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Where to set the idle on a mid size gas plane?

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Where to set the idle on a mid size gas plane?

Old 03-17-2011, 03:28 AM
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Default Where to set the idle on a mid size gas plane?

I've entered the world of an 18" prop and note that its braking ability is far greater than previously experienced. The modern gas engine can be adjusted to idle down in the low teens but I've noted some room for caution because the engine braking can cause an accelerated time frame to full stall while landing.

How do others deal with this? Do you simply practice more careful rpm management with the stick? Do you adjust the idle for a certain rpm? Do you use a dual rate idle switch? Do you stay away from the lower pitch wide blade props?

My problem is perhaps more acute because many of the planes in my hanger are floaters and I'm used to pulling the throttle stick fully to idle before touch down and the new Yak doesn't like that.

How do I avoid a mental mistake of seeing the new plane go into a full stall too high off the runway because of the higher braking action. Will I learn it before it deals me a nasty lesson?
Old 03-17-2011, 05:45 AM
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Default RE: Where to set the idle on a mid size gas plane?


ORIGINAL: AA5BY

How do I avoid a mental mistake of seeing the new plane go into a full stall too high off the runway because of the higher braking action. Will I learn it before it deals me a nasty lesson?
Most of us do, but it's one reason to consider buying a bit heavier airframe for a first gasser; it's more forgiving of, shall we say "less than perfect" landings???? lol

Short answer is that you land a gasser differently than a glow model.

Remember your basic aerodynamics? Set it up in a slow flight condition rather than a straight glide. The model will fly slower in a slow flight condition that it will glide. Throttle controls altitude and elevator controls attitude. So set it up with a decent rate you feel comfortable with designed to get it to the runway and vary that decent rate using mostly throttle, "DO NOT "PUMP" THE ELEVATOR" as one of my full scale flight instructors told me once during primary flight training and it applies to models. As it gets into the ground effect, pull the power out of it and it will slow it down letting you just paint it on.... [8D]

If it balloons on you, catch it with a throttle blip and let it settle again. If it gets really out of hand, don't fight it, go around. Flying a pattern usually helps you set up for a reasonably consistant approach.

It sounds easier than it is in practice, but it's not hard to do once you figure it out. Start by practicing slow flight while you're two mistakes high and get the feel of how the model responds and just how slow it really will fly before stalling. REMEMBER the rudder, it will keep working long after the ailerons have quit.

Hope that helps but you are 'gonna have a few hard landings along the way, just the nature of the beast.
Old 03-17-2011, 08:26 AM
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Default RE: Where to set the idle on a mid size gas plane?

ZEEB,
Great advice!

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Old 03-17-2011, 03:00 PM
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Default RE: Where to set the idle on a mid size gas plane?


ORIGINAL: chris923

ZEEB,
Great advice!

Chris923
+1 [sm=thumbs_up.gif]
Old 03-17-2011, 03:05 PM
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Default RE: Where to set the idle on a mid size gas plane?

And for the question on the actual idle. I like to set mine up at the lowest reliable idle the engine will maintain on the ground. As an engine wears in and the needles get fine tuned you will find that speed gets lower and lower.

A lot of the "IMAC crowd" here (in in the west) actually cut their engines when they are on the last few feet of their final.
I learnt by practicing circuits, it doesn't take long to get the hang of it and soon you will wonder what all the fuss was about.
Old 03-18-2011, 03:24 AM
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Default RE: Where to set the idle on a mid size gas plane?

Thanks guys... I'll work on it. I've flown this plane about a dozen times now but have had a couple of landings where it fully stalled in from 2-3 feet because of being at full idle and not anticipating the degree of engine braking. No harm was done but they weren't gentle landings.

btw, a few of the flights were with a higher pitch narrow blade prop that has far less braking action and the landings with it seemed too hot but of course no stalling issues.
Old 03-18-2011, 06:52 AM
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Default RE: Where to set the idle on a mid size gas plane?

ORIGINAL: AA5BY

btw, a few of the flights were with a higher pitch narrow blade prop that has far less braking action and the landings with it seemed too hot but of course no stalling issues.

Welcome to the wonderful world of gas aerobatic models and prop selection.... lol

Pitch has more to do with speed and braking than the chord (width) of the blade. Not all props claiming to be the same size from different manufacturers will perform the same since they all use different airfoils and in some cases, you'll find that they actually measure some distance shorter than the stated size. Some props are better for 3D, some make better IMAC props due to a bit more speed and better downline braking, then there's the whole wood vs. composite debate....

