Spitfire MK.XIV and La-7

[Soren]

Because the air is already slowed down 20% by passing through the prop.

RG may I ask, how did you calculate that ??

Also wasnt the prop normally developed to 'cut' through the air, and 'accelerate' it backwards at an even faster rate =Thrust ?

(Your in deep now RG, and your about to sink )

Not really. This was the belief in the pre-war years, when the blunt nose of a radial was simply stuck into the wind. But during WWII advancements in cowl and spinner design showed that radial designs could be just as streamlined as liquid cooled designs. Look at all the fastest props of WWII - except for the P-51H (which has significantly improved radiator thrust over the B/D), they are all radial type designs. Even the TA-152 is, for all intents and purposes, a radial type design.

Does this look like a Radial engine to you ?

Fw-190D-9 liquid-cooled inline engine.

 

[wmaxt]

Because the air is already slowed down 20% by passing through the prop.

RG may I ask, how did you calculate that ??

Also wasnt the prop normally developed to 'cut' through the air, and 'accellerate' it backwards even faster yet=Thrust ?

(Your in deep now RG, and your about to sink )

Not really. This was the belief in the pre-war years, when the blunt nose of a radial was simply stuck into the wind. But during WWII advancements in cowl and spinner design showed that radial designs could be just as streamlined as liquid cooled designs. Look at all the fastest props of WWII - except for the P-51H (which has significantly improved radiator thrust over the B/D), they are all radial type designs. Even the TA-152 is, for all intents and purposes, a radial type design.

Does this look like a Radial engine to you ?

Fw-190D-9 liquid-cooled inline engine.

Surprisingly it is propellar drag that limits Prop planes to subsonic speeds.

It was Luftwaffe practice to utilize Anular radiators to maintain the shape of an aircraft designed originaly for radial engines so they could go back if needed. It was also convienent because it limits plumbing and radiator modifications to a minimum. The Ju-88 was another example of this kind of thinking.

 

[Soren]

Surprisingly it is propellar drag that limits Prop planes to subsonic speeds.

.

Yes if you reach the max rpm for the engine (Depending on prop design), it will cause prop-drag, as then the propeller can't accelerate the air backwards no-more.

 

[GermansRGeniuses]

Even the TA-152 is, for all intents and purposes, a radial type design.

He's not saying it HAS a radial, he's stating it basically is one, due to its annular radiator which gives it a radial shape.

 

[wmaxt]

Surprisingly it is propellar drag that limits Prop planes to subsonic speeds.

.

Yes if you reach the max rpm for the engine (Depending on prop design), it will cause prop-drag, as then the propeller can't accelerate the air backwards no-more.

The limit is in the Propellar itself - ever wondered why all aircraft limiting RPM's are around 3,000rpm? When the tip of the prop exceeds the speed of sound it becomes a brake and produces no lift/thrust. Depending on diameter of the prop (the larger diameter of the prop the faster the tip speed is in relation to the RPM) that speed is around 3,000rpm.

A ducted fan helps in relation to the shock wave at the tip but is very cumbersome. The high bypass jet engine is an example of how the highspeed ducted fan can work.

 

[Soren]

The limit is in the Propellar itself - ever wondered why all aircraft limiting RPM's are around 3,000rpm? When the tip of the prop exceeds the speed of sound it becomes a brake and produces no lift/thrust. Depending on diameter of the prop (the larger diameter of the prop the faster the tip speed is in relation to the RPM) that speed is around 3,000rpm.

A ducted fan helps in relation to the shock wave at the tip but is very cumbersome. The high bypass jet engine is an example of how the highspeed ducted fan can work.

There's no disagreement on that.

 

[Anonymous]

Because the air is already slowed down 20% by passing through the prop.

RG may I ask, how did you calculate that ??

I didn't calculate it. It comes from a study I read of the FW190 and Tempest cooling system. Part of the reason for the bullet spinner and the fan is to counter the 20% drop in airspeed caused by the prop.

