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PostPosted: Mon Feb 14, 2011 7:36 pm 
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Killratio wrote:
ESzczesniak wrote:

3. Did you have any head wind? If you have any substantial head wind increasing airspeed over the wings (and through the radiator), your engine can put less power in to forward airspeed and more power in to the climb. If you using real world weather, you can find pockets with 60-80 kts winds aloft started just a few thousand feet off the ground.



An aircraft flys in the relative wind. The airspeed will not vary (except for short term..instantaneous..windsheer or gusting) with the "headwind". The only difference in performance with wind will be the ground speed. Climbing into a 10 knot wind will give exactly the same fpm climb performance as climbing into a 200 knot wind. What will vary is the distance covered across the ground.

Darryl


Post deleted: I need a chance to work out the math. I think I'm approaching this from pure physics while your approaching it as a pilot. Ultimately, I think these both work out to the same end point, but explained with different objects of reference(as long as we note that I was not refering to guage "airspeed", but airspeed over the wings), but I need a chance to work this out first.


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PostPosted: Mon Feb 14, 2011 9:49 pm 
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Location: The South West of the large island off the north coast of Tasmania
You have me at a disadvantage..the only interest I ever had in physics was how far a 3lb physics book could be thrown from my highschool balcony.

However...if an aircraft in a 100 knt cruise is flying into a 30 knt headwind, the air is flowing across the wings at the equivilent of 100knots....I say equivilent because naturally the air above the wing is moving faster than the air below because of the aerofoil section. The groundspeed will be 70 knts. If that wind then increases to 200 knts, the wing will be moving through the air at 100 knts. A fixed power and relative speed is putting a fixed amount of air to the rear in a given time. What will change then is that the groundspeed is now -100 knts ie, you are flying backwards.

An extreme example above...but I have flown a C152 "backwards" at about 10 knots which REALLY freaks out the people on the ground!!

So climbing into a stiff headwind the aircraft will appear, from the ground, to be climbing much more steeply (despite nose angle) but relative to the airmass in which it flys, nothing has changed,

regards


Darryl

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PostPosted: Tue Feb 15, 2011 12:56 am 
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ESzczesniak wrote:
Post deleted: I need a chance to work out the math. I think I'm approaching this from pure physics while your approaching it as a pilot. Ultimately, I think these both work out to the same end point, but explained with different objects of reference(as long as we note that I was not refering to guage "airspeed", but airspeed over the wings), but I need a chance to work this out first.


Quick note - there's a reason it's called Indicated airspeed on most gauges in planes, and Calibrated airspeed on rarer occasions. They simply measure the apparent airspeed through the dynamic static port (aka pitot tube) and convert it into a speed. There are various reasons it can be off, but as far as the airplane is concerned, whatever is said on the Airspeed gauge is what the airplane thinks it's doing, regardless of anything else going on around it. True airspeed simply takes Calibrated airspeed and adjusts for temperature and is the actual speed at which the airplane is moving through the air around it. This is only really useful for one reason - critical airspeeds and calculating ground speed without the presence of a drift meter or RNAV equipment.

When you consider aircraft performance, everything is set based on the indicated airspeed as that's what the aircraft is experiencing, so 100 knots Indicated into a 100 knot wind will still result in the same performance from the airplane as 100 knots Indicated into a 100 knot tailwind. The difference is how fast you're going across the ground (as was said before).

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PostPosted: Tue Feb 15, 2011 8:34 am 
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Joined: Sun Feb 24, 2008 9:33 am
Posts: 1035
Location: Germany
Yesterday I made my first flight in extreme heat, using the MkIIa, and I must say it wasn't much of a problem. The flight took me from Edwards AFB to Nellis AFB and I used archived real weather from August 1st of last year.

The OAT at Edwards was 35°C (95°F). I performed a normal start and runup (checking magntos and cycling the prop pitch lever) and had a long taxi to the runway (1.3 miles!). Because of the length of the taxiway my radiator temperature was just a tad above a 100°C when I reached the runway, despite my fast taxiing to keep the airflow up. Even though it was slightly exceeding the recommended take-off limit I decided to go through with the take-off anyway. Total time on the ground from engine start to take-off: 8 minutes.

After take-off the temperature slowly crept towards the dreaded 120° limit but I could counteract that by step climbing with a bit of low power/low RPM/open radiator level flight between climbing stints. Using that technique I kept the temperature below 115° until I reached cooler altitudes where the engine temperature wasn't an issue anymore. What followed was an uneventful but nice 50 minute flight at 20,000 feet, with Death Valley passing by on my left.

