Contra Rotating Propeller Drive for f3a 2m Pattern Planes
05-10-2013, 09:37 AM
#1101
RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
"PS ; I like your design It looks like you plagiarized one of Bryan Heberts "
LOL Now Brian, O`l Buddy,
You have been begging me for set up info for your Euphoria bipe,and New Oxai Bipe for years now, pm`ing me in private ,every time I join a disscussion. Just when I`m feeling a little week and Guilty for not telling you the answer, and on the verge of spilling the beans, you want to jab me
I guess your going to have to buy one of my designs for the answer now ,Or! ,see if you can get it out of Shane, LOL
Well, good luck with Your contra bipe in The F pattern![:@]
Bryan
LOL Now Brian, O`l Buddy,
You have been begging me for set up info for your Euphoria bipe,and New Oxai Bipe for years now, pm`ing me in private ,every time I join a disscussion. Just when I`m feeling a little week and Guilty for not telling you the answer, and on the verge of spilling the beans, you want to jab me
I guess your going to have to buy one of my designs for the answer now ,Or! ,see if you can get it out of Shane, LOL
Well, good luck with Your contra bipe in The F pattern![:@]
Bryan
05-10-2013, 10:05 AM
#1102
RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Dean,
I hope your model needs a little refinement. If our cg's are anywhere near the wing tube (like most) we have negative yaw stability. What do you think?
Jim O
I hope your model needs a little refinement. If our cg's are anywhere near the wing tube (like most) we have negative yaw stability. What do you think?
Jim O
05-10-2013, 10:11 AM
#1103
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RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Hi Bryan,
Ok, lets assume the wing is big enough (which it is, I won't tell you the design). The wing shape/area/fuz shape/tail plane area cannot be altered. CG can, but I like it where it is so lets leave CG out for the moment. Apart from using a rudder flare (which essentially is just altering the airfoil) is there anything else that can be done to increase yaw stability? Remember, no add ons. You say wing airfoil but this requires a new wing. Something I am not ready to do yet. However, please explain how airfoil can affect yaw stability.
Next question: Can the amount of lift a fuz produces at low angles of attack be increased without changing the shape or area of the fuz? No canalysers or add ons please. I'm thinking turbulators here. How about those little canopy strakes Naruke uses on the Asyuler Eternal? To prevent the air spilling over the fuz and reducing the lift I'm guessing? Add ons I know but I might just get away with it....
Ok, lets assume the wing is big enough (which it is, I won't tell you the design). The wing shape/area/fuz shape/tail plane area cannot be altered. CG can, but I like it where it is so lets leave CG out for the moment. Apart from using a rudder flare (which essentially is just altering the airfoil) is there anything else that can be done to increase yaw stability? Remember, no add ons. You say wing airfoil but this requires a new wing. Something I am not ready to do yet. However, please explain how airfoil can affect yaw stability.
Next question: Can the amount of lift a fuz produces at low angles of attack be increased without changing the shape or area of the fuz? No canalysers or add ons please. I'm thinking turbulators here. How about those little canopy strakes Naruke uses on the Asyuler Eternal? To prevent the air spilling over the fuz and reducing the lift I'm guessing? Add ons I know but I might just get away with it....
There is absolutly a way to increase yaw stability, I have been hinting at the solution for some time. But it is over looked as being useless info or rambelings from a poor speller, So I`ll "throw a bone" out.
Increase the wing square area! and or get a better airfoil.
If you reduce the area of your wing and then add a small wing, to compensate (canilizer) put negative in the small wing and make it work against the top wing (more speed More effect)Like the Nuance and the old Axiom is set up (small wing Neg Can) there is no way to get the lift required to do a ,verticle up line or knife edge and keep it from pulling to the canopy. For the airplane to fly horizontal, with inefficient wings the elevator needs up trim.( to lift the 1g required) In turn the up trim, makes the airplane go to the canopy in up lines, down lines and knife edges. Thats why the airplane has to fly with pos inc. in a up line, down line, knife edge.
