Tire drag

[Bruce Fenstermacher]

Well it's about time to maybe think about replacing my 8 x 600 tires. After all the tread is warn all off one side and I've rotated them to wear of the other half. And at probably 15 years of age, not all mounted on an aircraft, the tires have and pretty good checking(cracking) going on. Some folks lately, not knowing what a really nice Rat Plane tire looks like, have questioned their airworthiness.

Now I have to tell you that I like big tires. My wife likes big tires. And that is the only reason we have 8 x 600s. Can't even tell you if I've ever landed somewhere that I needed them except maybe in my dreams. Lately I've realized I've been burning a lot of gas on convention trips. 10.5 and 11 gph have been my average and I'm going nowhere fast either. Been wondering if the drag of the big tires is worth it. Or if they really have any effect at all. I've considered reducing size to 7 x 600 which to the eye you can't hardly tell the difference from an 8 x 600.

Been shopping for a nice set of 7 x 600s and holy cow those things are $110 a tire. Mean while those puny little 6 x 600s are half that price. Damn should I consider some of those little things?

So to the point to this post. Does any one have the differences in drag for each size tire? George you probably have that info right on the tip of your tongue.

And if no one has it I'd think if a fun little project for someone a bit more mathematically inclined than me to figure this out. I'm wondering if I might pick up a mile or two per hour or at least maintain the 110-115 mph my plane will do at a slightly lower power level.

And before anyone suggest it, NO I won't be putting on wheel pants next. I've never seen a decent Rat Plane with wheel pants.

[3958v]

Bruce Maybe you could put one 600-6 on and see if your plane wants to fly in circles. If it does its a good bet that putting two on will make quite a bit of difference. Course if your flying around with one big one and one small one they might mistake you for Old Gar.

[blueldr]

Bruce,
If you're burning 10.5 to 11 gph, you sure as hell must have the hammer really down at a relatively low altitude. I can't ever remember stuffing that much fuel through an O-300 at even medium altitudes in a cruising configuration. (Mayhap you have one of those funny Lycoming front end vibrators. That could account for it.) After all, fuel weight is directionally proportional to horsepower and the O-300 isn't all that powerful.

[Bruce Fenstermacher]

BL I rarily go above 1500ft MSL unless I'm escaping turbulance, clearing terrain or there is a tailwind or more favorable winds higher. I am after all a helicopter pilot.

Oddly the 27 hour trip I got 11 gph average I had actually reduced power from the year before when I averaged 10.5 gph over about 20 hours. And yes before reducing power this past year I would run about 2600 rpm with casual leaning. My plane performs in the lower speed ranges for power that the book says it should do. There are other reasons the plane might have more than the usual drag, the 8 x 600 tires are just one.

Oh and I only wish I had a Lycoming in the front but it's just the stock Continental with a semi cruise (54 pitch) prop.

[Harold Holiman]

Bruce,

7:00 x 6 have 140 sq in of frontal area vs a 8:00 x 6 with 176 sq in of frontal area so it should make a difference. 7:00's look good on a 170. At one time I had 7:00's on my 170A many years ago, but changed to 6:00's because I added wheel pants.

Harold

[Bruce Fenstermacher]

Harold comes through with some numbers. Thanks Harold. Now maybe someone can figure out what 36 sq inches of frontal area per tire really means as far as power required to move it. And of course we're still looking for info on the 6 x 600 tires.

Yes Harold I agree the 170 looks fine with the 7 x 600, much better that the 6 x 600 and if it were not for the price I'd probably just buy a set.

[blueldr]

I have tried six, seven, eight, and eight-fifty tires. I never noticed any difference between sixes and sevens, very slight difference with eights, and very little more with eight-fiftys. I did not like the ground handling with eight-fiftys. The eights requied brake rotor spacers to provide caliper clearance from the tires, however I've been told that this a function of the tire brand. I settled on sevens. As a mqatter of fact, I have a set of new Air Trac sevens for sale. See the "Trademart" section.

