TAS/GS weirdness

j225

Thanks for these tweaks. I noticed similar discrepancies with Little Navmap.

On the 25/25 rule, not everyone agrees. Came across this recently. See in particular John Collins’ comment and subsequent discussion.

https://www.pilotsofamerica.com/community/threads/the-25-25-myth.74115/

BernieV

@j225 said in TAS/GS weirdness:

@BernieV

Thanks for these tweaks. I noticed similar discrepancies with Little Navmap.

On the 25/25 rule, not everyone agrees.

Agreed. I think that 25X25 is specified in my POH (for a '67 Arrow, not a III like this model, but pretty close in most aspects and both have 4 cylinder IO-360s engines) for climbs, but I get your point. Get a bunch of pilots together on a non-flying day, add a few adult beverages, and mention either lean-of-peak operations or he "right" combination of MP & RPM for a given engine, and watch the fur fly ;) I think the conversation you referenced managed to work both topics into the conversation!

I've put 1850 hours on my engine since rebuild and compressions are still good, so I'll stick with what got me this far. Fingers crossed and hoping to get to 2000 hours on this engine. The nice thing with a sim is you can do whatever you like and you wont have to pull out your wallet and pay for a top end overhaul or rebuild.

BernieV

I've done one more round of testing and have have achieved a good balance of 75% power cruise TAS , climb performance, airspeed buildup in the descent, and service ceiling. They are as close to book at I can get for now.

In flight_model.config

parasite_drag_scalar =0.8
induced_drag_scalar =1.88

In engines.config

fuel_flow_scalar =0.73

j225

@BernieV

Ha, yes! If everyone has been following these rules of thumb for years without any problems, then it does seem a bit academic, but interesting nonetheless.

Martyn

Just to confirm, the ASI inaccuracy issue is now fixed and our flight dynamics creator is continuing their work on power/TAS performance.

RetiredMan93231

@BernieV , Thanks for doing this research, and the recommended settings for achieving more realistic performance. For reference, the Parasitic Drag of any aircraft (parasite_drag_scaler) affects both the cruise speed and climb performance, while the Induced Drag (induced_drag_scaler) affects mainly the climb performance. The speed and climb performance are both determined by the ratio of Total Thrust vs. Total Drag of the aircraft...

BernieV

@Martyn Good to hear, thanks Martyn.

BernieV

@RetiredMan93231 You are welcome RetiredMan. In practice, the induced drag scalar was the parameter that tuned rate of climb, the "slipperiness in the descent" (not a book value, but one I know from experience), and service ceiling. The final values posted gave me 137 kts level cruise at 75% power at 6500 ft (FT, 12 g/h, 2500 rpm as per book), an increase in TAS of 10 kts in a 500 ft descent at 75% power out of 6500 ft (from experience I expect 15 kts pickup, but I didn't want to detune ceiling or rate of climb for this subjective performance metric), and a service ceiling of 15000 ft (book).

RetiredMan93231

@BernieV , In the real world Arrow III what is the expected RPM / power level on final at 75 knots with full flaps? It seems to me that the landing gear drag and the flaps drag are both currently a little too high, requiring too much power (RPM) to maintain the glideslope on final. What are your thoughts?

BernieV

@RetiredMan93231 Between 15 and 17 inches of MP, prop level full forward, mixture full rich (unless operating at high elevations)

set3times

Thanks BernieV, will try these values!

BernieV

@set3times No problem. The last (3rd) notch of flaps is almost all drag. In real life, I delay using it until I am sure I'd make the airport if I lost power or unless I was high/fast on the approach and need to get down or slow down.

copper

@BernieV Just tested your settings and it feels SO much better now :) Thank you for your research, I can only imagine how much effort that is.

Let's see what JF brings in a future update, but until then I'll be using your setup! The EGT is a bit odd but I'll go with the fuel flow gauge, as you said. That works fine.

set3times

I have tested those values but I have an excessive RoC at around 1300fpm at 90knots at 2200lbs. (sea level)

EDIT: I just noticed I had a rapid headwind increase during initial climb-out, so disregard last transmission, will test in calm conditions ;)

sdvpilot

@BernieV
Thank you for the mods. Aircraft performance is so much more realistic as I remember it. I could not understand why it was so slow.

set3times

Tested in calm conditions with the values mentioned by BernieV but I find the RoC way too high (above 1200fpm) with full throttle and full prop (these are the values used in the performance manual reference, they might not be used in reality but that's the only reference we have).

BernieV

@set3times Are you testing at full gross weight (2750 lbs)? I suspect not.

copper

Just a quick question before I open a ticket, maybe someone can confirm this issue since the situation is basically impossible with the stock performance anyway:
When flying straight leveled with autopilot HDG and ALT HOLD on, say at 5000ft with full throttle and rpm, once reaching the yellow range of the IAS scale, my aircraft starts to jitter up and down with a very small deflection and high frequency.
From the cockpit you can see this by the sun visors rattling and from the external view you can see the whole aircraft jitter/rattle with very tiny deflection and high frequency.

It is almost like it's an overspeed indication but the way it looks it's more like a glitch.

RetiredMan93231

I have also noticed this "buffeting" behavior when the flaps are down... I think JF should look into it, so please open a ticket.

set3times

@BernieV You are right. The sim resets the weights at each flight reset and I keep forgetting it. The performance now is very close to AFM numbers, thank you.