Van's Air Force
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prop length
- Thread starter miyu1975
- Start date
John Courte
Well Known Member
It is if the nose is pointed up or straight. The only reason I can think of is that with a nosewheel, it's less likely the aircraft will somehow be in a nose-down attitude on the ground, resulting in a strike with a longer prop.
Add to that the fact that the pivot point of the ship rolling on the main gear is a bit aft of the same situation in a nosewheel, which adds length to the (whatever engineers call one side of a lever) resulting in more vertical travel for the nose for a given angle of rotation.
propeller magic
I just don't understand prop stuff.
Ed Sterba cut me a 68" (72 pitch) prop for my 150 HP -9A.
It seems to work great & I'm happy.
Some have looked at my prop and commented that it looks too small (short). It does look kinda short.
I have about 12" of ground clearance and I have not picked up any rocks & dinged the prop so far. That's a good thing..
If a 68" prop is good, why would a longer(72") prop be better?
I'm just glad there are people out there that understand these things
Dave (electrical engineer ignorant about things aeronautical...)
-9A
I just don't understand prop stuff.
Ed Sterba cut me a 68" (72 pitch) prop for my 150 HP -9A.
It seems to work great & I'm happy.
Some have looked at my prop and commented that it looks too small (short). It does look kinda short.
I have about 12" of ground clearance and I have not picked up any rocks & dinged the prop so far. That's a good thing..
If a 68" prop is good, why would a longer(72") prop be better?
I'm just glad there are people out there that understand these things
Dave (electrical engineer ignorant about things aeronautical...)
-9A
Neal@F14
Well Known Member
I think that if you're pitched so far nose down in a taildragger RV that you're counting on only an extra 1" of length on each blade to spare you from a prop strike, you're probably going to suffer that prop strike anyway. The RV-8 in my avatar has the 74" Hartzell blended airfoil prop and it is a fine fit on that aircraft. We've lifted the tail up in the hangar to see what the prop clearance is like, and the tail has to come up in the air to an astonishing height before the prop blade tip would ever touch. If you ever saw that kind of abnormal ground sight picture in the windshield while rolling down the runway, you would've already resigned yourself that you were already way too deep into the process of having a propstrike. And if you're hitting on the mains so hard as to compress them down enough to hit the prop without the tail being way up too high, then you weren't landing... you were already crashing anyway. Maybe with the springier main gear of -4, -6, -7, -9 this may be more of a factor to consider for deciding on the shorter prop, but the with the tall, heavy-duty rectangular main gear of the -8, I don't think the 74" prop poses any ground clearance problem at all.
The primary reason for the shorter prop for the taildragger is for reserve distance between the prop and runway during a botched wheel landing.
In a normal taxi, takeoff and three-point landing confguration, the prop clearance in a tail dragger is greater than the tri-cycle gear.
In a tri-gear, one can reliably depend on the prop to ground clearance because of the nose-wheel and fixed distance between the ground and prop. Naturally, a severe landing or porpoised landing, all bets are off.
However, in a tailwheel, when doing a wheel landing, one intentionally pitches the nose forward, as soon as the mains grace the runway. In the case of an over-zealous pitch forward, the only thing to keep the prop off the runway is pilot skill. In a normal wheel landing, there is ample room to support a 72 or 74 inch prop on a RV7. But, a 74-inch leaves less reserve should one over-compensate in forward pitch.
You can check the static ground clearance by raising the tail and setting zero degree pitch attitude. But, that is a static stance, envision some gear flex on landing, with a 20-knot cross wind and bumpy grass runway, you might see a little PIO as the intrepid aviatior guides his craft to a uneventful stop.
I think....
In a normal taxi, takeoff and three-point landing confguration, the prop clearance in a tail dragger is greater than the tri-cycle gear.
In a tri-gear, one can reliably depend on the prop to ground clearance because of the nose-wheel and fixed distance between the ground and prop. Naturally, a severe landing or porpoised landing, all bets are off.
However, in a tailwheel, when doing a wheel landing, one intentionally pitches the nose forward, as soon as the mains grace the runway. In the case of an over-zealous pitch forward, the only thing to keep the prop off the runway is pilot skill. In a normal wheel landing, there is ample room to support a 72 or 74 inch prop on a RV7. But, a 74-inch leaves less reserve should one over-compensate in forward pitch.
