Counter Rotating Props-how ?

[CESARNOGUEIRA]

Please, any clue :

1)Is this arrangement better than normal props in eletric airplanes?
2)How to do , please...:confused: :confused: :confused:

[mimai]

have a look on

http://members.aol.com/KMyersEFO/sitetoc.htm

I saw somewhere, either here or on the site that Keith Shaw was building a counter-rotating drive system.

mike i.

[nhoj62]

i have designed one an have built it an it runs nicely however it is a more complicated an heavier option, to go for o a plane.

how?
1) use to small motors, speed 400 for example, put them side by side one facing forward and one backward, get to gearsets large enough so that the prop shafts can run between the two motors.
so u will have a gearset at the back and one set at the front.
the gearset at the back will drive a shaft that goes between the motors an out to the front prop having passed throught sleeve shaft that is driven by the front gear set, the difficulty is getting props that run in different directions that are basically the same size, most common is the graupner grey nylon props.

2) one single larger motor with lots of gears to drive the props
an is a whole lots more complicated than the first system(this is the one i is working on right now, the problem is also to ge it to run smoothly so that power is not lost.

hope this helps u some wot.
if not email me an i can do simple diagrams for you.
later
nhoj62.

[CESARNOGUEIRA]

:confused: Once I'm a litlle bit not clever at all, could you kindly show me some diagrams about the one engined arrangenment?
TKS!!! :D

[nhoj62]

i will post you some diagrams later on today once i am home hope u can hang on that long!, lol
later
nhoj62

[nhoj62]

hi cesarnogueira
hope this pic helps to make things clearer to you.
first diagram is complicated version and the second diagram is the simple one the bottum one is even simpler.
if u need more help drop me an email or just get onto msn an chat.
later
nhoj62

[nhoj62]

forgot to mention the current copy of avaition modeller international has a good little feature a contra prop unit, if u can get hold of it, it is interesting reading.
if not let me know an i will scan it and send it to you.
later nhoj62.

[Terry Lyttle]

Many problems: gearing, 2 motors or one, as outlined above,
weight, interference in airflow over 2nd prop, on and on.
Benefits? a Short Seamew with more than one prop, or if you are truly into self-flagellation, a Brabazon...

[nhoj62]

and then there is the shackleton if u is so inclined or the gannet or later mark seafires or.... ok i`ll stop now:D

later
nhoj62

[CESARNOGUEIRA]

Originally posted by nhoj62
and then there is the shackleton if u is so inclined or the gannet or later mark seafires or.... ok i`ll stop now:D

later
nhoj62

Well, it's a pity...
I thought it would be better using this system.

Well, back to the drawing board!

THANKS A LOT !!! :D

[Willber-Force]

A differential gearset is awesome for counter-rotating props. A little modification is necessary but it is pretty much as-is. Think about it, it's surprisingly simple.

[All__talk]

Yep willber is right, I have designed a counter rotating system with differential gears for a helicopter (haven’t built it yet), it makes for a very simple system. Trouble is both props run at the same RPM which means the second one must have a much higher pitch to do its share of the work, could be hard to find a combination of left and right props with enough pitch difference to work right. A spur gear system with independently adjustable ratios would be more complex but would allow the use of matched (or unmatched), left and right handed props and be tuned through gearing so that both props do equal work. I see the idea of counter rotating props as more of a design challenge that any thing else, as it would never be as efficient as a single prop, It might be a solution for a model with an extreme torque roll problem or to gain ground clearance with a short undercarriage, of course a multi blade prop would be a more efficient solution to the latter.

Just some thoughts
Gary

[CESARNOGUEIRA]

I,ve been talking to Mr. Ramoser , from Varioprop, and may be I would calculate my AN-22 with 4 bladed Varioprops.
Visually, almost same effect...
Unfortunately Motocalc does not have any data about Varioprop...
HELLO MOTOCALC PEOPLE, WAIK UP!!!
THANKS!!!;)

[ssatoru]

Hi,

I would like to introduce a beautiful Macchi MC72 with contra-rotating propeller built by Mr. Sakamoto in Japan, back in 1999. I have only seen it static but the mechanism is very well done. The gear unit is custom built utilizing existing gear parts. Powered with Kontronik KBM42-20. Prop is APC 11x7 normal and reverse pitched.

