Simviation Forum - 1% Bombers Overheat immediately!

Reply #15 - Sep 27^th, 2003 at 8:55am

Hagar Offline
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Regarding variable pitch props - this is what I was taught over 40 years ago.

Fine pitch = revs. Revs = power. Power = heat.
You need full power (fine pitch) for take-off (& landing in case you need to go-around for some reason). An engine going flat out drinks fuel like it's going out of fashion & can also cause overheating. Coarse pitch is used for economical cruise.

Prop pitch is like a gearbox on a car. Use low gear for overtaking & going uphill, high gear for cruise.

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Reply #16 - Sep 28^th, 2003 at 1:48am

Professor Brensec Offline
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I might have all this back to front.

My understanding is this:

OK, you have a blade. It has the slightest 'twist' in it, just a couple of degrees. I consider this 'fine pitch'. It will cause the engine to turn faster, as there is less resistance to turn it. Result - higher revs but less air is 'displaced', so does the plane go faster or slower, with the same amount of power applied.

Then, you have another blade. It has a 'heavy' twist in it, say nearly 45 degrees. I consider this 'coarse pitch'.
It will cause the engine to turn slower because of the 'resistance' in turning it. Result - lower revs but more air is 'displaced', so again, does the plane go faster or slower with the same amount of power applied.

In the Sims, my understanding has always been that 'full pitch', i.e. with the lever up where full throttle is on the 'engine control quarter', is the fastest the plane will go (under a given amount of power), but more 'load' on the engine.
If you have the pitch lever right down, you are effectively 'straightening' the blade or reducing the 'twist' in it. This, I thought, was the slowest the plane would go (under the same power) but the engine would be under less 'load'.
The ultimate in 'fine pitch' in my book, is a blade with no twist at all, which would move no air and have no thrust effet at all.

Are these assumptions correct, or have I got it all stuffed up? ??? ??? Grin

Thanks

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Reply #17 - Sep 28^th, 2003 at 3:57am

Hagar Offline
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There seem to be a lot of misconceptions about variable pitch airscrews. The first Spitfires & Hurricanes used in the BoB had wooden 2 bladed, fixed pitch props. These were found inefficient & soon replaced with metal variable-pitch (2-pitch only - fine & coarse) airscrews. While improving performance they were difficult to use properly while in combat & the final improvement was the 3 bladed constant speed prop. (The difference between variable pitch & constant speed is similar to that between a manual & automatic car gearbox.) This all took place within a matter of weeks & drastically improved the performance of both aircraft. Note that prop pitch controls the revs of the engine for a given throttle setting which translates into power output. Most WWII piston engines were allowed maximum power (engine revs) for a limited time. Using emergency power for longer than recommended would risk serious overheating & maybe even engine failure.

This article gives a good explanation of the diifferent types of prop. http://www.airmasterpropellers.com/wa.asp?idWebPage=3637

Note this which confirms my previous statement.
Quote:

A constant speed propeller will automatically deliver you the advantages outlined above for variable pitch propellers, with almost no control required from the pilot. Once a propeller/engine speed is selected, the pilot is able to control the power purely with the throttle (actually controlling manifold pressure, which then determines power output) and the controller will act to keep the propeller/engine speed at the selected setting.

While allowing the pilot to ignore the propeller for most of the time, the pilot must still choose the most appropriate engine/propeller speed for the different phases of flight.

Take-Off, Go Around and Landing. A high speed setting is used when maximum power is needed for a short time such as on take-off. The high speed setting may also be used to keep the propeller pitch low during approach and landing, to provide the desired drag and be ready for a go-around should it be required.
Climb and High Speed Cruise. A medium speed setting is used when high power is needed on a continuous basis, such as during an extended climb, or high-speed cruise.
Economic Cruise. A low speed setting is used for a comfortable cruise with a low engine speed. This operation produces low fuel consumption and longer range, while the advantages of low noise and low engine wear are also enjoyed.

