Issues flying behind a 2-stroke

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Posted (edited)

Note: I wrote this to clarify my own understanding of how things work, and I think I have the essence captured.  I apologize in advance to those with more experience than I with these engines; I am not trying to install myself as the grand pooh-bah here.  If, by my relative lack of 2-stroke flying experience there’s some tidbit of physics I have missed, or have something wedged into my mind sideways, I certainly invite discussion.  The egoless scientist humbly seeks the truth for the benefit of us all, not the puffing up of his own self-esteem.  Good God, that’s tacky!  So here goes:

The 4-stroke engine has the luxury of a full half-rotation in which to extract the fuel’s heat energy.  For the 2-stroke, with the low-pressure, low-leverage bottom of the power stroke cut away, less time is available so the duration of the combustion event becomes more critical. 

 The time interval required for a combustion event to occur is shortest when the air/fuel ratio is close to stoichiometric, and cylinder charge pressure and temperature are high.  Too far from stoichiometric on the lean side robs heat from the combustion process, slowing the reaction, while operating on the rich side robs heat too, but also creates a host of intermediate products of the incomplete combustion, like CO, particulates, etc., expensively cooling and slowing the reaction.  High altitude or part-throttle operation lowers the charge pressure, molecular mean collision frequencies, and hence lowers the reaction rate.  In the absence of exhaust-pipe resonance effects, 2-stroke thermal efficiency has got to be maximized when full combustion of each charge occurs as far ahead of exhaust port opening as possible (without pinging, of course).  This explains why the 2-stroke works most efficiently when the engine is slowed down by load, and is why leaning back to near stoichiometric at high altitude is important, as is not over revving the engine at part throttle in cruise or descent.  Part-throttle over revving lowers charge pressure and reduces time available, causing the combustion process to finish up relatively late in the cycle, wasting fuel and raising EGT, which looks like, but is distinct from lean-of-stoichiometric operation.  This potentially delivers the thermal double-whammy of less cooling by charge expansion and combustion still ongoing as the ports open.  This thermally stresses the pistons, and favors ceramic-coating the piston tops as a protective measure.  Piston cooling only comes from the underside’s exposure to the cooler incoming charge and heat conduction through the piston skirts to the cooler cylinder walls.  The piston’s aluminum has very high thermal diffusivity, but a relatively low melting point.  Here, the EGT relative to limit serves as a guide.  Yes, these engines like to run with carb slides open!

 We’ve all heard of those model airplane 2-strokes that turn 30,000 rpm.  This works since the much smaller cylinder diameter substantially reduces the characteristic time required for the combustion event to occur (assuming the same speed for the flamefront).  So how about a greater number of smaller cylinders?  Would a different engine design help here?  As it turns out, holding engine displacement constant, characteristic combustion time varies inversely with the cube root of the number of cylinders divided by the two we have, while scrubbed cylinder area goes up by that same cube root.  So frictional losses go into this tradeoff.  Going to a 3-cylinder engine only reduces characteristic time by 13%, and it would take 16 cylinders to reduce it to ½ of its 2-cylinder value, while engine scrubbing friction would double.  There’s very little kindness there, unfortunately.  The additional complexity of more cylinders would increase cost and weight too. Oh well!  Let’s look instead at the propeller.

 If the (fixed-pitch) propeller is set up for brisk climb performance, it will be very much unloaded in cruise, and only able to absorb a fraction of the engine’s power potential.  I was surprised to find out by how much.  My prop calculations confirm that my 3-bladed 72” IVO prop, with the pitch set to absorb all 65 hp at Vy in climb will only be able to absorb about 15 hp in 85 mph cruise at 5000 rpm.  This may not be enough power to even go 85 mph, and is very likely an over-rev condition, the only cure for which is more prop pitch. (BTW I am running at a higher pitch than this!). The cleaner we make our airframes, the worse this problem becomes, as the cleaner airframe demands even less thrust power for a given speed in level flight.  Ground setting the prop to a higher pitch will obviously reduce climb performance as the engine’s max rpm drops, but thrust power available for cruise at, say 5000 rpm increases dramatically, increasing cruise speed a little (at high speeds, achievable speed varies roughly with the cube root of the thrust power.).  If you really want the best of both worlds, the answer is the IFA prop, but now you no longer legally have a light-sport category airplane.