Although text is in Japanese, the below link has many detail photo.
http://www.fortunecity.com/meltingpot/hornsey/147/mc72.html

Satoru
[photo from his web site]

[Oliver Leifeld]

There had been one on the latest electric flight meeting in Aspach, Germany, Sept. this year, built by A. Gassner from Switzerland. It used two home-built LRK-brushless (350/25) motors arranged inline, with coaxial shafts. No gears involved.

Motors drive a 16x9 prop each, using 24 cells Sanyo 2400.

Oliver

[Dave Harding]

Here are two pictures of a Shackleton at the Imperial War Museum's facility at Duxford in England.

Note the smaller diameter of the aft prop.

If Don Stackhouse is on-line maybe he will explain why.

Not quite relevant to the discussion but interesting pix.

Dave.

[Dave Harding]

Other pix.

[roballoba]

http://visionhobbies.com/1637131.html


I got one! now does anyone have plans or a kit for the mc72??

I would love to build this legend!

[JetPlaneFlyer]

Note the smaller diameter of the aft prop.

If Don Stackhouse is on-line maybe he will explain why.


I'd guess that the rear prop is smaller so that it avoids the tip votexes generated by the leading prop.

Steve

[Sparky Paul]

Shackelton motor in the Red Baron P-51 at Mohave, 1976

[Basingstoke Roy]

Hi

I have been reading your thread with interest.

A couple of year’s back I finished my attempt at contra props. It took me about 3 years of tinkering to produce a working unit. By the time I’d completed it, brushed motors were old hat.
Brushless is the way to go if your just starting your build, you need to Max out the power to weight ratio. For mine I used some aluminium box section (liberated from work) and two old VMAR aluminium engine mounts braised together. The total weight for the unit including motors & props is 25oz, after cutting off and drilling out as much metal as pos.

My set up uses two Cobalt motors each driving their own shaft. The shaft is a boat prop shaft mounted in bearings each end; so one motor drives the casing and the other the shaft. Peak power about 950 watts from a 5 cell Li-Po.
The only down side is the shafts are very soft, so one ground strike and that’s the end of playtime. Still I have the shaft overhaul down to a fine art now

I spent a lot of time with mine on a test rig measuring the thrust, and volts drop across each motor, with a host of different props until I was happy with the set-up.
I ended up with an APC 14x6 pusher and master airscrew 14x8. The lack of pusher props was a real problem.

This year I will use two, two blade scale VarioPROP. They would have made thinks a lot simpler if they were around when I started out.

If I were to start again I would go for 2 out runners back to back, it’s a simple set-up with the fewest parts (KISS rules). For a one motor set-up I would use bevelled gears driving the shafts, and the motor mounted at right angles driving the middle lay gear (if that make sense)

With two motors you can monitor the power consumption of each, and adjust the blade pitch to balance them out. But with a single I don’t know how you could tell if one prop was blanking the other.

I hope my rambling have been of some help.

[Sparky Paul]

That MB-5 is a really pretty airplane.
Too bad the full-scale didn't get any real attention until too late.

[vintage1]

You don't need any gears at all to make a contra rotation prop.

Just take a standard brushless motor and mount it on a rotating shaft..the WHOLE MOTOR..and attach one prop to the case, and one to the shaft. And take the power in via three slip rings.

The torque reaction of this combo has to be zero!

[Don Stackhouse]

...Note the smaller diameter of the aft prop.

If Don Stackhouse is on-line maybe he will explain why...

There are a few reasons, but the basic one is slipstream contraction.

A propeller makes thrust by grabbing air in front of it and accelerating it aft. The airspeed in the slipstream behind a prop that is making positive thrust is greater than in front of the prop. Half the acceleration occurs in front of the prop and the other half occurs behind it.

Now, consider the volume per second of the air flowing through the prop. The volume per second is equal to the cross-sectional area of the slipstream at that point (which is proportional to the square of the flow field's diameter at that point) times the airspeed at that point.

