rpm's

[Fianna]

what is the difference between lowering the throttle and lowering rpm's?

what effects does each one have on the performance of your plane?


thanks.

[TinmanX]

Lowering RPM's reduces fuel consumption. Lowering throttle doesn't

[GunnerCAF]

Originally posted by TinmanX
Lowering RPM's reduces fuel consumption. Lowering throttle doesn't

It's a good question and I don't have an answer.  Both lowering throttle and lowering RPM will reduce fuel consumption.  

Gunner

[OOZ662]

Literally, reducing RPMs is a whole different story from bringing down Manifold Pressure (Throttle).

Manifold pressure is how much the engine is working. When you push down on the gas in your car, you are increasing manifold pressure. Same thing with the plane.

RPMs have nothing to do with the engine. This adjusts the pitch of your propeller to take larger or smaller "bites" from the wind. Higher RPM means that the props are at more of an angle and provide more thrust. Putting them at a lower angle makes them more flat toward the air and results in less thrust.
I'm guessing (and it's a blind guess) that the reason fuel useage is lowered because less pressure is being applied to the engine from the prop when at low RPMs.

Raising either adjustment will increase the speed of your aircraft but will lessen your range. Lowering either will have the opposite result.

[Pooface]

well, your throttle, like in almost any engine, controls the amount of fuel that flows into your engine cylinders, the more fuel, the more energy, the more power.

now, rpms, revolutions per minute.

the rpm's are a measure of how fast the engine shaft, and therefore the propellor is moving. now, the only thing that controls the engine is the throttle. the reason that we can lower the rpm's in a plane is because we can change the angle of the propellor. if we tilt the propellor more, we can take more of a 'bite' of the air, and push the same amount of air backwards with less power, which means our fuel consumption goes down


normally, when you increase the throttle, you increase the rpm and vice versa, however, in a, we dont actually have propellor controls, just an rpm control. to be honest, im not entirely sure how it works.



but the basics are these:

throttle = fuel into the engine

rpm = revs per minute





the reason that pulling back rpms saves a lot more fuel is that when you reduce the rpm, you are also reducing your manifold pressure (watch next time you do it), and if we decrease both we are making a far larger saving on the fuel.



basically, if you only reduced the fuel flow, and kept the prop pitch quite low, you would be using lots of energy (fuel) to keep the rpms constant. if you adjust the prop so that you dont need to use as much fuel, and still get the same power, it's far better for you and your engine



that's kind of it, and im not a great fountain of knowledge on engines, so it's hard to explain, hope that i've helped in one way or another:aok

[GunnerCAF]

So are you saying lowering the throttle and lowering RPMs both reduce fuel consumption?

Gunner

[Pooface]

Originally posted by GunnerCAF
So are you saying lowering the throttle and lowering RPMs both reduce fuel consumption?

Gunner

well, lowering the throttle means less fuel flows into the engine.

if we had direct control of our prop pitch, then it wouldnt reduce fuel consumption if you changed the rpms. the reaons rpms make such a difference in AH, i think, im not entirely sure, is because out rpm controls are linked to the throttle also, so we are reducing the fuel we need to use.

i didnt program the game so im not sure how it works.


but really, the question, simple answer, rpm is the revolutions of the prop every minute, throttle is the amount of fuel used:aok





im really not entirely sure about the whole rpm stuff gunner, im not a big engine buff, and im not sure how AH does it. maybe someone else knows more?

[OOZ662]

Yes, reducing RPMs will lower your fuel useage, as will lowering the manifold pressure.

[Schutt]

In Ah2 most planes have a constant speed prop, which means that you do not select the prop pitch directly but the rpm you want and the prop then automatically adjusts the pitch so the engine stays at the selected rpms. The props can only adjust within a certain angle, so on some planes you overspeed the engine in dives and cant reduce rpms when fast (since prop already at max pitch)

Lower rpms with a higher prop angle gives more air pushed per revolution. This means for the same thrust the prop blades move less distance through the air.

The engine has a power curve just like car engines, so at a specific rpms it has the most power. But with an old engine its a little bit more straightforward, so roughly highest permittable rpm = highest power.

Since mainfoldpressure is generated by a charger reducing rpms sometimes will reduce mainfoldpressure too.

So you can reduce engine power by 2 means.
a) reduce mainfoldpressure
b) reduce rpms

When you reduce rpms the propellor gets more effective since it has less total drag to overcome, but at reduced rpms you are not at best engine horsepowers. The prop pitch is increased to get lower rpms.


