Propellers

Sac Arrow said:
That is, however a vacuum gauge, it measures the reverse of absolute manifold pressure. High numbers = high vacuum = low manifold pressure.

It's also a bit misleading, it just assumes low power = good economy which is generally true for a carbureted automobile, but all piston engine are most efficient at wide open throttle. In airplane terms, a high MP coupled with a low fuel flow (lean of peak condition) is far more efficient than the converse for the same power output.
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That is why I asked if he saw the relationship. An MP gauge is an "economy gauge" with the numbers reversed.

And for your second point, why then is most economical cruise at low altitude not full throttle? Anyway, I am just trying to show what an MP gauge is, not say that engine power vs. economy is the same in an airplane as a car. The conversation was not about fuel economy, it was about what an MP gauge is and how it relates to the prop lever.
 
alfadog said:
That is why I asked if he saw the relationship. An MP gauge is an "economy gauge" with the numbers reversed.

And for your second point, why then is most economical cruise at low altitude not full throttle? Anyway, I am just trying to show what an MP gauge is, not say that engine power vs. economy is the same in an airplane as a car. The conversation was not about fuel economy, it was about what an MP gauge is and how it relates to the prop lever.
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The density of the air. Down low there is enough air flowing through that with full throttle it will also take with it more fuel than it is rated for horsepower. BTW, for greatest efficiency you would go to leanest of peak you can smoothly run before reducing the throttle.
 
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>And for your second point, why then is most economical cruise at low altitude not full throttle?

It would be, if you were able to run sufficiently lean.

>Anyway, I am just trying to show what an MP gauge is, not say that engine power vs. economy is the same in an airplane as a car. The conversation was not about fuel economy, it was about what an MP gauge is and how it relates to the prop lever.

Points made and topic sufficiently digresssed. As your post count increases, so does your threadjack account limit.
 
Sac Arrow said:
It's also a bit misleading, it just assumes low power = good economy which is generally true for a carbureted automobile, but all piston engine are most efficient at wide open throttle. In airplane terms, a high MP coupled with a low fuel flow (lean of peak condition) is far more efficient than the converse for the same power output.
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Speaking of threadjacking, that reminds me, and this is for everyone, not just the OP.

You are on the xway in your 300+ HP BMW and just stopped to pay a toll. You are cruising at a legal 65 mph. What is the most economical (in terms of fuel only!) manner to accelerate to "cruising speed". Easy on the throttle or floor it?

If the overall distance matters, say the fuel used from the toll plaza to 20 miles down the road at speed.
 
alfadog said:
Speaking of threadjacking, that reminds me, and this is for everyone, not just the OP.

You are on the xway in your 300+ HP BMW and just stopped to pay a toll. You are cruising at a legal 65 mph. What is the most economical (in terms of fuel only!) manner to accelerate to "cruising speed". Easy on the throttle or floor it?
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Depends on the headwind/tailwind component.
 
Henning said:
It does... Again, another sample where things are not as clear when varibles are added.
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Yes, it is all so terribly complex. Go ahead and add any variable you want and answer the question that way. :nonod:
 
[FONT=&quot]I still have a lot to learn about this subject from a pilot standpoint. And, I forgot a lot of this from a mechanics standpoint (life's dilemma). But , from memory:

If you want a pretty good explanation on how constant speed props work, you can look at FAA Advisory Circular, AC 65-12A Airframe and Powerplant Mechanics Powerplant Handbook, a free download from the FAA site.

It’s constant speed, because once you set the RPM, if the engine has the power to make it, it stays there.

Understanding CS props and thrust can get a little deep because things change as you go from standing still to moving forward.

Almost all engine governors are based on the same principles as James Watts centrifugal engine governors (animations are on the internet).

One thing that can be confusing is the difference between PSIG and PSIA.

PSIG is a gauge pressure, zero PSI is indicated at 29.92 inches or 14.7 PSI. Automotive types use PSIG so pressure in an auto engine is a vacuum, less than 0 PSIG.

PSIA is an absolute pressure. Zero inches Hg., or 0 PSI is zero reference so in an airplane engine PSIA Manifold Absolute Pressure is always a positive number.

