pe reivers

continued:
from the same web page it shows the better cruise conditions at altitude. But that realy is well known, and is why airliners fly up there, and not down here.

Cruise Performance Comparison Chart

wiz310

This is actually more of an aerodynamic problem than a power problem. you will by all means see a change in power available, but the wing needs to travel faster through the atmosphere to "see" the same airspeed that it would at a lower altitude. there are less molecules per square foot/inch than at a lower alt. this is the lower pressure you see up higher.
Actually, you need to skinny up the fuel flow....... and lean out the mixture. less oxygen molecules per square foot/inch of air consumed and therefor fuel needs to be leaned to accommodate the now richer mixture. lesser fuel flow will follow the leaning.
Actually, the indicated airspeed will remain the same. it is measured by a pitot/static system and that is pressure operated. the actual Calibrated airspeed is higher. this is corrected for pressure/temperature. (this is most noticeable by the ground speed increase.... in fact sometimes even wrongfully referred to as airspeed)
Less molecules at work again.
Are you taking into account that this aircraft could (and probably is) using a constant speed propeller?. the RPM will remain at whatever you select with the propeller control. D'oh. I should have looked at the MP's. Its a turbo charged engine and will definitely have CSU. It will also have a wastegate to maintain manifold pressure at altitude (actually to prevent over boosting at lower altitudes and it closes as you get higher to maintain a constant pressure)
So far, semi retired aviator has nailed them all correctly. I love the user name SRA. . I bet your not a rich semi retired aviator though...... as thats usually part of the job description with aviator. I bet we both frequent another common site that specializes in our chosen profession. think rumors and networking. I'll be pretty easy for you to pick from the crowd from Auz.
Cheers, Wiz. (the still tirelessly working aviator who wishes he could afford to retire. )

Sport_Pilot

Pe,
That was a turbocharged engine, the turbo will maintain approximately the same manifold pressure. The turbo is not so much used to boost power at sea level but more to maintain the same power level at altitude. With a constant speed prop but no turbo charger the manifold pressure will drop at the same rpm. I only put those charts up there for comparison and to help show what is actually happening.

From my original post.


That manifold pressure of the turbo is a positive pressure or above atmospheric or above 29.92" of mercury, in the See Bee the pressure is a negative pressure or below 29.92".

pe reivers

At 4000 ft the boost pressure was lower for same rpm?
We do not fly at much higher altitudes, except some blokes near 'Vegas They all report about samish rpm with same props.
This is backed up by RCpilet, Lucien, and others like Dick Hanson, who also flies at altitude and reports frequently in this forum.
It also is backed up by plain theory. To have you state something different does not alter al these facts.
Maybe you just published for discussion's sake? The myth is long gone. Nitro-spoiled engines even gain rpm (not power)with altitude!

DarZeelon

Hugh,


That is pure hog-wash! (borrowing FBD's term)

If altitude density is half of the sea-level value, the cylinder volume will contain half as large a mass of air.
The mass of the oxygen in that smaller air-mass will also be half of that in sea-level air.

I.e half the altitude density - ~half the power, since there is half as much oxygen that can oxidize half as much fuel. Power goes down at a linear rate. It is a linear function of altitude density.

Since the prop's drag (as any drag) is represented by 1/2 Cd * A * � (Ro) * V squared; and '�' is the air density; drag is also a linear function of altitude density.

RPM will only change subtlely, due to efficiency issues, not directly related to the air density changes.


I am still surprized this thread is still alive for this long (~3 days...).

DarZeelon

Cyclic,


As I wrote before, in a previous thread, I am quite sure there is something amiss with your engine.
This is like what people get here on an MVVS .61 with a muffler...

Take it to near sea-level altitude and you will see that this engine cannot achieve very different RPM there either.

It is not an altitude issue and if your Jett is sub-standard, I am sure Dub will make good on his word.

----------------------
Compared to 14,500 RPM at sea-level (2.25 HP), your 12,600 RPM at 5,000 ft. is a 45% power loss.

Sport_Pilot

Dar,
You may be right, I didn't run through the calculations. I made a hypothesis on the known fact that a full scale fixed pitched propeller equipped aircraft will not go past redline when at altitude, but at near sea level it will be at part trottle at red line. This I have observed myself, very common in the smaller GA aircraft such as a C 172. This was verified in the Lycoming tech article. It may indeed be the fact that the friction stays constant while the engine power is reduced. Still that may not be a small matter. Automobile engines have a mechanical efficiency of between 80 and 90 percent at sea level. That means that of the total indicated HP only about 80 to 90% gets to the shaft. Drivetrain losses drive that down further. I don't know how much is lost in model engines I suspect it is about the same in the four strokes and a bit better in the two strokes. Then again because of the high RPM's it could be much worse.