 Another way to mitigate this disparity is to go to a smaller prop diameter or perhaps fewer blades.  This requires you to increase prop pitch in order to properly load the engine. At the higher blade pitch, the prop may be partially stalled at takeoff, although now it’s less sensitive to forward speed, and will unload less in cruise, allowing the engine to not overspeed as much.  This is due to simple geometry, as changes in blade element angle of attack corresponding to changes in forward speed are smaller at the higher blade angles and pitch.   But turning slower means less potential power from the engine.  There’s got to be an optimum in there someplace, but it’s likely different for different folks, depending on how they fly their airplane.  Hopefully the optimum is not at the sickly climb performance of a C-150, as this would totally destroy our street cred as a STOL airplane! 

 Incidentally, calculations for my 72” IVO show that going from 3 blades to 2 is impractical since this makes the prop too small, causing serious blade-element stall and horribly low efficiency when the prop is pitched to absorb 65 hp at Vy.  The alternative of cutting a prop down is risky in that you can’t glue it back together if you cut off too much.  Then there’s the sensitive prop-balance issue. I’m not too keen on this approach yet.

 At this point I will start feeling out the tradeoff between climb and cruise performance.  It occurs to me that I have a lot of climb performance to comfortably give away, but I have not yet flown my little Avid with two of us onboard.   My gut feel is that the STOL variant may not be as slow a cruiser as previously thought; it’s just that most of us are not keen on trading away much of our stellar climb performance.

 

 

 

 

Edited by Turbo
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Posted

I had to read it several times but it finally sunk in, I think.  I wonder if my reed valve engine (3202) has the same principals applied? Probably does since they are on the intake side.

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Posted (edited)

Allen,

Having four cylinders vs. the two of the 582 should provide a little mercy, but I would agree: the principles are the same.  I wonder if 4-strokes see any EGT rise under part-throttle overspeed conditions?  One would think this could be hard on exhaust valves!  Perhaps it's effectively masked (or overwhelmed) by continued cylinder cooling prior to and during the exhaust stroke.

Edited by Turbo

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Posted

In my experience on GA engines, they don't have the EGT problems that a two stroke has. The biggest issue on GA engines is shock cooling, when pilots "cut and drop" so to speak.  Flying along at 7K and then throttle back out of the green and glide down to pattern altitude is extremely hard on the front two cylinders. First sign of damage is warped exhaust valve stems. On the 0-200 it also cracks the heads out from the spark plug seat. Rental aircraft usually have at least the front left cyl replaced a couple of times before TBO.  I've replaced several cylinders due to this and its always the front left one. Students are pretty hard on aircraft, private owners, not so much, once they understand what is causing it. And it comes out of their pocket. GA engines have issues just like 2 strokes, mostly due to improper operation.  Engines don't fail, they are murdered.

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Posted

I can attest that you don't want to go from a 3 blade 72" to 2 blade.  It sucks at best.  If you want a little higher cruise go for a 68".  I have never flown mine cruising around at 5000rpm.  I like to keep the temps up as it helps to keep the carbon down getting a better cleaner burn, not to mention it make the muffler heat work much better.  With big tires, ski's or floats on the additional drag is enough to want to keep the RPM up around 58-5900 just to make the Avid "feel" right in the air.

:BC:

 

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Posted

Great points!  You Alaskans impress the hell out of me!  Down here at 46 deg N, I'm not flying.  Too cold!  Maybe after I finish installing a heating system... 

One thing that I forgot to include in my long-winded discourse is the issue of ignition timing.  4-strokes, at least the more sophisticated ones, feature some combination of vacuum and centrifugal spark advance.  Both are aimed at advancing the start of the combustion event as the engine speeds up, and/or intake manifold absolute pressure is reduced.  With the 2-stroke, things get complicated, and exhaust- system scavenging tends to drive the advance curve, often actually retarding the spark under these same conditions.  With the 582 the exhaust system is de-tuned for a more even power band, and the engine has no spark advance mechanism at all!  In either case, with a 2-stroke, we're stuck with essentially the same problem with high-speed, part-throttle operation.

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Posted

My Hirth 3202 does have a ditigal chip for limited spark advance. It is part of the PVL ignition. How good it does remains to be seen. I don't think its that much of a spread but does advance the timing based on RPM. I do know the reed valve system does idle smoother and throttle advances are smoother.  But they are another point of failure.

