frenchie79

I don't know if this is the right forum for this question! I built a Kobra without alot of ground clearance, and used a large engine. A 9" prop is about as big as i can go. 9x7 still turns to many RPMs-what would be a good choice for a 3 blade prop to give a equal RPM to a 10x7 or say a 11x5? Using a 46 size engine.

jmohn

They recommend you go down one inch in diameter and up one in pitch. This would mean going to an 8 x 8 or try a 9 x 7 three blade if you can find one. It depends on what you want speed or power? I use a 13 x 8 on my 46 and my plane flys great, but that's a two blade. I have an 11 x 7 three blade on another plane and get good performance with it, but it's faster. I have had better success just adding another blade and not changing dia. or pitch.

da Rock

What you really have to do to find the right 3bladed prop for your airplane starts out simply enough. First you actually have to find a 3bladed prop. They aren't lying about in heaps for sure. I think there are only two mfg's and they don't make but one pitch per diameter for most diameters.

I've been doing a lot of testing recently with 3's and have found that the conventional wisdoms don't seem to pertain to what's available versus how it works. But that aside..............

You say 9" is as big as you can go. OK, that pretty much stops everything. Master Airscrew makes a 9x7 3blader. That's it for them. Graupner makes one 9". Guess what pitch.

So you got the choice of either a Graupner 9x7 or a MA 9x7, right. The only way to get more pitch in that diameter is to buy a 10" or 11" and cut it down. So guess what pitch is available from MA and Graupner in 10" 3bladers. You guessed it, 7" pitch. Now, all is not lost, because you can also get 11" 3blades....... in 7" pitch.

The problem with 3blade props is that there aren't any.

The good thing about them is that everyone I've tried so far on my 46s, 55, 61, and 91s have worked great. They've all done as well as the best 2bladers on those engines.

frenchie79

Thanks a bunch-Guess which pitch I'm going with-Your right "7" 9x7 it is. Thanks again

da Rock

The Kobra was a .20-.35 size airplane wasn't it. Then it ought to have clearance for a 10" prop since .35s favored 10x6s pretty heavily. Seems everyone ran either a 10x6 or an 11x5 on .35s forever. Are you sure you can't get a 10" prop on the sucker? If you can, there is hope.

My 46AXs like 11x6 2bladers. And like a MasterAirscrew 11x6(3) I made out of a 12x6(3). They didn't like the stock 11x7(3) but might if it were cut down until the rpm came up where the engine wanted it to be for the load. Which is what I'd suggest you try.

OR.....................

Bending new wire for the Kobra gear wouldn't be very hard to do. The nose gear wire is probably availabe from any hobby shop. But the mains would probably not exist. They'd be dead simple to make from hobby shop piano wire. One piece the right diameter would make both and wouldn't cost but a buck or two. One inch taller all around would give you plenty of room and the airplane wouldn't look much different at all.

da Rock

The MA 3bladers seem to have less "wing area" than you'd expect for the diameter. It's why my 55s do as well with the MA 12x6(3) as they do with all the better 12x6 two bladers. They actually turn slightly higher r's with the 3blade than they do with most of the 2s.

So if that little 3 spins too fast, it's most probably because the surface area of the prop isn't loading the engine. And I'd then look at any of the larger diameter 3blades to provide more area even after the diameter is reduced.

Are you sure you don't have 1/2" more ground clearance? That's all you need to swing a 1" bigger prop.

SPLIT S

I ran a APC 9x6 on my Kobra with a ST G34 in it and I can tell you that if the grass was even a little high you were trimming it as you taxied out the field. Stock landing gear set up on the plane. I think I would have had to replace the gear to get even a little more clearance.

Not to hijack the thread with another question but I've read off and on to stay away from 3 blade props. Is that just a bunch of bull? I'd like to start a TF P40 but would love a 3 blade prop for the scale looks. But I've read no,no,no - you have to use a 2 blade. Planning on a Saito 100. Probably around 10-12 lbs all up weight.

Dan

CustomPC

Bolly (http://www.bolly.com.au) make carbon fibre 3 and 4 blade props. They're not cheap at around US$50 a pop, but they sure are nice. They claim they are much more efficient than the other nylon brands so you can run almost the same size 3 blade carbon props as you can a 2-blade prop.

They have a 10.5x6.25 3-blade and a 9.5x6.5 4-blade which might work for you.

Apc also made some small 4-bladed props. I know of one shop here in Australia that still have 9x6 and 10x6 APC 4-blade props in stock.

I have two Graupner 9x7 3-blade props in my stash that are still brand new in the box. If you can't find any in the US, i would be happy to sell them for a fair price and airmail them to you.

da Rock

got the answer then
longer gear
After all, what's more important, how the plane flies or how it looks sitting in the pits.

BS? You bet it is.
The real reason most people haven't found 3bladers that work for them is because there aren't any. And of the very few available, most people who want to try one do just that. One. They are expensive and hard to find. So most blow off the risk of paying the money to do the test. And with only one to try, what are the odds the one 3blader is going to work as good as a 2blader that you're using after trying 4 or 5 different 2bladers of different pitch and/or diameter.