I have things to do so I will run some of these numbers later.

DarZeelon

Hugh,


The amount of HP consumed to overcome internal friction in an engine is indeed nearly constant, at a given RPM.

Nearly, in a non-ringed engine, but it actually is not in a ringed engine.
The crankshaft and con-rod bushings and bearings do produce a constant (~) amount of friction, as does a non-ringed piston.

But compression and combustion pressures applied to the ring from behind (entering between the ring and the top of its groove) are larger, when altitude density and power are greater.

This may be why some aero-engines exhibit a slight rise in RPM as altitude is gained, even though power is reduced.

Sport_Pilot

I have flown as small as a C 150 to a T-6 and all of the fixed pitched had a RPM drop at altitude. if they had a RPM rise then they were using a constant speed prop which does sometime gain RPM at altitude. The differance in internal friction from ring expansion should not be that great on any well broken in engine.

Sport_Pilot

I think the critical factor is the HP asorbed by the prop, not drag. But I think density would have a similar effect.

DarZeelon

As to my quote, this is what some others wrote here.

The full-size T-6 Harvard/Texan has/had a variable pitch prop. I am sure of this.
The Cessna 150 does have a fixed pitch prop.

Semi Retired Aviator

It looks as though the T-6 does have a variable pitch prop just looking at the mechanicm in the pic.

Further to pe reivers post at the top of the page, most airline category aircraft give the best True Airspeed at about 28,000'. That level is where the efficiency of the jet engine starts to fall off dramatically and the density of the air doesn't decrease corrrespondingly to give an increasing TAS. Newer generation jets are improving on that marginally though.

The reason they fly higher than that optimum TAS level is to take advantage of generally increasing winds at height if flying east; if they are flying west, then it's generally lower to get out of headwinds. Winds tend westerly at altitude right around the globe, and you don't want to be caught tracking west into a jet stream with a velocity of 250 knots/475 kms per hr. But tracking east, they have the potential to increase the ground speed to 700+ knots or 1300+ kms/hr and that's good for fuel consumption figures. The company accountants are then happy to talk with pilots!

Thanks for the PM Cyclic; I responded.

Sport_Pilot

Did I say the T-6 had a fixed pitch? No, I said "of all of the fixed pitched types the RPM dropped at altitude". I didn't mean to claim these were all fixed pitched aircraft. In fact your quote has differant wording, I definately did not say "that all of them had fixed pitch". If you are going to quote me please quote me exactly.

I only mentioned the T-6 only because it was the most complex plane I have flown. Nor were these all of the aircraft I had flown. In fact I should have put the J-3 at the bottom of the list, though I never flew it at any altitude to speak of. The T-6 and a Piper Arrow are the only varible pitched prop planes. I think the T-6 was constant speed as well. I only flew from the back seat as the owner a (sorta) friend of mine flew in the front. So I did not have enough time in it to tell you how it performed at altitude.

Sport_Pilot

Per Marks Handbook propeller Static thrust is (Pi * p(rho)/2)^1/3 (cube root) * P (propulsion power) dp (Propeller diameter)^3. However, this does not seem to use a Cp or N for propeller efficiency that I would think should be part of the equasion. Not sure I understand this chapter, but I think this is the max possible thrust to use against the actuall thrust to determine propeller efficiency. Still it does change to the cube root. Dar's equation uses velocity and is a simpler F=MA type of equation. However V may change with altitude.

More later possibly next week.

Sport_Pilot

At altitude the propeller has less drag. Thus less power to turn it at altitude.

So you ignore those at higher altitudes who report otherwise? As well as the Lycoming tech article which explained the engine would no longer go past the red line? I never said the differance was huge as others have indicated.

Earlier you agreed that the RPM would drop at altitude, but because of the engine friction. So please explain the theory. I did not lay this out as a fishing lure, I really want to learn from this. So far no one has explained this sufficiently, or to my satisfaction.

pe reivers

Sport, you are hashing everything up in your attempt to make sense of things. This is not the place to discuss all existing formulae. Nor is everything written true, so I respect your ability to doubt.
Much of what you describe as reason for lower prop rpm in full scale craft is due to overly rich mixtures at altitude. Many pilots dare not adjust mixture for fear of damaging their engines. Prop rpm and power limits together with boost blow-off according to the books further cloud the issue.
There is however a very simple relationship between prop absorbed power and engine power.
Both are linearly depending on air density. The prop for the amount of torque asked from the engine to turn around in air of specific density, and the engine for the amount of torque it is able to provide due to the amount of oxigen in air of a given density. These two will keep each other in balance, provided the mixture is leaned out when at higher altitude. (at near stochastic values, but slightly richer)
Do a search on Gordon Jennings and Gordon Blair if you want to study two stroke engines. Buy the books.
If you want to study prop behaviour do a search for Jeff Lewis's propeller design. He had the theory all sorted out and put together again. It is placed in the public domain.
When done, combine the two, and you will agree with us.