[trake]

I recently removed the wheelpants, extended axle nuts and backplates and switched from 6 x 600 to 7 x 600 tires. Ive noticed no speed loss and the larger tires may be slightly more forgiving when landing. If youre not running wheelpants the 6 x 600 just look too small. I think the 7 x 600s are a good compromise.

[GAHorn]

..Now I have to tell you that I like big tires. My wife likes big tires....

Seems someone might be trying to make up for something....

[Bruce Fenstermacher]

..Now I have to tell you that I like big tires. My wife likes big tires....

Seems someone might be trying to make up for something....

I guess I'm trying to make up for the fact I have wheels at all instead of a good skid tubes like most good helicopters have.

[Metal Master]

Bruce,
I have 8:00 x 6 Tires on my 170A and I use 8 hrs/gallon for my cross country calculations. I generally cruise at 6, 500 to 7,500 to clear the mountains in the Cascades and Rockies. 8 Gallons / per hours is accurate enough that from my GPS time in flight I can figure how many gallons I have remaining from full tanks and then load fuel for best performance into the Idaho back country.

No mater what prop I use. If I was burning 10 gals per hour I would think something was drastically wrong. I do lean for peak performance at the altitudes that I fly. It sounds like you fly pretty low all of the time. Having been in Helicopters myself for 11 years myself I get it..

[blueldr]

Bruce,
If, as you say, you're burning 10.5 to 11 GPH, something has to be radically wrong. That's about the normal burn on a C180 or a C-182 with an O-470 which is a 57% larger engine. I just can't see how you can cram that much fuel through an O-300 even at low altitude. Are you sure about those figures?

[Bruce Fenstermacher]

Yes BL I know it is high and if I'm going to be burning that much fuel I want to go a hell of a lot faster. I suspect an error in my calculations but to this date I can't find any. The 10.5 gph was from my convention trip to Branson MS. That trip I probably ran the engine at 2600 rpm and 22 inch of manifold pressure over about 20 hours. I looked that spread sheet over pretty hard and never found anything incorrect about it. I assumed it a fluke.

Then this past summer we flew 27 hours on our Duluth convention trip and averaged 11 gph. This time I actually reduced RPM to about 2500 RPM and probably about 21 inches of manifold pressure and tried to lean a bit more aggressively than I had to save fuel. I'm not leaking fuel any fuel but I know that my engine runs on the rich side if I don't lean.

Never of these trips was a climb to some high altitude a lean and go. There were short hops about the airport and longer legs with perhaps multiple climbs and descents.

One of the things I want to do but always seem to forget is keep better records of my fuel usage each time I fly to get a better feel for this but keeping these kinds of records was never part of the fun of flying for me. In fact I really don't want to know how much it costs because I'm likely to quit.

My plane has always been slow and I account that to drag. The tires are just a small part of the picture. Long slow climbs with my semi cruise prop at high power without a corresponding long enough cruise to make up for the climb is probably a big part of the picture.

[hilltop170]

Bruce-
My O-300-D only burns 11.5 gph at full throttle rich mixture at sea level. Why don't you lean aggressively?

[GAHorn]

Interpolated data from the 170-B performance, standard engine/prop indicates the following altitudes vs fuel burn at recommended cruise power of 2450 RPM, LEANED:

Sea Level- 9.2 gph
2500 MSL- 8.3 gph
5000 MSL- 7.4 gph
7500 MSL- 7.0 gph
10K MSL- 6.5 gph

This is in level cruise and therefore does not take into account takeoff/climb data. I suggest adding 1 gph for every hour of flight to be conservative.

My own practice is to climb to 7500 MSL (plus/minus 1K westbound) for flights over one hour. I cruise 2450, leaned, and flight-plan 8 gph, but actual results are dependably 7.8 gph and 104 kts (117 mph) TAS.