There is no trickery in wing shape to make a Airplane snap better and fly the easy stuff.
You cannot overcome canopy pull and,yaw problems, if the wing is too small, because every time you add inc. you have to move the c/g forward.You cannot over come this with down thrust, because the more down thrust you run, the more up trim is needed to compensate for the down thrust, and you also have to move the c/g forward. Can you see the Circle? So, old school modelers figure this out as being nose heavy. They add tail weight, this in turn destabilizes it more! Yaw becomes uncontrolable and unreliable. Simple as that! this requires eccesive right thrust, (SSS debate)and the Big Brakes make it worse.
Although Dean has a great peice of math ,the problem is not with the placement of the wing it`s with the wing being to small.
If you move it forward you cannot get the c/g placement right because you can`t shove the battery farthur up the nose.
to get the stability back.
I was Lured to make smaller wings Because it was the cool thing to do! and if I wanted to be able to sell in the Market.
It`s hard to overcome trends when your thinking is in the opposite direction. And you don`t win a major event, But you will see all the top designers going back to 1050SQ wings or bigger shortly, SO I`ll fit in again the Myth that a big wing can`t snap is just that ,or a Bipe could never do it.
I tried to walk the tight rope of small enough to fit in and big enough to do the job. Small wings have Made all modern airplanes easy to snap ,but hard to fly the gravy stuff. and no amount of adjusting on a airplane with the wings to small will give you a happy airplane every where. The Contra drive Just makes it worse for this condition.
This has been Information I have sat on for some time , Take it or leave it, Laugh if you like (as Usual) But,if your smart you will run with it I hate to speak in parables,,But if I don`t, when the answers are discoverd they forget were they got um LOL, SO if I make the debate fun it`s remembered better
Now,This is Why I`m against Add ons , they cover up the real problems with trimming airplanes and we chase the effect not the root cause.
Bryan
Increase the wing square area! and or get a better airfoil.
If you reduce the area of your wing and then add a small wing, to compensate (canilizer) put negative in the small wing and make it work against the top wing (more speed More effect)Like the Nuance and the old Axiom is set up (small wing Neg Can) there is no way to get the lift required to do a ,verticle up line or knife edge and keep it from pulling to the canopy. For the airplane to fly horizontal, with inefficient wings the elevator needs up trim.( to lift the 1g required) In turn the up trim, makes the airplane go to the canopy in up lines, down lines and knife edges. Thats why the airplane has to fly with pos inc. in a up line, down line, knife edge.
There is no trickery in wing shape to make a Airplane snap better and fly the easy stuff.
You cannot overcome canopy pull and,yaw problems, if the wing is too small, because every time you add inc. you have to move the c/g forward.You cannot over come this with down thrust, because the more down thrust you run, the more up trim is needed to compensate for the down thrust, and you also have to move the c/g forward. Can you see the Circle? So, old school modelers figure this out as being nose heavy. They add tail weight, this in turn destabilizes it more! Yaw becomes uncontrolable and unreliable. Simple as that! this requires eccesive right thrust, (SSS debate)and the Big Brakes make it worse.
Although Dean has a great peice of math ,the problem is not with the placement of the wing it`s with the wing being to small.
If you move it forward you cannot get the c/g placement right because you can`t shove the battery farthur up the nose.
to get the stability back.
I was Lured to make smaller wings Because it was the cool thing to do! and if I wanted to be able to sell in the Market.
It`s hard to overcome trends when your thinking is in the opposite direction. And you don`t win a major event, But you will see all the top designers going back to 1050SQ wings or bigger shortly, SO I`ll fit in again the Myth that a big wing can`t snap is just that ,or a Bipe could never do it.
I tried to walk the tight rope of small enough to fit in and big enough to do the job. Small wings have Made all modern airplanes easy to snap ,but hard to fly the gravy stuff. and no amount of adjusting on a airplane with the wings to small will give you a happy airplane every where. The Contra drive Just makes it worse for this condition.