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Posted (edited)

For what It's worth, I translated the brochure Rotax data into a form perhaps more understandable to those of us who are not fully into the metric system as yet.  The power vs. RPM curve labelled "Propeller Curve" was initially a bit of a mystery to me, but ultimately seeing how it differed substantially from certain of my prop calculations, for conditions I have flown, I have decided it represents the maximum power available from the engine at a given RPM.  The thermal efficiency plot is interesting.  Many years ago in engineering school we were told that car engines are only about 25% efficient overall.  Although I'm sure that number has improved substantially since then, I was surprised that the 582,  a lowly 2-stroke, did so well.  So here they are.  Enjoy! 

  

 

R582_HP.JPG

R582_FF.JPG

R582_EFF.JPG

Edited by Turbo
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Posted (edited)

I pay close attention to power and torque curves. I find them interesting when comparing engines. I find that the 65 hp of a 582 is mainly for take off RPM and after that drops off rather quick. A few low RPM 2 strokes maintain a flat power and torque band for a thousand RPM's below take off power allowing low cruise RPM's which is less fuel, less wear, less heat, which relates to reliability. Horsepower sells engines, torque turns props. Yea, I know the relationship between horsepower and torque, but sometimes we are mislead by factory horsepower claims into thinking that's all there is to it. If we took a hard look at the curves, you might find we are flying along at very low horsepower and doing just fine. Truth is that the STOL ability of an Avid/Fox is what brought us here to start with, and nobody dislikes a scary short roll and a nosebleeding climb capable aircraft if one chooses to do it. If we did, we'd all be flying C-150's or Taylorcrafts. Both good aircraft for their purpose, but lackluster performance. My dream aircraft is a Super Cub but I'll never afford to own one. So I got the next best thing (my opinion). And I can fly on lunch money! But then, some could fly a C-130 on their lunch money! (big eaters) LOL!

Edited by Allen Sutphin
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Posted

For sure, that nosebleed climb could save a fool's life getting out of a box canyon some day!  

Now I'm wondering if with an IFA on front it might be possible to float about the sky at the 582's efficiency peak at 3500 rpm!  With my fixed-pitch where it is, my prop calculation estimates that the prop's absorbing about 20 hp at somewhere close to 80 mph in level flight.  Just a thought.  I also think Leni makes a good point regarding keeping things clean in the upper cylinder,  piston tops, and especially rings.  I just installed a fuel totalizer, and I'm keen to find out where the airplane is happiest, and what altitudes & airspeeds to shoot for to maximize range.  Then there's the issue of headwinds/tailwinds.  Please pardon me for waxing nerdy here, it goes with the territory!

Hey, let's hear it for lunch money!

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Posted

at 3500 rpm you will be looking for a place to land in very short order.  The engine is what it is.  Running it too slow and loading it up will only lead to early failure.  Get used to having a screaming demon out front or chose another plane to fly with a lycosuarus on the nose.  

:BC:

 

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Generally an IFA prop does its best on aircraft faster than 100mph. It can improve performance on slower aircraft, but not enough to justify the cost and trouble. Unless you have excess money. My old flying buddy had one on his model 4/NSI powered and it was a two position switch, takeoff or cruise.  He wasn't impressed with it. A properly matched prop on a Avid/Kitfox is about the best one can get. A ground adjustable one, that is. Doesn't sport pilot regs limit a prop to ground adjustable only, if that applies to you. As for RPM, AK is correct, low RPM will murder a 2 stroke unless an engine is designed for that range, which Rotax isn't. When I first started flying a 2 stroke, it was nerve-racking to listen to the engine screaming but you get used to the sound and get comfortable with it. After a hundred hours or so, you can tell pretty closely what the RPM is by sound only. And tell when something isn't quite right. My wife can tell by the sound of me that I ain't right!

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So, what's the weak link here in this non-screaming-meemie scenario?  Would I be lugging the engine?  Overstressing the conrod bearings?  Or would I just be running it so cold that it would crud up in short order?  Or have a seisure?   Clearly if I'm trying to actually go someplace I'd need to spin it up a bit more.  Spun up, it sounds cool, like a turbine engine.  I'm just thought-exploring the max endurance scenario.

