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Prop too big?

Old 03-26-2009, 02:57 PM
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Juice
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Default Prop too big?

A question for Keith Lawes:

First off... I think it's great to have the inventor of RCV technology frequent these forums. Thank you.

I have an RCV 90SP. I am putting a 3-blade 18x10 prop on it. I know that prop is too big, and it's evident in the 4400 RPM maximum at full throttle. The question is... Is that bad for the engine? I've flown about half a dozen times with this prop and I like it. It still has more than enough power to fly my H9 Corsair.

Here's a video of the first time I tried it...
[link=http://www.vimeo.com/1918254]Big Prop on RC Corsair[/link]

Can you also comment on how it will affect the longevity of the engine if I keep over-propping it like this?

Thanks,
Josh
Old 03-31-2009, 03:52 AM
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keithlawes
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Default RE: Prop too big?

Hi Josh

Thanks for the flattery!! Always susceptible to a bit of flattery.

On the overpropping, its a very interesting subject.

Firstly I am sure you are not doing the engine any harm at all, if anything you are probably subjecting it to little less stress than the more normal rev range of 5000+. If you think about it you are running at a more conventional crank RPM (i.e. 8800, which would be absolutely typical for a normal ungeared 4 stroke).

By revving the SP engine higher as we normally do you do get an increase in power (as power = torque x RPM). However as the engine breathes less efficiently at higher speed the torque drops of somewhat so the net increase in power is not that significant. So as long as its enough to fly the plane then you are probably doing the right thing.

Probably the main limiting factor would be the pitch. As the RPM goes down the effective maximum forward speed (4400 * 10 inches per minute = 40mph) gets a bit low. However the engine will unload in the air, the RPM will increase and prob give you enough in hand.

To be honest we have always traditionally run the engines in the 5000-6000RPM range, but aside from the pitch question its probably more efficient to run in the 4000-5000RPM range, but then you do start to run into forward speed v pitch problems on some model. Also the prop sizes required to load the SP engines down to this speed become a bit impractical.

So to sum up, it works well and it isn't going to do any harm so keep going!

Regards

Keith
Old 03-31-2009, 04:24 AM
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keithlawes
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Default RE: Prop too big?

Hi Josh

Had probs viewing your video (v slow computer), but have managed to do it at last.

Very impressive. Does look great, and I'd say the takeoff performance is pretty fair. How much does the model weigh?

Keith
Old 03-31-2009, 09:52 AM
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Michael211
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Default RE: Prop too big?

Correct me if I'm wrong guys, but a 10 pitch prop is quite a bit below the recommended pitch range for the SP series engines isn't it? I presume that by going up in number of blades and diameter that this is making up for this lower pitch? As I recall RCV's recomendations are for 12 pitch props, which is why I wanted to ask about that...

But hey, if it works then fly it right? [:-]
Old 03-31-2009, 10:00 AM
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Default RE: Prop too big?

That video player does NOT work well Josh. My computer (and internet connection) is not particularly slow, and I could only get a few frames of the video to come up over a 5 minute period. However your Corsair looks great and that prop is like OMG big! LOL!

Who's hub is that you're running and are you really able to adjust the pitch of the blades using that hub? I'd love to see a few pics of your engine setup, sans cowling???

- Michael
Old 03-31-2009, 10:06 AM
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Default RE: Prop too big?


[quote]ORIGINAL: Michael211

That video player does NOT work well Josh. My computer (and internet connection) is not particularly slow, and I could only get a few frames of the video to come up over a 5 minute period.

I was able to view tbe vidio clip just fine, it did stop for just a second two time waiting for the download.
Old 03-31-2009, 10:38 AM
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Default RE: Prop too big?

Well... I guess your mileage on that video player may vary by quite alot. [&o]

- Michael
Old 03-31-2009, 11:48 AM
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Default RE: Prop too big?

Josh ,
I watched that video on youtube, which is a much better connection. It certainly is very impressive mate! I'm also very interested to know the weight of that model, as as keith pointed out the pitch speed on a 10" pitch prop at 4400rpm can get a bit critical and I'm guessing that the coarsair is actually a lot lighter than It's size suggests.
I'm currently looking in to what 3 blade props are available other than the APC's for use on an RCV 120sp in a 71" Zero that will probably be around 12lb. It's a shame that the Varioprop only has IC blades up to 11" IIRC as I have the 12c hub with a set of 15" blades for electric use only. So far I've found Biela props & Engel props that do 3 bladed versions, But the nearest size to what I'm looking for is an 18" x 11" from Engel, which I think might still be a bit low on the pitch speed. Biela do an 18 x 10, I'm not convinced that will give a safe pitch speed on a 12lb model though.........It could get expensive experimenting

Walts.
Old 03-31-2009, 03:34 PM
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Juice
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Default RE: Prop too big?

Keith... My Corsair weighs 10 lbs. It's no speed deamon, but I have no problem keeping in the air. Thanks for your comments. It's good to know I'm not damaging the engine.

Michael... The hub is custom made. I reverse-engineered a Soloprop hub and sized it down to 1/8-scale. And yes, I can actually change the pitch (on the ground, during setup). I have more videos of it. See links below.

Walts... As mentioned, the Corsair weight 10lbs. With 10' pitch the theoretical max speed is proven to NOT be a concern. But with a 12 lb model, I wouldn't try anything less than 12' pitch.

More vids (on YouTube):

[link=http://www.youtube.com/watch?v=3pagp2fy97U]How to mount the 1/8 scale hub.[/link]
- See the inside of the hub.

[link=http://www.youtube.com/watch?v=oU0RrCmR7Sc]1/8 Scale Hub: First Run[/link]

[link=http://www.youtube.com/watch?v=-B1iBvwJcbc]1/8 scale Hub. RPM for 18x13 3-blades[/link]
- Note: My tach was incorrectly set to 2-blade mode getting 5760 RPM on 3-blades. The actual revs is 5760/3x2=3840 RPM, with an 18x13 3-blade.

[link=http://www.youtube.com/watch?v=cok9cPH4CYQ]First Flight with the 1/8 scale Hub[/link]
- Please ignore the bad landing. =(

[link=http://www.youtube.com/watch?v=fdA8_Rbj7oM]Big Prop on RC Corsair[/link]
- Same as the video in my original post (but on YouTube).
Old 04-01-2009, 12:15 PM
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Michael211
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Default RE: Prop too big?

Sweet! The YouTube videos work perfectly for me. That's really good to know that the 90SP can fly a H9 Corsair that well with that large of a prop too!

Many thanks for sharing!

- Michael
Old 04-01-2009, 12:28 PM
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Default RE: Prop too big?

The hub is custom made. I reverse-engineered a Soloprop hub and sized it down to 1/8-scale.
I remember that now! You had a bunch made up and were selling them.... I still don't have a Corsair, so I still can't justify buying such an expensive part for a plane I don't even have. LOL! I'm working on a Mustang with a 90SP engine still... if I ever get it done, then I'll be looking to do a Corsair.

The Soloprop hub maker is a fool for not doing what you did, there's a huge market there with the H9 Corsairs!

Regards!

- Michael

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