Soon I hit Nevada and it was time to descent. I did so with less than 2,000 feet per minute in order to keep the prop from overspeeding/windmilling and the engine from cooling down too fast (it was at a comfortable 85°C during level flight). After passing over Las Vegas and having a good look at The Strip I lined up for final approach on runway 3L at Nellis AFB with my radiator flap full open. The OAT at Vegas was even a bit hotter than at Edwards, 37°C (99°F), and the radiator temperature slowly crept towards 100° again when I was about to touch down.

Immediately after touchdown I raised the flaps and another 1.4 miles (5 minutes) of taxiing followed. When I finally shut down the engine at my parking spot the temperature was a tad above 115°C but never hit 120. The oil temperature never went higher than 85°. The following engine inspection showed everything to be in tip top shape and I was very pleased with the performance of my trusty Spitfire in hot weather. :D

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Last edited by Jigsaw on Tue Feb 15, 2011 9:25 am, edited 1 time in total.

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PostPosted: Tue Feb 15, 2011 9:11 am 
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Nice one Patrick. I must admit, with the help of Darryl on here, I haven't had a temps problem either. Very good advice on here.

I did however have a major setback yesterday. I've trashed my lovely spitfire because of a terrain mesh issue in FSX. I couldn't see it, but the airfield dropped off at the end of the runway straight down. The Spit went off the end and dropped about 6 feet, smashing the landing gear, and bending up the prop. I'm suing the airfield manager for not posting any signs :lol:

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PostPosted: Thu Feb 17, 2011 8:12 am 
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is there any difference in the accusim engine between the merlinIII and merlinXII engines "progress of the wear" codes?

meaning with same well under the max power usage (like max 2-3psi,same fuel)


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PostPosted: Fri Feb 18, 2011 9:35 am 
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He He another question good people 8)

When on very high altitudes like 25,000, 30,000+, where the boost is pretty much low, i tend to have very high rpm becouse of lack of air that would stop the engine from spinning. The question is : i heard that very high rpm and low boost don't go together, that it would stress the engine, but after checking, there is no visible wear, so is it possible that engine will not brake apart in this situation?

PS: there was a situation, when after a startup i forgot to turn off an electric starter in MKI and flew for an hour or so , and being uncertain of the way it works, im asking if it should stay together or brake apart after a while, becouse after checking there was no problem with it.


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PostPosted: Sun Feb 20, 2011 7:20 am 
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Senior Airman

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Caldemeyn wrote:
there was a situation, when after a startup i forgot to turn off an electric starter in MKI and flew for an hour or so , and being uncertain of the way it works, im asking if it should stay together or brake apart after a while, becouse after checking there was no problem with it.


Do you mean the starter magneto? If so I don't think it would do any harm

Caldemeyn wrote:
When on very high altitudes like 25,000, 30,000+, where the boost is pretty much low, i tend to have very high rpm becouse of lack of air that would stop the engine from spinning. The question is : i heard that very high rpm and low boost don't go together, that it would stress the engine, but after checking, there is no visible wear, so is it possible that engine will not brake apart in this situation?


As far as i'm aware, at those altitudes you should keep your RPM settings to standard, I tend to cruse with a setting of between 2200 and 2400 RPM.

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PostPosted: Sun Feb 20, 2011 7:53 am 
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Thanks for a reply

Yes, im aware of what settings i should have a this altitude, i just wanted to check, what effect will high rpm/low boost have at this altitude to an engine, but there wasn't really much to see, after forceful flying at max settings i could achieve in a climb without overspeeding a prop(+/- 3200rpm indicated) there was no real damage, so im just wondering if high rpm/low boost really can do any harm? maybe at lower altitudes? or maybe it depends on the position of the throttle?


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PostPosted: Fri Mar 04, 2011 11:41 am 
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Joined: Wed Feb 16, 2011 7:42 am
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Location: Finland
Hi all again.

If i want to maximize my engine lifetime on my spit, so should i get the oil temp to 30+, even if it would require an additional shutdown and startup procedure? In another words, would that additional shutdown/startup wear off the engine less than a takeoff with 15+ oil temp and let's say 6-7 psi pressure? (boost same in both cases). On a spit mk2 and 100 octane fuel.

So far i've flown in cold temperatures, (-13 to 0). On such temps, do i even stand a chance to have my engine work for a long time, or will the cold startups destroy the engine signifantly faster than let say at 15 celsius outside temp? I tend to be 30-50min in the skies per flight, so far i have little less than 4 hours and the engine is running smooth. (which means, i've undestand, it has already lost its peak condition since it's no more in "beautiful shape" or "great shape"?) All but two flights have been done with double-start procedure in order to have the oil temp up. And in reduced take off boost.