Add in different model types, different flying styles and different engines and it gets more expensive 'cause the ONLY way to find the best prop for you is to try 'em. Sorry but that's just the way it is and recommendations from others can help with which props to try but just because one guy likes a certain make and size for his model doesn't mean you'll like it for yours.

As for engine idle; yeah I kind of blew over that one but aussiesteve covered it pretty well. Essentially you want it as slow as it will go and still cover the transition. Most IMAC guys I know use what I call a "Flight Idle" which is actually a full scale turbine term meaning the engine remains at a higher RPM until a squat switch on the landing gear is triggered allowing it to go to idle. Setup either a throttle curve or a throttle/throttle mix on your tx and assign it to a switch. Setup the curve or mix to kick your idle up a few hundred RPM and use that to fly, kick it down again to land. Gassers are REALLY sensitive coming off idle so using the throttle trim to do the same thing like you would with a glow engine just doesn't work very well.

That'll make it so the model doesn't taxi off on pavement, has better downline braking and a more consistant landing since you know how much braking you'll have each time you land. Kicking the idle up in flight is mostly a "make you feel better" deal in that it pretty much removes the possibility of it quitting in flight due to the idle being too low. It's also usually best to set idle RPM right after you've landed as the gassers will usually slow down a bit when warm.
Old 03-18-2011, 02:14 PM
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Default RE: Where to set the idle on a mid size gas plane?

Zeeb... all made good sense and because this plane has no flaps and I'm comfortable with finding the flap switch easy, I just programmed a throttle to throttle mix activated with the flap switch. Tomorrow at the field will dial it in for a fast idle for flight and once touched down can flip it for slow idle. The hope is to prevent slow idle braking from stalling the plane too abruptly on flare when I forget and pull the throttle stick fully down when landing.

Thanks... I'll report how it goes.
Old 03-20-2011, 03:24 AM
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Default RE: Where to set the idle on a mid size gas plane?

The fast idle switch worked great. It allowed pulling the throttle stick fully back during landing without experiencing too abrupt of flare stall and was easy to switch off at touch down.

About five landings were made with ease until the cross wind got my number. I'd experienced once before on the Yak a cross wind gust hit that large rudder area and roll the plane sharply but that had happened when I was yet about ten feet up leaving a little more time to roll back level and deal with it. This time I was about three feet up when it hit, the plane rolled thirty degrees and I reacted to get the wings level but the process killed speed and she stalled to the runway fairly hard ripping the strut mounting T-nuts through the light ply.

Fortunately, the wings were returned level before it got to the ground and there was no damage to them or otherwise any damage to the plane other than the gear mounting piece, which was easy to saw out with a multi tool. I'll replace with aircraft plywood and use nylon bolts. I'm not the first nor will be the last to do that mod on an arf.

I think I'm a little wiser about that large rudder and gusty cross winds. It was really an Ultra Stick fly day... but I've been enjoying the Yak so much I couldn't resist.
Old 03-20-2011, 04:39 AM
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Default RE: Where to set the idle on a mid size gas plane?

Well sorry to hear 'bout your hard landing, but it's something we all have to deal with along the way. Biggest issue I see with that, is some folks will try to cross control it to straighten it out on short final like you would a full scale having a straight wing. That cause two problems; cross controlling raises the stall speed and at some point the ailerons will quit while the rudder still has authority, then when a wing drops they try to catch it with aileron but there isn't anything there due to the slow airspeed.

What I like to do is handle it just like the full scale jets with swept wings since you cannot cross control those without a wing stalling; let it crab into the wind and touch down, then kick the rudder to straighten it out once down. Also, if you get hit with a gust of cross wind or even a head wind that throttle blip will keep it flying but you have to remember the rudder for directional control, then you can try to get it down again or on a bad balloon or really out of shape from a cross wind gust, go around rather than try and fight it down to the runway again.

You'll get used to it, it just fly's more like a full scale than your smaller models due to the physics involved, bigger fly's better and you just need some practice now...
Old 03-20-2011, 06:25 AM
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Default RE: Where to set the idle on a mid size gas plane?

Just a thought, but if you're comfortable with the idea of moving your CG around, a slightly nose heavier condition might help you out? This can increase glide potential big time. If taken far enough can lead to a plane that will float just off the surface the entire length of the runway! Something to play with/keep in mind if you have the interest.

