Fuel consumption Update!

Thanks to everyone with their ideas on the fuel issue. I'm geting old or something. I had the two tubes in the front of the tank hooked up backwards. I had overflow hooked up to the uniflow and uniflow hooked up to overflow. Since I reversed it I have a 6.5 minute run. If I start clean and run to the handle I can just sneak in the full pattern. I think a 6.5oz tank is in order.

Thanks again and keep flying

>Thanks to everyone with their ideas on the fuel issue. I'm
>geting old or something. I had the two tubes in the front of
>the tank hooked up backwards. I had overflow hooked up to the
>uniflow and uniflow hooked up to overflow. Since I reversed it
>I have a 6.5 minute run. If I start clean and run to the
>handle I can just sneak in the full pattern. I think a 6.5oz
>tank is in order.
>
>Thanks again and keep flying
That sudden silence in the middle of the clover can be disconcerting

> . . . Since I reversed it
>I have a 6.5 minute run. If I start clean and run to the
>handle I can just sneak in the full pattern. . . .
> . . .

We have had several threads covering timing of a run, but I don't recall anyone stating their "target" time. Wouldn't 6.5 minutes be a good target for FAI 7 minutes? Any more, and you risk an over run. Any less means a risk of running out of fuel at a "bad time"?

We have a minute longer window under AMA. What do y'all shoot for to minimize the risk at both ends, including an over-run due to a less than perfect start?

Larry Fulwider

>> . . . Since I reversed it
>>I have a 6.5 minute run. If I start clean and run to the
>>handle I can just sneak in the full pattern. . . .
>> . . .
>
>We have had several threads covering timing of a run, but I
>don't recall anyone stating their "target" time.
>Wouldn't 6.5 minutes be a good target for FAI 7 minutes? Any
>more, and you risk an over run. Any less means a risk of
>running out of fuel at a "bad time"?
>
>We have a minute longer window under AMA. What do y'all shoot
>for to minimize the risk at both ends, including an over-run
>due to a less than perfect start?

People talk about it all the time, particularly in FAI, but to be honest with you I hardly ever pay attention to my actual flight time. I looked back at whatever old scoresheets I have a few months ago and noticed that I was pretty well centered on 6:30 - 6:45, and that really hasn't been dangerous either way. I may have timed a total of about 10 flights in my entire stunt career.

As long as your tank will run cleanly until the very end, it's not hard at all to hit the 6:30 mark and that will give plenty of time after the pattern to be safe and accomodate a few extra laps here and there. The trick is to get a tank that doesn't want to start acting funny at the 5 minute mark, and if you put in enough wedge and taper/cant angle, it's pretty easy.

Brett

p.s. Another thought, and most people know this already, but when using less than a full tank, it's more reliable to fill the tank every time, then suck out the excess, as opposed to adding a measured amount. The latter doesn't account for any excess fuel left in the tank from the previous run.

I also mentioned this before, but I never explained it. In FAI I usually cut off the flight using the "high overhead flight" maneuver. After the pattern, fly around at 5 feet until you think you are close, then go up to about 60 degrees and fly around and it will usually quit. It's really no different than a cutoff loop except you don't take much chance of getting blown out of it. In FAI, it also allows you to choose your starting point for the landing and dive to 5 feet and pick up speed, so that the "1 lap landing" becomes much easier. It's not very important but it's a simple matter to accomplish. I think the cutoff loop is probably better on average for AMA, since you don't have to care about how long the landing descent is. Touch down at warp 3 just before downwind and the entire thing becomes much easier to control.

BTW, I got the idea from watching RJ/Alphadog. We never talked about it but I saw him doing it and realized the total awesomeness of the idea.

(snip)
> Brett
>
>p.s. Another thought, and most people know this already, but
>when using less than a full tank, it's more reliable to fill
>the tank every time, then suck out the excess, as opposed to
>adding a measured amount. The latter doesn't account for any
>excess fuel left in the tank from the previous run.
>
(snip)

Agree with Brett 100%. This is the way we have done it for many, many years. We use a crank type pump and fill the tank all the way, then take out "x" number of turns (or partial turns) to get the run length we are seeking. And, as a follow-up to what Brett mentioned here, Matt's first run of the day is always just a little different from the rest. I attribute this to the fact that some amount of fuel was left in the tank and probably evaporated out, leaving the oil content and "whatever else" behind. This combined with the first batch of fresh fuel, but after that it is fuel only that is left behind to combine with the new refill. I wouldn't trust adding a measured amount and trusting this to give the same results every timel.

Leonard, Brett –

Excellent overview of all the factors involved, explained in a concise way that any bozo can understand! Thanks much! It also emphasizes that the “one flip start” is not just for show.

Brett also said:
> . . .The trick is to get a tank that doesn't want to start acting funny at the 5 minute mark, >and if you put in enough wedge and taper/cant angle, it's pretty easy.
> . . .