You can check the static ground clearance by raising the tail and setting zero degree pitch attitude. But, that is a static stance, envision some gear flex on landing, with a 20-knot cross wind and bumpy grass runway, you might see a little PIO as the intrepid aviatior guides his craft to a uneventful stop.
I think....
Efficiency vs diameter
One of the things that effects the efficiency of a propeller is the mass flow through the disc. Since thrust=mass-flow X deltaV, and deltaV is a loss, the more mass flow, the lower the deltaV, the downwash, and the more efficient a propeller can be. Since mass flow is the disc area X (forward speed + deltaV), the disc area goes up with the square of the diameter. So in going from a 68" to a 72" increases the area and mass flow 12%, and decreases downwash 12% which can allow the prop to be more efficient. However, a larger diameter prop will have a higher tip speed, and if the tip chord is wide, it will have a lot of tip loss. This is why, since the mass flow is also dependent on the number of blades, with a multi-blade prop you can have the smaller diameter for more ground clearance, lower tip loss, and still have good mass-flow.
One of the things that effects the efficiency of a propeller is the mass flow through the disc. Since thrust=mass-flow X deltaV, and deltaV is a loss, the more mass flow, the lower the deltaV, the downwash, and the more efficient a propeller can be. Since mass flow is the disc area X (forward speed + deltaV), the disc area goes up with the square of the diameter. So in going from a 68" to a 72" increases the area and mass flow 12%, and decreases downwash 12% which can allow the prop to be more efficient. However, a larger diameter prop will have a higher tip speed, and if the tip chord is wide, it will have a lot of tip loss. This is why, since the mass flow is also dependent on the number of blades, with a multi-blade prop you can have the smaller diameter for more ground clearance, lower tip loss, and still have good mass-flow.
F1Boss
Well Known Member
Steve Wittman said:
"Keep your prop as long as you can, as long as you can."
The 72 might climb a bit better, but the 74 will be more efficient in cruise. Since you spend more time cruising than you do climbing,....
Paul can jump back in, but it might be that the 72 would be more suited to any SARL activities?
Carry on!
Mark
"Keep your prop as long as you can, as long as you can."
The 72 might climb a bit better, but the 74 will be more efficient in cruise. Since you spend more time cruising than you do climbing,....
Paul can jump back in, but it might be that the 72 would be more suited to any SARL activities?
Carry on!
Mark
A shorter prop should give lower climb rate, rather than higher, all other things being equal. The reason for this is that the longer prop will have greater mass flow and so be more efficient. The same is true for multi-blade props because for a given diameter, a three-blade prop will have 50% more mass flow, so less energy gets thrown away in downwash. But all other things are not equal. Some propeller designs are more efficient than others. If your blades have a round tip or a wide tip is it less efficient than a blade that has a very narrow or zero chord tip. Do your blades have a clunky, non-aerodynamic shape in the root just outside of the spinner, with a flat, triangular area about 2"-3" long top and bottom? Then it is not as efficient as a blade that has the correct airfoil shape at the correct helix angle all the way to the spinner. Does your spinner have wide clearance around the blade or is it sealed up to the blade? Big holes make drag! All of these factors enter into a prop's efficiency, so comparing two blades of different diameters, planforms, and streamlining is futile!
This extends also to the back-and-forth about CS vs FP. Even though you can operate the engine more efficiently at high MAP and low rpm with a CS, if the CS blade has any of the drawbacks just listed, a well-designed FP can actually give more efficient cruise at a higher speed!
hevansrv7a
Well Known Member
Some questions for Paul
Please see embedded questions. Not arguing, seeking to understand.
Please see embedded questions. Not arguing, seeking to understand.
smokyray
Well Known Member
Practicality
Wow, if you think that was a cool write-up, you should see Paul's latest post in "Contact" magazine! Having never had the experience or knowledge Paul has, I learned everything he stated the hard way. Longer props give better takeoff performance! That's why the best Super Cub prop is the "Borer" prop or seaplane prop, the longest available. Ever seen a certified FP prop that the spinner doesn't fit closely around?