Myself, I prefer the way you're doing it now. I don't use dual rates though. Preferring instead to bring it in with a little power on and actually use power management until deep in the flare where it's reduced gradually to ease the plane down (ex Tri-Pacer pilot). That's when I have time to think though! I get the concept of a busy cross wind landing and what can happen there!
Old 03-20-2011, 06:25 AM
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Default RE: Where to set the idle on a mid size gas plane?

Thanks Zeep for the comments. The dropped wing was probably not a tip stall or the ailerons probably wouldn't have leveled it... I was lucky. Or... it was a tip stall and the huge ailerons can overcome some tip stall... I don't know.

After I posted, I checked my transmitter and am feeling kind of dumb. A factor that may have played a part in the hard landing was that I had been playing with some mixes and left the mix switch off. Backing up a few flights, the first mix that I'd messed with was some down trim at low throttle. The plane needed a fair amount of up trim at half to full power but at low throttle, the up trim didn't feel right, it needed a slight bit of down elevator on the final. It could be that the plane has too much down thrust and I know the plane require two more clicks of up trim after adding longer strut, larger wheels and doing away with wheel pants. At any rate, I'd programmed some down trim mix at low throttle but because I was unsure of the values, I put them on the mix switch so I wasn't locked in. I did have those numbers dialed in and should then have moved them to ON.

Then I started messing with a rudder to elevator mix and was comparing mix/no mix and forgot and left the mix switch off and therefore didn't have my down trim mix needed for slower flight. I'd aborted one approach prior to the hard landing and now suspect the lack of down trim was part of the cause of the hard landing.

That was my bad, I should have moved that mix to always on before messing with the other mixes as I think it somewhat critical to slow speed trim.
Old 03-20-2011, 06:40 AM
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Default RE: Where to set the idle on a mid size gas plane?

Ahicks.... I just saw your post after posting my previous. The plane does not feel snappy and in flight and static balance feels and looks good... but as noted above... glide trim without some down trim mix isn't good and leaves the plane floating too much as you suggested. I've little doubt it contributed to the problem yesterday... in part because I was no longer as vigilant thinking I'd cured that part of flight but it was on a mix switch that got left off.

Old 03-21-2011, 07:07 AM
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Default RE: Where to set the idle on a mid size gas plane?

Oh UGH.....

You've either got a CG problem (most likely) or an engine thrust line problem. It needs to be fixed before you start playing with mixes for different power settings.

The CG listed in the manuals usually gives you a range for the CG and that isn't always accurate but is usually safe to get the thing in the air, then most guys develop a preference for whether they like the thing a bit forward, neutral or a bit aft. A bit forward will be more forgiving but requires a higher landing speed. Neutral will be a bit more touchy especially on landing with ballooning and some slightly squirrely behavior there. Aft is what the 3D guys like 'cause it makes the models do high alpha manuevers more easily but they are tough to get back on the runway. There's an old axiom that goes something like this; "forward CG is stable, doesn't manuever as well and lands a bit hot. Aft CG fly's once....."

For CG, a lot of guys use what's generally known as an IMAC setting; mid range throttle, pull a 45 degree upline and roll the model inverted. Let go of the sticks and the model should continue on that upline for several seconds and then start to drop the nose a bit. I like mine set so it will keep going on that 45 degree upline which is just a bit forward of neutral CG.

For engine thrust lines; up/down is checked first. Flying level at a mid range on the throttle if you pull the throttle out of it, the nose should drop just slightly as it starts to slow down and the model should start to descend. Same initial setup only open the throttle up and it should stay level. If the nose pitches one way or the other it's got a vertical thrust line problem. Right/Left thrust lines are a bit different and you do that after establishing the correct up/down. For what it's worth, not many models have up/down thrust line issues when first assembled because most manufacturers set it at neutral and build them that way, but it does happen. How much right thrust you need is sort of like the CG question where personal preference has some influence on where it's set.

Let's hope that gets you close 'cause the next step if that doesn't work is checking the incidence angles on the wing and horizontal stab but again, those are not usually a problem.
Old 03-21-2011, 07:16 AM
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Default RE: Where to set the idle on a mid size gas plane?

An easy repair. Sliced out the remnants of the old gear mount (one thickness of 1/4 light ply) with a multi tool and replaced with two thickness of 1/4 aircraft ply + 45 deg support pieces. I'd added 45 deg support pieces to the original, but because the T-nuts were not recessed they couldn't be carried fully to the ends where the nuts were leaving those areas weak where they ripped out.

In the repair, a forstner bit was used to recess for the T-nut heads so that the 45deg supports are fitted fully around the gear mount. It will now suffer a much harder landing. The question is, will the gear block pull out before it rips up bulkheads? I of course hope I never get the answer to that question.
Old 03-21-2011, 12:07 PM
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Default RE: Where to set the idle on a mid size gas plane?