Thank you very much!


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PostPosted: Wed Jun 08, 2011 1:09 pm 
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Location: Pennsboro, West Virginia U.S.A. 30 mins from KPKB
Hello Gentlemen,

When I fly the Spit I'm able to be in a climb with 2800 rpm and 6 boost and it does just fine. I also cruise around 2800 rpm and 4 boost, the Spit seems to hold up really well to this kind of treatment, although there may be just a bit more wear and tear on the main bearings. Food for thought...

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PostPosted: Sun Dec 18, 2011 11:05 am 
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Caldemeyn wrote:
so im just wondering if high rpm/low boost really can do any harm? maybe at lower altitudes? or maybe it depends on the position of the throttle?

At high altitudes, low boost is less harmful since you run the engine with throttle lever firewalled: there's no pumping loss since throttle plate is full open. High rpms increase friction and as result heat (not a problem when flying high) and higher fuel comsumption with marginal performance gain.

There is a difference from running low boost with high(ish) rpm up high and down low. When you MkI with full coarse pitch, around 2200rpm -4psi up there in the stratosphere, it's pretty economic flying. If you were to fly low with -4psi you'd barely be able to produce any horsepower. Most of the power would be wasted just to get the prop spinning at 2200rpm. That's because -4psi at sea level is causing a lot of pumping loss (engine braking effect), it's not just friction from bearings anymore. At low level you'd get more horsepower out of -4psi at 1800rpm not just because less bearing friction but because supercharger won't waste as much energy pushing air toward a throttle plate that's almost completely shut. (EDIT: Supercharger is downstream of throttle plate. Still, pumping loss exist whenever throttle lever is not firewalled.)

Flying with high rpm with low boost is definitely more of a problem at low altitudes. While you can descend from stratosphere to lower altitudes with half-throttle and -8 psi, it won't cough. It could overcool but otherwise it wouldn't show symptoms of fouling. Do that at sea level and it fouls in no time, low rpm or high rpm. Obviously faster with high rpm.

So rpm and boost relation is highly dependent on altitude. At lower altitudes, lower rpms - but still never below the one's stated in the manual. Manual lists absolute minimum rpms for absolute maximum boosts. The cold air at high altitude also means that for the same boost setting, you get quite a bit more oxygen into the engine so despite the cold, engine is still likely to suffer detonation if boosted too much, with too lean mixture or too low rpm.

While answering to the question in the most vague and lengthy way imaginable, I started wondering myself and would like an answer to one question: Spitfire's variable venturi SU carburetor is altitude compensated but is it temperature compensated as well? Should lean mixture be avoided during winter, even when cruising, as cold air is more dense and contain more air for the same manifold pressure?


Last edited by whiic on Fri Feb 17, 2012 4:29 pm, edited 1 time in total.

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PostPosted: Sun Dec 18, 2011 12:26 pm 
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Alfredson007 wrote:
If i want to maximize my engine lifetime on my spit, so should i get the oil temp to 30+, even if it would require an additional shutdown and startup procedure? In another words, would that additional shutdown/startup wear off the engine less than a takeoff with 15+ oil temp and let's say 6-7 psi pressure?

It's better to shut down and restart. The next start wouldn't even be a cold start... merely a lukewarm start, which is much less stress to the engine. The biggest disadvantage is the time it takes for the coolant to cool down. The oil will also cool down a bit but not as much as coolant does in the same time period.

But, that only if you can't keep the coolant from overheating while waiting for oil to warm. I've noticed than on some mid-sized modern airfield, you can warm up the engine to 70 deg C and then start to taxi. If the ambient is cool enough (like 10 degrees below zero), taxiing will keep the coolant temperature below 90. The trick is to use short bursts of power, spending most time idling, and trying to avoid any braking, keeping it constantly in motion. If the taxiway is long enough, you'll have 30 degrees of oil temperature by the time you reach the active and there's no need to shut down the engine. Quick magneto check and you're still (slightly) below 100 deg C coolant and well acceptable ~35 deg C of oil. Since 35 deg C is still just lukewarm and because 100 deg C is borderline acceptable for take-off, I tend to keep 2000-2200rpm (full fine pitch on MkI) and -2 psi during take-off, if possible, and after take-off sustain a speed of 100...120 mph while retracting the gear, coarsing the pirch a tad as I level off, preventing it from revving above 2200rpm and setting radiator to neutral. I fly it that way until oil warms to 50...60 deg C, i.e a couple of minutes, then increase rpm and boost to climb values. It also allows the coolant to cool down from 110 to 90, so I have a cooler engine but warmer oils when I finally start to climb. I think it's working. 30 hours on the engine, much of which in wintery conditions and it's still in beautiful/great shape.