Good luck,
[/FONT]
 
alfadog said:
Yes, it is all so terribly complex. Go ahead and add any variable you want and answer the question that way. :nonod:
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There is an optimum point in zero wind that will neither be full nor minimal throttle. As the headwind increases so does the advantage to more throttle, tailwind favors less throttle. Thing is none of it really applies to the discussion yet you thought to interject it deflecting an argument into another direction. I just though I'd block the deflection to keep the main subject in play.

I call it "Troll Blocking".
 
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Henning said:
There is an optimum point in zero wind that will neither be full nor minimal throttle. As the headwind increases so does the advantage to more throttle, tailwind favors less throttle. Thing is none of it really applies to the discussion yet you thought to interject it deflecting an argument into another direction. I just though I'd block the deflection to keep the main subject in play.

I call it "Troll Blocking".
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There you go, bringing in the personal insults.

If you think I having been trolling in this thread you are one very confused fellow.

(Note to self. Is there an "ignore" button on this site - will have to look).
 
alfadog said:
There you go, bringing in the personal insults.

If you think I having been trolling in this thread you are one very confused fellow.

(Note to self. Is there an "ignore" button on this site - will have to look).
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Found the "ignore". Go to the offending user's Public Profile page (click on their name for drop-down list). Click "User Lists" on profile page. Add to ignore list. Do happy dance.
 
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alfadog said:
Speaking of threadjacking, that reminds me, and this is for everyone, not just the OP.

You are on the xway in your 300+ HP BMW and just stopped to pay a toll. You are cruising at a legal 65 mph. What is the most economical (in terms of fuel only!) manner to accelerate to "cruising speed". Easy on the throttle or floor it?

If the overall distance matters, say the fuel used from the toll plaza to 20 miles down the road at speed.
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I get the same number each way using simple calculations.

First- get the time for the work being done

v(f) = v(i) + a*t where v(f) = 30 m/sec, near 60 mph, v(i) = o = initial speed.

30 = 3m/sec**2 * 10sec

30 = 5m/sec**2 * 6sec

Now get the distance for the work:
d = v(i)t + (a*t**2)/2

for 3 m/sec**2 acceleration, distance = 150 m
and 90 m for the faster 5 m/sec**2 acceleration

Get a force for each acceleration:
F=m*a, let m=1kg since it's the same car (kg=kilograms)
for the slow 3 m/sec**2, F=3gm/sec**2
Faster acceleration, F= 5gm/sec**2

Work= force * distance
w= 3 kg/sec**2 * 150m = 450 kgm/sec**2
w=5 kg/sec**2 * 90m = 450 kgm/sec**2

The work done is the same in each case (unless i worked myself into a circular argument). You will need more power to accelerate faster as the faster acceleration needs more energy per unit time.

In a more practical sense, it is more complicated.

This study uses fuel consumption simulators for 15 late-model automobiles to determine how one ought to drive to maximize fuel economy. The simulation is based on extensive on-road and dynamometer testing of the 15 cars. Dynamic programming is used to determine the optimal way to accelerate from rest to cruising speed, to drive a block between stop signs, and to cruise on hilly terrain while maintaining a given average speed. The dependence of fuel economy on cruising speed is also characterized for various road grades. Findings include that optimal speeds are generally higher for larger cars and higher on downgrades than on upgrades, and that the relative fuel penalty for exceeding the speed limit is no worse for small cars than large cars. Optimal control for accelerate-and-cruise and for driving between stop signs varies considerably from car to car; in the latter case fuel economy is much improved by achieving a rather low peak speed. Optimal control on hills is consistent from car to car and can achieve fuel economy 7% to 30% better than that of constant-speed driving on 3% to 6% grades. Results that appear generalizable to other cars are reduced to advice for the fuel-conscious driver.
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Ref: http://www.sciencedirect.com/science/article/pii/0191260788900362
 
alfadog said:
Speaking of threadjacking, that reminds me, and this is for everyone, not just the OP.

You are on the xway in your 300+ HP BMW and just stopped to pay a toll. You are cruising at a legal 65 mph. What is the most economical (in terms of fuel only!) manner to accelerate to "cruising speed". Easy on the throttle or floor it?

If the overall distance matters, say the fuel used from the toll plaza to 20 miles down the road at speed.
Click to expand...