Now, consider that the mass flow in front of the prop, through the prop, and behind the prop all has to be equal; the prop does not have the ability to create or destroy matter (air in this case), nor convert matter to energy or vice-versa. Also, the pressures involved in the prop flow are low enough that the density of the air flowing through the prop is essentially constant. Because of this, the volume per second flowing through the prop must also be essentially constant.

Now (finally!) consider that the speed of the air flowing into the front of the propeller is lower than the speed behind, yet their volumes-per-second must be equal. The only way this can be true is if the diameter of the slower slipstream in front of the prop is larger than the prop, while the diameter of the faster slipstream behind it is smaller than the prop.

Now, if we have a contra-rotating propeller system, the first prop contributes some thrust, and the second prop behind it contributes some more. The airspeed of the slipstream though the second prop is substantially greater than the airspeed through the first, and therefore the diameter of the slipstream at the second prop is smaller than at the first prop. The diameter of the aft prop needs to be reduced to match this.

[Don Stackhouse]

One other point that is also a result of this: The airflow through the aft prop is faster than through the forward prop, so either the pitch or the RPM of the aft prop has to be greater to account for this.

I saw a graphic example of this a few years ago, when I was providing some technical help to a guy who built a 24' model of the Rutan Voyager. In his original setup, he had identical motors and batteries driving identical props at the front and rear (yes, even with the entire length of the fuselage in between them, they still can act as a contra-rotating propeller system). He couldn't get enough thrust to get up to flying speed. The slipstream of the front prop was driving the rear prop, which meant that the aft prop was making little or no thrust. In effect, he was trying to fly on the front prop alone.

He switched the aft prop to one with about 2" more pitch, and the problem went away.

[vintage1]

It's an interesting thought..two engines in normal arrangement gives you effectively twice the swept area at the same pitch and RPM..tow props one behind the other gives you nothing extra..so the rear prop has to act as a second stage turbine with coarser pitch.

[Don Stackhouse]

It's an interesting thought..two engines in normal arrangement gives you effectively twice the swept area at the same pitch and RPM..tow props one behind the other gives you nothing extra..so the rear prop has to act as a second stage turbine with coarser pitch.

Exactly.

There are a couple reasons for using contra-rotating props:

One is to cancel out gyroscopic, torque and P-factor effects. The current popularity of small electric helicopters with contra-rotating main rotors to eliminate the need for a tail rotor is a perfect example.

The other is to absorb lots of power in a relatively small-diameter prop.

The torque from the engine is applied to the air by the prop. Because of this, the air behind the prop has a rotation or "swirl" to it. In extreme cases of way too much power in too little diameter, the "helix angle" of this swirl might be 15 degrees or more. This rotation represents energy, and therefore efficiency loss.

By cancelling out the swirl of the first prop with the opposite swirl of the second prop, a contrarotating prop can recover this energy. Of course there are other efficiency losses associated with the second prop, so (just as is the case with winglets), you have to do a really good job in the aerodynamic design of the props to actually come out ahead on the deal, and even then the gains tend to be small.

For just about any power loadings you're likely to find in a model, with the possible exception of ducted fans (which are nothing more than a prop mounted inside a tube), the amount of efficiency loss is truly negligible, and certainly not nearly enough to justify all the weight and complexity of a contra-rotating propeller system.

Note that in the specific case of a ducted fan, the stator vanes are in effect a second, contra-rotating prop, designed to operate at zero RPM. There are efficiency losses involved, mainly the drag of the stator vanes themselves, but they do recover some of the swirl energy without adding a lot of mechanical complexity.

However, if you have practical limits on propeller diameter, and you need to absorb maybe 12,000 horsepower or more in that diameter, the losses could be enough to justify the extra trouble of a contra-rotating propeller system. Do not underestimate the amount of trouble involved. Quite a few airplanes, including the propeller driven Northrop XB-35 flying wing among many others, failed due to the problems of trying to get acceptable life and reliability from their contra-rotating props.