I built my short gear RV4 in the 80's when Van's was in North Plains, Reagan was President, RV's were cheap and props were predominately wood. The best length for my operations over time in my short gear 0-320 RV4 I found was 69" masterfully made for me by Ed Sterba. Later, I worked very hard with Craig Catto to develop an acceptable pitch/length number for a 2 blade prop. The final result (1999) was a 69" Catto 2 blade with 71" pitch. This gave me the best compromise in performance with ground clearance on unimproved strips. 80% of my landings are off pavement which for me is an important factor. Not too many of us left in the RV world from back in the days of "the RV4 being the highest number". The ones that are will tell you Paul is right on the money.
A properly rigged RV3/4/6 or any of the newer RV's with equal HP and the right prop will match or exceed any C/S out there. For the current generation "pour the clecos in the kit box, shake it and out pops and airplane crowd" (RV7/8/9/10/12) you don't have to spend a fortune on panel/engine/prop to have a nice, efficient airplane.
Nuff said...
Smokey
RVX
Wow, if you think that was a cool write-up, you should see Paul's latest post in "Contact" magazine! Having never had the experience or knowledge Paul has, I learned everything he stated the hard way. Longer props give better takeoff performance! That's why the best Super Cub prop is the "Borer" prop or seaplane prop, the longest available. Ever seen a certified FP prop that the spinner doesn't fit closely around?
I built my short gear RV4 in the 80's when Van's was in North Plains, Reagan was President, RV's were cheap and props were predominately wood. The best length for my operations over time in my short gear 0-320 RV4 I found was 69" masterfully made for me by Ed Sterba. Later, I worked very hard with Craig Catto to develop an acceptable pitch/length number for a 2 blade prop. The final result (1999) was a 69" Catto 2 blade with 71" pitch. This gave me the best compromise in performance with ground clearance on unimproved strips. 80% of my landings are off pavement which for me is an important factor. Not too many of us left in the RV world from back in the days of "the RV4 being the highest number". The ones that are will tell you Paul is right on the money.
A properly rigged RV3/4/6 or any of the newer RV's with equal HP and the right prop will match or exceed any C/S out there. For the current generation "pour the clecos in the kit box, shake it and out pops and airplane crowd"
(RV7/8/9/10/12) you don't have to spend a fortune on panel/engine/prop to have a nice, efficient airplane. Nuff said...
Smokey
RVX
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John Courte
Well Known Member
This is a good thread. Just to make sure, I checked my prop specs, because I'm too much of a noob to be flying with a 74" prop on a taildragger -7. 72 inches. I haven't thought about the prop in a while, good to get familiar with all those parts in the shop waiting to go on an airplane.
But the argument still stands that if you nose over far enough to hit the prop, you got bigger fish to fry. Didn't take into consideration the spring flex on the -7 mains though. Good point, especially for rough field operations.
But the argument still stands that if you nose over far enough to hit the prop, you got bigger fish to fry. Didn't take into consideration the spring flex on the -7 mains though. Good point, especially for rough field operations.
tomwebster
Well Known Member
Not to muddy the waters, but
I don't know about the 74" prop, but
I think my '72' Sensenich is actually
71" long.
Tom
I don't know about the 74" prop, but
I think my '72' Sensenich is actually
71" long.
Tom
Bill Dicus
Well Known Member
Prop length
If you're talking the Hartzell cs props one factor is that the 74 inch can be shortened for tip damage whereas the 72 inch cannot. Len Kaufman has the 74 on his beautiful RV-8 for that reason IIRC.
If you're talking the Hartzell cs props one factor is that the 74 inch can be shortened for tip damage whereas the 72 inch cannot. Len Kaufman has the 74 on his beautiful RV-8 for that reason IIRC.
F1Boss
Well Known Member
not so fast there...
Paul sez:
"All of these factors enter into a prop's efficiency, so comparing two blades of different diameters, planforms, and streamlining is futile!"
The initial question was should the user purchase the longer or shorter version of the Hartzell BA 2 blade CS prop. This was not a CS/FP question; a search of this site will reveal the many opinions to that particular question.
My guess is that other factors (streamlining etc) are not entered into this equation -- it is simply an A or B question.
My answer is B: go for the longer version. This will require a higher level of skill during any wheel landing activities, and on rough strips.
Carry on!
