Prop Question
Fri Jul 25, 2008 12:33 pm
What determines the number of blades on a propeller? Thanks in advance; I really want to know this.
Fri Jul 25, 2008 12:50 pm
horsepower, max RPM, and ground clearance,
They made a 1 bladed prop for 65hp T-crafts, all the way to the 6 or 8 bladed props for turbo props.
They made a 1 bladed prop for 65hp T-crafts, all the way to the 6 or 8 bladed props for turbo props.
Re: Prop Question
Fri Jul 25, 2008 3:43 pm
michaelharadon wrote:What determines the number of blades on a propeller? Thanks in advance; I really want to know this.
The number of holes in the hub.
Notwithstanding Matt's excellent answer, perhaps be a bit more detailed in the question, and you might get a better answer...
Fri Jul 25, 2008 4:01 pm
and here i was going to say whatever the TCDS says its suppose to have.........
i apologize as ive been in the last few weeks before i take my written test for my airframe/general and am a lil braindead from all the prep test we have been doing in school.
i apologize as ive been in the last few weeks before i take my written test for my airframe/general and am a lil braindead from all the prep test we have been doing in school.
Fri Jul 25, 2008 4:04 pm
JDK, good thought. I'd really like to understand what I'm looking at when I see a propeller driven aircraft: why did spits start out with two blades and end up with, I think, five? Why does a b-25 have three and a Marauder have four? Does a b-29 have four blades and a b-17 three because the 29 has more powerful engines? Why does a p-38 swing three per engine, but a black widow four?
I'm not looking for a mini course in aeronautics, but would like to know what the thinking behind the number of blades on a given prop is...
I'm not looking for a mini course in aeronautics, but would like to know what the thinking behind the number of blades on a given prop is...
Fri Jul 25, 2008 4:17 pm
michaelharadon wrote:JDK, good thought. I'd really like to understand what I'm looking at when I see a propeller driven aircraft: why did spits start out with two blades and end up with, I think, five?
Matt's answer is a good general one; to summarise that, it's the best compromise between the factors he mentioned.
Spitfire (and Hurricane) Mk.I aircraft started out with fixed pitch two-blade Watts wooden propellers; these were like driving a car with one gear, and a compromise between best for take off and higher speeds. Both Spitfire and Hurricane Mk.I aircraft were switched to DH or Rotol type three blade variable pitch constant speed propellers as soon as there were enough avaliable, and the engines they needed to have the prop mounted to (oil system pitch change); they could have been fitted with two bladed VP props, but the best conversion of available power was using a three bladed prop.
Likewise the step up to a higher powered Merlin called for a four blade prop (On both Spitfire Mk.VIII / IX etc, and Hurricane Mk.IV / V). When the Griffon came in (for Spitfires) more powerful still, a five blade setup enabled the power to be best deployed while using a shorter set of blades that allowed sufficient ground clearance - a set of four blades for that power would be too long.
Certain late Griffon Spitfires (Mk.21) and Seafires (46, 47) had two sets of three-blade contra-rotating props to remove the torque effect and ensure the aircraft tracked straight.
Rather like car tyres or gear ratios, there's no 'best' but 'best compromise' for the job.
Hope that's a) correct (corrections welcome) and b) useful.
Fri Jul 25, 2008 4:19 pm
michaelharadon wrote:JDK, good thought. I'd really like to understand what I'm looking at when I see a propeller driven aircraft: why did spits start out with two blades and end up with, I think, five? Why does a b-25 have three and a Marauder have four? Does a b-29 have four blades and a b-17 three because the 29 has more powerful engines? Why does a p-38 swing three per engine, but a black widow four?
I'm not looking for a mini course in aeronautics, but would like to know what the thinking behind the number of blades on a given prop is...
#1 is Horsepower.
#2 is size of the A/C in relation to the space available to swing the prop.
#3 is the anticipated speed.
Rich
Fri Jul 25, 2008 4:26 pm
51fixer wrote:#1 is Horsepower.
#2 is size of the A/C in relation to the space available to swing the prop.
#3 is the anticipated speed.
Rich
And this is precisely why you'll see the early Corsairs with three bladed units and the later -4 and -5 variants with four bladed props.
Fri Jul 25, 2008 4:41 pm
Also, one of the reasons for changing to a 4-blade prop on the Corsair wasn't as much of an issue with increased HP (it was all of 400 HP more), but an increase in torque. With the introduction of the -4 model, the torque had increased sufficiently that another blade was required to maintain the right amount of resistance to the engine and absorb enough of it to keep the engine spinning at the right speed.
Fri Jul 25, 2008 6:49 pm
Engineering speak ON:
To sum up the several excellent replies above: designing airplanes (or anything else) is often all about compromise. There is rarely "one right answer" - there are several "almost right answers", each with pluses and minuses.
The Hurricane, Spit and Corsair prop changes are also examples of "controlling the scope" of design changes. Yes, we want more horsepower, but no we don't want new landing gear to provide clearance for a bigger diameter prop (because that means a new and heavier wing, and then we need more horsepower, and then...).
Back in WW2 "controlling the scope" meant getting the changes into the front lines quicker, where people were being killed every day. Today, quick changes keep the accountants happy, but may sometime confuse or amuse the customers.
Engineering speak OFF.
