Wood or Warpdrive

[Bugwar]

Got this from the Cubby website

Cruise Speed

85 mph (power fin)
95 mph (P prop)

Can anyone confirm?
Difference between climb rate / take-off distance?

[KFA]

Hi Paul, that will be a very dangerous and I think expensive assuption. Your theoretical airspeed is determined as follows: prop rpm x pitch (inches) x 0.000947 = mph (this is theoretically how fast you can fly) You take this answer and you deduct the drag in the case of a BB 12-18% and the answer should be what the calculated topspeed will be. (courtesy oom Piet de Necker) the answer has got nothing to do with take of and climb calcs although you can make some logical deductions from it as to how the aircraft will perform with different pitch settings and prop lenghts.

[Bugwar]

KFA, thank you for the explanation. Just trying to gather as much info as possible and to understand the world of propellers a bit better.

I am trying to make a decision between a fixed pitch 2 blade P-Prop, a 4 blade powerfin and a 3 blade warp drive for my Koala.

According to the CUBBY website (http://www.cubby.co.za/tricubby.htm) there is a difference of 10 mph on the cruise speed of the CUBBY when the p-prop (95mph) is fitted and when a 2 blade powerfin (85 mph) is fitted. They mention nothing about pitch or length so I can only assume that it is different for a reason.

I am asking 1) Can anyone confirm that it is indeed so and why (pitch, efficiency, length)? and 2)What is the difference (if any) in the climb rate and take-off distance on the Cubby between the specific p-prop and powerfin as mentioned in their specifications.

I keep on getting "the p-prop is more efficient" (mentioned plenty in this thread) and trying my darnest to understand why.

If I look at the formula

Your theoretical airspeed is determined as follows: prop rpm x pitch (inches) x 0.000947 = mph (this is theoretically how fast you can fly) You take this answer and you deduct the drag in the case of a BB 12-18% and the answer should be what the calculated tops peed will be

the pitch will determine the top speed of the aircraft if all the other values are assumed as constant. This should then mean that there are no difference between propellers as long as the pitch (and surface area and length?) is the same. Why then is the p-prop deemed as being more "efficient" or am I missing the point completely?

[KFA]

The lenght of the prop is determined by the tipspeed. Wood and composite = mach .75 max and steel mach 0.85 max

[Morph]

When you start selecting props in the beginning for a new plane and especially for a new pilot you need to select a prop with the best possible climb performance. This is to safely get you off mother earth as fast as is possible for your plane. A way of checking this is to tie the plane to an immovable object and rev the moer out of the engine, watching your max RPM you get. This is called the static setting. Now you want the engine to rev up to and NOT more than 300 rpm below your maximum rpm for that engine. On a 582/503 this is about 6500 rpm maximum. This is achieved by reducing or increasing the pitch of the prop until the static rpm is reached. Now if you have a fixed pitch prop like a P-Prop you can put it on the plane and if you are getting a lower rpm such as 5800 at max revs then that prop is a cruise prop and not suited to your needs now. This way you will be able to check before even trying to get in the air and discovering that the damn plane won't lift off

The reason you set it lower than max rpm, is once you do take off, the rpm can easily exceed the 6500 and run up to 6800. If you set the static to 6800 on the ground you could easily overrev the engine in the air.

Later when you are much more comfortable with your plane and the airfield you fly out of, and if you need a cruise setting, then you can increase the pitch, which will load the engine and increase thrust. This will reduce your ability to climb. In the case of a Koala with it's high drag design I doubt very much that you will be able to get significant performance improvement w.r.t. speed. Your speed is governed to a large degree by the wing design and not the propellor. Additionally by loading the engine with a heavier pitch, it runs hotter (EGT) and heavier on fuel. It's like trying to take off in third gear.

Two, three four blade?

Two blades are usually nice and fat to give the maximum thrust. Also the two blades have 180deg travel before turning through the disturbed air from the other blade. This means that the two blade has the most use of clean air and therefore will produce the most thrust for that size blade. In fact a 1 blade is the most efficient, but would need a counter balance otherwise it would vibrate the whole lot off. Two blades always rougher than 3-blades. A 3-blade has only has 120deg clean air, a fourblade, 90deg etc. So the higher the number of blades the less clean air and the efficiency drops but they are smoother. The more blades you add the thinner each blade gets because you need to keep the entire weight of the propellor down, while maintaining the same total surface area, and these thinner blades normally mean the pitch has to be set higher to make up for the lost thrust due to the thinner blade. This means that they load the engine more than a 2 blade of the same length and will have a lower static rpm. To compensate for this they are usually cut the blades a bit shorter. Try spinning yourself on an office chair with your legs outstretched. Now bring your legs inwards, you naturally speed up. By shortening the blade slightly, it will rev higher and be closer to the static, with less loading on the motor.

A four-blade will be shorter still etc. So if you have limited prop clearance then going multiple blades will solve your problem, you will have a marginal reduction in efficiency, but a very smooth ride. The opposite is true as well, if you are using it in a tractor config with a large cowling, you need a prop as long as possible to extend past the cowling.

So you need to look at your specific needs and choose the approriate prop.

[Bugwar]

Thank you Morph, what a most excellent piece of advice.