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Reply #18 - Oct 1^st, 2003 at 1:02pm

B24Guy Offline
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One solution would be to use notepad to edit the Reality-X.cfg file in the aircraft directiory.

Adding 5 or 10 deg. to the fail point of the cyl. head temps might do the trick.

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Reply #19 - Oct 4^th, 2003 at 1:54am

Professor Brensec Offline
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Thanks for that tip B24. I wouldn't have touched a Reality-X CFG without some advice first. Now that I know it's ok to change simple settings such as this, I'll give it a go. Grin

Hagar,
I'll have to read the article you've given the link for, because now I'm totally confused.
My understanding was always that you had engine speed that was determined by the throttle, the same as a car. Then you had prop pitch which sharpened or flattened the angle of the blade. This would create more 'thrust' but also more drag, hence the engine revs would drop because of the increased drag on the blade. Sort of like changeing to a higher gear in a car.

Having seen your short explanation, I don't know if this applies now. I'll have to read up.

Thanks Grin

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Reply #20 - Oct 4^th, 2003 at 2:09am

Professor Brensec Offline
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Hey Hagar, I remember I did a test, when this first came up last week.

I was in a plane at a constant altitude, no wind etc.
(I bore in mind your instruction "fine pitch = power etc")

I was running full throttle at 100% prop pitch i.e. both levers to the firewall.
I took the prop pitch down to 50% i.e back away from firewall, and the plane speed slowed.
Now if 0% is full power, how come the plane slowed.
I have found a plane goes faster and accelerates quicker, if the prop is at 100% i.e to the firewall. So what's the story.

The link you gave doesn't explain clearly (in idiot terms) what fine and coarse are!
To my mind fine is 'almost no twist in the balde'. And coarse is the maximum 'twist' in the blade. Is this much at least, correct? Grin

Tell me this. If I had a propellor that had NO twist in the blade at all. It was just say, three flat peices of metal turning with the 'edges' cutting through the air at 90 degrees to the way it's facing. Would the plane move?
I'm quite sure it wouldn't. So how can fine pitch (i.e. less twist) be more powerful or efficient or faster, when the finest pitch is completely POWERLESS? Grin

Sorry, I'm getting frustrated, because it doesn't fiy into my preconceived little box.........lol Grin

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Reply #21 - Oct 4^th, 2003 at 8:32am

Hagar Offline
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To understand this I think you need to differentiate between engine power & airspeed. I was an airframe man & never had much to do with the sharp end, although I was taught the basics on engines & props. This was also more years ago than I care to remember. The Tiger Moths & Austers I learned to fly on had fixed-pitch props so I have no "real-life" experience of using the variable pitch variety. You already know my thoughts on the realism of the M$ sims & I don't mess around with prop pitch & all that stuff. I do know that it's possible to define different types of prop in the AIR file & Aircraft.cfg.

I'm not really sure how fine & coarse pitch translate to percentages. I would assume that 100% is fully coarse. This would take bigger "bites" of air per revolution compared with fine pitch. The different types of thread on an ordinary nut & bolt are described in the same way. Going back to the car gearbox analogy, think of the difference in acceleration between flooring the GO pedal in a high or low gear. The low gear gives higher engine revs which converts to power - acceleration, not speed.

All piston engines have an optimum power setting which is the most efficient for that particular engine in a specific installation. They will run for short periods at full power but this uses most fuel & can quickly cause wear & overheating. Fixed-pitch props were always a compromise between optimum & full power. They need a fine enough pitch to give sufficient power (engine revs) for take-off while being coarse enough to give a reasonably efficient & economical cruise. This is why the variable-pitch prop was introduced. In normal circumstances the only times you need full power available are during take-off & landing.