Also wasnt the prop normally developed to 'cut' through the air, and 'accelerate' it backwards at an even faster rate =Thrust ?

Not really. The purpose of the Prop is to cut into the air and pull the plane forward. The "thrust" is really just a byproduct. But also, props generally don't have much blade down that close to the spinner, and even if they did that'd be the least effective part of the prop since it is moving so much slower than the outer part.

(Your in deep now RG, and your about to sink )

Not at all. You do your best to prove me wrong, and then I'll spend the time to locate the document describing the 20% loss of airflow through the prop. 8)

Not really. This was the belief in the pre-war years, when the blunt nose of a radial was simply stuck into the wind. But during WWII advancements in cowl and spinner design showed that radial designs could be just as streamlined as liquid cooled designs. Look at all the fastest props of WWII - except for the P-51H (which has significantly improved radiator thrust over the B/D), they are all radial type designs. Even the TA-152 is, for all intents and purposes, a radial type design.

Does this look like a Radial engine to you ?

Fw-190D-9 liquid-cooled inline engine.

So what, it is mounted such that it has all the negative drag characteristics of a radial engine, right? Or do you believe there is something magical about the liquid cooled engine that makes its cooling system not make drag?

=S=

Lunatic

 

[Soren]

Not really. The purpose of the Prop is to cut into the air and pull the plane forward. The "thrust" is really just a byproduct. But also, props generally don't have much blade down that close to the spinner, and even if they did that'd be the least effective part of the prop since it is moving so much slower than the outer part.

As Newton stated, "actio est reactio". For the propulsion problem, this means that a device accelerating air or water in one direction, feels a force in the opposite direction. A propeller accelerates incoming air particles, "throwing" them towards the rear of the airplane, and thus feels a force on itself - this force is called thrust !

Not at all. You do your best to prove me wrong, and then I'll spend the time to locate the document describing the 20% loss of airflow through the prop. 8)

Go ahead, dispute what i just said above, if you can ! I am looking forward to seeing the "20% loss of air-resistance behind the prop" theory !

So what, it is mounted such that it has all the negative drag characteristics of a radial engine, right? Or do you believe there is something magical about the liquid cooled engine that makes its cooling system not make drag?

Not at all, but the front nose section is MUCH smaller than that on the La-7, as a result of the Inline engine.

 

[Anonymous]

Soren,

The blades create pulses of high speed turbulent air followed by periods of practically no airflow. Turbulent air is not good for cooling, requiring baffles, and of course no airflow = no cooling. Here are some sources:

http://www.geocities.com/donshoebridge/h-stab.html

http://www.ch601.org/resources/cooling_systems2.htm

http://www.bewersdorff.com/wankel/radiator/CoolingSystems1.html <== the definative source, is no longer online

You can also read through various NACA reports (<-- click) on the subject. In particular look at page 377 of the http://naca.larc.nasa.gov/reports/1943/naca-report-771/ document, which conviently is available as a sample in .gif form:

http://naca.larc.nasa.gov/reports/1943/naca-report-771/index.cgi?page0007.gif

As you can see, in this experiment they were only able to recover about 75% of "free stream" airflow, though later improvements got as high as 80%. That leaves 20% loss.

This cooling loss was why the P-40 and Typhoon/Tempest V needed such huge chin scoops.

Anyway, the partial solution for radials (and the Dora/Ta) was the large bullet spinner, which accelerates the airflow into the cowl opening enough to overcome the loss of ~20% of cooling flow efficiency created by the prop, but over a much smaller inlet area. This provided sufficient cooling for climb and cruise conditions.

The problem with the large bullet spinner is that at high speeds it generates very high speed air into the cooling system, which is not good for cooling either - it becomes highly turblent on striking the cooling vanes and does not sustain long enough contact for good heat transfer. The best solution is to slow down the air entering the cooling system without creating turbulence or periods of no airflow - as far as I know only the P-51 and to a lesser degree the Mossie achieved this in WWII.