Now, my specific need is to get away from mother earth as soon as possible with the least possible amount of vibration. I have done a static test on a 72x50 p-prop and get 6500 rpm. Only 2 things bother me weight and vibration. Vibration may be due to balance / tracking not being optimal, which leaves me with weight.

The powerfin I have an option on is almost the same length (68"/70" can`t remember). If I understand you correctly it means that the loading of the 4 blades on the powerfin will make my static rpm lower and if I set the pitch to compensate it will not be efficient enough.Not enough thrust.

Will there be any advantage if I take 2 blades off the powerfin like the weight? Assuming that it is the same length as the wooden prop?

[Morph]

Now, my specific need is to get away from mother earth as soon as possible with the least possible amount of vibration. I have done a static test on a 72x50 p-prop and get 6500 rpm. Only 2 things bother me weight and vibration. Vibration may be due to balance / tracking not being optimal, which leaves me with weight.

Don't worry about the weight. If you are getting a static of 6500 it means that your motor has enough power to drive the prop without loading the engine. When you talk about vibration it depends on how much. When I first used a two blade it felt wierd and I got a slight buzzing sensation in my hands when I touched the airframe. I got used to this very quickly. Having said that I first tried it without balancing and it felt horrible. Once I was convinced that the balance and the tracking was right I forgot about the slight buzz and flew happily with the prop.

The powerfin I have an option on is almost the same length (68"/70" can`t remember). If I understand you correctly it means that the loading of the 4 blades on the powerfin will make my static rpm lower and if I set the pitch to compensate it will not be efficient enough.Not enough thrust.

Yes in order to compensate for reduced surface area of each blade the pitch is usually higher which loads the motor. As you can see the 4-blade is shorter to get it spinning faster, 72" 2-blade, 68-70" 4 blade.

Will there be any advantage if I take 2 blades off the powerfin like the weight? Assuming that it is the same length as the wooden prop?

No, two blades of a 4 blade with not have enough surface area to provide sifficient thrust. You are going to have to set the pitch very high to try and get enough thrust and this is not a good idea

[Eyes]

Warpdrive is the only way to go.

[Abe]

Hi Flyers!
Yous guys are going to get me in trouble, the missus said I must be in bed by 24:00!
Yes and no, Eyes. Warp Drive is best for trikes because it is indestructible, not because it has best performance.
And I have news for Morph. Propellor surface area does not always equate to thrust. I found Powerfin terrible in this regard. Its BIG wide blades is outclassed in climb at the same revs by narrower Kiev blades (80% area) set to, can you believe it, 1.5deg LESS pitch! (at 90% chord) And yet the Kiev does not overrev in level flight like the Powerfin does, go figure...
FlySafe!
Abe.

[Nacho]

Gents. I bought a Cobra, two weeks ago. Had a NC Composite prop. I had ok power on my 582, till my instructor asked me just to try a P Prop. This is the cold hard facts:

- My take off distance decreased by about 30m
- I gained height like you can’t believe, with the wood prop. At least twice as high at the bottom of the runway as with my NC Prop
- The weight is the same, so don’t believe stories on extra strain on the gearbox.
- My NC prop was perfectly adjusted, so the avgas performance was NOT due to incorrect adjustment
- I was also sceptical, read many opinions, did research etc. But only after feeling the difference myself truly converted me.

All the best, with your decision. Fly safe

Nacho

[Greenman]

Paul

Entries from my FP Super Koala ZS-WHH logbook:Engine- 532 Rotax:

02 March 1998 (255 hrs)
-"Remove prop G Killey (66x34) due to crack at tip"
-"Fit new prop - 3 blade 70" - not performing"
-"Fit new P Prop (66x37) - test flight ok"

08 August 1998 (279 hrs)
-"Fit spare prop to check performance versus P prop fitted. Better climb overall
- P prop better cruise"

10 December 2007 (451 hrs)
-"purchased new P Prop @ R5500. 66x39 pitch. Now flies level @5500 rpm.Top cruise 75mph Awsome!"

My comment:
25 March 2008 (459 hrs)
Nose over at Freds farm- no more any prop Damaged aircraft sold to me.

Entries from my other FP Super Koala ZS-VTN manual and logbook:

Manual:
22 February 1987
Engine 503 Rotax
Prop- Geo Killey (68x30) Max thrust 134 kg
Gearbox 2.58

Logbook:
15 June 1988 (1 hr)
"Aircraft test flown and found satisfactory in all aspects."

Logbook:
10 May 2002 (248 hrs)
"New (68x30) Geo Killey Propellor fitted

24 September 2006 (377 hrs) last date in logbook no further mention of props.

Info from previous owner:
-New 582 Blue top engine was fitted somewhere after Sept 2006. Flown one hour with (68x30) prop - unsatisfactory.
3 blade Warp drive bought and fitted- flown one hour- Engine and instruments stolen. Fuselage sold to me.

Hope this helps some
Regards
Chris

[German]

Sorry for the hijack.Morph what size prop have you got on your 80hp bushbaby?

[Morph]

69 inch warp drive, 3 blade. Works absolutely perfectly

[German]

Thanks.You dont know of any for sale?