KB2 prop question

[Chopnotch]

I have a 52" Tennessee on this KB2 which won't work for me. Does the addition of a blade, going to a tri-blade increase the thrust the same as having a larger diameter 2 blade prop or am I missing something?

I am trying to stay in the same physical space as the 52" so I don't have to chop and drop the keel.

Thanks
Stu

[jcarleto]

The same as....no. A two-blade prop is more efficient than a three. A three more efficient than a four and so on. The more blades in the same air, the less the overall efficiency as each blade creates airflow issues for the next.

That said, the only way to add more thrust in the same "circle" is to either increase the pitch or to increase the blade surface. Adding an additional blade will increase the blade surface, but increasing the pitch of a 2 blade prop would be the most efficient. There is, of course, a limit to the amount of pitch you can put into a prop blade. In the case of a wood prop, changing to pitch is generally changing the prop.

You are better off staying with two blades if you can, with more pitch. Failing that, a 3 blade will give you more thrust in the same diameter with similar blade surfaces area and pitch. You might find that to get the same push in the same diameter with a composite prop, you'll have to go to 4 blades. Tennessee props are big and fairly efficient with lots of surface area. There isn't enough information in your initial post to make a determination of the best way for you to proceed.

What is missing here is the problem you are trying to solve. You say the prop you have won't work for you. Why is that? Engine, machine and pilot specifications would be helpful.

[Chopnotch]

Thanks for the detailed explanation Joe.

I guess my problem is I picked up a gyro that is questionable for my size and weight. I am 6'2" 220#. The bird is a KB2 which I believe was a glider first then converted to a Mac. Not sure of the history but, it has the thin plywood tail group so I am making an assumption. The engine is a 503 single carb with suggestions to get the second carb kit for extra HP.

Previous posts I have made, the advice was my size and weight are out of spec for this setup or a drop keel with a larger prop. I am trying to figure out a way to make it work and keep it as classic as possible because I like the way it looks now and I also want to continue to keep it in my standard home garage. ( I like having it here because it really makes me happy looking at it every day for some reason... probably because I dug these as a youth)

So, the 503 has 2" of clearance between the prop and the keel. The prop interferes with the rotor by about 1/2 inch now (the Mac to Rotax conversion has not been completed). I am guessing the prop location must be further aft than it's Mac location to interfere with the rotor so, i need to figure out how to keep the prop diameter the same and work, or drop the keel for a larger prop (not primary choice) or I don't know what... Maybe this bugger just is not going to work for me at my gorilla size and I should cut my losses now? I dunno. Trying to figure it out.

http://hphotos-snc3.fbcdn.net/hs302.snc3/28691_114677265240596_100000949698104_78439_719759 5_n.jpg

http://sphotos.ak.fbcdn.net/hphotos-ak-ash1/hs331.ash1/28691_114677121907277_100000949698104_78438_228839 1_n.jpg

[jcarleto]

You may consider replacing the mast with a taller one by a few inches. That wouldn't be a ton of work. It wouldn't hurt to move the rock guard back under the tail either. I fly a 503-powered machine at 6'2" and 255 pounds and have flown it in winter with a pilot weight of 300 pounds (with boots and winter gear). Granted, I have a larger 2 blade prop on a different airframe.

What kind of rotors do you have?

[scottessex]

Just make taller cheek plates. Do a hang test, the rotor may move back some.
Tim Chick put a 503 on his KB. and used a 52" prop with no problems.

[PW_Plack]

Is it me, or do those cheek plates look way too far forward? The clearance problem may be a non-issue after a hang test, as Scott suggests.

[Timchick]

I replied to your message before I saw this thread, Stu. If you're trying to keep the short mast and 52" prop I'd suggest swapping out the 503 for a 582. Your 503 is not even a dual carb engine so that's working against you with the current setup also. I'm not sure why you are have the clearance issues with the rotor blades. It could be the cheek plate you have on it are the glider plates and not the powered cheek plates. It looks like your rotor head is forward of the mast more than it should be, thus allowing the rotor blades to tilt down farther right above the prop. If the rotor head was moved back about 3 inches it would give you more clearance above your prop.

[Passin' Thru]

Is it me, or do those cheek plates look way too far forward? The clearance problem may be a non-issue after a hang test, as Scott suggests.
My thoughts exactly!
Those are glider plates to balance without the engine.
.

[Timchick]

Paul and I were typing at the same time regarding the cheek plates.

[WHY]

Hi Stu

Regarding "classic" I knew this gal, she was " classic" 38,24,36 she also had aids, you want classic or good design and saftey ???????

Tony

[Chopnotch]

Thanks for all the advice guys.

I was trying to make a hybrid Tony but keep the classic small footprint if possible. Dave Bacon suggested a tail group with the CLT H Stab which I am cool with if it's safer. To be quite honest with you guys, between the IRS and child support, I am limited on funds so, I have to pick what I can do and paying for storage or a hangar space is not in my budget even as a working mechanical designer making okay money.

That is one reason I am trying to make it work and fit in my garage and also keep it part 103 so, I don't have to pay another cent to this wallet vampire system we have here.

That's my wish and i am really trying hard to come up with something that will fly with me strapped to it without being doomed from the start and also, not filling the pockets of anyone else who doesn't deserve my cash.

Tim, I got your message and thanks a million for your advice. On the cheek plates, yes, i got two sets of rotors, glider and powered cheeks with this bird and the pics showed the glider plates. The powered ones have the interference and the glider ones don't. I figured I would build new cheeks with a hang test result and an extra two inches of height to get the clearance. Just wanted to make sure with my gorilla butt and prop constraints it was going to be workable in the end.

Jon... thanks for your specs bud. It gives me some hope that this stuff can be worked out for my butt somehow with sufficient brains on the job!

Thanks again all of you guys. I sincerely appreciate the advice, suggestions and time you spend offering the previous two commodities! (advice & suggestions)

Stu

Here is a pic with the powered cheeks with the interference.

http://c1.ac-images.myspacecdn.com/images02/105/l_324c07237e5849f28e0e9da248950508.jpg

http://c1.ac-images.myspacecdn.com/images02/141/l_45395b6358a34e89a815975ff416a948.jpg

[Chopnotch]

YouTube - Ron Awad flying his "Gyronimo" at Bensen Days 2008 (http://www.youtube.com/watch?v=i7MvB4ei6aM&playnext=2&playnext_from=PL)

That is freaking awesome!

[giro5]

With a mac the prop/engine rpm is 4000 wot. You do not say what reduction you have with the 503. What rpm are you turning the prop at? You need to be close to 4 grand with a 52 inch prop to be comprable with the mac. A 503 can use from a 60 in 3 blade to a 68 inch 3 blade and pitched to about 6250 static runup with a 2.58 ratio. The 68 inch is more optimum because you have to pitch the 60 inch too much and the prop is not at its most effecient operating condition. I once flew my trike with a 60 in 3 blade warpdrive but with a 4.00 ratio. It flew but just barely. With a 60 inch 3 blade airplast and the 4.00 ratio it flew fine. There are a number of factors involved in prop selection including brand.