The Brits have been reasonably successful in developing large, geared props, including contra-rotating props, and the Russians seem to do fairly well with the props on their 14,000 hp Kuznetsov turbines. However, those exceptions aside, contra-rotating propellers have historically been unmitigated disasters in the reliability department.

[Don Stackhouse]

Regarding the option of using more pitch on the second prop, vs. using more RPM to compensate for the higher inflow velocity:

Generally you are better off using more pitch. Props have profile losses that are proportional to the square of the local helical airspeed. If you try to make a linear increase in RPM (and a corresponding linear increase in the helical airspeeds all along the blade), your profile losses in that second prop will increase with the square of the RPM change.

It's like trying to run your car on the highway in second or third gear, instead of fifth. Yes, maybe the engine can run at the higher RPMs required without blowing up, but you will waste massive amounts of power to internal friction, and that will be reflected in your gas mileage. The engine life certainly isn't going to benefit, either.

Using the proper gear in your car, or the higher pitch in the second prop in your contra-rotating propeller system, avoids these problems.

[Don Stackhouse]

...Just take a standard brushless motor and mount it on a rotating shaft..the WHOLE MOTOR..and attach one prop to the case, and one to the shaft. And take the power in via three slip rings...

Yes, that would certainly eliminate the gearing required. However, you now have the weight and the (not insignificant) mechanical and electrical efficiency losses associated with the slip rings and brushes, not to mention the wear life issues. You just took your brushless motor and gave it back all the problems and compromises associated with a brushed motor, only worse, since you have to have three slip rings instead of two. My guess is that the overall efficiency of a well designed gearbox would probably be better.

[Basingstoke Roy]

I would second that

I found with my own experiments that varying the pitch has far grater effect on thrust than relying on the motor speed to compensate.

[markdrela]

Yes, that would certainly eliminate the gearing required. However, you now have the weight and the (not insignificant) mechanical and electrical efficiency losses associated with the slip rings and brushes, not to mention the wear life issues. You just took your brushless motor and gave it back all the problems and compromises associated with a brushed motor, only worse, since you have to have three slip rings instead of two. My guess is that the overall efficiency of a well designed gearbox would probably be better.

Nah. Vintage1 is correct. This is a very good solution, which I've used for hovering MAVs. One big advantage is that you get a "free" 2:1 gear reduction without gears, since the motor runs 2x faster than each prop. Also, slip rings are inherently less problematic than a brushed-motor commutator, since they don't have intermittent contact together with inductive voltage spikes, which is the major cause of arcing. So the slip rings need less contact force and can have less friction than a commutator.

This kind of counter-rotating drive has been used in torpedos some time ago, when they were still electrically powered.

[Salto]

... This kind of counter-rotating drive has been used in torpedos some time ago, when they were still electrically powered.Out of interest, how are they powered now?

Graham.

[Don Stackhouse]

...This is a very good solution... Also, slip rings are inherently less problematic than a brushed-motor commutator, since they don't have intermittent contact together with inductive voltage spikes, which is the major cause of arcing. So the slip rings need less contact force and can have less friction than a commutator.

This kind of counter-rotating drive has been used in torpedos some time ago, when they were still electrically powered.

No question that it's "workable", and from a complexity standpoint it has merit, I just question whether it's "better". From my own experience with aircraft propeller deicers, I know that in service the electrical efficiency of slip rings (not commutators) leaves a lot to be desired, especially at high current levels, and their wear life in a real-world service environment tends to be a significant problem as well. Maintenance of the electrical deicing system is one of the worst troublemakers in aircraft propeller maintenance.

For some applications (such as torpedos, which only have to show a service life of a few minutes at most), the simplicity issues might tip the scales in their favor, but in general there are some problems. Given the efficiency issues and the larger batteries required to make up for that, a gearbox might still be better.

Overall the option of using two brushless motors, one with a hollow shaft, isn't a bad approach. I'll defer to the brushless motor experts regarding whether combining these into a brushless motor with a single stator and windings sandwiched in between two opposite-rotation rotors could be made to work.

[Don Stackhouse]

Out of interest, how are they powered now?

