The A2A Simulations Community

Hello.

I am very happy to join your community after reading many topics here ...
I would like to thanks the developers of this amazing addon Spitfire, and also participants in this forum, which represents a very rich source of information.

I learn to use my new MK1, (with variable pitch de Havilland airscrew and three blades), and I take great of pleasure to fly over the Alps.
This is a different experience each time to take off, fly and reach land.
It's magic, so realistic!

There's something that I do not understand anyway, and i think that maybe can you help me:

When I fly at high altitudes above 20000 feet, it is impossible to maintain a low airscrew speed in cruise.
I would get about 2000 rpm, but I can not have less than 2600 rpm.
I do this:
Take off with fine pitch, and moderate throttle (+1 or +2 psi), and once reached 160 mph, I increase progressively the pitch to get 2600 rpm with +2 psi of boost. All is well, IAS: 160 mph. I continue my ascent by adjusting the boost and pitch as the atmospheric pressure decreases.
But at a certain altitude, the airscrew appears accelerate, and boost pressure becomes low!!
Rpm increases even though I still increasing pitch (full coarse). I find it's impossible to cross over 20,000 feet with a low airscrew speed of 2000 rpm.

For example, at 30000 feet, my Spitfire settings are:
2600 rpm, boost pressure: -4psi (full throttle), oil at 73°, coolant at 83° and an IAS of 200 mph ... It seems correct?

What do I not understand?
Thank you, and cordially

This behavior is realistic. The reason the prop is accelerating at higher altitude is that the thinner air just doesn't put as much load on the blades at maximum pitch. Normally under these conditions the governor would increase prop pitch to compensate but at this point the de Havilland prop has reached its limits. The Rotol unit has a much broader range of pitch which is why it can hit 3000 RPM on the takeoff run as well as maintaining very low RPM at higher altitudes.

Oh, and on a completely separate note, the Merlin likes boost. You can use at least 8 psi of boost for takeoff in the Mk I all day every day without fear of harming the engine and cruise on 100 octane can be as high as 4 psi for the duration of your flight. I usually climb around 7-8 psi.

_________________

First, let me thank you for taking the time to answer me...

I am reassured to read that the behavior of the airscrew is normal at high altitude.
I suspected that the low air density should play a role.
And I seemed to read on the web, pilot's reports tests who related flights with 2,000 rpm at 30,000 feet, but it must be with the unit Rotol, more efficient.
Could you please give me for my knowledge the range of pitch in degrees on DeHavilland and Rotol too?

And thank you for the advice of the extra boost, I'm too shy with power, but I can't manhandle this prestigious machine.
I actually use a 100 octane fuel with my Mark1, which gives me the red index (max pressure) gauge at 4 psi... I never approach it except acrobatics!
As to achieve a takeoff speed to take off not too long, 3 psi were enough for me. Then maintain a climb speed of 160 mph, 2 or 3 psi were enough for me too.
So now I know that Merlin appreciates the boost, I will force some of the load.

You say 8 psi of boost for takeoff?
I'm having some trouble finding the time I have to return the control column spade grip to the rear of the neutral once the aircraft reaches 30 mph, at the same time i need to increase the power...
I feel that there's more power there, the harder it gets! Too soon: it may crash the propeller on the track, and too late: the aircraft may take off before taking his horizontal attitude, with the risk of stall... Delicate work.
But I will try as you say, with more intake pressure.

Again thank you very much for your valuable advice.
Sincerely

Haha, yeah, taking off with a powerful tail dragger is a tricky business sometimes, you just gotta be smooth with it. That redline of 4 psi with 100 octane, however, is just the max recommended cruise setting (wikipedia mentions full power of the Merlin II and III on 100 octane at +12 psi!). In aerobatics you can really go to the end of the dial, the Spit's engines could run more boost than you could read on their gauges for the first few generations of planes.

_________________

Thank you Sergeant.
Your knowledge on the Spit is truly remarquable.

You are right, +12 psi maximum for brief moments like takeoff, and climb: +6.25 to 2600 rpm.
The challenge is to read the pressure when opening full throttle, the needle stops at the mark-4psi, which really corresponds to 5 psi ... How can I do after what real boost is applied beyond 5 psi?
I'm really shy.

