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Because the air is already slowed down 20% by passing through the prop.
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.
Soren said: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.
wmaxt said:Surprisingly it is propellar drag that limits Prop planes to subsonic speeds.
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RG_Lunatic said:Even the TA-152 is, for all intents and purposes, a radial type design.
Soren said:wmaxt said:Surprisingly it is propellar drag that limits Prop planes to subsonic speeds.
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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.
wmaxt said: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 said:Because the air is already slowed down 20% by passing through the prop.
RG may I ask, how did you calculate that ??
Soren said:Also wasnt the prop normally developed to 'cut' through the air, and 'accelerate' it backwards at an even faster rate =Thrust ?
Soren said:(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.
Soren said:Does this look like a Radial engine to you ?
Fw-190D-9 liquid-cooled inline engine.
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.
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)
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?
Soren said:After reading all your sources, I still see nothing to back up your "20% loss of air-resistance behind the prop" theory !
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
Soren said: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)
Soren said: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.
Soren said: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 ?
Soren said: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.
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.
Soren said:Also have a look at this Tropicalized Spitfire, it has an intake very close to the inner propeller, the same does the A6M "Zero".
Soren said: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