This has been Information I have sat on for some time , Take it or leave it, Laugh if you like (as Usual) But,if your smart you will run with it I hate to speak in parables,,But if I don`t, when the answers are discoverd they forget were they got um LOL, SO if I make the debate fun it`s remembered better
Now,This is Why I`m against Add ons , they cover up the real problems with trimming airplanes and we chase the effect not the root cause.
Bryan
05-10-2013, 10:19 AM
#1104
RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Bryan,
A joke at your expense
Pity you could not see that.
It's said don't give it if you can't take it
Re the PM ; I took PM to mean Private Message.
From your post it would seem you also understand it to mean 'private'.
As it seems you don't know what private means here you go ; 'Not open to the public' [X(]
Please don't confuse being asked politely with being begged.
While I'm at it I suggest you also refrain from exaggeration - that's the root of lots of trouble.
You spend a lot of time here telling us that you know it all / have all the answers.
I'm sorry for interpreting that as ; 'Maybe this guy can actually help'.
Brian
Brenner ; Apologies for the distraction.
A joke at your expense
Pity you could not see that.It's said don't give it if you can't take it
Re the PM ; I took PM to mean Private Message.
From your post it would seem you also understand it to mean 'private'.
As it seems you don't know what private means here you go ; 'Not open to the public' [X(]
Please don't confuse being asked politely with being begged.
While I'm at it I suggest you also refrain from exaggeration - that's the root of lots of trouble.
You spend a lot of time here telling us that you know it all / have all the answers.
I'm sorry for interpreting that as ; 'Maybe this guy can actually help'.
Brian
Brenner ; Apologies for the distraction.
05-10-2013, 11:06 AM
#1105
RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Brian
If you ask more than three times and are told No Sorry politely, would you not call that begging?
I have helped Many who have asked for Bipe help. A lot of them do what I tell them, But can`t explain it or even understand it so they keep quiet. But only the ones I trust that do understand will recieve it and keep it to themselves. They have believed and used my methods for years and appreciate the knowledge They are the one I help not the guys who aurge and poke me
You don`t want my help in public you want it in private.
In Public you act as tho I`m a dumb ass! and then in Private act as if We are best friends
I call that manipulation LOL.
You would rather look smart in public [&o]
Now Can`t you see I was only joking too[8D]
I never said I knew it all I have given the world everything I know Yelling it out! if you just read, all the while being aurgued with and ridiculed. I only hold back the bipe info I have learned because if I give it out, it will easily be forgotton where it came from.
Everybody will adjust the Bipe designs, and be on par with mine. Have you called SuZuki for Help with his Plane ,Or QQ? WHy should you expect the help from a guy Whos design your POO POO!
SO from now on When I learn from very Hard Work and Many Many hours at the feild and in the shop, I will sell the info.
Because when I give it away it`s ridiculed or not appreciated.
I don`t try to come off as a know it all. I only counter what I know to be wrong. If you ever try what I recommend ,and follow it to the end, you understand. With all the help I give world wide, have you ever seen anyone say "well I tried what you said,and your full of Crap!" Nope they say Thank you. I contribute my learning to the debate, If you know something in and out you have a edge in the debate, because facts can`t be debated. Yea may be you can aurgue terms and spelling with me But thats it. This is why I can trim airplanes over the internet from China to New York ,without flying them, only from good feed back ,I know my Business to the core!
Just because I don`t jump on board with speculation, equasions or theory, from the The real Experts does not mean I can`t contribute to the discussion with my tried and true knowledge.
Angus
I mention airfoil because it contributes to effeciencey and flight envelope of the wing. This flys the airplane the root cause of all problems is when we forget this.
I have not tried the fuse strakes I`m sure it does something But , reducing weight, adding fin/rudder ,or making a bigger wing is the only fix for yaw issues. The bigger the prop the more the yaw issues are manifested.
I say Tape some fuse fences on , and we will both learn something.
I will say this however , if the stab inc. is off even .1 tenth of a degree you will have yaw issues you will be amazed at this overlooked adjustment,and how it effects yaw problems.