A question comes then, is it appropriate to reduce the power, when after takeoff, I will increase the propeller pitch to reduce rpm?
An example: I take off with +6.25 psi of boost and 3000 rpm.
Once 140/160 mph reached (undercarrier up), I want to set up to climb power.
Normally, I have slightly to reduce the boost before change the pitch to 2600 rpm, and then applying +4 or 5 psi of boost, in order to not exceed the maximum torque.
But if my Merlin can support +6.25 psi at 2600 rpm, why not go directly to the right rpm, without decrease power juste before?
Although I feel that I would never inflict such a thing to my poor Merlin...

But in this case where I am a good student , what strategy to adopt when one wants to reduce the rpm?
This little following rule could be pertinent: reduce power before rpm, in the same ratio?
An example:
My set up after take-off is 160 mph, 3000 rpm and 6.25 psi.
I want to switch at 2600 rpm.
1) I decrease my absolute intake pressure (14.5+ 6.25) / 3000 * 2600 = 18 absolute psi. (14.5 for atmo pressure at sea level)
18-14.5 = + 3.5 psi of boost.

2) Once my boost to 3.5, I can quietly increase speed to 2600 rpm, keeping a constant torque in my mind.

3) Ajusting slightly power to get a correct couple rpm/boost
What do you think about?

Thank you for supporting these banalities of a beginner. And sorry for my bad English, I should have learned better in school ...

When INCREASING power and RPM:

Pitch, then Power.

When REDUCING power and RPM:

Power then pitch.

Remembering that once "set" at a pitch angle, you have to treat the airscrew as a "fixed pitch" prop, the your procedure in the example is spot on.

RPM affects boost slightly and boost certainly affects RPM..so there is a little "fiddling" to do to settle down at your chosen boost/RPM combination

The DeH takes a little care and practice to master but once you get the idea (and you are well on the way!) she is a joy to handle.

Oh, and if you are only going to use 3.5 lbs per sq in boost, try switching to "weak mixture" as well, you'll get better range.


Darryl

_________________

...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Thank you too Darryl, I think I understood!

I spoke to +3.5 psi, just the time to increase the pitch, and then increase to 5 psi. I was wrong.
But I understand, it's easier than I thought.

If I understand correctly (hum), when I want to leave the takeoff configuration to make transitions to that of climb, the goal is to change the twice rpm/power while keeping the same torque.

Torque: intake pressure (power) multiplied by pitch?
Decrease the power is equivalent to decreasing the torque.
Pitch increase (or decrease rpm) is equivalent to increase the torque.

So I must enchain my two actions so that the torque decreases temporarily by a lower value before recovering, and not vice versa (increase before recovering).

Example: My set in 3000 rpm / +6.25 psi.
So I power down a little below +6.25 psi, approximately +5 psi, and then I increase the pitch to get 2600 rpm.
Same reasoning for the cruise, I drop to 4 psi, and I increased the pitch even if the regime wants to go lower than 2600 rpm ...

Conversely to the descent, when I want to adopt the landing configuration, I must ask: what does reduce torque in the first?
And it's naturally a drop of pitch!
So I increase the rpm with fine pitch, and then the boost that is adjusted upward (with short bursts of full power to decongest after descent with low throttle?) ...

Do I understand, Chief Darryl?
Note: But I did not understand what makes the mixture weak in climb?

Thank you very much all for your valuable advice.
Cordially

Power is a factor of torque (which is proportional to intake pressure) and RPM (lb-ft of torque multiplied by RPM divided by 5252 gives you horsepower), prop pitch is only a factor in controlling RPM. As such, if you reduce your boost while holding constant engine speed you lose power because you lost torque. Conversely, if you lower your RPM but maintain boost you lose power but (theoretically) maintain torque.

It's a little more complicated than that in practice because a closed throttle plate causes a flow loss that costs you power meaning that 6 psi of boost with a closed throttle will not make as much power as 6 psi of boost with an open throttle at the same RPM, despite using the same amount of fuel. Because of that it is best to keep the throttle as open as possible during cruise without exceeding boost limitations, to do that you have to have a good combination of altitude and RPM.