In the last month, I decided to see how good or bad 3bladers would work on my fleet. I got 4 engine sizes in it. So I figured the 5 3blade props that were readily available and suited those engines might be less of a financial risk. I could try some of the props on bigger engines than recommended and on smaller. So it'd cut the risk of worthless props. Turns out I found those 3bladers are pretty good. Actually very good. And I've got at least one size that works excellently for each of the 4 engine sizes.

The usual expert reason for the ignorant advice is "efficiency". Ask how much efficiency is lost and you get more BS. Ignorant people aren't necessarily stupid, they're just ignorant of the subject. I seriously doubt that most of the experts who warn you away from 3bladers haven't even held one, much less put one on one of todays engines and flown the sucker.

When I do prop tests, I do them with a bit more "test" than most. Each prop gets more than one run. One test is the vertical climb. It's timed. And a minimum of 6 are flown. And best and worst thrown out. And somebody else does the timing. And more than one prop is tested in a session. And at least one prop from a previous session is run in the current session to provide some cross reference. And I record things like density altitude, temp, etc. And near as I can tell, I've found a 3blader for every one of the planes that got to test 'em. A 3blader that is better than the average 2blader was all I was hoping for. What I got was a couple of combos where the 3blader is as good as the best 2blader that'd been found in previous tests.

Avoid 3bladers? How can you avoid something you can't even find most places? chuckle...........

da Rock

So how do you find the best 3blader for your Saito on that 10-13lb airplane?

Look at what props Saito recommends and start from there and the 1"less/1"more rule. Or better yet, fly the finished plane with the Saito until the engine is broken in and you've found a favorite 2blader for it. Then try and find a 3blader that follows those dimensions. And expect that if you try a MA that's 1"less/1"more you might need the next size up. Depending. Check your rpm with the prop that didn't seem good enough. If it was spinning lots slower than the engine usually does with the favored 2blader, look for a 3blader that'll spin faster. If the failed prop was spinning too fast, actually probably won't show that but will spin the rpm you are afraid to go above, but didn't pull the airplane worth spit, then look for a "bigger" 3blader.

Expect to try a couple of 3's. If you can't afford to try but one, you probably won't want to fly the sucker 'cause you might break it. So stick with 2s.

SPLIT S

Thanks much for the reply. I don't mind parting with some cash to try and find a 3 blader that will work for the Warhawk. Afterall once you have so much invested in the kit, engine, retracts and all the other goodies that go along with it a few extra bucks is ok by me. It just bugs me having a 2 blade prop on a warbird. Kind of like the beautiful prom date with the 3rd eye, you are going to have some fun but "what's up with that"?

Dan

CustomPC

da Rock, what motor and prop is that on your Skybolt? I'm about to start assembling my Skybolt ARF this weekend and i have an OS .75AX for it.

I'd really love to see the results of all your testing. ie, what prop worked best with each of your motors.

frenchie79

I bashed the kit and made it a tail dragger. Put a forward bend in the wire and moved the mount a few inches forward, this dropped the plane about a 1/2 inch. Check out the Kit build forum under Kobra and i'll have some pictures of it, you will see what i mean. Thanks for everones advice

da Rock

That 75AX ought to be very good, probably a perfect one for the ARF. Mine has an OS61FX in it and is as fast as a little Ultimate40 (which is actually a 30) that has a 46 in it. Biplanes often have sort of a built in max speed they're going to do no matter what engine you stuff in 'em. I used to watch the kit Skybolts back in the day with their hefty weight and the 60s of that day, and knew this 7pound ARF was NOT going to need more cubic inches.

The prop in the Skybolt picture is probably the MA 12x6 before I trimmed it during testing on the 55AX. It did well after trimming on both engines. And that speaks volumes about the present sound byte wisdom on props. After all, MA's own literature on their 3bladers recommends the 12x6(3) for .90-1.08 engines. They don't mention 2C or 4C, so it's unclear what they think along those lines. But it's obvious they feel a 12x6(3) of theirs would be more load on a .60 that what has proven out in my hands on testing.

I looked at the blade area on the MAs I got and felt right away that they're not really paddle bladers. And saw how much area my latest crop of OS's were swinging and flat ignored the expert advice.

frenchie79

The picture shows about 3/4" but the tail is not level. 9x7 APC prop

da Rock

Do you plan to fly with those wheels?

Have you checked to see what a little flexing down on the mains does to that clearance?

What do you guess your clearance is with the plane level?

You may wish to land that combination with the engine off.

da Rock

Boy, if it were mine, I'd pick up a piece of piano wire at the LHS for a buck or two and bend some new mains. With nothing needed but right angle bends, it'd be no sweat.

That isn't a lot of clearance even for a paved runway, and any landing anywhere with the prop turning is going to be a grinding experience with those wheels on those struts.

frenchie79

The wire itself is fairly heavy-landings will have to be greased. Yes i plan on trying the wheels. I may need a good supply of props!!!
Longer wires may be the answer-it's quite abit more than just right bends.

woodworker30

I have a master airscrew 9x6 3 blade on a t 28 with a e rc 450 motor on and it works. I also have a 9x7 Master Airscrew 3 blade on a Horizon Hobbies t28 with an Eflite Power 10 motor. It flies really great. Landings are a little fast with out flaps. With flaps it slows down fairly well.

ejmorgan

My Feedback: (1)

I'd make new LG struts at least 1" longer. Could also replace the wheels with larger diameter wheels. You might use both new struts and larger wheels to get the ground clearance you need.

Ed