Quote:

There are two types of variable-pitch propellers adjustable and controllable. The adjustable propeller's pitch can be changed only by a mechanic to serve a particular purpose-speed or power. The controllable-pitch propeller permits pilots to change pitch to more ideally fit their requirements at the moment. In different aircraft, this is done by electrical or hydraulic means. In modern aircraft, it is done automatically, and the propellers are referred to as constant-speed propellers. As power requirements vary, the pitch automatically changes, keeping the engine and the propeller operating at a constant rpm. If the rpm rate increases, as in a dive, a governor on the hydraulic system changes the blade pitch to a higher angle. This acts as a brake on the crankshaft. If the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm can increase. The constant-speed propeller thus ensures that the pitch is always set at the most efficient angle so that the engine can run at a desired constant rpm regardless of altitude or forward speed.
http://www.sky-jet.com/propellers/001.htm

Going back to the original topic, we're talking about bombers. Most, if not all, WWII bombers were fitted with constant-speed props. A bomber is much the same as a transport aircraft in that economical cruising speed & range are the priorities. I assume there would be several marked pitch settings on the quadrant, take-off/landing, climb & cruise.

I'm re-reading Richard Hillary's classic biography "The Last Enemy" right now. Hillary was a Spitfire pilot, born in Australia, who was shot down & suffered terrible burns during the BoB. He wrote the book during his long convalescence. His brief reference to an incident that occurred during their early night-flying training on the Harvard jumped out & hit me - it seemed so relevant to what we're discussing. Hilllary had just passed the course after 3 solo night flights & was relaxing in the hut with his instructor.
Quote:

Outside, someone was coming into land. He was given a green on the Aldis lamp & throttled back, only to open up & go round again. We watched the glimmer of his nav lights as he made a quick circuit & once again throttled back. He was past the first flare, past the second, past the third & still not touching down when the engine roared into life & he was off again.
"Christ," said White "he's in coarse pitch."
Again we watched the navigation lights, but we soon lost them and we could just hear the hum of the engine headed towards the sea.

...... We found him on the shore, the machine half in & half out of the sea. The officer in charge of night flying climbed on the wing & peered into the cockpit.
"In coarse pitch," he said "as I thought." Then, after a slight pause "Poor devil."

« Last Edit: Oct 4^th, 2003 at 4:07pm by Hagar »

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Reply #22 - Oct 4^th, 2003 at 10:24pm

Professor Brensec Offline
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OK. It would seem to be sinking in, at least a little. Wink

I have this idea in my head that, if you want the plane to have the most thrust, then you set the prop to the 'coarsest' setting (i.e. it takes bigger bites of air or moves more air or creates greatest thrust).

OK, it also stands to reason that, if you want maximum thrust or power for a takeoff or 'possilbe' go 'round, you would want this maximum coarse setting along with the option of maximum engine power or revs available, i.e. more fuel, more throttle (if required).

I still can't see how making the pitch 'finer' i.e. less bites of air and less thrust, would make for more acceleration (which is what is need in take off and go round).
Yes, I see how the engine would have less drag on it because of less resistance on the blades, but the increased engine revs would be negated by the fact that, even tho' the prop was turning faster, it wouldn't be moving as much air.

I undestand the relationship as far as a car is concerned and how it translates. For instance, if you try to take off or go 'round with coarse pitch it's the same as trying to take off in a car in top gear.
I understand that you need a low gear, at low speed if you want to get maximum acceleration. I see how this must translate to a plane in so far as low gear = fine pitch and hence less load on the engine hence higher revs maintained. In thoery this makes sense. But in reality, all I see is a prop spinning (fine pitch), and not pushing much air, as opposed to a prop spinning (coarse pitch) pushing heaps of air and hence more acceleratiuon.

I know it's a mental block. I think it has to do with the fact that I don't appreciate the fact that the drag on a prop spinning in the air would have the same effect on engine revs and power as does wheels, sitting stationary on the ground. I just don't see how that much drag can be created by a coarse pitch (as opposed to fine pitch) as to actually slow down a 1500 HP or so engine. That's what it is. Grin

I think if I just keep in my mind the fact that the prop pitch is my gear stick (fine = low gear and coarse = high gear), I'll be OK.
P.S How many people in Sim's use fine pitch to take off? I wouldn't think very many, because as with me, I think with most people, coarse pitch = more power. Grin

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Reply #23 - Oct 5^th, 2003 at 7:49am

Hagar Offline
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I don't pretend to know all the technical aspects or fully understand it myself. This is a complex subject & I'm sure there are others here more qualified than me. All I know is what I was taught. My first engineering instructor drummed it into us from an early age. "Revs = Power & don't you forget it." I never did forget, in fact it's one of the few things that remain engraved on my memory from those days. Everything I've been told or read on the subject since then confirms that he was correct. I'm sure he went into more detail but my old notebooks have long since been lost.