Various means, including gas turbines and other internal or external combustion engines, electric, and even rockets (such as the Russian "Shkval" supercavitating torpedo, that has a top speed in excess of 200 knots and a range up to 13 Km!).

Wikipedia has a good article:

http://en.wikipedia.org/wiki/Torpedo

I did find another website on modern torpedo design that proposed a brushless electric motor, with stationary windings driving magnetic rotors that were integrated into the propellers themselves, eliminating shafts and their seals:

http://www.chinfo.navy.mil/navpalib/cno/n87/usw/issue_14/torpedoes.html

[skylark3f]

Don - thanks for the explanation. Have been looking for some decent literature on contra-rottating propellers for quite some time now but I've not been all too successful so your description was of use. Do you know of any www sites that cover the subject in detail or indeed any good books that cover the subject in detail?

Basingstoke Roy - love the MB 5. I would be interested to know the general specs for it if you have them to hand. Was it built from a plan and if so I was wondering where I might purchase a copy/set? :p I've been looking for a decent MB 5 plan for quite some time now but have not come up with much - searched Traplet as well but their version seems to be very 'stand-off' scale. Not even found any good 3-views :(


I have been working on the idea of contra-rotating props for quite some time now after deciding I'd like to put a unit into a Seafire Mk. 47 :) Found a source on the www that mentioned some full sized units used planetary gear setups so I started there and drew up rough sketches and calculated ratios etc. The problem here is, as I see it, finding the correct size (radius & teeth no.) annulus to fit the system: seems to me that the general dimensions could also be too large to fit a model comfortably?

Bevel gears seem like the easiest and simplest option but are there not huge efficiency losses in turn the power through 90° twice? I'd be interested to know what people 'feel' about that. Then there are spur gears which seem nice and easy and quite efficient but would there be a space issues here too? The problem I have come accross so far is finding gears that are small enough as well as being sturdy enough.

I'm quite interested in using just the one motor, can't give a reason why but I think I like the idea of a gearbox... seems cool to me anyway :cool:

There are plans for a C-R prop unit using torque reaction (ala V1's suggestion) over on the plan plage. Koolhoven Pursuit is the name of the machine: free flight though but interesting nonetheless.

Bit of a ramble I know, sorry :o

Cheers then,

Jack

[vintage1]

One fairly simple but weird way to gear is to use a spur gear and a belt drive, running off the same shaft and onto concentric shafts.

[JetPlaneFlyer]

Basingstoke Roy - love the MB 5. I would be interested to know the general specs for it if you have them to hand. Was it built from a plan and if so I was wondering where I might purchase a copy/set? :p I've been looking for a decent MB 5 plan for quite some time now but have not come up with much - searched Traplet as well but their version seems to be very 'stand-off' scale. Not even found any good 3-views :(


I've got a couple of free flight plans for the MB5... a Peanut scale and a 21" span version. Both are intended for ribber power but the larger one could easily be modified for lightweight RC.

Here they are:
http://www.fileden.com/files/2006/11/11/368005/Martin%20Baker%20MB-5%20with%20counter-rotating%20props%20by%20Mark%20Drela.pdf

http://www.fileden.com/files/2006/11/11/368005/Martin%20Baker%20MB-5%20with%20counter-rotating%20props.pdf

[skylark3f]

JetPlaneFlyer - you sir are a scholar and a gentleman. Thanks so much, those plans were just what I have been looking for. I don't suppose you have a set of Mig 3 plans too do you?! :rolleyes: :p very cheeky of me to ask i know :o I have the Earl Stahl plans but they aren't all too scale (I guess he drew them in war time when things were still classified?!)

Sorry to hijack the thread,

cheers again,

Jack

[JetPlaneFlyer]

Jack,
I’ve got the Larry Kruse and Pres Bruning versions of the Mig-3... both look quite accurate:

http://www.fileden.com/files/2006/11/11/368005/MIG%203%20-%20by%20Larry%20Kruse.pdf

http://www.fileden.com/files/2006/11/11/368005/Mig%203%20-%20by%20Pres%20Bruning.pdf

[Dave Harding]

Eduardo's site is a treasure trove of 3-views;

http://www.fortunecity.com/marina/manatee/272/mig-3.html

Dave

[Stef]

Try this gearing

[skylark3f]

JetPlaneFlyer - wow, thanks so much again! I now have some really ace plans for some of my most favourite aeroplanes... I owe you an ale! :) Is there a facility for beer exchange on here?! :rolleyes: Have I seen that plane on your display picture on ffscale.co.uk? Is a good site, have wasted many an hour at work perusing the pictures and reports.

