First cross country...a few engine ops questions

[Pavewlc]

Thanks again to everyone who responded. I should know try and remember my aero a little better, I was just thinking of temp alone, I forgot about density.
I'll do some more searching on the list for oil and gauges. Interesting reading.
I'll keep everyone posted. Again, thanks much.
Lee

[cessna170bdriver]

How about in a dynamic situation? i.e. if you have the same indicated airspeed at different altitudes, wouldn't the cooling be the same? If so wouldn't it be more accurate to say that cooling varies with indicated airspeed?

As you know, indicated airspeed is measured as dynamic pressure, which is proportional to the density times the SQUARE of the true airspeed. The mass flow of cooling air through the cowling is proportional to the density times the true airspeed (not the square). So, even at the same indicated airspeed, the mass flow through the cowling will decrease as altitude increases.

At 100 knots indicated at sea level let's say, for argument's sake, there is 10 pounds per second of air across the cooling fins. At 9500 feet and 25 degrees F, the density is 75% of sea level, and the true airspeed for 100 KIAS would be about 115 Knots. The air through the cowling is 15 percent faster, but it's 25 percent less dense. So the mass flow rate of the air through the cowling would be 1.15 x .75 x 10, or about 8.6 pounds per second, or only 86 percent of what it was at standard sea level conditions. Assuming that a given indicated airspeed takes a given amount of power, the amount of heat that needs to be rejected by the engine will also be the same. To compensate for the lower mass flow, the temperature RISE of the air as it flows through the cooling fins would have to be 1/0.86 or about 1.16 times whatever it was at sea level to carry away the same amount of heat.

Miles

[lowNslow]

Miles, thanks. I was wondering about the mass flow but could not remember how it was derived. It's great having an engineer here for these insights. I hope you didn't waste the whole afternoon on this, you have an engine to rebuild.

[cessna170bdriver]

Miles, thanks. I was wondering about the mass flow but could not remember how it was derived. It's great having an engineer here for these insights. I hope you didn't waste the whole afternoon on this, you have an engine to rebuild.

My derivation may not be 100% rigorous, as as there is SOME pressure recovery in the cowling, but I don't think I'm too far over in left field.

I'd rather be building my engine, but they won't let me bring it into the office. What would be even better would be for them to let me build a piston engine test cell, and use one of our electronic data systems in it. Hard to justify at a rocket lab, though...

Anyway, thinking about how air-cooling systems work is MUCH more interesting than having a hands-on type like myself count beans...

Miles

[cessna170bdriver]

My derivation may not be 100% rigorous, as as there is SOME pressure recovery in the cowling, but I don't think I'm too far over in left field.

I may be further over in left field than I thought... My assumption was that volumetric flow across the engine could be determined by multiplying the entrance area by the true airspeed. Then it was a simple matter to multiply volumetric flow by density to come up with mass flow. It turns out it's not that simple, and the loss of cooling effectiveness with altitude seems to be less than that assumption would predict.

I just did an internet search on air-cooled engines and high density altitude, and came up with a NACA report written in 1947, using a well-instrumented P-47 powered by a P&W 2800. http://naca.larc.nasa.gov/reports/1947/naca-report-873/

Up until that time, the experimental data had shown that cooling air "weight flow" was proportional to the density of air at the entrance to the cooling system times the pressure drop across the cooling system. The 1947 experiment showed that this was valid up to about 20,000 feet where compressibility becomes a factor, and the density of the air exiting the cooling system starts to have a significant effect. No where in the report was there a mention of "true" airspeed, although I'm sure that both density and true velocity play a part in the pressure drop, although not nearly as significant as I had assumed. I did find one article discussing Volkswagen engines that states that a major factor in cooling is the velocity of air across the fins, although this article was nowhere near as rigorous as the NACA article.

According to the data in the NACA report, at constant engine power (and I would assume from that a fairly constant indicated airspeed at the lower altitudes), the cylinder head temp rose no more than 10 degrees at 20,000 ft. It seems that Karl's assumption of cooling correlating to indicated airspeed comes closer to the mark.

I'm now suspecting that in or 170's, the loss of cooling we see with altitude may result from losing indicated airspeed faster than we lose power at higher altitudes. It would be an interesting experiment...