At the risk of boring you here's another quote I found while researching this. It's from an article by AVM James "Johnnie" Johnson describing his first flight in a Spitfire.
Quote:

The instructor bellowed in my ear: "You're trimmed for take-off. Don't forget your fine pitch or you'll never get off the ground! Good Luck".

Quote:

P.S How many people in Sim's use fine pitch to take off? I wouldn't think very many, because as with me, I think with most people, coarse pitch = more power.

I'm sure more people realise this than you might think. I copied this from the FS2002 default Corsair checklist.

BEFORE TAKEOFF

[ ] Brakes -- SET
[ ] Canopy -- OPEN
[ ] Wings -- SPREAD

Do not run up engine past 1,500 rpm with wings folded.

[ ] Flight Controls -- FREE and CORRECT
[ ] Flight Instruments -- CHECK and SET
[ ] Primer -- LOCKED
[ ] Mixture -- RICH
[ ] Fuel Quantity -- CHECK
[ ] Fuel Selector Valve -- RECHECK main
[ ] Elevator Trim -- SET for takeoff
[ ] Rudder Trim -- SET for takeoff
[ ] Throttle -- 30 in. Hg.

[ ] Magnetos -- CHECK
[ ] Propeller -- CYCLE high rpm/
low rpm/high rpm
[ ] Engine Instruments and Ammeter -- CHECK
[ ] Throttle -- 800-1,000 rpm
[ ] Flaps -- SET for takeoff (0-20 degrees)
[ ] Cowl Flaps -- OPEN

« Last Edit: Oct 5^th, 2003 at 10:03am by Hagar »

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Reply #24 - Oct 5^th, 2003 at 11:37am

Hagar Offline
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Quote:

I still can't see how making the pitch 'finer' i.e. less bites of air and less thrust, would make for more acceleration (which is what is need in take off and go round).
Yes, I see how the engine would have less drag on it because of less resistance on the blades, but the increased engine revs would be negated by the fact that, even tho' the prop was turning faster, it wouldn't be moving as much air.

I think this explains your mental block. Forget all about moving air. The prop blades are basically aerofoils & act like a wing. This goes back to the principles of flight itself. Changing the pitch of the prop blades increases or decreases the angle of incidence (angle of attack) of each blade in relation to the direction it's moving, the prop blade - not the aircraft. The efficiency of an aerofoil is related to airspeed & angle of incidence (Thrust & Lift). Usually, increasing airspeed at a given angle of incidence will also increase lift, up to a certain point. High angle of attack (or coarse pitch) increases drag. Increasing AoA too much causes the airflow to break up & results in a stall.

Variable-pitch props would be designed to take full advantage of this. The pitch settings would be carefully worked out to suit the engine they're fitted to.

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Reply #25 - Oct 5^th, 2003 at 11:51pm

Professor Brensec Offline
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I must admit, it seems a little clearer, but I'm not 100% clear on the whys and hows. But then who is?

I'm just the type that doesn't accept a given 'law' or rule. I have to know why it is so (Prof. Julius Sumner Miller - if you're familiar with him.......... Grin

). The study of Physic is simply - "Why things are as they are, and not otherwise"...........I love that.

But, thanks for the help and the explanations, Hagar. Wink

I can't wait to get home (at work again - public holiday here, so I got a shift at $40-00 an hour - couldn't pass it up!) to get my favourite 1% plane and see if I accelerate on the runway more quickly with 100% prop pitch or 50% pitch etc etc. I'll try a few settings and see if there is an optimum, at all, other than 100% (coarse).