SO if you are running dif in elevator halves or stabs are miss aligned, fix that first,than me later
Bryan
If you ask more than three times and are told No Sorry politely, would you not call that begging?
I have helped Many who have asked for Bipe help. A lot of them do what I tell them, But can`t explain it or even understand it so they keep quiet. But only the ones I trust that do understand will recieve it and keep it to themselves. They have believed and used my methods for years and appreciate the knowledge They are the one I help not the guys who aurge and poke me
You don`t want my help in public you want it in private.
In Public you act as tho I`m a dumb ass! and then in Private act as if We are best friends
I call that manipulation LOL.
You would rather look smart in public [&o]
Now Can`t you see I was only joking too[8D]
I never said I knew it all I have given the world everything I know Yelling it out! if you just read, all the while being aurgued with and ridiculed. I only hold back the bipe info I have learned because if I give it out, it will easily be forgotton where it came from.
Everybody will adjust the Bipe designs, and be on par with mine. Have you called SuZuki for Help with his Plane ,Or QQ? WHy should you expect the help from a guy Whos design your POO POO!
SO from now on When I learn from very Hard Work and Many Many hours at the feild and in the shop, I will sell the info.
Because when I give it away it`s ridiculed or not appreciated.
I don`t try to come off as a know it all. I only counter what I know to be wrong. If you ever try what I recommend ,and follow it to the end, you understand. With all the help I give world wide, have you ever seen anyone say "well I tried what you said,and your full of Crap!" Nope they say Thank you. I contribute my learning to the debate, If you know something in and out you have a edge in the debate, because facts can`t be debated. Yea may be you can aurgue terms and spelling with me But thats it. This is why I can trim airplanes over the internet from China to New York ,without flying them, only from good feed back ,I know my Business to the core!
Just because I don`t jump on board with speculation, equasions or theory, from the The real Experts does not mean I can`t contribute to the discussion with my tried and true knowledge.
Angus
I mention airfoil because it contributes to effeciencey and flight envelope of the wing. This flys the airplane the root cause of all problems is when we forget this.
I have not tried the fuse strakes I`m sure it does something But , reducing weight, adding fin/rudder ,or making a bigger wing is the only fix for yaw issues. The bigger the prop the more the yaw issues are manifested.
I say Tape some fuse fences on , and we will both learn something.
I will say this however , if the stab inc. is off even .1 tenth of a degree you will have yaw issues you will be amazed at this overlooked adjustment,and how it effects yaw problems.
SO if you are running dif in elevator halves or stabs are miss aligned, fix that first,than me later
Bryan
05-10-2013, 12:30 PM
#1106
RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Bryan,
You got two queries from me. Straightforward requests - no acting as anything .
Each time you said yes to helping re my request 'to be pointed in a direction only'.
Each time you then withdrew the offer of help. This go / no go resulted in extra PM's ! Ah well.
Again I remind you they were 'PM's'.
I don't do begging.
I'm sorry you feel that 'I act as tho you're a dumb ass' - the feeling is yours , not mine.
Until recently I held your views in fairly high regard actually.
I'm comfortable with my public persona - I don't claim to be something I'm not.
If I have something to say I will say it. I don't do bones or crumbs.
Just so you know over here that's fodder for dogs or rats & mice.
Edit; I have never argued terms or spelling with you - that's somebody else - wires are getting crossed here.
Brian
You got two queries from me. Straightforward requests - no acting as anything .
Each time you said yes to helping re my request 'to be pointed in a direction only'.
Each time you then withdrew the offer of help. This go / no go resulted in extra PM's ! Ah well.
Again I remind you they were 'PM's'.
I don't do begging.
I'm sorry you feel that 'I act as tho you're a dumb ass' - the feeling is yours , not mine.
Until recently I held your views in fairly high regard actually.
I'm comfortable with my public persona - I don't claim to be something I'm not.
If I have something to say I will say it. I don't do bones or crumbs.
Just so you know over here that's fodder for dogs or rats & mice.
Edit; I have never argued terms or spelling with you - that's somebody else - wires are getting crossed here.