Mark
Paul sez:
"All of these factors enter into a prop's efficiency, so comparing two blades of different diameters, planforms, and streamlining is futile!"
The initial question was should the user purchase the longer or shorter version of the Hartzell BA 2 blade CS prop. This was not a CS/FP question; a search of this site will reveal the many opinions to that particular question.
My guess is that other factors (streamlining etc) are not entered into this equation -- it is simply an A or B question.
My answer is B: go for the longer version. This will require a higher level of skill during any wheel landing activities, and on rough strips.
Carry on!
Mark
I don't claim to be an expert, but I can't see any advantage to going with a 74" prop. The 72" props we use on most RVs are plenty big enough to absorb 180hp. I'd go for the extra ground clearance. Of course you and I would never do it, but a certain combination of PIO with a hard touch down would bring your prop closer to the ground than you could imagine.
Also, on our airport with sloping taxiways (needing a shot of power in places) rock chips are a major annoyance.
Also, on our airport with sloping taxiways (needing a shot of power in places) rock chips are a major annoyance.
hevansrv7a
Well Known Member
How much efficiency change?
I used the program written by Andy Bauer to test some typical numbers:
RPM=2700
Altitude=8000'
BHP=135 (75% of 180)
TAS=200 mph
2 blades
If prop diameter is 72" then the program can produce a prop with a design efficiency of 89.95%,
If prop diameter is 74" then the program can produce a prop with a design efficiency of 90.35%.
The program does show clearly that the smaller prop must accelerate the air more to produce the same TAS with the same BHP. Paul is, of course, correct on that.
The efficiency delta is a little less than 1/2%, prop to prop.
In real life, the prop efficiency would be less. I would expect the delta % to be relatively constant, though. Just my guess.
Just thought you might want to know. Given such a small delta, I'd go for the 72" prop which would be lighter and stronger and less liable to be damaged. You may see it differently.
I used the program written by Andy Bauer to test some typical numbers:
RPM=2700
Altitude=8000'
BHP=135 (75% of 180)
TAS=200 mph
2 blades
If prop diameter is 72" then the program can produce a prop with a design efficiency of 89.95%,
If prop diameter is 74" then the program can produce a prop with a design efficiency of 90.35%.
The program does show clearly that the smaller prop must accelerate the air more to produce the same TAS with the same BHP. Paul is, of course, correct on that.
The efficiency delta is a little less than 1/2%, prop to prop.
In real life, the prop efficiency would be less. I would expect the delta % to be relatively constant, though. Just my guess.
Just thought you might want to know. Given such a small delta, I'd go for the 72" prop which would be lighter and stronger and less liable to be damaged. You may see it differently.
Captain Avgas
Well Known Member
Shooting yourself in the foot.
Yes true, but with the 74" prop you're much more likely to have a prop strike in the first place. In this particular case maybe an old adage might be applicable: Prevention is better than cure.
Also consider this....in most cases a constant speed prop strike is an insurance claim. If you have a 72" prop you are guaranteed to get a nice sparkly new prop from the insurance company (it can't be repaired if there's any tip damage). If you have a 74" prop you might just end up getting the damaged prop repaired if that's the cheapest option for the insurance company. In other words you might just be shooting yourself in the foot.
There has been talk on this thread of prop strikes and landing techniques but most of my friends who have had prop strikes did so while taxiing across unmade areas. One went into a dirt mound. another went into a dip, and yet another hit a tie down. You just can't have too much prop clearance. It's easier to have a prop strike than you think.
I don't think builders should get obsessed about absolutely piddling theoretical efficiency gains from a 74" prop (that may or may not exist). The only thing that is certain about a 74" prop is that it will significantly increase the chances of having a prop strike....and a prop strike will REALLY ruin your day and your prop (and perhaps your engine as well).
Yes true, but with the 74" prop you're much more likely to have a prop strike in the first place. In this particular case maybe an old adage might be applicable: Prevention is better than cure.
Also consider this....in most cases a constant speed prop strike is an insurance claim. If you have a 72" prop you are guaranteed to get a nice sparkly new prop from the insurance company (it can't be repaired if there's any tip damage). If you have a 74" prop you might just end up getting the damaged prop repaired if that's the cheapest option for the insurance company. In other words you might just be shooting yourself in the foot.