To sum up the several excellent replies above: designing airplanes (or anything else) is often all about compromise. There is rarely "one right answer" - there are several "almost right answers", each with pluses and minuses.
The Hurricane, Spit and Corsair prop changes are also examples of "controlling the scope" of design changes. Yes, we want more horsepower, but no we don't want new landing gear to provide clearance for a bigger diameter prop (because that means a new and heavier wing, and then we need more horsepower, and then...).
Back in WW2 "controlling the scope" meant getting the changes into the front lines quicker, where people were being killed every day. Today, quick changes keep the accountants happy, but may sometime confuse or amuse the customers.
Engineering speak OFF.
Fri Jul 25, 2008 7:39 pm
Let's not overlook harmonics.
While certain engine/ prop combinations work well with one airframe...it may be unhappy in another.
Two engines of different makes of identical horsepower, intended for the identical airframe, and designed for the same operating parameters may require two completely different propeller configurations due to prop/crank shaft strain. The two different engines could have entirely different orders of balancing and gearing causing these harmonics. It appears inlines with long busy cranks are more sensitive to propeller selection and matching, and reap bigger benefits (increased reliability, longer TBO, etc) when chosen carefully.
While certain engine/ prop combinations work well with one airframe...it may be unhappy in another.
Two engines of different makes of identical horsepower, intended for the identical airframe, and designed for the same operating parameters may require two completely different propeller configurations due to prop/crank shaft strain. The two different engines could have entirely different orders of balancing and gearing causing these harmonics. It appears inlines with long busy cranks are more sensitive to propeller selection and matching, and reap bigger benefits (increased reliability, longer TBO, etc) when chosen carefully.
Fri Jul 25, 2008 8:35 pm
Good point, Wheels Up. Prop/engine harmonics and resonances are what causes the red "avoid zones" on a tachometer. Operating in the red zone can reduce your fatigue life by a factor of 100 or more. Changing the number of blades results in a big change in the frequencies that things are shakin' at.
Fri Jul 25, 2008 9:33 pm
A case to study in regards to engine and props is the B version of the R-2800- its been a while since I have worked on this engine so I don't recall all the dash numbers in exact detail.
The engine was built up in different configurations of nosecase and accessory sections. However the power section, that is the cylinder and crankcase with supercharger was basically the same. The different nosecases had various gear ratios to deal with the length of blades required and the speed the blade needed to turn to manage torque. Remember the prop can only spin so fast as supersonic speeds deteriorate efficiency. The accessory section bolted on the back of the engine has various setups. Carb on top, carb on bottom, extra 2 speed supercharger, ect.
That is why a similar engine was in the early Corsair and Hellcat which used the same prop. The P-47, C-46, P-61 all had different props.
Later the R-2800 evolved into a different engine with the same cubic inches. The later version R-2800, when compared side by side to the earlier version doesn't look the same. The prop shaft went from 50 spline to a larger 60 spline so not even the same props as earlier would even fit.
If the aircraft is flown in the US as a certificated aircraft the manufacture will authorize what prop(s) are allowed to be used. Based on their testing and engineering the FAA will stamp its approval and you can't use anything else unless someone goes through a process that requires engineering and testing of a new prop and then proves to the FAA that it meets certain requirements and it is given an STC. This is all in the data that the FAA keeps and is used to inspect and maintain flying aircraft.
Rich
The engine was built up in different configurations of nosecase and accessory sections. However the power section, that is the cylinder and crankcase with supercharger was basically the same. The different nosecases had various gear ratios to deal with the length of blades required and the speed the blade needed to turn to manage torque. Remember the prop can only spin so fast as supersonic speeds deteriorate efficiency. The accessory section bolted on the back of the engine has various setups. Carb on top, carb on bottom, extra 2 speed supercharger, ect.
That is why a similar engine was in the early Corsair and Hellcat which used the same prop. The P-47, C-46, P-61 all had different props.
Later the R-2800 evolved into a different engine with the same cubic inches. The later version R-2800, when compared side by side to the earlier version doesn't look the same. The prop shaft went from 50 spline to a larger 60 spline so not even the same props as earlier would even fit.
If the aircraft is flown in the US as a certificated aircraft the manufacture will authorize what prop(s) are allowed to be used. Based on their testing and engineering the FAA will stamp its approval and you can't use anything else unless someone goes through a process that requires engineering and testing of a new prop and then proves to the FAA that it meets certain requirements and it is given an STC. This is all in the data that the FAA keeps and is used to inspect and maintain flying aircraft.
Rich
Fri Jul 25, 2008 10:18 pm
Matt Gunsch wrote:They made a 1 bladed prop for 65hp T-crafts
Do you have pictures or a source on that!
Ryan
Fri Jul 25, 2008 10:32 pm
RyanShort1 wrote:Matt Gunsch wrote:They made a 1 bladed prop for 65hp T-crafts
Do you have pictures or a source on that!
Ryan
Even stranger, one of my former employers made a one bladed helicopter. The Bo 102 was a tethered trainer, while the Bo 103 actually flew.
http://sandbox.plone.av.hu/PS1/exhibition/bolkow-bo-103
One blade is lighter and cheaper, even with the balance weight. The one bladed propellers are the same. The total swept disc area is bigger than a multi-blade disc, but for a light helicopter (or dinky little airplane) that doesn't matter. In technical terms, 150% of not very much is still not very much.