Sorry, I know I'm way of topic :o

Jack

[Don Stackhouse]

Don - ...Have been looking for some decent literature on contra-rotating propellers... Do you know of any www sites that cover the subject in detail or indeed any good books that cover the subject in detail?

Good luck. Props are rather specialized devices that are poorly understood by a large portion of the aeronautical community, including a lot of the ones who write books. Contra-rotating props are even more specialized.

Start with basic propeller theory, then work from there. Once you understand the inflow and wake characteristics of a single propeller, it will be easier to understand how two of them interact.

I have been working on the idea of contra-rotating props for quite some time now after deciding I'd like to put a unit into a Seafire Mk. 47 :) Found a source on the www that mentioned some full sized units used planetary gear setups so I started there and drew up rough sketches and calculated ratios etc. The problem here is, as I see it, finding the correct size (radius & teeth no.) annulus to fit the system: seems to me that the general dimensions could also be too large to fit a model comfortably?

This is a situation where you need to step back and consider the difference between the full-scale example and the small model you're working with.

Planetaries give a lot of teeth in mesh in a small volume, so they're good at transmitting lots of power, assuming your design is clever enough and your machining and assembly precise enough to get good load sharing between the multiple planet gears.

You're not transmitting lots of power in your application. Also, the efficiency of the gearbox depends in part on how many different components are in the drive train. Each time you hand power from one component to another, a little bit of energy gets lost in the transfer. Typical numbers are 2-4% per gear stage. Part of that is the loss in the gear teeth, but there are also losses in the bearings supporting the shafts, shaft seals, etc.. Also, there are "windage" losses from the churning of lubricants in the bearings and between the gear teeth (which of course depend on the type of lubrication used, among other things).

So, assuming you aren't trying to transmit many thousands of horsepower through as small and light a gearbox as possible (the problem faced by a full-scale contra-rotating prop), then to keep your efficiency as high as possible, you need to keep the entire drivetrain as simple as possible.

Bevel gears seem like the easiest and simplest option but are there not huge efficiency losses in turn the power through 90° twice?...

Turning through 90° in itself is not a problem. Properly done, a well designed and built set of bevel gears should have efficiencies similar to an equivalent set of spur gears.

The key here is that spur gears, bevel gears, and even helical gears (assuming they have parallel shafts) are all cases of involute teeth with what's called "pure rolling contact", like the contact in a ball or roller bearing. The friction from that is far less than sliding friction, such as the motion in a worm gear assembly. Consequently, worm gears tend to have relatively poor efficiency per stage. Belt or chain drives tend to be more efficient than worm gears, but somewhat less efficient than pure rolling contact gears.

Assuming the gear box is built well, with accurately machined gears set at the correct center distances and with near-perfect alignment, and a simple design with a minimum number of bearings and meshes, a set of spur, helical or bevel gears should all have excellent (and roughly equal, assuming the same number of stages) efficiency.

With that in mind, and assuming you can make it fit in the cowl (look for a motor with a short overall length, a "pancake" would be ideal, and set it well aft in the cowl), you can make a contra-rotating gearbox using bevel gears and a total of only three gears and three shafts, one for each prop and one for the motor. The gearbox will have one stage and one gear mesh to each prop, so the efficiency should be very similar to a single-stage spur gear gearbox like one of Astro Flight's Superboxes.

Just turn the motor 90° to the concentric propshafts. The propshafts, one inside the tubular other shaft, carry two identical (other than their shaft bores) bevel gears facing each other. The motor is set to one side, with its shaft centered between the prop gears, and the single drive pinion meshing with both of the prop gears.

