a weather change.

Suppose you have a perfect engine setup for 70 degrees and 50% humidity. What do you do on hot dry days? What do you do on cold damp days? What do you do on hot humid days? What do you do on cold dry days? What algorithms are in your head that moves you to change the fuel mix? How much of a change from "normal weather" would cause you to make a change? How do you make the change -- blending of stock fuels or adding specific ingredients like oil or nitro?

The top flyers know how to do this. I don't have a clue as to which way to go with humidity or temperature changes.

Now on the 53 and the pipe set up, we run these on 5% nitro in cool to "normal" weather, go to 7 1/2% in warmer (somewhere in the lower 80's) weather and then 10% when it is warmer. Sometimes we just run the 10% when we should be running 7 1/2%. We use commercial fuel and blend 5% and 10% together for the 7 1/2%. Randy Smith had said he would sometimes go to a contest and put in a practice flight with 5% in the cool of the morning, switch to 7 1/2% for the first official, and go to 10% in the heat of the day. If your engine is set up properly, you can get away with this.

I would like to caution that you need to have your head clearances set properly if you are adding nitro. (Lower compression for more nitro). Adding nitro is a little like going to a larger venturi. Decreasing nitro is a little like going to a smaller venturi. Adding a filter is like going to a smaller venturi. If you are using a filter and not getting the power, get rid of it. Or expect to need a larger venturi if you need to use a filter.

I agree 100% that you don't want to play with head shims to compensate for weather changes. Venturis, maybe. But merely adding nitro as the day heats up is the simplest solution.

Just remember, also, some people get carried away with overkill. They come from the "if a little helps, let's try a lot" school of thought. These are the guys who end up getting their diplomas in summer school after everyone else has graduated.

LAST EDITED ON Jun-02-01 AT 10:05 PM (EST)

LAST EDITED ON Jun-02-01 AT 10:00 PM (EST)

>Gang --
>
>In other threads there are references
>to adjustments for high /
>low humidity and high /
>low temperatures. Let's make a
>huge (no data) leap here,
>and assume that the only
>input variable we change for
>weather is the fuel. That
>may not be true, but
>I don't want to change
>gaskets or plugs because of
>a weather change.
>
>Suppose you have a perfect engine
>setup for 70 degrees and
>50% humidity. What do you
>do on hot dry days?
>What do you do on
>cold damp days? What do
>you do on hot humid
>days? What do you do
>on cold dry days? What
>algorithms are in your head
>that moves you to change
>the fuel mix? How much
>of a change from "normal
>weather" would cause you to
>make a change? How do
>you make the change --
>blending of stock fuels or
>adding specific ingredients like oil
>or nitro?
>

To first approximation, changing the nitro is straightforward and obviously very easy way to tune the engine for different air conditions.

A prerequisite is somehow being able to determine the effective air density. More humidity, higher temperature, higher altitude or lower pressure lower the air density. Dryer, cooler, lower altitude, or higher pressure raises the air density.

The net effect is usually expressed in terms of "density altitude". The density altitude is a reference to the standard atmosphere. A density altitude of 0 feet means that the air is equivalent to an air density of the standard atmosphere at sea level and 59 deg F, The density altitude goes up 1000 feet for a 15 degree F temperature change, so at sea level and 104 degrees, it's like you are flying at 3000 feet in a standard atmosphere. Similar relations relate pressure changes to altitude. This is standard information for light aircraft operators. Searching on Altavista for "density altitude" will get you a lot of hits for explanatory links.

Humidity is slightly different. An water molecule (H2O) is lighter than an oxygen (O2) molecule, so the presence of water means that some oxygen has been displaced and the net result is a less dense gas. What you need to know is how many water molecules are floating around. Unfortunately, relative humidity doesn't directly relate to the absolute number of water molecules in the air, but instead how many molecules are present relative to how many you could possibly hold. The "carrying capacity" of water molecules is heavily dependent on temperature. Hotter air can hold more water molecules. The "dew point" is a much better gauge, but there's no really easy way to make use of it without resorting to computer programs.

Fortunately for all of us, master modeler (and master aerospace engineer) Pete Soule has written such a program. His website has some of the best information yet available for modelers.

The main site is at:
http://www.geocities.com/ama271

and the program is linked from:

http://www.geocities.com/ama271/Aeronotes/pwscol.htm

The ATMO99 program will calculate the effective air density after you put in the current parameters. I use it almost exclusively. It's only for WINTEL machines, but I run it in Windows 98 with VirtualPC 4.02.1 on the Mac. ATMO99 is not a real demanding program in any event.