Miles

[HA]

hey Miles- do you think that the increased angle of attack at higher altitudes (just to stay up there) would affect the cooling airflow significantly as well? not that the cooling inlets/outlets are at all optimized in these aircraft for any particular airspeed or altitude regimes anyway. to my eye the cooling inlets at least are mostly positioned to get their air straight on, so any angle would reduce the ram air effect, adding to the less-dense-air and reduced power efficiency problems.

on the fuel tank / flow question, I can report that my trusty 172 burns within a gallon between tanks on XC trips, but I always run one tank at a time, timing my fuel use. I don't trust those gauges at all, when they show 1/4 they really seem to mean about 1/2 etc. I have noticed what appears to be uneven useage between tanks when on BOTH, but I always attributed that to either my fat butt on the LH side skewing the balance, or just not flying particularly straight (laziness). PLUS, I have that old AD that directs me to choose one tank or another above 5000' too, and I wouldn't want any (more than usual) FAA guys mad at me. AD72-07-02 doesn't apply to the 170 though - why? no reason.

Hans

[cessna170bdriver]

hey Miles- do you think that the increased angle of attack at higher altitudes (just to stay up there) would affect the cooling airflow significantly as well? not that the cooling inlets/outlets are at all optimized in these aircraft for any particular airspeed or altitude regimes anyway. to my eye the cooling inlets at least are mostly positioned to get their air straight on, so any angle would reduce the ram air effect, adding to the less-dense-air and reduced power efficiency problems.
Hans

Just thinking out loud (sort of) here: If you assume the inlets are perpendicular to the airflow at zero angle of attack, and a maximum angle of attack before stall of about 18 degrees, then the area of the inlets perpendicular to the airflow is still about 95 percent of the head-on area (cosine of 18 degrees is 0.951). However, both the true and indicated airspeeds at 18 degrees AOA are less than half of what they would be at zero (or at least very low) angle of attack.

I’m hypothesizing that above a certain altitude you’ll be cruising at full throttle, and the higher you go above that, the slower (indicated) the 170 will cruise. As the speed slows, induced drag rises at an ever increasing rate, and above some altitude the speed will start to drop off faster than the power available drops off. If cooling depends on speed, and speed is dropping faster than power, at some point the engine is going to get warmer. How does that sound?

Miles

[Joe Moilanen]

You guys are making my head hurt. If I drink colder, denser beer at at a higher altitude will my angle of attack increase at a faster rate until I perform an accelerated stall or will I just mush out and experience the falling leaf affect???

Joe

[cessna170bdriver]

If I drink colder, denser beer at at a higher altitude will my angle of attack increase at a faster rate until I perform an accelerated stall or will I just mush out and experience the falling leaf affect???

Joe

Joe, I don't know the answer to that question, but come on down to Tehachapi sometime and we'll give it a try. I have the altitude (4000 ft) AND the beer (Lobotomy Bock, 10.8%)

Miles

[GAHorn]

As altitude increases OAT decreases due to the fact that the less-dense air consists of air molecules farther apart.

It's only a very minor point in this discussion, and it is not my intent to start an argument, but the anal engineer part of me thinks a bit more explanation is in order . The air is not necessarily cooler due to the fact that it’s less dense (molecules are further apart). In fact less dense air can be warmer, as in inversion layers. The phenomenon George is talking about is the "Adiabatic" (constant heat) Lapse Rate, which only applies to a parcel of air in the process of rising, and therefore in the process of expanding, like letting compressed gas out of a closed container. This is due to the fact that as a constant amount of heat occupies a larger volume (expansion), the temperature will decrease. The cooling rate due this phenomenon is just under 5.5 degrees F/1000 feet in dry air.

In an atmosphere with little or no vertical movement, the air at higher elevations is cooler mainly because it’s further from its heat source, the earth. This phenomenon is called the “Standard” Lapse Rate, and runs just over 3.5 degrees per 1000 feet (especially in the lower portions of the atmosphere where 170’s can go). The fact that density decreases with altitude is mostly attributed to its lower pressure.

The part about less dense air (regardless of how it got that way) having less cooling CAPACITY is absolutely correct. The amount of heat a given volume of air is capable of carrying away from the engine depends much more on its density than its temperature

Miles

Well, since this is primarily for the fun of discussion....