Yes, but you have to make your own tank. It is amazing that no one manufactures a tapered tank! And on many 35-40 size kits or ARFs (ARF Nobler, for example) a shy eighth cant on a straight (Brodak, et al) 1 ¾ series tank is all you can get without minor surgery. Mine burps for two laps, which I hate, and I really squozed it in there. When an engine quits it should be like someone drove a wooden stake through its heart, and you hear only one short hungry protest and a flutter.

At least with Brodak tank kits, they will supply a 1 ¾ end with a 2” tank so you can build your own by unbending the seam and the two inboard bends. Sheet metal in hobby shops is not well sized for tank building. I know, there are other sources of tin sheet, but the mainstream hobby business does not sell either tapered tanks or handy materials to build one from scratch.

Oops! I rambled off topic.

Larry Fulwider

> Sheet metal in hobby shops
>is not well sized for tank building. I know, there are other
>sources of tin sheet, but the mainstream hobby business does
>not sell either tapered tanks or handy materials to build one
>from scratch.
>

I use the regular K&S sheets straight from the hobby shop. You have to make the tank body in two parts but I prefer than anyway.

Brett

> . . .
> I use the regular K&S sheets straight from the hobby
>shop. You have to make the tank body in two parts but I prefer
>than anyway.
>
> Brett

OK, guess I had an old plumber's prejudice against that. Prejudice removed.

Larry Fulwider

>> . . . Since I reversed it
>>I have a 6.5 minute run. If I start clean and run to the
>>handle I can just sneak in the full pattern. . . .
>> . . .
>
>We have had several threads covering timing of a run, but I
>don't recall anyone stating their "target" time.
>Wouldn't 6.5 minutes be a good target for FAI 7 minutes? Any
>more, and you risk an over run. Any less means a risk of
>running out of fuel at a "bad time"?
>
>We have a minute longer window under AMA. What do y'all shoot
>for to minimize the risk at both ends, including an over-run
>due to a less than perfect start?
>
>Larry Fulwider
>

Actually I was thinking of commenting here. We usually shoot for 6 minutes, 20 seconds with Matt. That is total run time from start of the engine to when the plane stops its roll out after the landing. Makes no difference if we are talking AMA pattern or FAI. Flying a 5.3 to 5.5 second lap time he completes the pattern in about 5 minutes, so there is ample time in the run. We try for that first or second flip start, and when we get it he has over 30 seconds margin in FAI, over a minute and half in AMA. If it doesn't start immediately, that would be where the extra time is allotted for. But once you get things settled down, you should be able to get that start very quickly, and 6:20 from engine start to end of roll out is totally adequate. I don't see any problem with his 6:30 runs.

I seldom disagree with Brett publicly but I have to disagree about the value of timing engine runs on a fairly regular basis (every flight, however, I think is a bit anal).

Assuming you're using a "given" amount of fuel every time, knowing how long the engine runs on that amount of fuel is valuable information. This is especially true if you travel around a lot to different climates or if there is apt to be a big change in weather conditions at a given site. Tucson can be a good example of a place that can be very cool for a morning needle flight and be easily 20 or 30 degrees warmer by the time you fly your official flights.

Run time on a given amount of (identical) fuel is a pretty accurate measure of how much power you're getting out of the powertrain. The amount of power can be affected by changes in density altitude. When temperature goes up or barometric pressure go down (or when you fly at a higher altitude site than your home field) will reduce density altitude and the engine will run longer and produce less power.

The reverse is also true. Us sea level west coast guys fly in generally ideal atmospheric conditions most of the time. Whenever we go elsewhere where it will be predictably higher density altitude we either take nitro to doctor our stock fuel or we bring a can with five or 10% more nitro content than our stock.

If your engine runs 6:40 like mine does at home and 7:30 at Muncie or Lake Charles or 8:30 at Denver, you're facing a serious power drain. Increasing nitro content of the fuel until you've regained that 6:40 run time will get your power output back to darn close to what you're used to at home.

Yes, you can regain the power in other ways but pretty much all of them will simultaneously alter the way the engine runs and will require changing the engine configuration. Bigger (or smaller) intakes, higher or lower compression, etc. If I had my druthers, I'd rather suffer a bit of power loss in order to retain the "character" of my engine run. Using the nitro injection and a stop watch you get both.

Ted

A quick comment about other aspects of changing density altitudes.

It isn't only the engine that feels the loss of O2 in the air.

Everything that couples the airplane to the atmosphere through which it flies is also affected. Expect to use more pitch on the prop along with the nitro/stopwatch equation. The nitro will get your power back but in order to accelerate the same number of air molecules you'll need to take a bigger bite with each rev.

You can also get that bigger bite per rev with more diameter but it isn't generally the best approach. Because the air is thinner the wing and tail are also going to have fewer air molecules to work with. The only way to get more of those is to move through the air a bit faster. Pitch will get that for you; more diameter is unlikely to do so.