Also, if you look at them from the front, the La7's nose profile is not much if at all larger than the Spit XIV's.

=S=

Lunatic

 

[Soren]

After reading all your sources, I still see nothing to back up your "20% loss of air-resistance behind the prop" theory !

The job of a propeller is to accelerate incoming air backwards=Increasing airflow. (The density of the air might be decreased, but thats another matter)

Also:
Propellers have also been called airscrew in the past, but this term may be misleading, because a propeller does not move like a mechanical screw through a rigid medium. You don't call a wing knife or slicer because it also does not slice through the air in the direction of its inclined mean line. Each section of a propeller (or of a wing) has a certain angle of incidence and is moving through the air at its unique angle of attack - both are independent. On the other hand, a mechanical screw or a knife moves through a rigid material exactly in the direction which is given by its pitch or angle of incidence - a screw with different pitches along its radius would get stuck, a propeller does not.

So thrust is NOT a by-product !

Thrust is what drives the airplane forward ! The planes propeller does not eat its way through air, like was it some kind of rigid structure. No a propeller accelerates incoming airflow/inflow backwards at an even higher rate creating Thrust wich drives the airplane forward.

There will be a point in high speed flight, where the propeller can't ceate thrust nomore, thus the prop acts more like a brake.

For a typical, fixed pitch propeller, the largest induced velocity(Thrust) occurs under static conditions, where the efficiency is small. It decreases with increasing flight speed, until it reaches zero= no thrust is generated. When the flight speed is increased even more (e.g. by diving), the propeller acts like a windmill= it tries to turn the engine, which might be fatal for the engine. This is why piston-engined aircraft don't reach beyond subsonic speeds.

 

[Anonymous]

After reading all your sources, I still see nothing to back up your "20% loss of air-resistance behind the prop" theory !

Ummm... It's not my theory. I've read that figure when studying aircraft cooling systems 2 years ago. And if you don't see that in the NACA document then what can I say. It clearly shows that pressure drops by over 25% even with the best of the 4 cowls tested, as compared to "free-stream" airflow.

Did you not read the following:

The Germans thought that if the radiator was moved to a point right up behind the propeller, that the cooling capacity of the radiator would increase and they could keep the engine cool. What they found is totally opposite. The radiator had to be moved farther away from the propeller, not closer. The reason this problem occurred in the first place is because each time that a propeller blade passed by the radiator, the radiator would see a pulse of high velocity air, but the dwell time between blades, where there was very little flow, negated the high velocity air flow.
http://www.geocities.com/donshoebridge/h-stab.html

The job of a propeller is to accelerate incoming air backwards=Increasing airflow. (The density of the air might be decreased, but thats another matter)

The density of the air is what matters when the air is measured by such a sensor, which is measuring an average. A higher air-flow would mean higher pressure reading. Less air molecules means less cooling.

Also:
Propellers have also been called airscrew in the past, but this term may be misleading, because a propeller does not move like a mechanical screw through a rigid medium. You don't call a wing knife or slicer because it also does not slice through the air in the direction of its inclined mean line. Each section of a propeller (or of a wing) has a certain angle of incidence and is moving through the air at its unique angle of attack - both are independent. On the other hand, a mechanical screw or a knife moves through a rigid material exactly in the direction which is given by its pitch or angle of incidence - a screw with different pitches along its radius would get stuck, a propeller does not.

So thrust is NOT a by-product !

Thrust is what drives the airplane forward ! The planes propeller does not eat its way through air, like was it some kind of rigid structure. No a propeller accelerates incoming airflow/inflow backwards at an even higher rate creating Thrust wich drives the airplane forward.

There will be a point in high speed flight, where the propeller can't ceate thrust nomore, thus the prop acts more like a brake.