Brian
05-10-2013, 12:45 PM
#1107
Senior Member
RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
ORIGINAL: flyncajun
I was Lured to make smaller wings Because it was the cool thing to do! and if I wanted to be able to sell in the Market.
It`s hard to overcome trends when your thinking is in the opposite direction. And you don`t win a major event, But you will see all the top designers going back to 1050SQ wings or bigger shortly, SO I`ll fit in again the Myth that a big wing can`t snap is just that ,or a Bipe could never do it.
I tried to walk the tight rope of small enough to fit in and big enough to do the job. Small wings have Made all modern airplanes easy to snap ,but hard to fly the gravy stuff. and no amount of adjusting on a airplane with the wings to small will give you a happy airplane every where. The Contra drive Just makes it worse for this condition.
This has been Information I have sat on for some time , Take it or leave it, Laugh if you like (as Usual) But,if your smart you will run with it I hate to speak in parables,,But if I don`t, when the answers are discoverd they forget were they got um LOL, SO if I make the debate fun it`s remembered better
Now,This is Why I`m against Add ons , they cover up the real problems with trimming airplanes and we chase the effect not the root cause.
Bryan
I was Lured to make smaller wings Because it was the cool thing to do! and if I wanted to be able to sell in the Market.
It`s hard to overcome trends when your thinking is in the opposite direction. And you don`t win a major event, But you will see all the top designers going back to 1050SQ wings or bigger shortly, SO I`ll fit in again
the Myth that a big wing can`t snap is just that ,or a Bipe could never do it.I tried to walk the tight rope of small enough to fit in and big enough to do the job. Small wings have Made all modern airplanes easy to snap ,but hard to fly the gravy stuff. and no amount of adjusting on a airplane with the wings to small will give you a happy airplane every where. The Contra drive Just makes it worse for this condition.
This has been Information I have sat on for some time , Take it or leave it, Laugh if you like (as Usual) But,if your smart you will run with it
I hate to speak in parables,,But if I don`t, when the answers are discoverd they forget were they got um LOL, SO if I make the debate fun it`s remembered betterNow,This is Why I`m against Add ons , they cover up the real problems with trimming airplanes and we chase the effect not the root cause.
Bryan
Larger wings do flybetter producing a fuller envelope, easier; ......butneed to be designed right to get the maximum benefit. Large wings snap and spin fine when done right. The little iddy bitty wings of current stuff in my view don't really allow the planeto snap.....but do help create the illusion because are overpowered by ailerons
Large stabs on the other hand, sized upwards of 30% of the wing, also don't extract the max out of any design. Drop that stab down to 23% and realize a fuller envelope, easier (easier means less pilot work, less radio work, less servo power required).I've said this before too....large stabs are great on trainers
I believethat workingwith a Tail Volume Coefficient in pitch of around 0.60-0.65is best for all around aerobatics of the type we strive for in pattern. That's where I design my stuff and it works great there. Of course I'm not competitive so guys tend to ignore these ideas( par for the course, but does not make the claims and ideas less valid)
What is less clear is where to place the TVC for the vertical stab. The standard single prop drives worked well with a TVC in yaw of around 0.8. The contra seems to require a TVC in yaw of well over 1. The easiest way to get that is with additional vertical area on the main fin, keeping all else the same. Fin shape also plays a role I believe: less chord and greater span,while increasing areaby 15% at least
05-10-2013, 12:47 PM
#1108
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RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
ORIGINAL: OhD
Dean,
I hope your model needs a little refinement. If our cg's are anywhere near the wing tube (like most) we have negative yaw stability. What do you think?
Jim O
Dean,
I hope your model needs a little refinement. If our cg's are anywhere near the wing tube (like most) we have negative yaw stability. What do you think?
Jim O
Do you mean the location of the wing in my crude little model? If so, please ignore that: I drew a wing in there just to lend credence to the notion that you were looking at the side view of an airplan(k).
All my example shows is that the addition of a contra probably makes the airplane fly 0.3" more tail-heavy in yaw.
