Scott Smith

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A little off topic but still cool…Precious Metal at the 2012 Reno Air Races. She lost a gear door in the final Gold heat and had to pull out. Surprisingly, it didn’t sound that different in the air.

Scott Smith

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One more interesting photo showing a unique prop design for Brenner and team to consider…

The crew of Phantom reported a 35 mph increase with this design and it proved to be very quiet (no prop noise even though they were probably running over 4000 rpm.) This plane dominated the Biplane class (mostly Pitts) and set a course record of 260 mph in qualifying.

Brenner

Here's a link to more information about this propeller:

http://www.eaa.org/experimenter/arti...02_elippse.asp

Put simply, it seems that the lift distribution with this prop is designed to have an eliptical distribution along the length of the blade. This is what you get with an eliptical wing, (like the Spitfire for instance...) except that in the case of a propeller the tangential velocity at any radial point along the blade is proportional to the radius at that point times the rpm of the prop, and the lift at any radial point along the blade is proportional to the square of the tangential velocity, which is what gives this propeller it's unique blade shape.

The propellers that we have designed for the Contra Drive also have a wide blade near the hub that tapers down to a very narrow blade near the tip, but this narrowing of the blade is linear, not curved like in this prop. This results in a more constant distribution of lift along the length of the blade as opposed to an eliptical distribution of lift.

In any event, both designs are a big improvement over propellers that use blades that don't taper in width towards the tip, (For an example, take a look at the propellers on the contra rotating Mustang in the picture above ..) because the lift distribution with these blades is more concentrated at the tip of the blade where velocities are the highest. This means that the blade sections near the tip have a higher power loading, and an uneven pressure distribution that causes radial flow off of the tips of the blades resulting in the formation of vortices at the tips of the blades, all of which results in reduced efficiency.

Brenner ...

MTK

Scott,

I built a couple props that looked similar to the one in your picture. These were wood carbon laminates. The main reason for trying the experiment was to reduce tip area and hence lift and drag at the tips. The thought was that reduced tip load would reduce tip vorteces and improve prop efficiency. If true, noise suppression should have been superior. Well, so much for theory....turning these props at 7-8K increased noise footprint. It wasn't horrible noise, just different than we are used to. I scrubbed the experiment since I'm not turning my props at 4K

There is quite a bit of info on such blade shapes and there's definitely something to them, but for lower rpm

serious power

Hi Matt,
Reading just the article it's clear that there is a lot more to the design than reduced tip area - though that is an aspect he talks about as key to not 'throwing' away energy as noise.
He seems focused on the root section in particular, then section,camber,pitch and taper.
The elliptical shape seems to be a derivative of the relative velocities along the span of the blade.
This ,the efficiency at the root and the reduced tip noise seem to all have merit.
Did you use a program to derive the shape you used.
Interesting stuff.

Brian

MTK

Brian,

There is a very interesting book on "Propellers" written by Jack Norris. Mr Norris bypasses much of the math and tries hard to make the reader understand the "logic" and the "horse sense" of it all. It might be worth a read for anyone interested in prop design philosophy. The prop shown on the racing bipe by Scott uses the principles taught by Mr Norris. His point was that most anything can be used to propel....but certain shapes are superior in terms of amount of thrust per HP expended

BTW- Jack Norris was quite the character. He loved to talk shop. He gave high praise to APC props for their well thought out noise suppressing design for higher rpm. I found that quite interesting as well

Brenner

I haven't read his book, but Jack Norris has a website, and in it he seems to be confirming that the best props have tapered blades that narrow towards the tips in order to minimize the effects of radial flow along the length of the blade.

www.propellersexplained.com

Brenner ...

serious power

Hi Matt,
I will look that up.
Scott's post prompted me to do some reading on the subject.
A bit of a bottomless pit. Lot's of guys claiming to be ahead of their time etc.
There are many philosophies, mostly driven by specialist needs eg very high speed.
We/Brenner are operating in a somewhat unusual environment - with the contra - low rpm/modest speed.
We want good rate of climb (relative).
This is one of the areas where the contra excels.
We nearly have more in common with wind turbines than Reno racers .
Some of the German F3A team are doing some 'out there' stuff - the Grieve's I think.
They are using wingtip type fences on the prop tips.
Our guys that have seen them fly report good power and super super quiet, while the guys using them claim a 15% efficiency increase.
The same guys are playing with dorsal fins and even pectoral fins.
They have access to a wind tunnel.

