Question for high (ant)arctic Twin Otter dudes

[lostinthebattle]

Can anyone tell me the dimensions of the tundra tires used in the high arctic, the manufacturer and the tire pressure used for soft off-strip work? Not interested in the intermediates but the full size tundra's. Are they off a DC-3 as I have heard speculated?

[just curious]

I'll look on the rack. Still ski season here (sigh). But 18 PSI. Whether they are still DC3 I dunno, they used to be.

[lostinthebattle]

I'll look on the rack. Still ski season here (sigh). But 18 PSI. Whether they are still DC3 I dunno, they used to be.

Thanks, JC. I figured you were probably my best bet.

[jet a1]

i'm not sure about the exact details but they're just the right size for resting your coffee during the walkaround.

[wabano]

Old man Bradley(First Air)started up with DC-3 tires on single Otters, shaved to just a few plies with a chisel,
looked like a ball of strings!!! only four of five pounds in there...feel like sitting on a bouncing ball
but will swallow huge rocks ever sooo softly, but watch out to keep the tail
wheel clear of them rocks!

[PanEuropean]

What AFM supplement addresses operation with tires bigger than the three standard 11.00 x 12 wheels, all fitted with 15.00 by 12 tires, that make up the intermediate floatation gear installation? I know that AFM supplement 5 speaks to intermediate floatation gear, but I cannot find a supplement that speaks to any larger wheel or tire combination.

[lostinthebattle]

These are the ones I mean:
Intermediates

Big Wheels

[PanEuropean]

Lostinthebattle:

Uh, I think there is a bit of confusion with the two photos above.

The top photo (larger photo) appears to be an aircraft with standard 11.00 x 12 wheels (rims) on all three positions, and the wheel (rim) on the nose position has been fitted with a tire that is approximately the same diameter as the standard main tires.

The lower photo appears to be an aircraft with standard 11.00 x 12 wheels (rims) on all three positions, with 15 inch tires (intermediate flotation gear) on the main wheels and the same wheel and tire on the nose as is fitted to the aircraft in the upper photo.

In other words, the upper photo is a hybrid (standard mains but a larger nosewheel), and the lower photo is intermediate flotation gear.

[lostinthebattle]

I guess the size of the nose wheel has been throwing me off. I thought there were three sizes for the main wheels/tires. Thanks for the clarification.

[ettw]

The top picture SHOULD show 3 intermediate tires all around. I think the optics are making the mains look like small tires. If they are actually small tires on the mains it would be the first time I've seen that done.

As for the bottom picture, those are the TUNDRA tires that Borek operates on. Their rubber I believe is different than the ones that First Air used. Looking at my pics of FA tires, they appear to be narrower but larger diameter than Boreks. And I was always told that First Air's were originally DC3 tires that had reached the end of their useful life on the 3. Later on First Air used a new tire but shaved it to reduce the weight by about 75 pounds per shipset. If I remember correctly you were allowed to see 4 plys before you had to pull them off.

Bear in mind that the rims are modded with a flange that mates to the original rims and enlarges the bead area to accept the larger tires as per below.



Cheers,

ETTW

[PanEuropean]

I can't blame you for being confused - there is a lot of confusion in the industry about what wheel and tire sizes are available for the Twin Otter.

The AFM only provides one supplement for larger tires, and that addresses the use of 15 inch mainwheel tires on standard 12 inch mainwheel rims. In this configuration, which is referred to as "Intermediate Flotation Gear" (IFG), the two mainwheel rims are fitted with large 15 inch tires. The standard nosewheel rim, tire, and nosewheel fork are removed and a larger nosewheel fork is installed that can accept a standard mainwheel rim fitted with a tire that is larger than a normal nosewheel tire. The lower of the two photos above shows this installation, which is addressed by Supplement 5 of the AFM.

Rates of climb are slightly lower when IFG is fitted. This normally does not create a problem at Canadian temperatures, but the degradation in single engine rate of climb can create a serious problem when operating in warmer temperatures and from high elevation aerodromes (e.g. Nairobi, or the Angolan planalto). Some operators have elected to only increase the size of the nosewheel - as shown in your upper photo above. This helps prevent the nosewheel from digging in to softer surfaces.

I have seen only one photo (below) of a Twin Otter that was fitted with larger tires than the 15 inch IFG, and this is a very old photo, dating from the 1960s. I have never seen an AFM supplement or an engineering order that speaks to this configuration. It appears that DH experimented with using much bigger tires, but I don't believe that DH pursued this idea beyond initial experimentation. My guess is that the aircraft would not climb single-engine if it was fitted with tires that were larger than 15 inches. Note that the photo below shows the aircraft operating in the winter. Even in the winter, I think single engine ROC would probably be zero or worse than zero, which would prevent such a fitment from ever being approved for use.