For a typical, fixed pitch propeller, the largest induced velocity(Thrust) occurs under static conditions, where the efficiency is small. It decreases with increasing flight speed, until it reaches zero= no thrust is generated. When the flight speed is increased even more (e.g. by diving), the propeller acts like a windmill= it tries to turn the engine, which might be fatal for the engine. This is why piston-engined aircraft don't reach beyond subsonic speeds.

Let me be more clear. The explusion of air to the rear, which is what we normally call thrust, is not what drives the aircraft forward. That is the equal and opposit reaction to the prop pulling the plane forward through a gasous-liquid medium.

=S=

Lunatic

 

[Soren]

RG we are talking "Air-resistance" Not "Cooling properties"

The Air-resistance is NOT decreased by 20% behind the prop, that is nonesense !

Here, read how a propeller works: http://www.grc.nasa.gov/WWW/K-12/airplane/propeller.html

Fact is a spinning propeller sets up a pressure lower than free stream in front of the propeller, and higher than free stream behind the propeller.

If NASA aint right, then who is ?

 

[Anonymous]

RG we are talking "Air-resistance" Not "Cooling properties"

The Air-resistance is NOT decreased by 20% behind the prop, that is nonesense !

Here, read how a propeller works: http://www.grc.nasa.gov/WWW/K-12/airplane/propeller.html

Fact is a spinning propeller sets up a pressure lower than free stream in front of the propeller, and higher than free stream behind the propeller.

If NASA aint right, then who is ?

Stop being obtuse. We are not talking about the whole area behind the prop, we are talking about the small area of the prop in front of the cowling. At this point, most props don't even have much blade to them to drive air and they are moving at a slow rate compared to out by the tip.

You are trying to compare the effect of the whole prop to the inner 6-12 inches. In that inner zone, the air resistance is reduced by the prop behind the prop. Of course, it is encounterd by the prop itself so it all balances out.

=S=

Lunatic

 

[Soren]

Sure the inner part of the propeller doesnt spin as fast as the outer part, but it still sets up a pressure lower than free stream in front it, and higher than free stream behind it.

Also have a look at this Tropicalized Spitfire, it has an intake very close to the inner propeller, the same does the A6M "Zero".

 

[the lancaster kicks ass]

great pic...................

 

[delcyros]

cool pictures, indeed.
Hmm, I checked some books, too.
First of, I found no proof that the La-7 was inferior compared to any Luftwaffe fighters. A few are even credited with Me-262 kills.
There is a difference between "german pilots did not feel inferior to soviet planes" and the fact that La-7 killed experienced Luftwaffe units (including ace units). And I think the La-7 kills are partly bouncings and partly because of the superior tactics of energy fighting (look at Hartmann). Very few Luftwaffe pilots tried to outmanouvre a soviet plane, in general it was even forbidden in the most common altitudes (esspecially against Yak-3 and later La-fighter).

 

[Anonymous]

Sure the inner part of the propeller doesnt spin as fast as the outer part, but it still sets up a pressure lower than free stream in front it, and higher than free stream behind it.

Well, think about your previous logic. At lower speeds the prop pulls the plane by creating a low pressure area in front of the prop and a high pressure area behind it. But the prop is not of uniform effect - the outer part of the prop moves more air than the inner part.

Let's consider a prop turning at 1000 rpm (makes the math easy).

At a point measured 1 foot off the axis of the prop, which for simplicity we will assume is entirely blocked by the spinner, the circumfrence of the sweep is 2 x Pie x 1 ft = 2 x 3.14 = 6.28 feet. At 1000 rpm this means the prop at that point is turning at 6280 fpm = 1.19 miles/min = 71 mph. Since 2 x Pi x r is a simple linear equation we can see that the speed at 2 feet is about 142 mph, at 3 feet it is 213 mph, at 4 feet it is 284 mph, and at 5 feet it is 355 mph.

Again to keep things simple, lets assume the spinner blocks off the inner most foot of the prop blade. Then lets divide up the props sweep into 1 foot thick curcles and assume the thrust for that region is the average of the inside and outside boarder speeds. Rather than bore you with the math, I've made the following diagram:

So by your own previous statement...