Since we tend these days to fly with a bare minimum of yaw stability margin, in order to have lots of yaw and knife edge power, that small movement puts things annoyingly close to unstable, especially when the prop is delivering lots of thrust.
I hope I answered you,
Dean
05-10-2013, 12:48 PM
#1109
RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
ORIGINAL: Dean Pappas
The plank?
Naw, that was Nat Penton's ''UM'' series of planes. Nat said it stood for Ultimate machine, but I know it was really ugly M.... !
As far as the effect during cruise is concerned, it depends: a draggy airplane requires lots of thrust to cruise fast.
Oh yeah, and my penguin friends (at the Zoo of course) are convinced that wings are just add-ons.
Dean
ORIGINAL: serious power
Hi Dean,
Thanks,
I assume that the CG is to move with the wing.
Re your PS and it being at it's worst while accelerating from slow speed am I correct to assume vary little adverse effect from the extra prop at steady state cruise.
Brian
PS ; I like your design It looks like you plagiarized one of Bryan Heberts
Hi Dean,
Thanks,
I assume that the CG is to move with the wing.
Re your PS and it being at it's worst while accelerating from slow speed am I correct to assume vary little adverse effect from the extra prop at steady state cruise.
Brian
PS ; I like your design
It looks like you plagiarized one of Bryan Heberts
Naw, that was Nat Penton's ''UM'' series of planes. Nat said it stood for Ultimate machine, but I know it was really ugly M.... !
As far as the effect during cruise is concerned, it depends: a draggy airplane requires lots of thrust to cruise fast.
Oh yeah, and my penguin friends (at the Zoo of course) are convinced that wings are just add-ons.
Dean
Dean,
Thanks again.
Ok I get the point re a draggy model.
That what I'm trying to get at is;
If we were to say a draggy model at cruise speed was half way between the 0.3in and zero we would be talking about 4 or 5mm only for the wing to move forward to negate the effect from the extra prop.
Sorry for switching to metric.
Anyway this seems small to me - but all this is delicately poised on these models.
Brian
Edit;
Dean;Was posting while you were. That's making it a lot clearer.
05-10-2013, 02:54 PM
#1110
RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
ORIGINAL: Dean Pappas
Hi Jim!
Do you mean the location of the wing in my crude little model? If so, please ignore that: I drew a wing in there just to lend credence to the notion that you were looking at the side view of an airplan(k).
All my example shows is that the addition of a contra probably makes the airplane fly 0.3'' more tail-heavy in yaw.
Since we tend these days to fly with a bare minimum of yaw stability margin, in order to have lots of yaw and knife edge power, that small movement puts things annoyingly close to unstable, especially when the prop is delivering lots of thrust.
I hope I answered you,
Dean
Hi Jim!
Do you mean the location of the wing in my crude little model? If so, please ignore that: I drew a wing in there just to lend credence to the notion that you were looking at the side view of an airplan(k).
All my example shows is that the addition of a contra probably makes the airplane fly 0.3'' more tail-heavy in yaw.
Since we tend these days to fly with a bare minimum of yaw stability margin, in order to have lots of yaw and knife edge power, that small movement puts things annoyingly close to unstable, especially when the prop is delivering lots of thrust.
I hope I answered you,
Dean
Jim O
05-11-2013, 01:03 PM
#1112
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RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
ORIGINAL: OhD
No you covered yourself by saying the actual NPs were semi junk but just for kicks I looked at my 2M models and all the wing tubes are about 20 to 21 inches back of the prop or rear prop. The CGs are usually no more than a half inch in front of the tube. It would be a good drill to actually find the yaw stability margin. It might be surprising.
Jim O
ORIGINAL: Dean Pappas
Hi Jim!
Do you mean the location of the wing in my crude little model? If so, please ignore that: I drew a wing in there just to lend credence to the notion that you were looking at the side view of an airplan(k).
All my example shows is that the addition of a contra probably makes the airplane fly 0.3'' more tail-heavy in yaw.