There will be some optimal acceleration rate one could calculate comparing inertial power losses to engine efficiency and it wll generally be light on the gas. But understand that in this example and the other example you gave with the airplane cruising at sea level it's always a trade off between aerodynamic efficiency and engine efficiency. Airplanes are most aerodynamically efficient at their best glide speed. Cars are most aerodynamically efficient at zero. Both car and airplane engines achieve their best BSFC at somewhere near full power. Most cars, by design, achieve their best steady state economy somewhere around 55 mph.

If you were to ask what engine to put in your BMW for the best fuel economy, it would have a whole lot less than 300 hp.
 
Sac Arrow said:
There will be some optimal acceleration rate one could calculate comparing inertial power losses to engine efficiency and it wll generally be light on the gas. But understand that in this example and the other example you gave with the airplane cruising at sea level it's always a trade off between aerodynamic efficiency and engine efficiency. Airplanes are most aerodynamically efficient at their best glide speed. Cars are most aerodynamically efficient at zero. Both car and airplane engines achieve their best BSFC at somewhere near full power. Most cars, by design, achieve their best steady state economy somewhere around 55 mph.

If you were to ask what engine to put in your BMW for the best fuel economy, it would have a whole lot less than 300 hp.
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:confused: Drag = 0 for both standing still. Perhaps you mean to say the car is more efficient because the plane has induced drag which is greater than the rolling resistance a car has?
 
Henning said:
:confused: Drag = 0 for both standing still. Perhaps you mean to say the car is more efficient because the plane has induced drag which is greater than the rolling resistance a car has?
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Well, yes except for the induced drag factor in the airplane. Get below best glide and induced drag severely hinders aerodynamics.
 
Sac Arrow said:
Well, yes except for the induced drag factor in the airplane. Get below best glide and induced drag severely hinders aerodynamics.
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Exactly, a normal car has very little induced drag, it's mostly just rolling resistance and form drag. However some high performance cars will have more induced drag than a small plane which continues to rise to terminal speed. I read somewhere that the spoiler on the back of the Veyron is eating 200hp at speed creating down force, Top Fuel dragster way more.
 
Cap'n Jack said:
I get the same number each way using simple calculations.

First- get the time for the work being done

...
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Good job on the calculation and yes, it would not make any difference except for one big factor and this is the factor that ties into our discussion of MP and fuel economy.

Taking the 3000 lb Beemer to 65 mph involves the addition of energy of the m*(v-squared) type. Energy in this case as horsepower*time. That energy is provided by a RICE. A RICE is most fuel efficient at full power. The specific fuel consumption between full throttle, and, say, 1/4 throttle is significant, prolly 30 - 50% more fuel per bhp at the lower power settings.

That is how you explain to your significant other that it is really economical to have a heavy foot as you tear away from the toll booth. Just don't spin the tires, that wastes energy, not to mention attracting unwanted attention from folks with lights on top of their cars.
 
Yep- and that's why I posted the other citation- it's not as simple as the calculations would suggest.
 
Cap'n Jack said:
Yep- and that's why I posted the other citation- it's not as simple as the calculations would suggest.
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Right, I've never calculated it, only seen it metered in the exhaust, it's there whether you believe in it or not.

Since you're a chemist may I suggest you take a sample of gasoline from the pump to the lab and see what's in your fuel for real rather than what's supposed to be there by law?
 
Henning said:
Since you're a chemist may I suggest you take a sample of gasoline from the pump to the lab and see what's in your fuel for real rather than what's supposed to be there by law?
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What does that have to do with this thread?
 
Cap'n Jack said:
Yep- and that's why I posted the other citation- it's not as simple as the calculations would suggest.
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I cannot access that paper but they seem to be talking about accelerating for a short run between stop signs. That would be a complex problem and it seems that it varies widely from auto to auto based on that bit you posted. My premise is simpler - what it the most economical way to get to crusing speed for a long run at speed. My suggestion is that the most fun way is the most economical fuel-wise, which is a bit counter-intuitive.
 
alfadog said:
Good job on the calculation and yes, it would not make any difference except for one big factor and this is the factor that ties into our discussion of MP and fuel economy.

Taking the 3000 lb Beemer to 65 mph involves the addition of energy of the m*(v-squared) type. Energy in this case as horsepower*time. That energy is provided by a RICE. A RICE is most fuel efficient at full power. The specific fuel consumption between full throttle, and, say, 1/4 throttle is significant, prolly 30 - 50% more fuel per bhp at the lower power settings.

