Descents in the Comanche 250

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gpbarth
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Descents in the Comanche 250

Post by gpbarth »

I'm getting too old for this...I could probably fly the real thing all over the sky, but for the life of me, I can't get the descent nailed on this A2A Piper. And I can't find anything in the literature to explain it, either. What I have found is: full rich, max prop, about 15" - 17" MP, and trim for about 100-110 kts. This thing keeps flying at those settings. I have to cut the throttle completely and oull the nose up to get any sink rate, and then it goes to about -200 fpm. I can't power it back any further.

I'm using a Saitek X-55 controller, set up and calibrated through FSUIPC. All of the controls work fine, it seems. No throttle whatsoever idles the engine at about 300-500 RPMs. Prop control works fine, as does the mixture. Right now, I'm just climbing to about 5000' and practicing descents, and I can't get it smooth. Any advice from the experts?
-= Gary =-
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NAA551WB
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Re: Descents in the Comanche 250

Post by NAA551WB »

Have you tried putting out a notch of flaps?
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AKar
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Re: Descents in the Comanche 250

Post by AKar »

High speed descends are indeed a bit difficult in A2A Comanche, often one wants to slow down before going down, and if necessary for some reason, add some flaps or gear.

A quick general notes: I'd avoid using that full RPM, it just makes unnecessary noise and stresses the engine. Cruise RPM should be just fine or you can back off from that a bit too when you reduce power. Also, why the full-rich mixture? That just serves to cool the engine further when you actually want to keep it warm and throws away large amounts of fuel.

It depends on the airspeed obviously, but I don't think the RPM should go that low even if you shut the engine down. This should be tested in real thing to be sure, but at least at highish cruise (=descend) airspeeds and RPM selections of around 2000 or less I wouldn't expect the prop to go into the stops at all.

-Esa

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Re: Descents in the Comanche 250

Post by bullfox »

I have to get down to less than 15 inches mp to descend.
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Re: Descents in the Comanche 250

Post by renaissanceman »

I leave the TPM at cruise settings, push the nose down and trim for 250 - 300 FPM descent. This keeps the guages in the green. As I descend I adjust the throttle to maintain cruise MP and adjust the mixture to maintain 55% power.

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Re: Descents in the Comanche 250

Post by gpbarth »

I use flaps to slow down, but you have to be within that white arc before adding them. When you're at cruise of around 135 KIAS, at cruise level, you want to slow down gradually and let the IAS bleed off into that white arc. SO - if you're trimmed for level flight at cruise power setting, and you reduce the power, the aircraft should start to descend because of lack of airspeed. And then when it reaches the desired altitude, you can re-apply power.

Anyway, I found this over at Langley Flying School, which seems to explain things better:

To descend, use “PAAT.” Smoothly reduce power to the target reduced power setting (PAAT). As the power is reduced, use your feet to keep the nose straight and simultaneously adjust the aircraft pitch to maintain altitude until the targeted airspeed (PAAT) is reached. This is important—keep the aircraft level at the initial altitude until it slows to the descent speed. When the airspeed slows to the target speed, gently pitch forward (attitude—PAAT) sufficiently to maintain the speed—again rudder control will be required as the pitch is changed. Finally adjust the trim (PAAT) to maintain the descending speed hands-free. When the target altitude is reached the cruise power setting should then be re-established with the throttle and the pitch adjusted to the cruise attitude—while this is being done footwork will of course be required. Finally, re-trim for hands-free flight in the cruise attitude. When you level out after a climb or descent, be sure you perform the “cruise checks.” If descending, ensure that the mixture is advanced, either before or during the descent. Flaps are sometimes selected during descent—on final approach, for example. In such cases, the flaps are selected after the power reduction occurs, and prior to trimming. During climbs, the obstacle clearance flap setting is sometimes used, and you want to become precise at pitch and airspeed control. All flap changes should be done smoothly without allowing the nose to flop about. When recovering (retracting) the flaps during a climb, ensure the pitch is properly adjusted first.