Since we tend these days to fly with a bare minimum of yaw stability margin, in order to have lots of yaw and knife edge power, that small movement puts things annoyingly close to unstable, especially when the prop is delivering lots of thrust.
I hope I answered you,
Dean
Hi Jim!
Do you mean the location of the wing in my crude little model? If so, please ignore that: I drew a wing in there just to lend credence to the notion that you were looking at the side view of an airplan(k).
All my example shows is that the addition of a contra probably makes the airplane fly 0.3'' more tail-heavy in yaw.
Since we tend these days to fly with a bare minimum of yaw stability margin, in order to have lots of yaw and knife edge power, that small movement puts things annoyingly close to unstable, especially when the prop is delivering lots of thrust.
I hope I answered you,
Dean
Jim O
Hi Jim,
I'll bet it would scare you.
take care,
Dean
05-14-2013, 04:48 AM
#1113
RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Hi Brenner,
Just to keep up to date with Peter Hasse developments.
I was not there so no idea as to sound or performance.
Brian
Just to keep up to date with Peter Hasse developments.
I was not there so no idea as to sound or performance.
Brian
05-14-2013, 05:01 AM
#1114
RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Pity somebody didn't remove the driver from the canopy retaining screw before they took that second shot, I spent 2 minutes trying to see how a counter weight was attached to the rear prop!
Is this a Brenner Contradrive with folding type prop blades or something else?
Malcolm
Is this a Brenner Contradrive with folding type prop blades or something else?
Malcolm
05-14-2013, 05:25 AM
#1116
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RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
You know that old saying if it looks right it will fly right, well Hmmm.
I'm officially Old School.
Cheers
Greg Hede
Australia
I'm officially Old School.
Cheers
Greg Hede
Australia
05-14-2013, 05:40 AM
#1117
Join Date: Jan 2004
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RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
The blades look like the ones used by Grabbe in France last year. They were designed for efficiency in a wind tunnel.
05-14-2013, 05:41 AM
#1118
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RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Impressive setup...
You can tell that Peter is an innovative guy.
We considered props with seperate blades way back when, but at the time I was concerned about the rear blades moving forward and the front blades moving backwards during snap rolls. My thinking was that there was a risk that the blades could touch, causing a catastrophic inflight failure.
Brenner ...
You can tell that Peter is an innovative guy.
We considered props with seperate blades way back when, but at the time I was concerned about the rear blades moving forward and the front blades moving backwards during snap rolls. My thinking was that there was a risk that the blades could touch, causing a catastrophic inflight failure.
Brenner ...
05-14-2013, 05:48 AM
#1119
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RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Is it possible that they are hinged in a way that the movement is limited and so cannot touch no matter what?
05-14-2013, 11:25 AM
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RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
I've read some technical reports that stated that a different number of blades between the front and rear would be better. Interesting to see someone doing that.
05-15-2013, 07:05 AM
#1122
Thread Starter
RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
There is a specific benefit to having a different number of blades from front to rear, and it's related to blade passing frequency.
If the number of blades is the same for the front and rear props, then all of the blades pass by each other at the same time, but if the number of blades is different, then the blades all pass by each other at different times. This reduces the time between passes, which increases the blade passing frequency, and reduces the amplitude of any shock that results from each passing, both of which have the potential to reduce prop noise.
However, there is a potential downside in that increasing the blade passing natural frequency can potentially raise it up so that it is close to resonant natural frequencies in the props, which can also cause noise, and in a worst case scenario there could potentially be structural damage done to the prop.
There is one more thing to consider as well. Props are somewhat unique in that they experience something called "stress stiffening" due to the centrifugal forces from spinning. This is the same effect that causes the tone of a guitar string to change as the tension in the string is adjusted.
What this means is that the natural frequency of the props increases as a function of rpm, (actually the square of rpm..) which is a good thing as long as the blade passing frequency is significantly less than the natural frequencies of the props, but if the blade passing frequency gets high enough there is the potential for some sort of resonance problem, and since both the blade passing frequency and the prop natural frequency increase with rpm, this kind of resonance isn't something that you can easily drive through.