_________________

Thank you Sir!! I was not aware of that....but it certainly explains a large part of the "LOW revs HIGH boost will bring you safely home to roost" philosophy which is SO essential to good Spitfire operation. (hint hint to all of you out there who go doodling around at -2 and 1800 rpm!)


Licancabur,

First off, I thought you were saying 3.5 in the cruise, sorry..climb should not be performed at anything less than 4, you are correct...please DON'T climb with weak mixture!!

For climb, the goal is to get the boost and rpm to settle where you want them. This may require a bit of double handling, yes... but with experience you can allow for the boost drop when reducing rpm and therefore avoid the "second bite at the cherry". So, for example, if you want to climb with 4, you may reduce boost to 4.5 THEN reduce rpm to 2600 and find that the boost drops back to 4. this would avoid the double handling. I can't quote the exact figures/changes because I very much fly the DeH (or more correctly changes to power and prop) by "feel" rather than follow hard figures. Airspeed, engine health and a few other factors will vary the amounts of drop in boost with rpm drop etc..but in the end, what you are chasing is a climb of +4 at 2600rpm (note, I NEVER climb at less than 6 1/4 lbs per sq in, personally...the time to height improves by doing so, as does ultimate fuel economy. Temp management is not the problem for me that some people say it is for them. I use "normal" plus one (radiator position 3) on a cool day or if I "get off" relatively cool and normal plus two (rad pos 4) if it is hot or I get off slowly/hot. Usually by 15,000ft the radiator can be brought back to pos 2 "normal".

For a high power, maximum rate climb (this is performed at some 15-25mph slower IAS than "recommended" climb) I use between normal plus 2 and fully open (rad pos 6), actively adjusting it as I climb, usually closing it to "normal"on a progressive basis and by 20,000ft.

On descent clearing the engine is vital, yes.

And ANY time the engine is operated below 2150rpm for any amount of time, it should be cleared at 2600rpm for 1 minute in every 20 minutes.


The DeH is a real skill to master and many will fly her and not even realise that they could do better as if you fly her as a two pitch she will perform perfectly well (after all, it IS what the DeH was designed for!!). But used as a variable pitch there is great scope. The subtle differences and advantages between settings are not always noticable unless you are specifically paying attention...this is what the real pilots quickly learned to do ....

Darryl


[EDITTED due to reversing low and high!!!! ]

_________________

...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

You mean LOW revs HIGH boost?

_________________

Uggg...............editted...yep, you got me!!!!!!! How many *&^&*%$% times have I quoted that phrase?? and NOW I reverse it

D

_________________

...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Haha, no biggie, that's what we're here for, proof reading each other's work!

_________________

Ah well ... I better understand "Low Revs and High Boost".

It must be said that my English is poor and your last answers gave me much trouble...
Thank you both for your time and expertise on this topic.
I'll try to recall, if you allow, some hard points for me:

Sergeant, I do not understand in your example, how it's possible to obtain +6 psi of boost and closed throttle. I understand that we can have little boost with full throttle (in altitude), but the reverse? ...
"Conversely, if you lower your RPM goal malade boost you lose power goal (Theoretically) malade torque" ... This disturbs me, intuitively I would think that the torque increases ...
I can not to relate specifically to them the concepts of torque, power, rpm and throttle ...
I perceive without really understanding the reason, that it's necessary, within the limits of tolerance, to focus on high power (boost), even for cruise.

Chief, I understand the trick: to anticipate the drop of power due to increasing pitch before decrease rpm. That requires practice to anticpate correctly, sure!
But why climbing with +6.5 boost allows to save fuel? And what is your usual climb speed with +6.5?
I remember to have read that we must obtain, regardless of altitude, a target speed of 200 mph IAS ... But it's possible with low boost, not with high boost.
For my part, with +3 psi to 20,000 feet in cruise, I reach a speed of 250 mph IAS.

Thank you again for your patience.
Even if I give the impression of not assimilate everything, I read each of your answers with passion, hoping to improve my skills.
Cordially

Sorry I am Australian and we tend to use a lot of slang/colloquialism without even realising (my sister in law, a German, speaks perfect English... but had a devil of a time understanding us when she first moved out here!!). Your English is fine!