I remember when I was learning to fly, the old power v. pitch argument was used. Power controls altitude, while attitude controls airspeed. And the argument continues. I will take the advice of you guys up here and try to find a happy medium. When you're flying the heavy iron, it's simply cut the power to idle and maintain airspeed with trim. In the little guys, it's a bit more touchy.
-= Gary =-
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Re: Descents in the Comanche 250

Post by DHenriques_ »

gpbarth wrote:
I remember when I was learning to fly, the old power v. pitch argument was used. Power controls altitude, while attitude controls airspeed. And the argument continues. I will take the advice of you guys up here and try to find a happy medium. When you're flying the heavy iron, it's simply cut the power to idle and maintain airspeed with trim. In the little guys, it's a bit more touchy.
As a basic rule, power/altitude.......pitch/airspeed is fine for all aircraft big and small. Just remember a more advanced explanation of this basic principle includes the simple fact that when you adjust either one of these it affects the other.
So to be perfectly accurate, you adjust one and EXPECT a change in the other. This "trade off" explanation is the one I teach and encourage pilots to understand COMPLETELY.
There is one exception to the rule, and that exception addresses extremely high performance jets on final approach. With these airplanes, if and when you ever start flying them, flying the approach manually you will find it advantageous to stabilize the aircraft in the landing configuration at approach airspeed, point the airplane (or the velocity vector if using a HUD) at the touchdown point on the runway and use modulated power to control the sink rate and the airspeed. This is basically the same thing as before but the approach speed is high enough that you will be adjusting pitch without thinking as you use power to control the approach.
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AKar
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Re: Descents in the Comanche 250

Post by AKar »

In my mind, the energy conservation is one of the most fundamental principles to be applied to flying among many other things. In a few words, the energy must come from somewhere (thrust) and it must go somewhere (drag). When those two are equal, the energy the airplane has must remain equal too, it only can change forms (airspeed, altitude). In straight and level flight, the energy 'flowing' into the system due to thrust force acting just offsets the amount of energy flowing out of the system due to drag force of the same magnitude acting to oppose the thrust. Equilibrium is maintained.

Just pushing forward (without bringing in any discussion about flight mechanics) just translates our potential energy (altitude) into kinetic energy (airspeed). Initially the 'inflow' and 'outflow' of the energy (thrust and drag) are not altered, but the energy must still be maintained. Therefore, as we start to loose altitude, we start to pick up speed. We are just translate energy, not getting rid of it. Eventually, as the airplane accelerates, the drag increases. If the thrust is not altered, we'll be loosing energy. That energy lost can come either from airspeed or altitude (or both!) - they must decrease. The airplanes, in that sense, are not different from roller coasters.

The secret of descends is in understanding that we must get rid of the amount of energy that matches our height over our target altitude. The energy, not the altitude, is what we must get rid of! There are a few ways to do the trick. We can reduce thrust and take what is needed to offset the drag by descending. We can maintain the thrust but increase our drag (by increasing airspeed, extending flaps, gear ....), that extra amount of work done by drag is again offset by energy taken from our altitude by loosing it. We can, loosely speaking, even keep the energy equal by just translating our altitude into airspeed by diving, and getting rid of the energy only after reaching our desired altitude - all this without touching the throttle (by accelerating during the dive and then slowing down in level flight after the descend - then the energy remains constant during the descend and is gotten rid of after it!).

When flying gliders these are fundamental things to understand because we don't have any thrust and by default have very little drag. But no matter how little the drag is, the energy lost into it must constantly be replenished by loosing altitude. Drag can be altered by changing airspeed and/or configuration - in powered airplanes, thrust can be altered by power plant settings. In nowhere above the pitch is mentioned - it is coincidental thing, not fundamental one! It is important to flying airplanes but not to elementary physics that are needed to be known to understand what's going on.

-Esa

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gpbarth
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Re: Descents in the Comanche 250

Post by gpbarth »

Well, in real flying, my application by experience has been to reduce thrust (throttle) and then trim the aircraft to match an airspeed that will result in a specified descent. When I reduce power, drag begins to overcome thrust and the A/C slows. As it slows, lift diminishes and it begins to sink. Pushing the nose down slightly maintains a descent speed and set trim for that speed. If needed, throw in some flaps (at the recommended speeds, of course).

And so when I start my descent in the Comanche, the first thing I do is reduce power and trim nose up to keep the speed bleeding off. Of course, now I also have to deal with MP, so I guess the best thing is to leave the prop alone at cruise setting and just manage the throttle and trim. I'd just like a descent rate of about 400-500 fpm without building up airspeed. Either that, or start the descent further out. Trying to just chop the throttle and nose it over only results in massive airspeed build-up, which then has to be bled off somewhere.