Another link; http://www.supercoolprops.com/

Edit; I've often thought that insect wings are natures propellers. They are around a long time. Though they don't exactly go around in a circle.
Brian

bobwalden

Fascinating discussion guys, I made a Bugatti 100P from info on http://www.bugattiaircraft.com/plane.htm at just 32 inch span. The original 1937 design hadcoaxial contra rotating props driven by a pair of 450+ BHP Bugatti Racecar engines. The intent was to use this aircraft in European Air-Racing, but it did not fly before war broke out, and the plane was stored. It was found inthe 1960's I think, and after restoration is now in the EAA museum at Oshkosh. While the typical contemporary racers were doing a respectable 300 to 350 MPH, recent computer analysis suggested that this amazing plane would do a good 500MPH.
The designer found he could reduce the tail sizes, for less drag and more speed.
I tried flying my model with a single prop/motor but it rolled in left, so eventually tracked down and fitted a small twin contra motor.(Axi make a larger one if someone fancies making a bigger model). This was a little tricky, since the front motor drives the rear prop, but only had a stubby bit of the motor rotating can to fit that prop. I had to make a sleeve nut for this, then bodged a pair of spinners tocomplete the front end.
Hope to get it airborne very soon. I made an identically sized Depron model that flew OK on a single motor, so should be a good un!

bem

My Feedback: (1)

Brenner,

Could You tell what version the Contra is now that You sell and maybe some enhancements that may have been done to the latest version.
Is still the target about 1500 flights for the Contra unit to work or is it anychange to that goal?

/Bo

Brenner

Hey Bem,

The latest version is version 3, which has been available since January of this year. This version has a lot of enhancements over the version 2 drive that we had last year:

1/.. We added a second main bearing in the gearcase. This was because the single bearing in our V2 Drives wasn't very robust to prop strikes. Adding the second bearing has solved this problem, and considerably extended the life of both bearings. Right now we can't make any specific claims about life, because the main gearcase bearings in all of our V3 Drives are all still silky smooth, without any evidence of wear. We've even had some units survive nose first crashes without any damage to the main bearings. (We don't make any claims about crash resistance though..)

2/.. We increased the size of the planet gear bearings and removed the bearing shields. This was done to increase bearing life, and eliminate the possibility that the bearing shields might come off during operation. So far this change appears to have helped a lot. The V3 Drives that we know about are all still running with the original planet gear bearings, and the bearings still don't show evidence of wear.

3/.. We added an O-ring around the pinion gear between the rear clutch plate and the back of the gearcase to keep grease inside the Drive. This eliminates the need for the plastic disks that sealed up the bolt holes on our V2 Drives. This change has worked well. To date, we haven't seen any of these o-rings becoming dislodged, and they are very effective in keeping grease contained inside the gearcase. Now the only path for grease to escape is through the pinion gear bearings, and this only happens in any significant amount if the Drive is overgreased.

4/.. We changed the material used for the clutch spring, and increased the wire diameter to increase the spring force. This was done to prevent the clutch from slipping when pilots draw more than 3500W from their motors.

5/.. We increased the planet gear bearing dowel pin diameter from 3mm to 4mm, and changed the material to hardened steel. This was done to accomodate the larger planet gear bearings, and to eliminate any wearing and scuffing of the pin after an extended numbers of flights.

We still have 1500 flights as a design objective. We figure that this is probably three years for an average pilot, and maybe two years for pilots who have a lot of extra time to practice.

However, 1500 flights is an objective. Proving conclusively that we have accomplished this is a little difficult because we only have V3 units in the field for a single pattern season, but right now we think that the bearings are going to be pretty close to this, depending on what failure rate is considered appropriate. Industry standard practice is to consider the L10 life, which is the life reached when 10% of the bearings have failed, and our L10 life should exceed 1500 flights by a considerable margin. Currently we expect to see a 1% failure rate after about 650 flights.

The next failure mode will be wear of the pinion gear. Predictions here are a little harder to make, because failure is a little harder to define. However, based on typical usege, we expect to get close to 1500 flights before pinion gear wear starts to significantly increase the noise coming from the Drive. One thing to note however, is that this is for failure modes associated with wear. For Drives that are run hard, there is the possibility that the gear teeth might fracture as the involute profile loses significant thickness due to wear, and this could happen before 1500 flights, but predicting this failure mode accurately is a little harder than just predicting wear.