Actually, Miles, You may have lept to an assumption. I was deliberately not addressing lapse rates, inversions, vertical movement, etc. In my own example, I was attempting to address only air density, with regard to heat absorption and cooling capacity...and completely without regard to the complicating matters of lapse rates (either adiabatic, which does not always mean dry but can also be saturated or moist... or environmental, etc. etc.) I was trying to keep the discussion simpler, and address the ability of those air molecules to pass heat one to another (the process by which we all, inaccurately perhaps in the engineering world, but nontheless in the real world, refer to as "cooling" etc.)
If we are talking ONLY about the ability of an atmosphere to absorb heat from an engine... a less dense packet of air will absorb less heat from an engine than a more dense packet will. That is regardless of any other issue provided all other conditons are the same.... but purely as a function of individual molecules of air to be available to absorb heat.
(And just for arguments sake, ... I don't think the earth is the source of atmospheric heating. I believe the sun is the source. The atmosphere absorbs heat from both the sun, and that reflected/convected/radiated from the earth (having been in turn also heated primarily by the sun.) But you know far more than I about such matters..... I just had to increase the verbage of this discussion.)

[cessna170bdriver]

As altitude increases OAT decreases due to the fact that the less-dense air consists of air molecules farther apart. The molecules are less able to transfer heat to each other. This helps our TAS, of course, because fewer molecules impact the airframe to cause drag.

I took that first sentence to mean that you were saying that the air at higher altitudes was cooler just because the molecules are further apart. Sorry I misunderstood you. It is true that if the molecules are further apart, they transfer heat to each other more slowly, but given time, they would all eventually come to the same temperature. It also true that when there are fewer molecules in a given volume, then that volume will hold less heat at a given temperature. Both of these phenomena help to account for less efficient engine cooling at higher altitudes.

(And just for arguments sake, ... I don't think the earth is the source of atmospheric heating. I believe the sun is the source. The atmosphere absorbs heat from both the sun, and that reflected/convected from the earth (which in turn has also been heated primarily by the sun.) But you know all that. I just had to increase the verbage of this discussion.)

Here's an analogy, for the fun of discussion of course : What's the "source" of fuel to our engines? Is it the fuel tanks in the aircraft? Is it the refinery that made it? Or is it the dinasaurs that died millions of years ago?

Admittedly, I could have more clearly made my statement about the atmosphere's "source" of heat. My point was that the atmosphere is more "directly" heated by the earth than the sun, which expains the "standard" lapse rate.

[GAHorn]

Here's an analogy, for the fun of discussion of course : What's the "source" of fuel to our engines? Is it the fuel tanks in the aircraft? Is it the refinery that made it? Or is it the dinasaurs that died millions of years ago?

Admittedly, I could have more clearly made my statement about the atmosphere's "source" of heat. My point was that the atmosphere is more "directly" heated by the earth than the sun, which expains the "standard" lapse rate.

Ha Ha! We've got WAY TOO MANY ENJUNEERS, especially those in the OIL FIELD to let that one pass, Miles!! Dinasaurs? Did you mean Dinosaurs?

Dinosaurs (Cretaceous period, ca 65 Million years ago) were not even alive yet when the earth's oil was created (Carboniferous period, ca 300 Million years) by diatoms, bacteria, etc..

http://www.thomhartmann.com/dinosaur.shtml for my favorite discussion on this, but http://www.energyquest.ca.gov/story/chapter08.html for a more scientific one.

Ye olde dinasower, gahorn.

[Bruce Fenstermacher]

Ya know Miles it is a very rare day when you can slip something past George.

However if you were to check all my past posts you'll see I was successful once.

[cessna170bdriver]

Ha Ha! We've got WAY TOO MANY ENJUNEERS, especially those in the OIL FIELD to let that one pass, Miles!!

Some would say that one engineer is one too many, but engineering is what makes MY airplane fly. (Really its money, but money has to come from somewhere too, doesn't it? Mine comes from engineering. )

Dinasaurs? Did you mean Dinosaurs?

Here, where the official language is a matter of debate, dinasaur is the feminine of dinosaur.

Dinosaurs (Cretaceous period, ca 65 Million years ago) were not even alive yet when the earth's oil was created (Carboniferous period, ca 300 Million years) by diatoms, bacteria, etc..

Guess I need to ramp up my time spent watching the Discovery Channel, eh? You seem to have gotten my point anyway.

Miles