Once you know your effective air density, you still have to do something about it. Given that we are going to limit to fuel changes, nitro content is the key parameter. Think of nitro as an oxygen contributer. More tends to make up some of the oxygen you weren't getting otherwise. There's no hard and fast rule, but I have found that for my PA61, 5% more nitro with no other changes more or less compensates to about 1000 feet of density altitude change. My rule of thumb is to alter the nitro until you get the same fuel consumption you had when it was working.

Example - Before the NATS last year, I was using 7.4 oz of 10% SIG for a full pattern. When I got to Muncie (using fuel I pre-ordered, not the same jug), I found that to get the same lap time, I had to lean the motor out to the point that it was only running about 5.8 oz for a full pattern. Of course, the thing was seriously lean and was two-stroking about half the time, instead of a constant 4-stroke. I fiddled around, but with little useful effect, and was in the middle of the pack on my circle. After a few days of this, I went over and got a jug of 15% SIG. Immediately, I had to turn the needle back to where it had been, it ran in a constant 4-stroke again, and it was using about 7.2 oz. I later came to the conclusion that the original "10%" fuel was probably really 5%, since it was the same color and had originally been priced like 5%.

for comparison purposes:
home/Muncie
altitude (feet) = 10/1000
temperature = 95/85
humidity = 20%/80%
wet density alt = 2417/3689

meaning an effective air density difference of about 1250 feet

I would point that 5%/1000 feet is a pretty sensitive setup.

Note that if you are simply trying to sort out the air density difference, you don't necessarily need to change the head clearance when you change the nitro. If it was right at the baseline density altitude, and you add 5% to compensate for lower air density, you really aren't increasing the combustion pressure, you are simply returning it to the previous level, so you don't necessarily need to alter anything else. This is completely different from changing the nitro in constant conditions. If you do this, you do generally have to lower the compression when using more nitro, and raise it with lower nitro, although stunt motors are pretty insensitive to this effect. Usually, adding nitro makes the needle less sensitive. If adding nitro makes the needle more sensitive, then you have too much compression. There's no danger of blowing it up, so it's still worth a try.

So this is how to do it. Figure out your "baseline" air density and remember what it is. If you just want to recreate the engine performance at some other temp/pressure/humidity/altitude, run it so that you get the same lap times, and check the fuel consumption. If the air is less dense, you should get more mileage, and if it's more dense, you should get less fuel mileage. Change the nitro by mixing or changing the fuel until you get about the same fuel consumption at the same lap time. Keep track of this and the air density. Eventually, you'll have enough information to make a sort of plot of nitro vs. air density, or at least a feel for how much more or less nitro it takes for different density altitudes.

Sometimes in real hot weather, more oil will help. This is pretty obvious, since oil is a big part of the cooling. If this really helps a lot, I suggest you didn't have enough oil to begin with. Once you find oil helping in hot weather, just use more oil all the time.

Some engines and particular setups are more sensitive to atmospheric effects than others. I usually try for a relatively insensitive system. There are really no reliable rules of thumb, the only one that even comes close is that running the engine near it's peak seems to make it less sensitive.

This all sounds like it's all figured out, but this is only part of the story. Humidity, in particular, seems to cause occasionally unexpected effects that don't really follow the formula. There's also the "state line effect", meaning that if you cross state lines, the engine will run differently, even if everything else is the same. The most notable is the "schneurle effect", meaning the engine running differently in inside and outside manevuers. For unknown reasons, this seems to be much more pronounced on the West coast, probably due to some unpredicted effect of humidity vs. temperature. Air at 104 and dry may have the same calculated density as 85 and high humidity, but it doesn't act exactly the same. Usually, schneurle effects are less in midwest/southeast conditions.

This analysis completely ignores the extensive effect of air density on the prop and aircraft performance.

Brett

(edited 6/2/01 for spelling and typos, and AMTO99 data from example - bb)

Wow!

Brett, I am not disputing the fuel consumption changes that you are experiencing with your set up, but you are also running something "different" and your set-up may be exaggerating the changes in fuel consupmtion that you mention. You are one of the very few using that amount of fuel, for instance, on a PA-61. You are also running a small prop (less load) in a constant 4-cycle.

Matt has been running a PA-61 for a year now, and after trying nearly every prop on the planet as well as different pipe lengths, different venturis, and making compression changes, as well as fuel changes--the whole gambut (except for changing the pipe...you are using the Eather Pipe, Matt is using Randy's) we never experienced the need for using that amount of fuel. In fact, we have a 6 1/2 ounce tank in Matt's plane and have never found the need to go to anything bigger.