There will be a point in high speed flight, where the propeller can't ceate thrust nomore, thus the prop acts more like a brake.

For a typical, fixed pitch propeller, the largest induced velocity(Thrust) occurs under static conditions, where the efficiency is small. It decreases with increasing flight speed, until it reaches zero= no thrust is generated. When the flight speed is increased even more (e.g. by diving), the propeller acts like a windmill= it tries to turn the engine, which might be fatal for the engine. This is why piston-engined aircraft don't reach beyond subsonic speeds.

... isn't it obvious that as the speed of the plane increases the thrust generated nearer the center of the prop will diminish to zero and then go negative, and that this phenomna will expand outwards along the prop from the root toward the tip as speed increases? Won't this create a low pressure area around the cowl, reducing both the effectiveness of cowl inlet cooling systems and reducing the drag of the aircraft cowling?

Also have a look at this Tropicalized Spitfire, it has an intake very close to the inner propeller, the same does the A6M "Zero".

That's not a cooling intake, that's an air intake for the engine. Just bad design is all - it was an improvisation intended to resist injestion of dirt more than anything else. The loss of engine power from such a design was probably noticable but fairly minor. Also, they probably blamed the power loss on filters rather than the true cause. Lots of planes have the air intake up near the prop - it's a convienient place to put it.

=S=

Lunatic

 

[wmaxt]

I've read that the loss was fairly significant and yes they did blame it on the filters.

 

[Soren]

RG you can't just math it up like that, you've not even taken into considderation the inner prop design or how its formed !
The amount of thrust created is depending on the AoA of the prop blades, and as we all know those blades are twisted, so the thrust created is about the same on the inner prop as on the outer prop despite the speed difference.

Each section of a propeller has a certain angle of incidence and is moving through the air at its unique angle of attack. The angle of attack of the airfoils at the tip is lower than at the hub because it is moving at a higher velocity than the hub. Also the lower airfoils at the hub are thicker, creating more lift/thrust pr revelution.

Also as the speed of the aircraft increases, different pitch settings can be made to gain more thrust at high speed.
(The "Fixed pitch" propeller can't do this, but most WWII fighters inc. the La7 used the "Controllable Pitch" Propeller )

If it was as simple as you put it RG, then there would be planes flying in 1803 or even sooner !

RG why don't you take a look at this: http://www.grc.nasa.gov/WWW/K-12/airplane/propth.html

 

[Anonymous]

RG you can't just math it up like that, you've not even taken into considderation the inner prop design or how its formed !
The amount of thrust created is depending on the AoA of the prop blades, and as we all know those blades are twisted, so the thrust created is about the same on the inner prop as on the outer prop despite the speed difference.

Each section of a propeller has a certain angle of incidence and is moving through the air at its unique angle of attack. The angle of attack of the airfoils at the tip is lower than at the hub because it is moving at a higher velocity than the hub. Also the lower airfoils at the hub are thicker, creating more lift/thrust pr revelution.

I know this Soren, but it still does not change the overall effect. Yes the prop has some twist to it, but the difference in overall angle of attack is not enough to equal out the difference in speed. Also, the inner part of the prop is of a somewhat laminar flow design, generating little or no airfoil lift - and it is thicker not to provide more lift, but rather for strength.

Look at your image - the inner two cross-sections have no lifting foil to them at all! And the inner 6 inches (assuming a 5' prop) or so is ROUND! 8) This is because that far in on the prop there is no real value in trying to obtain any thrust. The angle of attack reduces slightly near the tip because the blade is becomming thin out there, and you don't want the prop tips to bend forward.

Also, if you're going to post a link, post one that goes somewhere meaningful, not just to the image you've arleady presented

As for planes flying in 1803... no engines means no self powered planes - period. Flat blades like on a cieling fan were sufficient for the earliest planes.

=S=

Lunatic