The IFA, behind a 4-stroke, I agree, is more the provence of the faster bird.  The sensitivity of this 2-stroke to oversped, low-throttle conditions, not to mention the hit in fuel consumption, seems to change the rules a bit.  A class-3 medical is $100 and a pair of glasses away.  Glasses are on order.  Booga-booga!  Note that at least two of our most experienced members fly with the IFA.  Just sayin'

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At low RPM you wont have enough thrust to fly unless your playing in a thermal.  It will also crud the engine up in short order.  you can't lug a 2 stroke like you do a 4 stroke and expect it to last.  Trying to reinvent the wheel is not always prudent.

As far as the IFA argument.  When I was flying mainly on floats it was a huge help.  With the prop set for cruise, I would barely turn 4800 RPM when starting from a dead stop in the water.  Since I sold the floats and am only on wheels or skis, the IFA is pretty much a mute thing and I hardly ever touch it.  There are a few times where it has saved my butt but that was going from 10 degrees at altitude to -38 when I dropped into the river bottom and my IFA mixture needles were froze up.  The only way I could get the EGT's under control was to really load the engine up using the IFA prop.  

As far as getting best economy and cruise at certain altitudes you will only be able to truly do this if you have an inflight mixture adjustment.  Going from stock jetting at sea level then trying to cruise at 5 or 6 K you will be running really fat.  If you throttle back to try and get fuel burn down your just going to load the engine with more carbon and not have it hot enough to burn it out.

If your just playing local none of this really comes into play.  If you are wanting Xcountry you must really consider the bladder as the limiting factor most of the time.  I highly doubt that one could dream up a mousetrap that has not already been built for the 2 strokes that has either been proven to be useful and helpful or has put people into the trees.  You are better off learning from others what works in the 2 strokes versus trying to learn on your own.  It only takes a few seconds of trying something dumb to melt one down.  Yes, this is the voice of experience :lol:  

My answer to longer range out of my bird is to hang football tanks on the struts and have and extra 10 gallons I can use in flight and have a couple 5 gallon jugs or a 15 gallon barrel in the seat next to me.  Have done it both ways.

:BC:

 

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Thanks for that advice, Leni.  I do have a leaning setup on my engine, basically a copy of Cowlove's approach, using a small, 12V diaphragm vacuum pump to lower float-bowl pressure.  I have a rheostat knob on my dash labelled "chemtrails", that energizes the tiny vac pump.  Pump-off, it reverts to ambient.  So far I have not done anything but pattern flying, trying to wire my landings before my highly critical wife climbs aboard.  I have only used the leaning circuit to shut the engine down, and to smooth the idle in summer when DA is high..

Regarding the IFA, any extra weight on the schnozz is to be avoided, as I'm clear my bird is noseheavy already.  And I just reinstalled the OI system & oil tank.  $1000 more in my pocket is good too.  Like I mentioned earlier, my next step will be to investigate what might be the best prop pitch for how I fly the bird, which currently is verrry conservatively, and mostly from tarmac.  If you don't use IFA, maybe it's due to the skis' drag hit?  I would, however, assume that floats are draggy too, and that both argue against the IFA, since lower L/D tends to require the engine to work harder at a given flight speed.  But I get it:  water takeoffs were a bitch without it!

I have only the single, 14 gal tank, and am, in my old age bladder-limited, so I think I have the right setup.  Not trying to re-invent the wheel; just trying to work things out for what I think will be my kind of flying.

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At low RPM you wont have enough thrust to fly unless your playing in a thermal.  It will also crud the engine up in short order.  you can't lug a 2 stroke like you do a 4 stroke and expect it to last.  Trying to reinvent the wheel is not always prudent.

As far as the IFA argument.  When I was flying mainly on floats it was a huge help.  With the prop set for cruise, I would barely turn 4800 RPM when starting from a dead stop in the water.  Since I sold the floats and am only on wheels or skis, the IFA is pretty much a mute thing and I hardly ever touch it.  There are a few times where it has saved my butt but that was going from 10 degrees at altitude to -38 when I dropped into the river bottom and my IFA mixture needles were froze up.  The only way I could get the EGT's under control was to really load the engine up using the IFA prop.  

