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Old Sep 05, 2009, 12:39 AM
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Longmont, CO
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Need help finding the right Propeller

hey, I'm trying to build a model rc plane, but I've run into an obstacle. I've been searching for about a week now, and I can't find a 4-blade propeller that has a diameter less than 8 inches. Does anyone know of a place or website where I can get one that small? (7 inch would be preferred, but 8 inch will fit). at the moment, the price doesn't matter, nor does the type of propeller it is (folding, variable pitch, fixed). Any help would be greatly appreciated, thanks.
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Old Sep 05, 2009, 12:58 AM
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Windsor,N.C.
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can you cut the big one down to what you need? I cut two blads down from 8x4 to 6x4.
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Old Sep 05, 2009, 07:07 AM
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eflightray's Avatar
South Wales U.K.
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A four blade prop does look good, but only on the ground when it isn't going round.

Two blade props are more efficient, so why not put a dummy 4 blade on for show, and swap to a 2 blade for flying. ?
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Old Sep 05, 2009, 08:12 AM
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Dr Kiwi's Avatar
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http://eam.net/Varioprop/Vario%20Prop.htm

I believe that <8" props are available for the 4-blade hub.
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Old Sep 05, 2009, 06:16 PM
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Quote:
Originally Posted by Donald Byrum
can you cut the big one down to what you need? I cut two blads down from 8x4 to 6x4.
I don't know why I didn't think of that, thanks!
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Old Sep 05, 2009, 06:20 PM
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Quote:
Originally Posted by eflightray
A four blade prop does look good, but only on the ground when it isn't going round.

Two blade props are more efficient, so why not put a dummy 4 blade on for show, and swap to a 2 blade for flying. ?
Wait, I'm not really up on the whole propeller design stuff, how is a 2-blade more efficient than a 4-blade? wouldn't having the extra two blades per prop make it give more thrust?

And if it really is more efficient, I will probably still go for a 4-blade because I'm trying to make a scale replica of a plane. But that's still a probably, not a definite, I may go 2-blade, so thanks for the advice.
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Old Sep 05, 2009, 06:24 PM
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Quote:
Originally Posted by Dr Kiwi
http://eam.net/Varioprop/Vario%20Prop.htm

I believe that <8" props are available for the 4-blade hub.
thanks for the website. although, I still have a question: do u know if when it says 6.0 inch diameter for the blades, is it talking about the total diameter or the length of that blade? or do they sell the blades in pairs? thanks.
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Old Sep 05, 2009, 07:55 PM
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What they mean by, for example, 6" diameter is exactly that... 6" diameter... so each blade is of course approximately 3" long. They sell blades individually, so you can buy a single blade as a replacement, or a pair for a new set-up.

It is well known that 4-blade set-ups are less efficient than 2-blade (additional drag offsets any additional thrust, and the extra blades just screw up the airflow anyway [dirty air]). The most efficient are single blade, counter-balanced set-ups... ask the control line speed guys.
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Old Sep 06, 2009, 12:33 AM
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Quote:
Originally Posted by Dr Kiwi
What they mean by, for example, 6" diameter is exactly that... 6" diameter... so each blade is of course approximately 3" long. They sell blades individually, so you can buy a single blade as a replacement, or a pair for a new set-up.

It is well known that 4-blade set-ups are less efficient than 2-blade (additional drag offsets any additional thrust, and the extra blades just screw up the airflow anyway [dirty air]). The most efficient are single blade, counter-balanced set-ups... ask the control line speed guys.
thanks for the clarification, that's the first place I've found that sells anything smaller than 9" diameter.

With the reduced performance of a 4-blade, will four 4-blade props be enough to pull 2-3 lbs into the air?
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Old Sep 06, 2009, 09:12 AM
Jack
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"..With the reduced performance of a 4-blade, will four 4-blade props be enough to pull 2-3 lbs into the air?..."


It takes 40-50 Watts per pound or more to get airplanes to fly. At 200 Watts per pound you can have unlimited performance. But it is also possible to have a plane with 200 Watts per pound of all up weight and have wings so small that the wing loading is too high and it still will not fly.

So share some info about the plane and the power system and we can venture a guess on your plane's chances of getting into the air.

Jack
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Old Sep 06, 2009, 09:34 AM
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Quote:
Originally Posted by firefly762
Wait, I'm not really up on the whole propeller design stuff, how is a 2-blade more efficient than a 4-blade? wouldn't having the extra two blades per prop make it give more thrust?

And if it really is more efficient, I will probably still go for a 4-blade because I'm trying to make a scale replica of a plane. But that's still a probably, not a definite, I may go 2-blade, so thanks for the advice.
Two reasons full scale goes with 3 or 4 blades, to avoid
the prop tips from breaking the speed of sound and ground clearance.
Your plane will fly better with 2 blades, get a scale prop for show.
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Old Sep 07, 2009, 12:03 AM
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Thanks, Jack, for the Wattage to Weight ratio, that helps a lot. well the estimated specs of my plane are about a 51 inch wing span and a 35 inch length, I'm using a motor that says it gets 381g of thrust with a 7x4 prop at 9900 RPM and 14.4 amp current. there are going to be a total of four of those motors on it as well. I don't know what the max wing load is because I'm building the frame from scratch, I'm not using any factory made diagrams or anything. I don't know if this affects the ability to get into the air, but I'm also using a 11.1 Volt 2200mah Li-poly battery. hope this helps, thanks again.
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Old Sep 07, 2009, 08:48 AM
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I am puzzled by the figures you give for the motors you are going to use..If that is a 7x4 APC SF prop then 381g @ 9900rpm is about right... but drawing 14.4A?... even if you are using only 2s that seems awfully high (>100W to get 381g).... and thus the motor must be very inefficient.