When you keep the same rpms and reduce mainfoldpressure the engine will run at reduced power too but at the same speed, which makes it more responsive to add throttle again. The prop pitch will automatically reduce to stay at the same rpms with less power (reduced prop angle reduced resistance)

[GunnerCAF]

You had a good explaination Pooface, I'm just kidding you about your longer answer  

In AH, when I look at the E6B clipboard, lowering RPM and Throttle (manifold pressure) will both result less fuel consumption.  I think of reducing RPM (incresing pitch, bigger bite of air) is like overdrive.  Once you are level at altitude, the loading on the engine can be reduced to maximize your range.  I use the settings are listed on the E6B when I am trying to get max range.

[Widewing]

Gentlemen, this is really simple. Reducing throttle reduces fuel consumption. Reducing RPM reduces fuel consumption as well, but not nearly as much as by pulling back throttle.

Here's an example. With fuel burn set at 1.0 (normal), a P-38G with 50% internal fuel, flying at 1,000 feet.

Max power, 47" MAP, 3,000 rpm: Fuel consumption is 310 gallons per hour (GPH) for a max endurance of 28 minutes.

40" MAP, 3,000 rpm: Fuel consuption is 239 GPH, max endurance is 36 minutes.

35" MAP, 3,000 rpm: Fuel consumption is 193 GPH, max endurance is 44 minutes.

35" MAP, 2,300 rpm: Fuel consumption is 175 GPH, max endurance is 49 minutes.

For MA use, cut endurance times in half (2.0 fuel burn). If you take 100% internal fuel, the above endurance times are applicable to the MA.

The simple act of reducing manifold pressure by 7" adds 4 minutes to MA endurance.

Be sure to use the E6B tool (available on the clipboard while in flight) as this provides fuel range at your current power setting. It also tells you the fuel burn rate in GPH.

As soon as I take off, I open E6B and leave it open for the whole sortie. Anytime I want to look at the map, I pull up the clipboard and I can check the MAP and my fuel and range status at a glance.

E6B is a valuable tool, be sure to use it.

My regards,

Widewing

[Schutt]

For sake of comparison i use a P38G too. I used 50% fuel and 1k altitude, measured the lvl speed. I list the resulting miles per gallon for the better combo only.

I did set full rpm with reducing mp and reduced rpm with reduced mp so they use the same gallons per hour. The combo that gives a faster speed is more effective. since i did not test diffrent reduction i cant say if i did the best possible reduction, might be another reduced rpm setting is better.

35 MP-3000rpm : 291mph -  1,46 mpg (198GPH)
40 MP-2000rpm : 282mph

32 MP-3000rpm : 279mph
35 MP-2300rpm : 280mph - 1,61 mpg (173GPH)

28 MP-3000rpm : 225mph
31 MP-2000rpm : 250mph - 1,85 mpg (135GPH)

24 MP-3000rpm : <130mph
28 MP-1500rpm : 189mph - 1,89 mpg (100GPH)

1.) So in the p38G at 1k alt when you pull back MP to less than 32 it pays off to reduce rpm as well since that gives you better speed for the same consumption.

2.) If only thing that counts is the distance coverd flying at 28MP-1500rpm gives you the biggest range at 1k altitude, from the combos i tested. I have no clue if the optimum is at another setting, but the ones tested that is best.

3.) From other planes i ran through reducing rpm measurements and this test i think it is save to assume that if you want long cruising range at medium to low speed at a certain altitude reducing rpms and mp gives best range.

4.) If the altitude is not set climbin with mil power to altitude high above FTH and then setting cruise setting gives a lot more range than flying at 1k, but you need to go pretty high. Some planes its 25k others its over 27k.

5.) i can not tell which setting is the best range wise. might be higher or lower than a 100GPH setting, might be at diffrent rpm than listed. Maybe i do some more testing later, but since im not real intrested in p38G at the moment with another airplane.

6.) These measurements show the way it is modeled in ah2. It might be diffrent in real world application.

[Widewing]

For the P-38L, Flight Operation Instruction Chart. Zeno has it posted, so I don't have to scan the manual...  



My regards,

Widewing

[Schutt]

According to the chart and the e6b our p38G is pretty thirsty at 1k altitude, if i read that correctly, or is it due to the diffrent engine in the 38G?

EDIT:

Checked it out, ah2 p38l seems to be exactly like in the charts but p38G is thirstier at lower throttle/rpm settings. You dont have that chart for the G too somewhere?

[Widewing]

Originally posted by Schutt
According to the chart and the e6b our p38G is pretty thirsty at 1k altitude, if i read that correctly, or is it due to the diffrent engine in the 38G?

EDIT:

Checked it out, ah2 p38l seems to be exactly like in the charts but p38G is thirstier at lower throttle/rpm settings. You dont have that chart for the G too somewhere?

I don't have a chart for the P-38G. I have them for the H, J and L. All three of those have the same consumption rates for MAP@rpm.

Perhaps, there is problem with the fuel burn tables used by HTC.

My regards,

Widewing