Brenner ...

pippy

Hi Brenner , I did notice that in the services I have done on the V3 unit, practically no grease got in behind the rear clutch plate hence you say we can eliminate the 4 plastic plugs in the casing screw recesses. One less problem to worry about.
Could you post a link to the latest manual please?

Brenner

Hey Paul,

Sure, here's the manual..

Brenner

Angus Balfour

Just received my V3. The quality of the machining is out of this world Weather permitting I will get to try this before the winter sets in, can't wait.

Thanks Brenner to you and the rest of team contra.

Cheers,

Angus

pattratt

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You will love it!
Dick

serious power

Hi,

Another model for the Contra stable

http://www.oxai-rc.com/main/Detail.aspx?ID=91&DID=528

serious power

Hi Brenner,
I see they have changed their props some.
More of this elliptical distribution stuff ??


Brian

Brenner

Hey Brian,

It looks like an attempt to reduce sound more than an attempt to create an eliptical prop lift distribution. I say this because the blades are still pretty wide close to the tip, so a lot of the lift is going to be generated in these outer sections of the blades.

I love the plane. The fuselage is slim and slippery, like the Wind S Pro fuse, and the wings are razor thin, so the plane should have a lot of speed range, and draw a minimum load from the battery packs. Also, I'm thinking that the biplane configuration should make this plane a real pleasure to fly the knife edge maneuvers that are in the f3a unknowns catalogue.

It also takes an 82mm spinner. I'd love to be flying one next year.

Brenner ...

wattsup

My Feedback: (1)

Brenner, it's certainly possble. All you have to do is get your heart and your checkbook RIGHT! At least that's my philosophy ie, it's when your emotions and ability to pay find themselves in complete agreement. Personally, I found out these truths 40 years ago while sitting in the lotus position with the Dalai Lama seated to my right and a "chillum" in my left hand! Before this gets too deep, let's all take the time to realize it's only money___ Everette

serious power

Hi,
I see the rudder flair on this is very like that what we have been doing.

wattsup

My Feedback: (1)

Brian, you've got to admit that is one beautiful biplane! BTW, I found my problem with my lipos, most of the Dean's connectors were corroded. I replaced everyone of them rather than take a chance and now I'm back in business. I appreciate your help and once again, you were correct. Regards, Everette

Brenner

The Wind S Pro has a similar flair. I'm guessing that Seba figured out this trick a long time ago.

Brenner...

serious power

Hi Everette, Brenner,
Yes a beautiful plane.
Very like the Miss Wind but with a change to the wings and tail.
The rudder flair just seems to be more pronounced and abrupt than the Wind S - may be the photo - encouraging anyway.

Everette the contra is a big prop load, or can be with gearing and prop selection.
I really do not think that deans are rated high enough for it.
It will work but at a cost - to your packs in the end !!.
See here; http://shop.rc-electronic.com/e-vend...t=12&c=44&p=44
and http://shop.rc-electronic.com/e-vend...t=12&c=44&p=44
I recommend these for the DC side ; http://shop.rc-electronic.com/e-vend...t=12&c=44&p=44
These are not cheap but they add no effective resistance and last for hundreds + of cycles.
They also have a lightweight high Amp connector but the fit matches are not quite as good. (I use the light one because I have to - so I buy extra and match them up.)
I would also add that their 4mm bullets are very good and that is what I use on the motor side.

Saving money (or weight) can be done elsewhere - this is not the place to do so.

Brian

wattsup

My Feedback: (1)

Thanks again Brian. Part of my problem was the knock-off Dean's look alike I and many others have been using. They are exactly as you described "CHEAP"! I am familiar with EMCOTEC and will contact them directly. Also let me add that I am probably guilty of bringing the outrunner mentality with me when I stepped up to the Contra Drive system earlier this year. In other words, what you were able to "get by with" while running an outrunner will just not give you long run durabilty and performance especially when the CD is added to the Neu F3A motor. But, the difference of the quality of flight along with the unbelievable performance was worth more than the cost of the ride! Regards, Everette

TonyF

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I've used nothing but genuine Deans plugs with my Contra power systems. No problems that I can tell after 605 flights with the Contra in the Onas and 361 in the Nuance.