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Exchange Autopilot options for a TB20

There is very little difference in the ceiling between AoA=8deg and AoA=9deg.

I think an E300 will fly with an AoA of 20deg only when there is a load of power i.e. at low levels. Most aerofoils start to stall around 10deg.

Plus you usually have dual GPS box for redundancy thus lowering the possibility that both fail at the same time which leaves only the option of system failing.

If the AP is wired to get GPS data from either GPS and auto-selects. Does the GFC500 have that capability?

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

Most aerofoils start to stall around 10deg.

I think 15 degrees is the typical number people use to approximate.

Silvaire wrote:

I think 15 degrees is the typical number people use to approximate

14deg AoA to be precise the associated power off PIT can be at -2deg though

Last Edited by Ibra at 12 Apr 21:05
Paris/Essex, France/UK, United Kingdom

Sure, but try it for real

Past about 10deg the plane won’t be climbing anymore, presumably because the parasitic drag has eaten all spare power, so in the context of a safe management of a hands-off climb on autopilot…

OTOH 10deg showing on the AHRS, whose installation aligns it in pitch and roll with the airframe axes, (and controlled for by the AP in PIT mode) is probably something like 11+deg of actual wing AoA.

Administrator
Shoreham EGKA, United Kingdom

Mooney_Driver wrote:

Pitch mode will keep a fixed pitch. That means in a climb with max power (or climb power) set, IAS will gradually reduce until it eventually stalls. Same as with VS, only a different parameter.

Not if the pitch is set below the pitch for stalling. A typical airfoil used in GA aircraft stalls around 15° AoA. The AoA at zero pitch shouldn’t be more than a few degrees. So the 8° pitch typically used for climb should be safe.

(sorry, I wrote this without having seen the other replies.)

Last Edited by Airborne_Again at 13 Apr 06:43
ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

Not if the pitch is set below the pitch for stalling. A typical airfoil used in GA aircraft stalls around 15° AoA. The AoA at zero pitch shouldn’t be more than a few degrees. So the 8° pitch typically used for climb should be safe.

Of course, yes. My point is simply that I contest the statement that PIT mode will not stall the airplane, it will. Let’s say, you are in initial climb with a constant power setting, low level and set the pitch for a climb. The higher you climb, a constant pitch with decreasing engine power will sooner or later lead to a decrease of speed to keep that pitch and consequently increase AOA and eventually stall.

IAS or Level Change or whatever manufacturers call it keeps speed as opposed to power. In the same scenario as above, it will set a certain pitch at low level to maintain selected speed (vy or cruise climb speed) with set climb power, resulting in a certain climb rate. As you climb, engine power decreases gratually. IAS mode will keep the IAS selected, which will gradually reduce pitch and subsequently vertical speed up to the point where the airplane will de facto level off or, in case of decrease of power, start a descent. The only way you can stall a plane using IAS mode would be if you select a speed below the relevant Vs.

What we currently do with most AP’s is to observe a climb profile with “safe” VS. E.g for the Mooney and the Stec 55x, we select 500 fpm up to 10’000 ft and 200 fpm above it in climb and VS mode. That however is not efficient really, as low level the airplane can climb with up to 800 fpm so you will simply climb with too much IAS and generally climb below the best performance.

In airliners, climb profiles are IAS/Mach based. Normally, 250 kts below 10’000 ft (for the ATC limitation), 290 kts above 10’000 ft and from a certain Mach number change to Mach.

Where FL Change does not work similarily well for our piston engines in in descent, as we have to keep our engines reasonably high power in descent. Usually, FL Change means going flight idle in descent. Here, VS is probably the better variant.

Last Edited by Mooney_Driver at 13 Apr 07:03
LSZH(work) LSZF (GA base), Switzerland

Airborne_Again wrote:

So the 8° pitch typically used for climb should be safe

I wondered why 8deg PIT just works for “stable & safe climb” in most pistons, maybe it’s half way between 14deg AoA? or maybe near low power cruise pitch? or maybe near max available power cruise pitch?

The first, is sensible but an irritating explanation: PIT value half way between cruise AoA & stall AoA should be just stable & safe on max power, as PIT<AOA for 100% power cruise and PIT> AOA for 100% power stall neither of these AoA states have stable speed-power dynamic, so setting PIT half way should just work to get AoA half way

The third, I highly doubt it, as it depends on altitude where available power decreases & Vy decreases, one has to pull nose pitch up to keep going

The second, seems way more plausible, it’s about pitch where one shifts from reducing power to adding power to maintain level flight as they slow down from cruise to stall?

Also, it’s where FD goes on most AP go-arounds…

Last Edited by Ibra at 13 Apr 09:12
Paris/Essex, France/UK, United Kingdom

It is just geometry,what ever the angle of attack is when the AI is at zero, for e.g. 4degrees plus the pitch attitude will be the new angle of attack when the pitch attitude is held constant and the flight path is level because the excess power is no longer excess.

E.g. 4 degrees plus 8 degree pitch attitude = 12 degree AOA which is often less than the stalling AOA.

If the power subsequently fails or is reduced then of course you will need a descent, and the numbers will be different.

If your descent angle is say 1 in 12 which is about 6 degrees, then you might get something like 6 (for the descent) + 4 (incidence) plus another 2 degrees pitch attitude to hold about 12 degrees AOA.

Of course flying at a different speed or config than the last example will vary the descent angle.

Last Edited by Ted at 13 Apr 11:04
Ted
United Kingdom

Yes it is a geometry thing. We discussed it briefly early on this thread:

NA you could typically climb at 3 degrees path at low altitude and perhaps 1 degree high up: a delta of 2 degrees.

Peter’s system is to fly same pitch 8degrees all the way up, so for example that is 5 deg AOA low down and 7 degrees high up, slowing down from say 120KIAS to 95 KIAS.

Even though it is not the most efficient it works fine due to the higher cooling requirements at 80% pwr low down vs 45% pwr higher up.

In summary: it is quite practical in his particular operation. And yes of course he can stall the airplane in attitude hold/pitch hold mode, just a matter of the right combination of pitch/power at that altitude

Antonio
LESB, Spain

Mooney_Driver wrote:

IAS mode will keep the IAS selected, which will gradually reduce pitch and subsequently vertical speed up to the point where the airplane will de facto level off

The issue with this mode is not the progressive pitch adjustment to maintain IAS as power is reduced, which always works great.

The issue is adjusting for minor shear and/or turbulence, where maintaining the set IAS requires quite heavy pitch (and accompanying G) excursions, depending on how accurate you want to be (system control gains in the IAS loop) . You can clearly see that in t he depicted video where pitch changes up and down 20 degrees to maintain IAS.

In airliners these pitch excursions bring pax (or even pilot) discomfort which is why a lot of times VSI mode is used instead.

Last Edited by Antonio at 13 Apr 13:47
Antonio
LESB, Spain
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