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I would not worry about the 90 size engine it will work fine. You will not get the horse power out of either of those motors because you will not turn them that high. Can you get enough Clarence to run a 14" prop? If I could I would run a APC 14x3 or 14x5. It would give you the thrust you need with out to much speed.

Might want to read the engine recommendation part of the manual. This plane was designed to be built light and fly slow in close. I have an os 52 four stroke on mine, verticle is unlimited and it will hover it. If you do overpower it with the 61 prop up in size down in pitch.

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HMMM, with my os 52 I have to use a wood prop or it balances nose heavy. The supertiger is a hunk of lead. I would go with the 0s 61 or a webra speed 61 or a saito 72....something light using the 13x4. You can put one size larger wheels on it and still use the pants.

BTW do not be fooled by hp #'s. My webra speed 61 is about 20% stronger than my supertiger 61. Based on what I have seen on the os 61 it would probably not be much different than the supertiger 90. My os 40 fx is about equal to my supetiger .51.

I agree with caf2461.
I flew mine with a FX46 and it flew fine, very fast rolling and nice snapper great KE, dont know about hovering, I wasn't hover back in those days...G

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Sorry, no clue about taller gear, but a ST 90 and a 15x4 (or 16x4) APC should be a beast. Plan on a bunch of tail weight. My Ultimate FF flew with a 16 oz (with muffler) Rossi and needed 1 ounce of lead to fly nice. Not a 3-D set up at all. The RX is intended to be in the LE of the right wing, behind the engine, but put it where ever you can. The batt goes in the left wing LE (or elsewhere). A Saito 91 is probably a better choice, maybe even a Saito 72, but I understand you already have the ST. The OS 61 FX is a bit lighter than a ST 90 too. Either way look for a Jett Engineering Muffler to enhance power to overcome your altitude problem. While building, entertain the thought of increasing spacing of ribs to make the wing bigger, to help with altitudte too. Altitude affects the way wing loading works too. The Ultimate FF flies great at 4.0 to 4.5 pounds and 6 to 8 pounds of thrust at sea level. Work numbers on thrust and wing loading to your altitude. Maybe try to cut an inch or so off of your nose too. The ST 90 is much more motor (heavier) than intended. Good luck!

If you ditch the pants you can probably go up in wheel size to accomidate larger props. If all else fails the gear should be very easy to make with a longer peice of alum. As for three blade props I cannot help you there.

I sympathise with the problem with the altitude in your area. I think the trouble with getting through should be considered on your end. You are asking for advice to go down a path that others with this plane are cautioning you against. The 20% deficit would be better made up with a lighter motor and even with the 61 fx you would be advised to shorten the nose about 1/2". The weight you listed earlier on the S90 does not include the heavy monster muffler that comes with the supertiger. They do not like to advertise the weight of it.

We all like power, I currently run a Webra speed 120 on a 60 size extra. If you believe hp #'s the 61fx produces 1 ful hp more than the os 52 on my ultimate (over twice the hp).

Some planes can handle the weight and power better than others before flight performance and overall satisfaction with the model diminishes. Perhaps to avoid adding tail weight you can come up with something more creative. How about a 2 foot tail boom to mount the battery pack on?

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"I'm thinking 6" tall tires on this plane is gonna look awful funny"
- this is from your post.

Can't somebody else provide humor?

Conservative I'm not. You asked for input. I appologize if you took offence. I hope it works out for you.

Moderator please edit the profanity iand name calling in the previous post. I'm out of this one for good.

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I have to look at this thread as someone just taking a look at it for the first time as I am. It doesn't make much sense at this point and isn't going anywhere. I will close it. If anyone takes offense to that, contact me privately_bob