Zeep... I know the numbers. First, the wing is not movable because it is inset so lets call it zero. The pitch thrust is zero to the wing. The stab is +0.5 deg. The first flight required someone to come click the up trim because to relax the stick the plane would dive. It required about six clicks of up trim. My take on that was that it was the positive in the stab that needed countering.

The stabs are easily adjusted as they are on a tube and have single pegs. I'd started doing that in 1/32 shim increments at the peg when the local IMAC guru and 33% Yak flier told me that was normal on the Yak and to leave it. Before that first shim, it had 1/4 up trim at the TE of elevator. That one shim reduced up trim by two clicks or about one third. However, the plane needed a different gear strut that angled forward some to deal with our rough field and I used a CF strut that is about four inches higher and changes from 3" to 4.25 wheels and no pants. I expected it to need some more up trim because of that lever and was right, the two clicks I gained was lost. So, I'm currently running 1/4 up trim at TE of elevator.

It stands to reason that those two clicks might not be needed at low power because the lever of that longer gear and larger wheels diminishes at lower power.

Personally, I'm for continuing to adjust the stab until the trim is neutral... as it isn't hard to do. Just have to pull the stabs, glue a shim strip on one side of the peg and relieve the other side.

Regarding balance.... it is right where it was called out to be and the inverted up line test shows it to be correct. The local IMAC guru did the test as well and confirmed CG was good.

He did not land it or get a feel for it at low power but by everything I've ever felt, it is trimmed up too much at low power and correct at mid to full power. Of course that goes against what I've always observed... in that a plane is trimmed for low power and then if it has a pitch issue at high power, that is corrected by thrust or incidence changes. So... according to that school of thought, I've got a pitch down problem with power on and with the thrust line zero to the wing, and positive in the stab... that is where I lay the problem.


My take is that the positive in the stab needs a lot of correction under power but when power is off, not as much correction is needed. But... I don't know why Yak's are supposed to have positive in the stab.... to counter the drag of a large vertical stab and canopy I guess... but this is a smaller Yak at 72" and if that is the reason, I don't think it needs it... especially with the undercarriage I'm running.
Old 03-21-2011, 05:34 PM
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Default RE: Where to set the idle on a mid size gas plane?

Removed the stabs today and shimmed another 1/32" in the direction towards zeroing it. Will test tomorrow if the weather is suitable.
Old 03-22-2011, 06:54 AM
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Default RE: Where to set the idle on a mid size gas plane?

Perhaps a new thread in the IMAC forum for trimming suggestions is in order?

Have you ever seen the Peter Goldsmith trimming article? It's generally considered one of the better references on the subject and I'll attach a .pdf as well as a kind of nifty little form someone over on FG came up with....

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Old 03-22-2011, 07:07 AM
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Default RE: Where to set the idle on a mid size gas plane?


ORIGINAL: AA5BY

Removed the stabs today and shimmed another 1/32'' in the direction towards zeroing it. Will test tomorrow if the weather is suitable.
What YAK model are you flying? How much wing area and overall weight? Could you just be trying to land too slowly?
Old 03-22-2011, 07:35 AM
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Default RE: Where to set the idle on a mid size gas plane?

I like to mount my gear with nylon bolts. That way the bolts shear instead of tearing out the mount Just my approach that has saved me a lot of repair. I use 10-24 for smaller models and 1/4-20 for the latger ones. Usually four bolts for a one piece strut or three per leg for a 2 piece gear. You can also drill trough the center of the 1/4 -20 bolts to tailor the shear resistance to the size and weight of your model and your landing technique. Even good fliers have some bad landings. Good luck.

Richard
Old 03-22-2011, 09:23 AM
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Default RE: Where to set the idle on a mid size gas plane?


ORIGINAL: Truckracer


ORIGINAL: AA5BY

Removed the stabs today and shimmed another 1/32'' in the direction towards zeroing it. Will test tomorrow if the weather is suitable.
What YAK model are you flying? How much wing area and overall weight? Could you just be trying to land too slowly?
The plane is a ntiro models 72" at 12.5 lbs with a 28cc at 1077 sq in.

I'm not trying to land too slow, but two factors seem to be forcing slowness. The first is the greater engine braking that requires a very narrow rpm range to suit a good landing and the second is a model that doesn't drop its nose adequately at low power.

If a low braking prop such as the APC Sport 18x8 is used, landings are hot and long.