The drive pinion on the motor shaft can be smaller than the two prop gears, so you can have up to about a 5:1 reduction ratio in this single stage gearbox (that ratio is a good rule of thumb for the maximum amount of reduction per stage before you start having problems with gear mesh, tooth engagement and power transmission). Note that for bevel gears, the pinion and the driven gears have to be made for that particular ratio; i.e.: if you want to change to a different ratio, you will need to change all three gears, you can't just change pinions like you can with a spur gear set.

Also, it's a good idea to pick ratios where the ratio ends up being some oddball number with a bunch of digits after the decimal point. If you pick a simple ratio such as 2:1, then the same two teeth come into mesh with each other every second (in the case of 2:1) revolution. If there are errors in the shape of some of the gear teeth (and no gear is completely perfect), having them mesh the same mating teeth repeatedly like that will cause the imperfections to get bigger. If the ratio is such that each tooth has to mesh with all of the other teeth on the mating gear before it meshes again with the same one it began with, then the errors will tend to average out, and mesh quality will actually improve as the teeth wear.

Gearboxes are an entire field of their own, just like airplane design. Just as with airplanes, engineers can spend their entire career mastering the fine points. However, for starters, check for basic mechanical engineering texts, such as "Mechanical Engineering Design" by Shigley.

For some insights on how the Brits managed to build very successful propeller gearboxes for very high horsepower piston engines (an extremely severe application), try to find a copy of "The Power to Fly" by L.J.K. Setright. It's about the history of aircraft piston engine development. A bit dry in places, but very enlightening, and punches some fairly large holes in some commonly believed myths.

[skylark3f]

Don - once again a fantastic explanation, thank you! You seem to have a real depth of knowledge on the subject: if it's not too impertenant a question, what's your background? RAF/USAF? Engineer? etc

I'm seriously considering doing prop theory and maybe C-R prop theory as my university final year project so I will definately do the reading up that you have suggested: this seems to me like a fascinating mixture of aerodynamics and mechanical engineering that is quite irresistable :)

Thanks again everyone,

Jack

[Don Stackhouse]

Don - ... what's your background? RAF/USAF? Engineer? ...

Generally I shy away from this sort of thing, one of my personal pet peeves is when threads shift away from discussing the merits of the ideas and more towards the credentials of the people proposing them (in fact I bailed out abruptly from another thread on this forum recently for exactly that reason, the moment it looked like it was heading in that direction). In my experience, there is no monopoly on good ideas. However, since you ask, just this once:

Mechanical engineer by degree, lots of aero experience, lots of very low Reynolds number experience, about a third of a century working as an engineer. Lots of experience with real-world systems. Not quite 50 years building models, about 37 years in R/C. A variety of experience with full-scale airplanes, sailplanes and hang gliders.

Head of R&D for one of the "big 3" propeller companies for 12.5 years. Worked on everything from blimps to regional airliners to military trainers to unconventional aircraft to record-setting airplanes and UAV's. Spent a lot of time doing things like FMECA's (Failure Modes and Effects Criticality Analysis, i.e.: how likely is it to break and in what way, what happens when it does, and what should the pilot do about it).

I happen to have my signature hanging up in the NASM (in the company of quite a few very good people). All of us who worked on the props for the Voyager signed the inside of the prop spinner. That was a little tradition I started there on special props for unusual applications, inspired by the way Ryan Airlines had all their employees sign the main wing spar on the Spirit of St. Louis just before they covered the wing.

Co-founder of DJ Aerotech back in '92, full-time in the model airplane business since the mid 90's. I'm the "D", Joe Hahn is the "J". Spent the last few years developing some unusual props and reduction drives (still in development, but getting close to production) for giant-scale models, in conjunction with Joe Boyd's CS Systems.

Props have profound effects on both the performance and the handling and personality of the planes they are mounted on. They influence the airframe and engine, and the engine and airframe influence the prop. Unfortunately, few aero engineers truly understand them, and it almost requires a few years actually working in the propeller industry to fully comprehend the finer points. I do know of a number of historical cases where the handling characteristics of a propeller interacted adversely with the handling of the aircraft, resulting in a crash. Also, studying the prop can tell you a lot about what the airplane is doing. When the NTSB is analysing a crash, one of the first things they want to look at is the prop, since it can tell them a great deal about what the airframe and engine were doing. I made it a point to attend the accident tear-downs so I could see what broke, and why.