lucien

[quote]ORIGINAL: wiz310
This is actually more of an aerodynamic problem than a power problem. you will by all means see a change in power available, but the wing needs to travel faster through the atmosphere to "see" the same airspeed that it would at a lower altitude. there are less molecules per square foot/inch than at a lower alt. this is the lower pressure you see up higher.
[quote]

True.. Actually, according to a quick calculation on one of the web based E6B's, I was actually truing out around 7 to 8 mph faster than my IAS. And I believe it....
This was really noticeable when I was close to the ground like on climbout and during landing. I remember in the flare I just seemed to really be screaming down the runway compared to what I was used to at sea level with the stick where it was .

Now if I can just get some more flyable weather and time I can try some more experiments

LS

rmh

When I did modified car engines - the power differences at sea level and 4000 ft and 10000 were OBVIOUS to say the least
noting what happened with the propeller equipped engines quickly made sense.
I knew the engines were rapidly loosing power and the fact that the props did NOT loose a lot of rpm showed that the power and load losses went along fairly neck n neck
Engine losses dictate that it will loose the race -----
there may still be some prop load but there will be no power to turn it.

Cyclic Hardover

My Feedback: (3)

I remember back in 78. I bought a brand new Z28- Drove it from NH to KY, then 6months later I drove it to Alaska. So I drove staright across to Colorado to visit a buddy for a coupl e days. Pull "up" to Denver and that was it. I recall getting off the interstate into a Hamburger hole and that was it. Had to get Roadside Assist to come and retune it for me. Then when i got to Alaska, Had the same thing again. What a mess.

Sport_Pilot

You will likely not get your pilots license if you do not lean the engine for cruise on your check ride. You would certainly lose points for that, though I doubt that alone would flunk you. What is not tested and sometimes forgotten about by pilots who have not taken off from high altitude airports for some time, is to lean out before take off. However, I am refering to straight and level cruise after leaning to the highest RPM. As is the Lycoming tech article BTW.

Sport_Pilot

What I think may be confusing is that it is hard to compare engine RPM at elevation. You might get a higher reading in the mountains on a cool day compared to the low country on hot day due to density altitude effects. Another factor is humidity. I suspect that high humidity would effect the engine more than the prop or wing lift, because it displaces oxygen. However I suspect this to be a very minor effect. What I have found to be more noticeable is maybe a littler more than a 50 RPM drop from a cool morning to a hot high humid afternoon. It may be less than that as I don't think my tach is stable enough to be sure.

B.L.E.

If wind drag and power are reduced equally, then a car or motorcycle should have about the same top speed on the Bonneville salt flats as it would on a sea level road, though the acceleration would be slower.

A combination of less air drag and an effectively smaller engine at high altitudes resulted in us getting 27 mpg in a vehicle that usually gets only 22-24 mpg at sea level when we crossed northern New Mexico a couple of years ago. The thinner air also reduces the engine's octane requirements which is why unleaded regular is only 85 octane over there instead of the normal 87 octane.

lucien

Of course, if you have fixed jetting like I do......... well you gotta start taking stuff apart and ordering stuff.......

I remember several times taking my last plane (which also ran a 503) up to high altitudes when I lived in TX. At about 4000', depending on the weather and density altitude at the time, the motor would start 4-stroking at certain throttle settings, typically below 1/2 throttle. That was always quite a surprise even though I knew it was going to happen...

And for what it's worth, when I moved my current plane to new mexico I had to go down something like 5 sizes on the main jets (148's from 158's) according to the chart for the Rotax motors. Thats quite a bit of leaning (the hole in the new jets is visibly smaller too).

So the altitude makes a big difference there and if you have the mixture knob, you do need to crank it when flying at higher elevations.

I'm still surprised a bit at the results with the prop, even though it makes sense to me. The power loss in the motor and the easier turning of the prop at a given rpm being pretty much neck-and-neck - I didn't know that was going to end up being the case.

Oh I should mention the power loss in my pickup is also hugely noticeable. It's got FI which seems to adjust the mixture, but its power is down. When I drive into the mountains to the ski area, which is at 10,000' here, it's like its V8 was replaced with a Datsun 4 cylinder......

LS