An el cheapo GPS/HXT/XYH C28-26-B (~1620kv) with a 7x4 APC SF got me this:

7.9v, 9.4A, 74W, 9900rpm, 413g, 5.58g/W


Here is an example using a lower Kv motor on 3s getting close to 9900rpm:

Komodo KH2204-15 (1361Kv) 7x4 APC SF:

10.9v, 7.2A, 76W, 9960rpm, 418g, 5.50g/W
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Old Sep 07, 2009, 09:23 AM
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As usual, though, be aware that unless you are planning to spend a lot of time in the hover as with a 3D type, the static thrust is of no help whatsoever in helping the aeroplane fly. You can have all the static thrust in the world with a fine pitch prop, but if you do not have enough pitch speed it will not deliver enough actual thrust at flying speeds.

What is the proposed wing loading and flying speed of your model?
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Old Sep 07, 2009, 10:01 AM
Jack
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We can close in on the numbers a little bit. The online calculators for aircraft are a little terrifying but useful. They can be ungodly complex:

http://www.geistware.com/rcmodeling/cg_super_calc.htm

or very simple:

http://fwcg.3dzone.dk/

We can take the second one there and input your 51 inch wingspan and guess at the chord length as 10" (for about a 5:1 aspect ratio, not uncommon or radical). I'll also use 10" for the tip chord and 0 degrees as the sweep, so we have a rectangular wing with of 510 square inches.

Any values can be used there, you apply your desired units (inch, metric, etc.) to the results. And you can ignore any of the other stuff that you get there for now.

So right now you have a 51" wing span, and 510 sq. in. of wing area. I am going to guess that this is a more or less conventional high wing aircraft and a 51" wing span and 35" fuselage length are not a radical departure for a plane like that.

You don't know the weight yet. My SIG Rascal 40 electric conversion is a adequately powered nice flying high winged plane. It has a 72.5" wingspan and an all up weight of 8.25 lbs. It we compared the wingspans on those two planes, your 51 inch wing span would be 70% of the Rascal's. So let's guess that your plane would fly at 70% of the Rascal's weight too. That makes your target weight to be 70% of 8.25 lbs or 5.75 lbs.

Now we can get to some wing loading numbers. A common way to measure that for airplanes is in ounces per square foot of wing area.

Your theoretical plane4 at 5.75 lbs is 92 ounces, your theoretical wing at 510 sq. in. wing is 3.54 sq. ft. in area, so you have an approximate wing loading of 26 oz./sq. ft.

If I crunch the same numbers for my Rascal 40, I also come up with a wing loading of 26 oz./sq. ft. also. So I'd say that your theoretical plane would fly nicely.

As a comparison, gliders have much lighter wing loadings (down around 15 oz/sq.ft. or less) so they will float down slower and planes that are intended to fly very fast would have higher wing loadings because less wing area means less drag and more speed.

One of the final numbers to consider here is the power to weight ratio. You know that you know that your power system is capable of 4 x 381 Watts or 1524 Watts. With the theoretical weight of 5.75 pounds you would have 265 Watts per pound with all four motors at full throttle.

And that will certainly fly a plane of the type I am speculating about. If fact it would turn it into a rocket ship if the wing stayed in one piece. Here are some commonly used rules of thumb for powering RC aircraft:

Approximate power requirements:

50-70 watts per pound; Minimum level of power for decent performance, park flyer/slow flyer models
70-90 watts per pound; Trainers and slow flying scale models
90-110 watts per pound; Sport aerobatic and fast flying scale models
110-130 watts per pound; Advanced aerobatic and high-speed models
130-150 watts per pound; Lightly loaded 3D models and ducted fans
150-200+ watts per pound; Unlimited performance 3D and aerobatic models

I've found that the 600 Watts or so (72 watts per pound of AUW) I get at full throttle with the Rascal 40 makes for a quick and confidence inspiring takeoff and climb out. I am typically slowly backing off on the throttle as soon as I have a little altitude and I use half throttle or so for just cruising around and flying the pattern power.

In a 12 minute flight with the Rascal I had an average power consumption of 116 Watts so you can see that a lot less than 40-50 watts per pound is needed to sustain smooth level flight in calm air. I'd guess that my Rascal will fly on as little as 120-150 watts or so which would be less than 20 watts per pound. Of course, as soon as you start maneuvering, turn into a breeze, or want to have any margin of safety against stalling, you want to apply more power.

I'm new to the Rascal and planes that large and of that type. I have a lot to learn there yet but it is really a lot of fun to fly.

Jack
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