And, of course, all the time I'm doing this, I will have uppermost in my mind that the Sim configs may very well not be true representations of what would happen in reality.............just for Hagar..... Grin

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Reply #26 - Oct 6^th, 2003 at 6:38am

Hagar Offline
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Quote:

I'm just the type that doesn't accept a given 'law' or rule. I have to know why it is so (Prof. Julius Sumner Miller - if you're familiar with him.......... Grin

). The study of Physic is simply - "Why things are as they are, and not otherwise"...........I love that.

I'm much the same Brensec. I once knew the explantion for all this which satisfied my innate curiosity but admit I forget it now. I'm afraid to say too much in the present company in case I make a complete fool of myself. Maybe I already have. That would be nothing new. LOL

Quote:

And, of course, all the time I'm doing this, I will have uppermost in my mind that the Sim configs may very well not be true representations of what would happen in reality.............just for Hagar..... Grin

I'll be interested in your findings. I do this for fun & never took this realism stuff too seriously. Wink

PS. I decided to do what I should have done much sooner. I checked this out on the CFS2 default Hellcat. This shot of the pitch control shows that my assumption that 100% is Full Coarse pitch was incorrect. It is in fact Full Fine. I can only think it indicates 100% Power. The instructons themselves confirm what I've been telling you. PUSH - INCREASE RPM - DECREASE PITCH

« Last Edit: Oct 6^th, 2003 at 12:40pm by Hagar »

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Reply #27 - Oct 6^th, 2003 at 12:05pm

B24Guy Offline
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I was re-reding this discussion and I agree with Hagar the game is a game and I am all for realism as long as it does not spoil the fun.

The 1% aircraft fly really well but they seem to lack one thing Power!

Just try and jump in your Wildcat and take off from the stock starting point on a carrier with a full drop tank. Crashing every time I take off is not too much fun.

What do you all think?

Wink

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Reply #28 - Oct 6^th, 2003 at 12:34pm

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Hi B24Guy. I have every respect for the AvHistory team & their dedication. They have set the standard for realistic flight dynamics in CFS2 & the M$ sims generally. I'm told by those in the know that the 1% certified AIR files are as realistic as possible within the limitations of the M$ flight model. Unlike Brensec, I've never been too worried about messing around with stuff like prop pitch & mixture controls or wondered if a representation of an aircraft in CFS2 behaves exactly like the real one throughout the flight envelope. I would be very surprised if this was possible. Undecided

As I mentioned, I do this for fun. Others enjoy the challenge of "full realism". I'm sure there are plenty like me & I'm prepared to sacrifice absolute "realism" for playability. I've been around aircraft all my life & flown several types myself. I was never convinced that any aircraft is really as difficult to fly as some examples are in the sim. Wink

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Reply #29 - Oct 7^th, 2003 at 6:04am

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Hi guys,
I've been away from the forums for a bit. This is developing into a very interesting conversation.

There was a similar chat over on avhistory on this same subject of prop settings.
Hopefully this link will work
http://www.avhistory.org/scripts/MegaBBS/thread-view.asp?threadid=1095
If not search there furum for a subject line of 'Dumb question time'. It was started in August this year.

Brensec, and anyone else. I do most of my flying online and I would love to do a hop or fight most evenings UK time.
Brensec I seem to remember that you are in Sydney, Aus. Looking forward to the world cup? Perhaps we could arrange a Saturday/ Sunday morning UK time which I think should be evening for you. I did fly at what would be evening for you two weeks ago when I was off sick and there was a busy 1% game for a couple of hours so I guess there are several Aussies flying the flag.

As for the question of prop settings. I'm not very up on reading excact settings, etc. I usually rely on trial and error to get things right. I have however flown the B17 a few times since reading this post and am having no proplems having reduced the prop setting to 80 - 90% (whatever that may mean. lol) I can get off the ground and not overheat. Same for landing. However on taking off I don't drop the flaps untill I have some speed up.

As always, I hope this helps.

Don't shoot me!!

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