However, I prefer the APC 18x6w because it pulls much better at mid range and will slow the plane for landing much better, though it does require careful rpm management to avoid over braking. With it, most of flight is at about half power whereas with the higher pitch, most of flight is near or at full throttle but down lines have little braking.

It could be I need a prop between these two characteristics.
Old 03-22-2011, 09:33 AM
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Default RE: Where to set the idle on a mid size gas plane?


ORIGINAL: spaceworm

I like to mount my gear with nylon bolts. That way the bolts shear instead of tearing out the mount Just my approach that has saved me a lot of repair. I use 10-24 for smaller models and 1/4-20 for the latger ones. Usually four bolts for a one piece strut or three per leg for a 2 piece gear. You can also drill trough the center of the 1/4 -20 bolts to tailor the shear resistance to the size and weight of your model and your landing technique. Even good fliers have some bad landings. Good luck.

Richard
Richard, thanks for the response. I'd actually planned to use four 1/4 nylon bolts when reinstalling the gear and have done so on both my Ultra Stick Lite and a 1.20 size Hanger 9 Taylorcraft with no problems of premature failure but the idea got kaboshed at the field Sunday when everyone there opined that it wasn't a good idea for this model. There argument mostly hinged on a growing issue that nylon bolts have had too much variance of strength with some of them being quite weak and that my normal soft landing touch that applies to those planes hasn't been the typical landing on this plane.

So... I got talked into going back with the metal screws.
Old 03-22-2011, 09:57 AM
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Default RE: Where to set the idle on a mid size gas plane?


ORIGINAL: Zeeb

Perhaps a new thread in the IMAC forum for trimming suggestions is in order?

Have you ever seen the Peter Goldsmith trimming article? It's generally considered one of the better references on the subject and I'll attach a .pdf as well as a kind of nifty little form someone over on FG came up with....

Zeep... thanks, I've printed out the pdf and will park in the little casa for later consumption.

Perhaps you are right that this should be in another location. I'm not an IMAC flier so don't have their forum tagged so didn't think of posting it there.
Old 03-22-2011, 12:44 PM
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Default RE: Where to set the idle on a mid size gas plane?

"The first is the greater engine braking that requires a very narrow rpm range to suit a good landing "

18x6W fan here too. I had a lot of trouble when first into the gassers because I didn't know how non linear the gasser throttle response really was. Actually I had a LOT of trouble with this. To flair like I wanted was nearly impossible - until I figured out I needed to set up a throttle curve. That got rid of that "touchiness" on final and during the flair. Could that be some of what you're running into?

Roughly, I have the first 1/4 of the throttle travel = first HALF of the stick travel. This gave me the control I was after....
Old 03-22-2011, 04:06 PM
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Default RE: Where to set the idle on a mid size gas plane?


ORIGINAL: ahicks

''The first is the greater engine braking that requires a very narrow rpm range to suit a good landing ''

18x6W fan here too. I had a lot of trouble when first into the gassers because I didn't know how non linear the gasser throttle response really was. Actually I had a LOT of trouble with this. To flair like I wanted was nearly impossible - until I figured out I needed to set up a throttle curve. That got rid of that ''touchiness'' on final and during the flair. Could that be some of what you're running into?

Roughly, I have the first 1/4 of the throttle travel = first HALF of the stick travel. This gave me the control I was after....
Yes, I think it is a big issue.

Unfortunately my JR 7202 does not have expo for throttle. And, the mechanics of the ball joint on the engine throttle is close to the shaft so servo travel is reduced so I've not given much thought to how it could be done mechanically. Actually, for some reason the servo travel display for the Yak, shows a non linear movement with the lower half of the stick yielding only 1/4 of servo travel. That might be a function of reduced numbers of the low limit compared to the upper limit... I don't know the why actually? It is not so with other models.

I did program a throttle to throttle mix to create a fast idle point for landings as an effort to avoid too much engine braking on landings. I liked the way that worked and even in the gusty cross winds that day, had made several nice landings and it was no problem to hit the lower idle switch upon touch down. I was fine up until I forgot to have the mix switch on for the down trim at low power.

Ideally of course, the goal would be to keep it simple and not have required switches for landings that might be forgotten. Generally, I can jockey a throttle to deal with head wind variations on landings without a problem but I'm definitely not used to the braking that takes place if one gets too low on rpms with this prop/engine and combined with a model that is not in trim for low power... it it challenging.

If I can get the model in low power trim, I think the throttle stick jockeying will come.

I did not fly today... strong gusty winds kept everyone home.

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