[PanEuropean]

ETTW:

I'll bet you a cup of coffee that the main wheels in the upper photo of the Borek aircraft are standard (Goodyear) rims fitted with standard landplane gear. I'm not sure enough that I would bet the rent money , but I will stick my neck out for a coffee.

I've seen that hybrid configuration (big nosewheel, standard mainwheels) often. In fact, there was even a set of performance charts created to address that specific configuration, although it was never published in the AFM.

What size is the tire that you posted the photo of? Is that a 15 X 12? The person standing beside it appears to be a child.

[ettw]

PanEuropean,

I can tell you from personal experience that the Twin Otter flies just fine single engine with the tundra tires on it.

The Tundra tires are installed on Twin Otters IAW with an STC. That would be why you can't find any references in the DH stuff like EOs etc.

Cheers,

ETTW

[PanEuropean]

Ah, that would explain it then (the STC). Thanks for that information.

Below is a photo I took some years ago showing an aircraft with standard main wheels and a IFG specification nosewheel. I think it compares with the upper of the two Borek photos. This aircraft was based in Kenya, and would have had a negative single engine ROC if all three wheels were fitted with the larger tires.

[lostinthebattle]

So to summarize, there is more than one size for the main gear. They are:11X12,15X12, Borek STC (unknown size but bigger than 15X12), and First Air STC (similar to Borek but slightly different). As for the nose gear there appears to be a selection of the standard small gear and 11X12. Or is the 15X12 also used on the nose?

I think I see the light at the end of the tunnel.


As for the Kenya picture above I take it that all 3 tires are 15X12? How come you omitted the call sign?

[Ref Plus 10]

Cheers,

ETTW

Ahh, so THAT'S where my other Aero Classic tire went....I want it back, the Goodyear is just making me look bad haha

[PanEuropean]

So to summarize, there is more than one size for the main gear. They are:11X12,15X12, Borek STC (unknown size but bigger than 15X12), and First Air STC (similar to Borek but slightly different). As for the nose gear there appears to be a selection of the standard small gear and 11X12. Or is the 15X12 also used on the nose?

Let me paraphrase what you wrote, in an effort to be more precise:

You can fit either standard landplane wheels or intermediate flotation gear on the main wheels. The rim used is the same for both, only the tire size varies.

You can fit either a standard landplane nosewheel or a intermediate flotation nosewheel. The rim used for the IFG nosewheel is, in fact, a standard landplane main wheel rim.

I am not familiar with any STC for any other gear configuration (excluding, of course, the STC for fitting Parker Hannifin brakes, which requires that the main wheel rims be changed, although the size of the rims remains the same). If anyone happens to have a STC number for these larger tire fitments handy, please post it, I would like to look at the STC.

Michael

[ettw]

So the Tundra tire could be sized at 17.00x16 12 Ply Rating. Depends on the STC. Not sure of the other size.

Cheers,

ETTW

[just curious]

17 x 16 & 16 x 16

[PanEuropean]

By any chance would you have a STC reference for that fitment?

Michael

[railroad]

I believe Willy used to operate on intermediate mains with a small wheel up front. Not sure of the advantages except maybe a little easier to handle in a crosswind. And, if you know how to drive the thing, you really wouldn't need the extra ruggedness up front (but I think it is handy somedays for myself).

[craige]

The break down on the wheel tire combo for the twin otter is as follows

2 std. mains 1 std. nose

2 int. mains 1 int. nose + large nose fork

2 tundra mains 1 int. nose + large fork

and the 2 above pics which are quoted as having std. mains and int. nose wheels are incorrect. the pics are showing the correct intermediate wheel set up all the way around. the taller nose fork makes it sit lower on its tail.

[PanEuropean]

Craig, what pictures are you referring to above?

Michael

[craige]

micheal the pics i refer to are the upper borek aircraft and the icrc aircraft
regards craig e

[Meatservo]

Hey guys, that intermediate installation causes confusion even when you look at the thing in real life. The nose wheel is actually the SAME size as the mains in that configuration, but it always looks like the nose wheel is bigger than the mains for some reason. It's an optical illusion. Just to check, get out a ruler and measure the wheels in the photo of the ICRC otter... you'll find that the wheels are all the same size. The aircraft in this photo does not have standard "street" wheels on the main gear, it just looks like it does.

I'm not sure, but I believe the "big wheels" Borek uses have the same nosewheel as the intermediate setup, with even bigger main wheels. I think this looks better, because with the intermediates the plane tilts back and looks funny IMO.

Craige is correct.