As far as getting best economy and cruise at certain altitudes you will only be able to truly do this if you have an inflight mixture adjustment.  Going from stock jetting at sea level then trying to cruise at 5 or 6 K you will be running really fat.  If you throttle back to try and get fuel burn down your just going to load the engine with more carbon and not have it hot enough to burn it out.

If your just playing local none of this really comes into play.  If you are wanting Xcountry you must really consider the bladder as the limiting factor most of the time.  I highly doubt that one could dream up a mousetrap that has not already been built for the 2 strokes that has either been proven to be useful and helpful or has put people into the trees.  You are better off learning from others what works in the 2 strokes versus trying to learn on your own.  It only takes a few seconds of trying something dumb to melt one down.  Yes, this is the voice of experience :lol:  

My answer to longer range out of my bird is to hang football tanks on the struts and have and extra 10 gallons I can use in flight and have a couple 5 gallon jugs or a 15 gallon barrel in the seat next to me.  Have done it both ways.

:BC:

 

Didn't Rotax design the rotary valve to address this exact issue? Old two smokes do foul at low speed but the Rotax is supposed to reduce oil at lower speed to prevent this. As far as the temperature thing goes, your not going to seize an engine from cold seizure if it's been running sustained at a given speed long enough to insure all the engine components are evenly heated. That's the whole point of having a water cooled engine. The thermostat if setup properly should regulate the engine temperature at all speeds. Anyone that's driven a snowmobile can attest to the fact that a 2 smoke engine will run at any speed since they are almost never run at one speed in a sled. The old 2 smokes in sleds hated three things, idling for too long, running wide open for too long or getting your oil mix wrong. Everyone I rode with carried an extra set of plugs to fix the first problem  of idling too long. We all carried a tow rope to fix the other two problems.

Granted the limiting factor may have more to do with the carburetor being  jetted for just two ranges. Idling and  cruise speed in our case. Any speed in between these two points is an area that changes the air/fuel ratio to a point other than optimal. The rotary valve for oil mixture knows nothing about these settings in between and therefore cannot feed the right oil under all conditions. Until they start putting oxygen sensors on these engines and incorporate an oil injection and fuel injection systems that can ebb and flow with the numbers coming from an oxygen sensor and the various temperature sensors this debate will never end. I say the two stroke still has room for improvement. With engine design improvement I think the in flight adjustable prop would be very practical. Like most things, no simple solution since the problem itself isn't simple.

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Posted

slow RPM equals high EGT that's what kills them.

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I said nothing about a cold seizure. 

 

The rotary valve only replaces reeds, has nothing to do with oiling at all.  The oil injection pump does that.  Yes it meters it at a lower feed rate, something to the tune of 50:1 wide open throttle and 70:1 at an idle IF you have the injection cable set right and its operating freely.  

The engine has 3 jets... idle jet, main jet and needle jet.  The needle and needle jet are what affect the cruise range of the engine.  Hence having in flight adjustable like the arctic sparrow needles or the hacman system that does it off the pressure in the bowl to keep EGTS in the optimum range and thereby having best fuel efficiency.

Of course the 2 stroke IS improved when going to fuel injection.  Not something that is reliably available for the rotax 582.

After going the route of attaching 02 sensors on sleds and dicking around with a boondocker box to tune the fuel injection most people learned expensive lessons and melted pistons.  Not only that, its another gauge to stare at and get lost in trying to tweedled in the best most efficient settings.  The engine is what it is, tune it to be good all around then keep your head out of the cockpit and pay attention to whats outside the windshield.  It is stupid easy to melt the pistons on a 2 stroke if you want to push the envelope.  On a sled you can do that and only have to tow it back to the truck or the trailer.  On a plane you run the risk of it being your last flight and I pray you don't take an innocent passenger with you.

I am off the soap box, I wont say anything more unless more incorrect information is posted.

:BC:

 

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Thanks for that advice, Leni.  I do have a leaning setup on my engine, basically a copy of Cowlove's approach, using a small, 12V diaphragm vacuum pump to lower float-bowl pressure.  I have a rheostat knob on my dash labelled "chemtrails", that energizes the tiny vac pump.  Pump-off, it reverts to ambient.  So far I have not done anything but pattern flying, trying to wire my landings before my highly critical wife climbs aboard.  I have only used the leaning circuit to shut the engine down, and to smooth the idle in summer when DA is high..