My problem is that I have spent too much time flying the heavies, and now that I'm back in the recip GA stuff, it's hard to re-program my habits. I will learn, though. I love the information gleaned from these conversations. I could yak on this all day. And BTW, I haven't flown a real aircraft in years now, but I recently learned about Rusty Pilots program (http://www.aopa.org/Pilot-Resources/Get ... nto-Flying), and I am about to get myself back in the air again. For any of you older gents (and gals) who had a certificate but haven't flown for awhile, and wish they could, please check into this.
-= Gary =-
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Re: Descents in the Comanche 250

Post by DHenriques_ »

gpbarth wrote:I'm getting too old for this...I could probably fly the real thing all over the sky, but for the life of me, I can't get the descent nailed on this A2A Piper. And I can't find anything in the literature to explain it, either. What I have found is: full rich, max prop, about 15" - 17" MP, and trim for about 100-110 kts. This thing keeps flying at those settings. I have to cut the throttle completely and oull the nose up to get any sink rate, and then it goes to about -200 fpm. I can't power it back any further.

I'm using a Saitek X-55 controller, set up and calibrated through FSUIPC. All of the controls work fine, it seems. No throttle whatsoever idles the engine at about 300-500 RPMs. Prop control works fine, as does the mixture. Right now, I'm just climbing to about 5000' and practicing descents, and I can't get it smooth. Any advice from the experts?
If you have excess airspeed when you level off after a descent, just come back anywhere below cruise manifold pressure unless you are already there as you well might be and raise the nose a few inches above the horizon. You should begin to lose airspeed steadily. You can also S Turn the airplane slightly. Any bank will help the process.

Dudley Henriques

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Re: Descents in the Comanche 250

Post by bullfox »

I reduce the throttle to 15 inches and leave the autopilot in altitude hold while speed bleeds off. When the autopilot asks for more up trim, I take it out of altitude hold. I manage the descent rate with small changes in throttle. I am not slowing down much but that's ok because it gets me where I'm going. When I get about there I engage altitude hold until I'm slow enough to deploy gear and flaps. Some times a bit of a climb at low throttle is required to slow down. Most of this is hands off and a bit of trim maybe required in addition to small throttle changes. Not sure if this is best practice, but it works for me.
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Re: Descents in the Comanche 250

Post by William Hughes »

17" and 115 knots is where I fly straight and level instrument approaches. Aircraft feels mushy and rather unpleasant at this speed, though.

For a descent I'll rich it up a smidge and reduce power a few inches from cruise, and leave airspeed alone. This will result in about 500 fpm descent rate. Then keep an eye on the EGT so that as it leans out during descent the temps stay below 1,450 and the CHT is over 300.

When I first started flying CF-NZB I had very real difficulty in getting it to slow down. I botched several approaches, and once flew right over CYXE while number 2 to join downwind. I was looking for the traffic and before I knew it I flew over the airport. It was just a matter of having 50% less time to make decisions.

I had to make a series of S-turns yesterday on long final as number 3 to land. There were some training 152 in front of me and a Comanche's 80 knot approach speed is faster than their cruise. I hung out all the junk and added power but really, I'm not going slower than 70 knots if I can possibly help it, making slow turns near the ground, and tower had us spaced at minimums. Even then the last one was a bit rushed getting off the runway, and I was very short final before cleared to land. That was less drama than it sounds, its all routine for a busy training airport.

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gpbarth
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Re: Descents in the Comanche 250

Post by gpbarth »

Great input, William and Bullfox! I'm all "ears" to get this to work right. Everything I've flown in real life, with the exception of the C182, has been a fixed pitch, non-complex aircraft. And now here I am flying the Turbine Duke and the Comanche 250. It's keeping me on my toes, especially during these approaches. I'll keep playing with this and let you know how it works out for me.

Thanks!
-= Gary =-
MSFS2020, DCS World, A2A Accu-Sim Military & Civilian P-51, Cessna 172, Comanche 250, Win 10 64-bit, I7-7700, ASUS GTX108Ti, 32Gb RAM, 2 2Tb drives.
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Re: Descents in the Comanche 250

Post by DHenriques_ »

gpbarth wrote:Great input, William and Bullfox! I'm all "ears" to get this to work right. Everything I've flown in real life, with the exception of the C182, has been a fixed pitch, non-complex aircraft. And now here I am flying the Turbine Duke and the Comanche 250. It's keeping me on my toes, especially during these approaches. I'll keep playing with this and let you know how it works out for me.

Thanks!
Glad you found some
people on the forum who could be of help.

Dudley Henriques

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Re: Descents in the Comanche 250

Post by William Hughes »

Another note: if you attempting to slow down with low power settings make sure to full fine the prop. It makes quite a difference. You should do this on final anyway in case of a go around - some circumstances might be different eg: in an engine out practice you will want that full coarse. You've probably read about GUMP checks by now, but don't just tap the handles while muttering. Think about why that particular control is set the way it is and is that correct for the current situation. I like doing it three times: downwind, base, and final.

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