That is how you explain to your significant other that it is really economical to have a heavy foot as you tear away from the toll booth. Just don't spin the tires, that wastes energy, not to mention attracting unwanted attention from folks with lights on top of their cars.
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Not to be nit picky, but he did allude to the fact that increased acceleration consumes energy. I didn't critically go through his calcs no but the bottom line conclusion is very dependent on vehicle specifics. In a light vehicle you want to accelerate quickly to get in to a more efficient engine regime. In a heavy vehicle you don't want to waste excessive energy on inertial losses. In either case heavy transient accelerations are inefficient from the engine's standpoint.

Just tell your wife a moderate acceleration to 130 mph is efficient and time saving, and the money saved on gas will finance that ten foot Easter bunny thing she's wanted to get for her nephews for the last ten years.
 
alfadog said:
I cannot access that paper but they seem to be talking about accelerating for a short run between stop signs. That would be a complex problem and it seems that it varies widely from auto to auto based on that bit you posted. My premise is simpler - what it the most economical way to get to crusing speed for a long run at speed. My suggestion is that the most fun way is the most economical fuel-wise, which is a bit counter-intuitive.
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Researching your question, I've seen some links that suggest.

The paper I cited also discusses efficient speeds going down hill, and the effects of going fast (beyond the speed limit). I'll try to get a PDF, since they seem to cover a lot of situations. I just can't do it from here.
 
I just wanted to understand CSP's. Not pay a toll, ride a bike, or a beemer. Just fly a HP aircraft and not damage the airplane. :D

There is NO WAY the average pilot understands all of this. :confused:

I have several articles, several links, and a great discussion.

I will drop back by this thread in a week and try to explain what I have learned. :idea:

Thanks everyone,

Terry
 
Cap'n Jack said:
Researching your question, I've seen some links that suggest.

The paper I cited also discusses efficient speeds going down hill, and the effects of going fast (beyond the speed limit). I'll try to get a PDF, since they seem to cover a lot of situations. I just can't do it from here.
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Here is how Wikipedia puts it:

"Fuel efficiency varies with the vehicle. Fuel efficiency during acceleration generally improves as RPM increases until a point somewhere near peak torque (brake specific fuel consumption.[20]) However, accelerating too quickly without paying attention to what is ahead may require braking and then after that, additional acceleration. Experts recommend accelerating quickly, but smoothly.[22]"

In my scenario that is no upcoming need to brake so rock and roll! I was not making this up .:D
 
Terry said:
I just wanted to understand CSP's. Not pay a toll, ride a bike, or a beemer. Just fly a HP aircraft and not damage the airplane. :D

There is NO WAY the average pilot understands all of this. :confused:

I have several articles, several links, and a great discussion.

I will drop back by this thread in a week and try to explain what I have learned. :idea:

Thanks everyone,

Terry
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Terry, I tried to keep it to the basics for you. I think any pilot that flies a complex aircraft needs to understand at least the amount of material I presented. As far as the Beemer, was it you that said to lighten up or was that someone else? We is just havin' some fun. :D
 
alfadog said:
Terry, I tried to keep it to the basics for you. I think any pilot that flies a complex aircraft needs to understand at least the amount of material I presented. As far as the Beemer, was it you that said to lighten up or was that someone else? We is just havin' some fun. :D
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I think it was me.

I am good. But seriously, I need time to read and study the information.
 
Ron Levy said:
No, it's kinda typical. As any CFI knows, not every trainee has your astonishing, instantaneous grasp of everything.
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He's not anybody, he's a mechanical engineer and this is mechanical engineer 101 stuff in my opinion, it's basically 150 year old technology, I would think a degreed mechanical engineer would have enough education that they would be able to work the system out by looking at it. There's only one way to do this mechanically really.
 
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Henning said:
He's not anybody, he's a mechanical engineer and this is mechanical engineer 101 stuff in my opinion, it's basically 150 year old technology, I would think a degreed mechanical engineer would have enough education that they would be able to work the system out by looking at it. There's only one way to do this mechanically really.
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He's also many years from his degree. Time does take it's toll.
 
PFM, Porsche F-ing Mooney, real POS, you couldn't make power with the thing.
 
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