Propellers tend to be underappreciated by the folks who don't understand them. I remember when one of my co-op students went back to school for an academic quarter. The first day of aircraft performance class, his professor announced that "Nobody uses props anymore, so we won't be covering them in this class." My associate proceeded to stand up and "read him the riot act." Turns out that part of the problem was that the professor didn't understand props very well. John ended up serving as "guest professor" for several days that quarter.

The aero questions are of course a big issue with props, but the basic theory for analysis of propeller performance is fairly straightforward and well established. The tricky part of that is the interactions between the prop and the rest of the airframe and engine. The other tricky part is managing the tradeoffs between different flight conditions. Predicting the performance in one flight condition is fairly simple, but figuring out how much to give up at one flight condition to help performance at another, in order to achieve the best overall mission performance, is a skill that doesn't come so easily.

I will say that developing that skill has been a huge help in my design abilities for complete airplanes, both from the standpoint of managing tradeoffs between different flight conditions, as well as in understanding things that are not as simple as they appear, such as the helical flow field surrounding an airplane in a turn. That condition shares some surprising similarities to the helical flow field around a propeller blade, and some similar design approaches can be useful.

However, the mechanical issues can be even more complex, particularly the vibrational issues. The structural design of propeller blades tends to be driven more by natural frequency issues than strength issues. I'd suggest taking all the courses on mechanical vibrations you can find. Then there's the control systems, in the case of variable pitch props. Those can result in accidents too, if you don't get the failure modes or the integration with the airframe and engine controls just right.

Lots to think about, and lots to learn.

[skylark3f]

Don,

I really am so sorry if I have caused any offence, it really was not my intention and I was I no way trying to discredit or undermine your knowledge on the subject. It seems to me that for someone to comment as clearly and in the "lay-persons" terms (that is to say, not using technical language that some of us might not understand but is nevertheless used in the business) must know their subject well: good explanations like yours are not easy to come by in my experience.

I really do just find it fascinating listening to experienced people, especially in the engineering field, talk about their careers and the interesting things they've done and seen.

I realise however that this is neither the time nor the place for such enquisitions and that this thread is way off track and I won't do it again :(
I will say that I am very grateful for the advice though :o

sorry again,

Jack

[Don Stackhouse]

Don,

I really am so sorry if I have caused any offence...

No, none at all. I just do not want to see the thread get into discussing who said it rather than what is said. Just because I, or anyone else, does or doesn't have some given level of experience relative to a particular topic does not mean they can't be wrong, or can't be right, and it certainly does not establish any "value" (or lack thereof) for their contributions.

One of the most valuable members of my team back in that "previous lifetime" was a high school grad with a 2-year degree in drafting. He didn't have any training in calculus or differential equations. He didn't know how to do stress analysis, or calculate the kinematics and dynamics of a 4-bar mechanism. He wasn't much good at figuring hydraulic flows and pressures or equivalent orifice diameters. (or, at least he couldn't when he joined the group; after a couple years of working with the rest of us he could! I tend to ignore diplomas and let folks try stuff they don't think they can do, and with a little coaching they usually discover that they can do a lot more than they thought.)

However, in our group discussions I learned to watch his reactions very closely. The other engineers could be fairly deep into esoterics, but if he looked like he was getting a bit lost, more often than not it meant that we had overlooked something. His input was truly invaluable, and saved us from falling into quite a few of our own self-dug pitfalls.

Everyone has their own set of strengths and weaknesses, and each one has something valuable to contribute. Often it's the ones who know the least about a given subject who question the things the more experienced have already accepted as a given, and in the process they sometimes unmask a hidden problem or opportunity.

My only concern here is that if we give anyone the idea that you have to have a certain level of experience or knowledge to be "qualified" to participate, then we risk suppressing potentially valuable ideas.

[skylark3f]

I understand and except your point that you are making, it's a good philosphy to follow! :)

Many thanks once again,

Jack