There has been talk on this thread of prop strikes and landing techniques but most of my friends who have had prop strikes did so while taxiing across unmade areas. One went into a dirt mound. another went into a dip, and yet another hit a tie down. You just can't have too much prop clearance. It's easier to have a prop strike than you think.
I don't think builders should get obsessed about absolutely piddling theoretical efficiency gains from a 74" prop (that may or may not exist). The only thing that is certain about a 74" prop is that it will significantly increase the chances of having a prop strike....and a prop strike will REALLY ruin your day and your prop (and perhaps your engine as well).
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Bill Dicus
Well Known Member
Prop length
Good points Bob. I hadn't thought through the insurance scenario you describe. When I had a prop strike due to my own stupidity, being 72 inches long wouldn't have saved the prop. Insurance did pay, less deductable and depreciation, for a shiny new prop. The prop on my -8 (still in garage) is a 72 inch BA Hartzell and I fervently hope never to damage it!
Good points Bob. I hadn't thought through the insurance scenario you describe. When I had a prop strike due to my own stupidity, being 72 inches long wouldn't have saved the prop. Insurance did pay, less deductable and depreciation, for a shiny new prop. The prop on my -8 (still in garage) is a 72 inch BA Hartzell and I fervently hope never to damage it!
Captain Avgas
Well Known Member
Check the archives
The 72" vs 74" debate has been extensively covered in the archives. Probably the best thread is this one: http://www.vansairforce.com/community/showthread.php?t=4366&highlight=prop+length
In Post #6 of that thread gmcjetpilot (good ol' George) makes a convincing argument based on mathematics and discussions with Hartzell that a 72" prop may provide a VERY SLIGHT top speed advantage and that a 74" prop may provide a VERY SLIGHT climb advantage. But a full reading of the post should convince anyone that the advantages either way are truly miniscule and are not worth worrying about in the real world (unless maybe you plan to race at Reno and desperately need half a knot....in which case you'd opt for the 72").
I would propose therefore that the only real difference between the Hartzell 72" prop and the 74" prop is their propensity for a ground strike....and in that category the 72" prop with its greater ground clearance clearly wins.
But there will always be builders out there who believe that at the same price the 74" prop gives you more metal and therefore must be the better bargain. I guess it's the bigger must be better (size really counts) philosophy.
I was a passenger in an RV that suffered a prop strike a couple of weeks ago. The strike caused the crank shaft to completely shear off and the prop departed the aircraft taking the starter ring gear with it. Needless to say the prop and the engine (only 100 hours since brand new) were totally decimated. If I wasn't a believer already that was enough to convince me that CLEARANCE IS KING. It's like money...you just can't have enough of it.
The 72" vs 74" debate has been extensively covered in the archives. Probably the best thread is this one: http://www.vansairforce.com/community/showthread.php?t=4366&highlight=prop+length
In Post #6 of that thread gmcjetpilot (good ol' George) makes a convincing argument based on mathematics and discussions with Hartzell that a 72" prop may provide a VERY SLIGHT top speed advantage and that a 74" prop may provide a VERY SLIGHT climb advantage. But a full reading of the post should convince anyone that the advantages either way are truly miniscule and are not worth worrying about in the real world (unless maybe you plan to race at Reno and desperately need half a knot....in which case you'd opt for the 72").
I would propose therefore that the only real difference between the Hartzell 72" prop and the 74" prop is their propensity for a ground strike....and in that category the 72" prop with its greater ground clearance clearly wins.
But there will always be builders out there who believe that at the same price the 74" prop gives you more metal and therefore must be the better bargain. I guess it's the bigger must be better (size really counts) philosophy.
I was a passenger in an RV that suffered a prop strike a couple of weeks ago. The strike caused the crank shaft to completely shear off and the prop departed the aircraft taking the starter ring gear with it. Needless to say the prop and the engine (only 100 hours since brand new) were totally decimated. If I wasn't a believer already that was enough to convince me that CLEARANCE IS KING. It's like money...you just can't have enough of it.
The same is true for multi-blade props because for a given diameter, a three-blade prop will have 50% more mass flow. (Doesn't this assume equal blades area, shape, CL, etc., which would not be feasible?), so less energy gets thrown away in downwash.(But aren't longer blades better for induced loss, if all else is equal?)