Matt's current set up, and what we now call "base one" because this is what we have found works best to our liking, is strictly Randy, and as supplied with the following exceptions:

Venturi was increased from a number 12 to a number 9. Head clearance was changed from .021 as supplied to .026 by adding an additional .005 head shim. (Randy said later this is what he was currently running on his as well.) 13 inch 3 blade Bolly repitched to give lap times of about 5.3 to 5.4 seconds on 68 foot lines and a pipe setting of 17 1/4. The engine is set to run 4-cycle with a break to 2-cycle at just about 45 degrees (just a slight break at the very top of the loops.) And with this set up, we have not found the need to go beyond 10% nitro (at least not yet) flying in Muncies altitude and in the heat of the summer.

I totally agree with you that adding nitro is the simplest way to compensate for heat or altitude, except that we switch between 5, 7 1/2, and 10% to compensate for the heat of the day. But fuel consumption stays relatively the same--even if we don't switch nitro. We have found a greater difference in fuel consumption switching from 22% all castor to 22% 50-50 (the 50-50 gave poorer mileage) than switching nitro content. (This latter statement is relating to different non-pipe set-ups.)

Now the mileage question. Last Sunday Matt flew at the first regional contest at Muncie. It was cool (he said "cold"--60 degrees and LOTS of wind. Did you see that, Ted, lots of wind at Muncie.) (Inside joke) We use a hand crank fuel pump and remove fuel as needed. He filled the 6 1/2 ounce tank and removed three "turns" for a 7 minute run. A few days later, back in Vincennes, Matt put in some practice flights on an 80 degree muggy day, and, with the same fuel, had to remove the same three "turns" for a 7 minute run. He will be paring that down to about 6 minutes and 20 seconds for the run, which means removing more fuel, but the upshot of all this is some of us aren't using as much fuel, and some of us aren't experiencing the deep fuel consumption changes through altitude density that you are.

True, Matt is running the engine closer to the limit with the bigger prop. But it is still running mostly 4-cycle and certainly is not overcompressed.

You have a lot of good information here, however. I just wanted to add this bit about fuel consumption which may not work as a test for density altitude for everybody. When the engine doesn't break properly, when it lacks "oomph" in hotter weather, that's when we go to the higher nitro.

Not quite as scientific. But, it works.

Actually, I'm using the original Randy/Werewage pipe (the one with the exhaust straight out the back, not the side). My fuel consumption is a little more than David, but not a lot. For reasons none of us understand, my particular PA61 is a whole lot stronger than all the others. Additionally, engine setup is among the strongest, and you would love it since it has the most static thrust of any stunt plane any of the locals have flow (B-17 excluded). Of course, even with 2-3 times the static thrust, it flies the airplane almost the same as the 40VF, but that's a different argument...

If you want to try my setup, the details are:

stock engine, .023 head gaskets
#5 spigot venturi (made from SIG rivet, ~.020 projection)
Smith/Werewage standard 60 pipe with stright exhaust
Brian Eather 12-3.5 3b with "Red" undercamber, cut to 11.5 and pitched to 3.6
Pipe length 17.5"
Fuel - SIG 10% (other brands 15%) in local conditions, 15% suggested in midwest.
7.6 oz uniflow tank
ground launch RPM - 10,000:10,100

> A few days
>later, back in Vincennes, Matt
>put in some practice flights
>on an 80 degree muggy
>day, and, with the same
>fuel, had to remove the
>same three "turns" for a
>7 minute run. He
>will be paring that down
>to about 6 minutes and
>20 seconds for the run,
>which means removing more fuel,
>but the upshot of all
>this is some of us
>aren't using as much fuel,
>and some of us aren't
>experiencing the deep fuel consumption
>changes through altitude density that
>you are.
>
>True, Matt is running the engine
>closer to the limit with
>the bigger prop. But
>it is still running mostly
>4-cycle and certainly is not
>overcompressed.
>
>You have a lot of good
>information here, however. I
>just wanted to add this
>bit about fuel consumption which
>may not work as a
>test for density altitude for
>everybody. When the engine
>doesn't break properly, when it
>lacks "oomph" in hotter weather,
>that's when we go to
>the higher nitro.
>

You guys are dealing with *micro-tiny* atmospheric changes compared to what I was talking about. Moving a few hundred miles and over a tiny range of humidity and temperature would lead you to beleive it's a very minor effect. I was talking the gamut - something that you have probably never experienced. I used to live there, too, and my experience is that moving from one place in the midwest/southeast to another does not expose you to the kind of huge changes I am talking about.

