fixed pitch props

[Bill Marsh]

Re: fixed pitch props / R Barron

Surfing around this site......... geez lots to see!

On the "going flat" deal specifically on the unlimiteds and specifically on the R Baron / Hinton "moment"....

Seen enough crashes, all of them in motor sports / air-ground-water / was from a split second to a blurr in duration.

The Red Barron final deal made me go in shock.... a dragged out 10 to 15 miute mightmare before the PA blasted out "he is OK"...... amid a distant black plume over the horizon.

1979?

BM

[Hawker 73]

Re: fixed pitch props

The contra-props seem to have come about because of a World War 2 problem peculiar to the British Royal Navy. They were flying Seafires off from small escort carriers and having problems with torque roll and yaw which meant that either the airplane rolled over the side of the ship into the water or the pilot could not apply full power until the airplane had some forward speed and a bit of control authority. Bad news on a small deck in the North Atlantic!

Then came the Griffon powered Seafire, still more power and what was worse, the engine rotated in the opposite direction just to confuse the pilot. Going to a contra-prop fixed all that, at the expense of a lot of weight up front. The equivalent land-based Spitfire with a Griffon kept the single rotation propeller.

So why keep the contra-prop for airplanes like the Shakleton bomber/patrol plane? Well, many British props were made from densified wood/phenolic laminate and therefore had thicker airfoil sections than the US metal propellers. When tip speeds get up to the transonic range the airfoil critical Mach Number becomes an issue due to the combination of blade thickness and blade lift coefficient. So the thicker blades can't generate as much thrust as the thinner metal blades without getting local shock-wave formation on the forward face and that means loads of drag (i.e. the "sound barrier"). To get more thrust without the high drag (power requirement) you need to add more propeller blades. Thats why you see some British airplanes with 5 blades. Using the double 3-blade contra-prop kept the blade loading down to acceptable levels. In the case of the Shakleton, Avro was able to take the existing Griffon/contra-prop combination off-the-shelf.

Another detail I seem to have noticed is that with contra-props the aft set is slightly smaller diameter than the front set. Maybe this is to prevent the aft set from intersecting the tip vortices from the front ones which woud surely create all sorts of noise and vibration.

Chris B

[Bill Marsh]

Re: fixed pitch props

Chris

What is the efficiency % of the first vs. second row of blades ( on the well documented UK V12s that we see in our world of race planes)

And of course, my ugly duckling favorite, tu95 with those huge gas turbine double row massive CR props.... any % age efficiency relationship gains from the brit 40 to 50s era technology


BM

[Hawker 73]

Re: fixed pitch props

More on contra-props relevant to race planes.

The bottom line is "is it faster with a contra-prop"? It is hard to get a really accurate answer but I have found some comparative data for a couple of virtually identical aircraft with and without contra-props. This is from "Spitfire - The Story of a Famous Fighter" by Bruce Robertson, Garden City Press Ltd, 1961.

Seafire F47, Griffon 87 with 2x3 blade Rotol contra-prop.
Max speed 451 mph at 20,000 ft
4,800 ft/min climb SL at 10,200 lb

Spitfire F24, Griffon 61 with 5 blade Rotol propeller.
Max speed 450 mph at 19,000 ft
4,900 ft/min climb SL at 9,900 lb

As you can see, there is virtually no difference but the contra-prop might just have a very slight advantage. There is data in that reference for the Seafire F/FR46 with Griffon & contra-prop which gives its climb rate as 5,300 ft/min at Sea Level at 9,900 lb (same weight as Spitfire F24) so it is indicating that the contra-prop has an advantage at the lower speed end of the envelope.

The seafire is navalised, with a tail hook and extra gaps and steps in the wing surface for the fold mechanism but looking through all the performance figures for all the different Marks of Spitfire/Seafire, there is no discernable penalty for the naval version.

So what's going on here?
Well, I think that at lower speed the contra-prop is doing better because it has more blades and each blade airfoil is operating at a better lift/drag ratio also the single-prop pilot is holding rudder on to counter the "p-effect" and this is causing drag on the airframe. At higher speed the p-effect is reduced, the pilot can relax on the rudder and the drag goes away. The 5-blade single prop is now doing as well as the 2x3 blade contra-prop so any improvement in theoretical efficiency of the contra-prop must be offset by mutual interference between its two blade rows.

I can't directly answer you question as to which row, front or back is the more efficient, I didn't find any research papers on the subject on my quick visit to the University library. In my opinion, the back row must be running in "dirty" air and be suffering some kind of performance loss.

IMHO the way to aproach this is to build the most efficient single-rotation propeller and to put the effort into matching the blade-loading to the blade airfoil to get it running at best lift/drag ratio. You then fit as many blades as it takes to hit this sweet-spot in the race. That might give us a Griffon with a single rotation propeller of 5 or 6 metal Hamilton Standard (see earlier comments on blade airfoil thickness) blades. Dowty and others are routinely building 6 blade propellers for the turbo-prop commuter airliners, presumably to get best efficiency.

Chris B.

[Bill Marsh]

Re: fixed pitch props

Ok Chris...

Where i am leading to is this... from my first exposure with the RB51 w/ CR props all the way to the present day World Jet / PM deal.