6.25 lb per. sq. in. allows you to get to altitude quicker where you can cruise at a much lower temp, rpm and at a higher ground speed than at low level...so, as I said, ultimate fuel economy is better The actual climb itself, at 6.25 uses more fuel than a climb at lower settings BUT at lower settings you are climbing for longer. There is a break even point, just as there is in a car. If you travel for 400km at 20kph, the fuel economy is worse than if you travel at 80kph...but if you do the same thing at 160kph, the fuel economy is worse than at 80kph. In many cars 80kph is about the optimum. I fly long distance and the high cruise altiudes are better. In the Spit getting to cruise alt of 20,000ft as quickly as possible is about the optimum.

Now, usual climb speed....it is the "normal" published speed for each height range (these are in the manual). The critical thing with performance is to set up your desired profile and then take the performance that RESULTS from these settings.

So you set 2600 rpm, +6.25 Boost, 180mph (while, say, at low level) and TAKE the climb RATE that that gives you. The trick is to know WHICH figure is to be left as "resultant". Usually performance "goals" are stated as being a set of figures..eg "cruise at 200mph at 10,000ft and 2600 rpm" Ok..we are told (or want) 3 figures...now we just need to know the thing that will give us those..and of course that is boost. Another day we may want to cruise at 15,000ft...so you will find that something must change...if you still NEED 200mph 2600 rpm, then obviously boost will change ...but if you want to use the same BOOST as you used at 10,000ft then something else must change...you can change rpm or speed.

The "resultant" is usually (but not always) the figure that will change with altitude.

You will often see cruise parameters stated as x boost and y Rpm, without explicit mention of SPEED..as that will change with alt.

Likewise, climb settings will usually say x mph, y rpm, z boost..without explicit mention of RATE ...same reason.

Descent stated as x fpm rate, y rpm, z speed...boost being the one you manipulate to maintain those...same reason (but also changes with weight etc in this case)


Finally, target speed 200mph.....well, not MY target. That is really the minimum speed you want for good cooling but again that depends on altitude. We often cruise at 180 ASI up high (20,000ft) where it is cold, if we are just flying formation for fun. At 33,000ft you couldn't cruise at 200 ASI even if you wanted to. If I am flying one of my photo recon missions (at 500ft below the condensation trail level of the day) I NEVER cruise at less than 240mph.


best regards


Darryl

_________________

...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Well, it's the exact opposite condition, really. Yes, if you fly high enough you will not be able make positive boost even with the throttle wide open, however the inverse is also true, if you fly at low level (sea level) the throttle does not have to be anywhere near wide open to give you max boost (think take off conditions). Yes, you're giving it considerable throttle to achieve take off power, but the engine is still far from wide open and at climb power it is even more closed. So basically, at any given RPM, to maintain boost during your climb you must open the throttle more and more. This means at the start of your climb (low altitude) you are making slightly less power for any given boost level than you are at rated altitude (the maximum altitude you can achieve peak boost) because while boost and RPM stayed the same the whole time you were also opening the throttle as you gained altitude. The difference in power is not much but it is something to keep in mind when setting cruise power at various altitudes since you burn roughly the same amount of fuel in either condition but you make that little bit more power at wide open throttle, thus fuel economy improves.

Another thing to think about is how cold it is at high altitude. If you run your engine at high RPM the water pump is spinning fast and moving a lot of coolant through the engine and at low boost the engine is not making as much heat per revolution. Combine the two with the cold outside air and it becomes difficult to maintain coolant temperature even with the radiator closed. With high boost/low revs you are making more heat per revolution and slowing down the coolant flow to keep temps up. Not sure how advisable the procedure is with these aircraft but as far as the math is concerned you actually want your coolant temps higher when cruising than when climbing for maximum efficiency. New Audi 1.8 liter TSI engines will actually cruise at 107º C and accelerate from 85º C or so all the way up to the aforementioned 107º C.

Oh, and torque output follows a curve. Yes, you might get more torque at lower revs with the same boost, but then again, you might not... Depends on numerous design factors and is difficult to predict, especially without any real test data.

_________________