Regarding the IFA, any extra weight on the schnozz is to be avoided, as I'm clear my bird is noseheavy already.  And I just reinstalled the OI system & oil tank.  $1000 more in my pocket is good too.  Like I mentioned earlier, my next step will be to investigate what might be the best prop pitch for how I fly the bird, which currently is verrry conservatively, and mostly from tarmac.  If you don't use IFA, maybe it's due to the skis' drag hit?  I would, however, assume that floats are draggy too, and that both argue against the IFA, since lower L/D tends to require the engine to work harder at a given flight speed.  But I get it:  water takeoffs were a bitch without it!

I have only the single, 14 gal tank, and am, in my old age bladder-limited, so I think I have the right setup.  Not trying to re-invent the wheel; just trying to work things out for what I think will be my kind of flying.

Turbo, learn to fly it well, go airport hopping, make a lot of new friends, and enjoy the plane as it was meant to be. Then in a couple years you might want to step up a bit, maybe not. You will get everything dialed in within a few hours, until then fly safe and have fun.

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Jumping sideways on the topic a bit, but Turbo, in your last post you mentioned the drag hit from skis.  I've flown quite a bit on skis (straight skis, not wheel penetration skis) and if anything, I get just a bit more speed at the same rpm as with wheels.  No drag hit at all that I can see.  And maybe part of it is the flat bottoms probably produce a certain amount of lift as well.  Just my observations.  YMMV  JImChuk

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Es buen consejo.  That's exactly where I'm at.  Just putting around, visiting friends, making new ones, enjoying the scenery.  Tailwind?  Fly slow and let God do the work.  Headwind? Crank up that overstuffed chainsaw, and scream your way along.  These engines are generally regarded as robust, if flown right.  But sure, ya gotta know how to not murder them, as it may decide to take you along with it.  As Allen pointed out, even lycosaurs are vulnerable to being murdered.

My bird has the bellyrad, wheel pants, and an IVO prop.  The bellyrad partially shields the unfaired LG struts just behind, tucked up against the belly.  According to my prop code, the IVO in cruise produces no thrust at all from the inner 1/3 to 1/2 of the blade radius.  In fact, the inboard sections act as an inefficient windmill to help turn the prop so the outboard sections can produce thrust.  This may sound bad, but also doesn't add propwash to magnify the drags of excresences near the fuselage,  in fact those contributions to drag might even be diminished.  The bird feels slippery to me, and I know I can improve it.  This is a great toy to play with, both in the shop and in the sky.  We are so lucky to have such fine toys!  Gotta love it!

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Posted (edited)

Thanks, JimChuk.  I guess even an L/D of, say, 1/2 is a whole lot better than zero!  Lots of guys favor those monster tires, bigger than our golf-cart Carlisles!  These have got to be draggier, but even the Carlisles are draggy.  So I guess Leni is saying that he sees little benefit for the IFA prop, outside of floatplane ops.

 

Edited by Turbo

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I've heard stories of big bush tires taking so much force to get them rolling on landing that it dang near put them over on their nose. I guess it would be close to landing with the brakes on.

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Interesting!  For sure that's a lot more rotational inertia!  Definitely something to consider.

One thing some folks are saying that mystifies me is the degree to which wider gear stabilizes the airplane on the ground.  Other than less roll to the outside of a ground turn, and less sideways movement of the tires when the gear squats on hard landings, I don't get it at all!  The CG is still behind the mains, so the propensity to groundloop can't be that different.  What physics tidbit am I missing?

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Posted (edited)

Maybe it's the combo of the two.  As a swerve initiates, weight shifts to the tire on the outside of the swerve. The gear leg on that side squats in response to the increased load, shoving the tire on that side outward.  Since that tire now is carrying more load, its firmer grip on the pavement makes this really manifest as shoving the airplane in the direction of the initial swerve, exacerbating the swerve.  A wider track lessens the weight transfer.  A suspension that does not shove the tire outboard when it squats might not have this problem at all.  Hmmmmm...  Thoughts, guys?

The guy who sold me my bird advocated for 10 psi for the Carlisles.  I see that this, in addition to the mercy of a little  lateral squishiness, makes the tires a more significant part of the suspension.  Of course tight new bungees have got to help too.

Edited by Turbo

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