Soooo
The physical result.... mo power, mo weight, mo macho, mo bite

The cost... mo $$$ and a real chopped up 51

='s ????

Am waiting for the maturing of something that looks great .... but if it has not (performance justified) all that tweaking by now....then what. ??

BM

[First time Juke]

Re: fixed pitch props

The original Cri Cri flew once 5 km with just one engine( 2 x 9 hp ) and made it back..lost 50 meters of altitude..that is why they nowadays fly with more powerful engines. The props are fixed pitch.

Cri Cri site: http://www.flight.cz/cricri/english/

[Skyracer]

Re: fixed pitch props on twins ?

"Before I develope any further idea of my new twin of non-category racer with fixed pitch props....I want to know if there is anyone flown a twin with one engine of this kind with fixed pitch on props ?"



Suggest you go read up on the Aeronca Lancer. A two place twin with Cont O-200 engines and fixed pitch props. WARNING - This may cause you to rethink the whole concept.

[First time Juke]

Re: fixed pitch props on twins ?

Suggest you go read up on the Aeronca Lancer. A two place twin with Cont O-200 engines and fixed pitch props. WARNING - This may cause you to rethink the whole concept.

Could you provide a pic of the Aeronca Lancer so we can all see what is wrong in the Lancer ?

One would have to abort the race anyway if the other engine quits...quitting the remaining engine as the other has gone will make it a glider..just watch that there is plenty of room to land the twin in that case...and lotsa air below you...or plenty of airspeed.

[First time Juke]

Re: fixed pitch props

Continetal 0-200 is rated 100 hp...and Nemesis went 290.08 mph with it.

Continental 0-200: http://www.zenithair.com/zodiac/xl/o200.html

Lets say one is able to double the hp output...makes a twin with retracts and smooth and light as Nemesis...would 360 mph be plausible speed ?

Nemesis F-1 racer: http://www.nasm.si.edu/research/aero...ft/nemesis.htm

I'd like to point out that this twin would be just marginally heavier than Nemesis ( unable to compete in the unlimiteds ) but would posses 4 times the power...and with retracts only marginally more drag, if not less.

I estimate it could go 500 mph...but I don't because I would be labelled as nuts ( with two Orendas it just might..but then it would be bigger and heavier ).

[First time Juke]

Re: fixed pitch props

Fixed vs. variable pitch props has been an issue before..Here is the DH88 Comet;

http://www.pjcomputing.flyer.co.uk/comet/

http://en.wikipedia.org/wiki/De_Havilland_DH.88

[Skyracer]

Re: fixed pitch props

Here are a few links I found for the Aeronca/Bellanca/Champion Lancer. Yes, I've had the privilage of actually sitting in, carressing, and exploring one (but feigned a trip to the restoorm when offered the opportunity to actually fly it).

http://www.planeandpilotmag.com/content/2004/feb/it_slug.html

http://1000aircraftphotos.com/GeneralAv/1359.htm

http://1000aircraftphotos.com/GeneralAv/ChampionLancer.htm

Single engine service ceiling at gross wt: 2000' BELOW Sea Level

Single engine climb rate is -120% of multi-engine rate

[440_Magnum]

Re: HP and cubes?

What kind of power are these unlimited motors making?? What kind of displacement and RPMs do Merlin and Griffon motors turn?? Well ok, RPMs on the merlin, and both for the griffon.

Parrotting back a lot of stuff you can find on this site and others:

The Merlin displaces 1650 cubic inches, and the RPM level that is run depends on the engine builder's philosophy. Jack Hovey used to build "high revving" Merlins that had to turn upwards of 3700-3800 crankshaft RPM to build the needed boost using -7 blower gears. Obviously light rotating mass was essential, so those engines used stock Merlin rods. Dwight Thorne's Mystery Aire Merlins evolved toward -9 blowers that could build the boost at a lower RPM, and used the heavier, stronger Allison rods to withtand the higher torque loading. I'm not real sure what horsepower a full-tilt-boogie Mystery Aire or Sparrow Merlin running with a -9 blower can put to the propeller. There's not any real good way to *measure* it either, since Merlins don't have torque meters.

On the other hand, the round-engine guys know with pretty good certainty how much power they're putting to the prop at all times, since the commonly raced radials all have torque meters (if you know torque and RPM, you know horsepower). The Dreadnought gang has always said that they run an honest 4000 horsepower to the prop (and can do it all day, every day) out of their 4360 cubic inches. I think its fair to say that September Fury's 3350 cubic inches put a fair bit more than that out this year, and the Bear has run in the 4000 horse ballpark as well in years past, also with an R-3350.

Griffons displace 2240 cubic inches. I don't really know what kind of power they're good for. In theory they could top the Merlin, but they haven't been developed nearly as far for air racing as the Merlin has and there aren't as many mix-and-match parts available to play with different combinations. The "good" blower Griffons are extremely rare. The common ones that were used on the Shackleton can't build racing levels of boost at practical RPM levels.

[First time Juke]

Re: fixed pitch props

Here are a few links I found for the Aeronca/Bellanca/Champion Lancer.

http://www.planeandpilotmag.com/content/2004/feb/it_slug.html

Single engine service ceiling at gross wt: 2000' BELOW Sea Level

Hilarious reading...let John Sharp redesign it !