There have been times when I "crossed the border" to the NATS and found very little change in the engine (and aircraft) performance, and other times the changes have been very significant. The only significant variable seems to be the humidity (or as Brett suggested, dew point). I have traveled several times from Seattle (~70 deg F, 50% humidity, Sea level) to Vincenses (spelling?)(~ 90 deg F, 80-90% humidity, ~1000 ft alt.). Both times the difference was as described, but the dew point had to be different. Both times the same plane was used. The first time it was plug and play. No changes to anything, just fly. The second time, it was a terrible dog. I opened the venturi, upped the nitro, re-adjusted the plane only to have to un-adjust it later in the week when the "dew point" changed. I have not spent much time trying to quantify this effect. I just adjust as necessary.

The 35 FP was the most forgiving when it came to temperature. When the termperature got below about 70 degrees it developed a little too much power (translated: we had to set the engine too rich to maintain lap times) and our "fix" was to go to a little smaller diameter venturi (from a .278 to a .274). But anything above 70 degrees and it was plug and play. Even when it got to 100 at Muncie it was "happy" running on 5% nitro.

On other set ups we have noticed a drop in performance as the temperature heats up. With the pipe we go from 5% to 7 1/2% to 10% as mentioned in order to compensate. At last Sunday's contest with its 60 degree weather Matt said he had to run a constant 4-cycle. Later in the week when it warmed back up to 80, it was breaking into a very brief 2-cycle again at the top of the loops (which is how we normally like to run it). So the temperature effect was there (and high humidity later in the week as well.) No quarrel that density altitude makes a difference, and that going to sea level would make a difference, too.

My only comment, question, call it what you will, was that we have not seen as drastic a change in fuel consumption as you described. Matt was using the same amount of fuel at 60 degrees and 4-cycle all the way as he was at 80 degrees and with it running on the break. Since you and Matt are running the same engine, I was wondering it it had to do with the way they were set up.

Matt has the Smith/Werwage pipe with the stinger coming out the bottom, but they should be, essentially, the same. You are running a little more compression, smaller prop, longer pipe length (not much) and launching about 1200 rpm higher, which makes me think you are set up to "fall off" the pipe as the control.

Matt's, by the way, would sound pretty close to Ted's in the air with that slight break at the top of the loop. I am sure there are differences in set up, however, as I know he was using the Eather pipe and a different prop (and the same 15% nitro at Muncie that you were using).

Still, I think the moisture goes in as vapor (humidity) and comes out as vapor unless there is water in your fuel. Thus its major effect is in density altitude and, perhaps, how it affects the combustion process. It won't, of itself, cause the engine to overheat.

To give perspective, at Roseburg the dew point was actually only about 45 degrees on Saturday. The humidty went down drastically (from around 55-60% to about 20%) during the day, but the dew point only went down about 5 degress. Right before I went to dinner on Saturday night, it was 42 degrees. The temp went from about 70 something at the start of OTS, but did get up into the low 90's peak.

The reason I point this out is that a typical summer dewpoint in the midwest is more in the 65 degree range, and the humidity rarely drops below 60%. This is hugely different from the conditions we would normally experience on the West coast. That's why, lots of times, different engine setups gain popularity in particular regions, but don't necessarily travel well. I've been lucky in being able to find systems that really only need minor changes (like a 5% change in nitro) to work acceptably well everywhere.

For what it's worth, in WWII vintage fighter aircraft engines, water injection was used to cool the charge coming from highly tuned superchargers. The water absorbs heat is the process of turning to steam. Have to agreee with Leonard on this one.
K

Might be a bit of a conjugation problem here - you're launching at 8900?

My error, Brett. That's what I get for sitting at the tail end of the airplane all the time while Matt gets all the fun of getting his finger in the prop. I forgot what rpm he was launching at.

We launched the 53 Magnum (on a muffler) at 8900. He is launching the pipe ship at 9700. So there is about a 400 rpm difference at launch between your set up and ours. But there is undoubtedly a difference in prop pitch and prop load so I don't know what the in air rpm difference would be. (Gotta get one of those sonic tachs.)

Your pipe length, which I think you said the last time was 17 3/8, would be a minor 1/8 inch longer. You are launching--with a different prop--at 400 rpm faster. I still don't know what that translates to in the air--and it is in air rpm that matters with the pipe length. And you are striving for a 4-stroke all the way, while we want that slight break at the top. You are also running a number 5 spigot which, by my calculations, comes out to about half a thousandth square inch less choke area than the number 9 true venturi Matt is using (essentially immaterial).

How all that comes out then I don't know. Just trying to make comparisons to see why you are using as much fuel as you are and why you are experiencing a greater variance in fuel consumption with changes in pressure altitude.

I guess we are a little more like Paul. We add nitro when we feel we need a little more zing because of the heat. You add nitro for the same reason but do it on the basis of fuel consumption changes which I don't think we would be able to do.