Obviously, I couldn't baby engine would the runway be shorter (like the grass fields RAF used, Digby for example). If the runway was shorter a shutdown and restart would make me more comfortable applying regular take-off power (2600rpm, +6 psi), and shortened warm-up on the second engine start would also give me cooler coolant temperature on take-off so that I wouldn't even need to fly level to cool it down before starting my climb.

If you have problems with engine durability in winter conditions, are you paying attention to oil pressure after start-up? Try to keep it below 120psi even in the coldest environment. It requires a very low, rough idle but that's something that just has to be dealt with, hoping it won't foul and stall before your oil temperature reaches safe limits to increase to regular warm-up rpm. (And obviously, that 120psi is absolute maximum. When you see it drop as oil warms a bit, it doesn't mean you apply more throttle to keep it barely below 120psi. You let it drop to below 60 psi and even then, assume oil it soo cold for revving. If you want to taxi, taxi with idle rpm but if you want to skip the extra shutdown and restart, warm up before starting the taxi. You cannot get cooling benefit to engine coolant if you start taxiing before engine coolant has warmed up. Still, whatever decide to do, don't increase rpm above idle until you see a reading on both gauges (coolant 40+, oil 0+). And you need coolant 60+ and oil 15+ for magneto check.

MkII is a bit trickier in sub-zero conditions as regardless what the propeller rpm setting is (1600-3100) when you idle, your propeller is practically flat. It's much flatter than finest pitch on MkI. This means that whatever the rpm selector is set to, your engine idles at higher rpm for same throttle opening on MkI. This means either: higher rpm and higher oil pressure after cold start, or low manifold pressure and severe risk of plug fouling during warm up. Also, while you can start taxi without raising rpm on MkI, MkII with a flat prop won't give any mentionable forward force for any rpm below 1600. Warm it up whilst stationary, I'd recommend. At least until above 5 or 10 degs of oil temperature, then using low boost of max -4psi and 1600rpm. The higher rpm during taxi, combined with a higher pressure supercharger less optimal for low altitude, and the need to stay still until your engine has warmed up combine to make ground operations during winter more difficult and adds risks more risk to overheating. The cartridge starter system is also somewhat demotivating toward the idea of shutting it down just to let it cool down a bit.

Sub-zero start-ups always have potential to wreck an engine but if done properly it's not going to cause any serious damage, just some accelerated wear. Mishandled cold-start or warm-up, or cold take-off can cause something much much worse.


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PostPosted: Fri Jan 06, 2012 4:58 am 
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Airman Basic

Joined: Sun Jun 19, 2011 1:40 am
Posts: 4
Since it is my first post here, I want to say hello to all and thank A2A for that great forum and their outstanding products!!

So, now my problem/question (which is more a request for some advice):
I've the Spit now for a couple of months and I like it very much. But I'm not realIy satisfied with my input settings. I want to set up the rudder, aileron and elevator inputs for the Spit regarding sensitivity and steering force ('ll do that via FSUIPC and FS-Force). I want to have it as close as possible to reality because in my opinion thats pretty much the only "direct" repsonse from an airplane in FSX. Since I belong to the majority of pilots who have never flown the Spit in RL ( :( ) I would appreciate some help/info from the lucky guys who have (Dudley maybe you can help here). Maybe there is also someone who wants to share the FSUIPC curve settings or even an FS-Force profile but comparing steering forces and sensitivity to a reference plane (e.g. Cessna 172 or 152) may be helpful too.

Cheers and Thanks
CAT-III


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PostPosted: Thu Jun 14, 2012 1:18 am 
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Joined: Thu May 31, 2012 1:36 am
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Hi guys, a quick one hopefully, what oil pressure should I be seeing in flight? I tend to fly at 2600 rpm and zero boost (seems to work) but the oil psi is up around 50 to 60, does that seem high? The engine is 3 hours in and in beautiful shape so I guess it's not quickly doing any harm but I did wonder.
On the earlier comment about wearing in the engine, I wondered why my compression checks went down then back up a bit! Just how bloody good is this model?! Loving having the persistent state, I now worry about weather far more than I ever did after shearing the gear in a crosswind landing...


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