Mass-flow should not be confused with blade area. Lift, or thrust, is the product of area, Q, and CL. If you have more blades, you reduce the area on each blade by the ratio of the number of blades, but you keep the CL the same, so the overall thrust is the same. Induced loss is a function of mass-flow which has to do with disc area, which is what was stated in the first sentence. If you keep the diameter the same and increase the number of blades the mass flow goes up, the downwash, which has to do with induced loss, goes down, and the efficiency, at least at low speed, goes up. The latter is true since at high speed the delta-v only contributes a very small portion to the mass flow since most of it comes from forward speed. That is why a multi-blade prop has better static thrust and its attendant better take-off and climb, where speed and mass flow are low.
It's unfortunate that in the aviation community the mis-conception that multi-blade props aren't as good in cruise as a prop with fewer blades was based on the fact that the aerodynamic shape of the blade roots at that time contributed so much drag to the overall plane's drag; more blades, more drag. Unfortunately, the "experts" attributed this to some mythical "tip loss" fuction, and every one nodded their heads in affirmative unison and repeated "I see, I see!".
Even with Tom Aberle dominating the biplane races with a four-blade prop, the skeptics still attribute this to enough engine power to overcome the supposed multi-blade drawback. Just look at the 8-blade props on the A-400M, the same on the new C-131J, and the 18 to 24 blade fixed pitch fan (prop) in the front of a jet engine. Are these designers laboring under some delusion? It is time for the aviation community to put this misunderstanding to rest!
Ron Lee
Well Known Member
I was a passenger in an RV that suffered a prop strike a couple of weeks ago. The strike caused the crank shaft to completely shear off and the prop departed the aircraft taking the starter ring gear with it. Needless to say the prop and the engine (only 100 hours since brand new) were totally decimated. If I wasn't a believer already that was enough to convince me that CLEARANCE IS KING. It's like money...you just can't have enough of it.
Was that in a taildragger?
Captain Avgas
Well Known Member
It was an RV6A.
Kevin Horton
Well Known Member
Tell us more. What were the circumstances that lead to the prop strike?
Captain Avgas
Well Known Member
Kevin, it's not my plane. I was just a passenger. The damage is the subject of a pending insurance claim by the aircraft's owner and therefore it is not appropriate for me to be making any public comments about the circumstances.
The point I was trying to make however is that a prop strike, for whatever reason, can cause very serious damage to the engine and is therefore best avoided.
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A person has to really work to hit the prop on a wheel landing. Maybe with a VERY hard drop or a deliberate dive into the runway as some newbies do when trying to force a bouncing plane on the ground. Langwiesche correctly illustrates in S&R that airflow over the horizontal stabilizer will counter any attempt to hit the prop when doing a wheel landing. Once slowed down and with less flow over the stabilizer I suppose one could tip the plane over and hit the prop but that would take either some very deliberate intention or some extreme ineptitude.
I once had, with a KR-2 taildragger, about 4.5 inches of clearance in the level position and I was constantly worried about hitting the prop when wheel landing. It was a Maloof CS metal prop - quite pricey and not as forgiving as wood. This led to quite a lot of apprehension and less than graceful landings when I first got the plane.
Re-reading Langewiesche's chapter on Landings, I decided to trust his wisdom as I was tired of worrying about a prop strike. Many years later I continue to fly a taildragger with not a great deal of clearance in the level position and never worry in the slightest about a runway strike. Langewiesche is correct. Relax about prop strikes when landing. It can be done, but you really need to screw up badly to do it.
I once had, with a KR-2 taildragger, about 4.5 inches of clearance in the level position and I was constantly worried about hitting the prop when wheel landing. It was a Maloof CS metal prop - quite pricey and not as forgiving as wood. This led to quite a lot of apprehension and less than graceful landings when I first got the plane.
Re-reading Langewiesche's chapter on Landings, I decided to trust his wisdom as I was tired of worrying about a prop strike. Many years later I continue to fly a taildragger with not a great deal of clearance in the level position and never worry in the slightest about a runway strike. Langewiesche is correct. Relax about prop strikes when landing. It can be done, but you really need to screw up badly to do it.