However, it should also be mentioned that the lowest prop natural frequencies are going to be related to bending modes, and since Peter has hinged his props at the hub, these natural frequencies are no longer going to be a problem, and the remaining natrual frequencies are going to be related to blade twisting modes, which should be much higher, but on the other hand, blade twisting modes are probably not going to be as affected by stress stiffening as bending modes will be.
Brenner ...
If the number of blades is the same for the front and rear props, then all of the blades pass by each other at the same time, but if the number of blades is different, then the blades all pass by each other at different times. This reduces the time between passes, which increases the blade passing frequency, and reduces the amplitude of any shock that results from each passing, both of which have the potential to reduce prop noise.
However, there is a potential downside in that increasing the blade passing natural frequency can potentially raise it up so that it is close to resonant natural frequencies in the props, which can also cause noise, and in a worst case scenario there could potentially be structural damage done to the prop.
There is one more thing to consider as well. Props are somewhat unique in that they experience something called "stress stiffening" due to the centrifugal forces from spinning. This is the same effect that causes the tone of a guitar string to change as the tension in the string is adjusted.
What this means is that the natural frequency of the props increases as a function of rpm, (actually the square of rpm..) which is a good thing as long as the blade passing frequency is significantly less than the natural frequencies of the props, but if the blade passing frequency gets high enough there is the potential for some sort of resonance problem, and since both the blade passing frequency and the prop natural frequency increase with rpm, this kind of resonance isn't something that you can easily drive through.
However, it should also be mentioned that the lowest prop natural frequencies are going to be related to bending modes, and since Peter has hinged his props at the hub, these natural frequencies are no longer going to be a problem, and the remaining natrual frequencies are going to be related to blade twisting modes, which should be much higher, but on the other hand, blade twisting modes are probably not going to be as affected by stress stiffening as bending modes will be.
Brenner ...
05-15-2013, 10:26 AM
#1123
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RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
There is a specific benefit to having a different number of blades from front to rear, and it's related to blade passing frequency.
05-15-2013, 11:27 AM
#1124
Thread Starter
RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
Hey Neko,
The blade natural frequencies will be a funtion of material stiffness, material density, and the shape and construction of the blades.
When we designed our props we did an FEA modal analysis to calculate natural frequencies and mode shapes, and the lowest natural frequency was about 200 Hz. This was a beam bending mode.
With two bladed props the blades pass each other twice per revolution, so if the props are spinning at 4000 rpm, then the blade passing frequency is 8000 rpm/60 = 133 Hz. Based on this, I wouldn't want to spin our props any more than about 5500 rpm.
With a three bladed rear prop that has a blade passing frequency of aboout 1000 Hz, we would pass through five harmonics of the lowest natural frequency of the blades in order to reach 4000 rpm, which is not something I would feel comfortable designing into the system.
However, like I mentioned, Peter's props are hinged, so the lowest natural frequencies aren't beam bending modes, so they are much higher.
Brenner ...
The blade natural frequencies will be a funtion of material stiffness, material density, and the shape and construction of the blades.
When we designed our props we did an FEA modal analysis to calculate natural frequencies and mode shapes, and the lowest natural frequency was about 200 Hz. This was a beam bending mode.
With two bladed props the blades pass each other twice per revolution, so if the props are spinning at 4000 rpm, then the blade passing frequency is 8000 rpm/60 = 133 Hz. Based on this, I wouldn't want to spin our props any more than about 5500 rpm.
With a three bladed rear prop that has a blade passing frequency of aboout 1000 Hz, we would pass through five harmonics of the lowest natural frequency of the blades in order to reach 4000 rpm, which is not something I would feel comfortable designing into the system.
However, like I mentioned, Peter's props are hinged, so the lowest natural frequencies aren't beam bending modes, so they are much higher.
Brenner ...
05-15-2013, 11:35 AM
#1125
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RE: Contra Rotating Propeller Drive for f3a 2m Pattern Planes
I was thinking of vibration modes (like a string), rather than beam bending. That's probably where I went astray.