Slow Flight and the Red Arc

[Ibra]

On 2/3/2021 at 2:40 AM, Brian E. said:

Fellow Mooniacs - with the Winter upon us I'm on the slow train for completing my CFI training in our beloved C.  One of the challenges I am facing is slow flight and the prop no continuous operating range (65C, B hub, 2 blade hartzell, 2000 - 2250 rpm red arc).  If I recall correctly, configuring the plane for  75 MIAS (1.3 * VSO), gear extended, full flaps and prop full forward requires ~16.5 or 17 in. MP giving me just about  2150 - 2200 RPM on the prop--solidly in the red arc.

  When I did my commercial training in the Mooney I never hung out in that zone, I just transited it on the way to power off stalls.  Now I'm having to instruct mock students on slow flight and find that it requires sustained flight in the red arc to demonstrate proficiency while in this configuration.  Anything I'm missing?  Do you just not perform slow flight training in your aircraft? 

For slow flight & stalls,

- For power OFF stalls it should be out of the prop bad harmonics arc but I tend to manage the engine before getting there

- For power ON stalls, you can still have straight & level slow flight with max 100% power or ideally on 55% power but it may require slower flying on very positive pitch & high power first maybe after slowing down on steep (turn) climb first than starting with descent or level flight states

A more challenging task is to leave it at 100% power (WOT throttle & max RPM) and ask the student to fly it straight & level in slow flight for a long period or even on -500fpm descent without even reducing the throttle, this requires a steep climb first (maybe on 20deg turn to drop nose quickly without gaining height) then lot of pitch & rudder coordination to keep it that way on straight & level/descent ! 

Long story short, you don't need to slow engine to fly slow airspeed in straight & level (basically hang it on max prop and slow speeds from a steep climb first)  

If the stall horn wiggles at VS+30kts or 6 deg AoA, get it fixed to VS+5kts or 1deg near max AoA it will be taken far more seriously rather than getting ignored everytime !

PS: bonus point at least one knows what to do if trottle get jammed after takeoff and/or prop governor fails on fine pitch as long as they keep the ball in the middle they can even fly it (slowly) to destination, without climbing to no-oxygen levels 
   

Edited February 11, 2021 by Ibra

[Nukemzzz]

I too noticed the OP's concern while practicing slow flight last week in my E.  My instructor still likes flying around with the stall horn on some and so that's what we were doing.  In the clean configuration, around 13in MP, is what my plane needs for slow flight like this at 5,500ft.   This has the Tach right in the "no sustained operation" section on the.  We had a conversation about if this is "sustained" or long enough to worry about...

Strictly speaking I think slow flight here is sustained...it's not passing through it. Is 10 minutes here a few times a year an issue?  Only people with the fatigue analysis know this.  I suspect that this isn't a real issue when at 13in MP and the real concern is trying to cruise here at 50% power or more.  I'm thinking of this area as a "don't cruise here" area.  But I guess they should have said that if this is what was meant.

[Ragsf15e]

1 hour ago, Nukemzzz said:

I too noticed the OP's concern while practicing slow flight last week in my E.  My instructor still likes flying around with the stall horn on some and so that's what we were doing.  In the clean configuration, around 13in MP, is what my plane needs for slow flight like this at 5,500ft.   This has the Tach right in the "no sustained operation" section on the.  We had a conversation about if this is "sustained" or long enough to worry about...

Strictly speaking I think slow flight here is sustained...it's not passing through it. Is 10 minutes here a few times a year an issue?  Only people with the fatigue analysis know this.  I suspect that this isn't a real issue when at 13in MP and the real concern is trying to cruise here at 50% power or more.  I'm thinking of this area as a "don't cruise here" area.  But I guess they should have said that if this is what was meant.

Try a different configuration.  
1. Clean

2. gear down, flaps up

3. gear down, t/o flaps 

4. gear down, full flaps 

 

one of those will likely get you out of the rpm band in level slow flight.

[Immelman]

I can't help but just wanted to chime in to jump on and say the FAA sucks and is wrong with this one (when they changed how slow slow flight should be).

Done the right way, stall horn blaring, an occasional buffet, right on the edge, the induced drag is sufficient to avoid the red RPM arc.

And I agree about the 99 knots thing. I mean a vintage mooney cruises in the yellow if you want to. What is "slow". Sorry. Ranting!

Edited February 11, 2021 by Immelman

[Hank]

2 hours ago, Immelman said:

And I agree about the 99 knots thing. I mean a vintage mooney cruises in the yellow if you want to. What is "slow". Sorry. Ranting!

My traffic pattern speed is 90 mph. See the warning in my Owners Manual: 

[0TreeLemur]

14 hours ago, Raymond J said:

The red arc mainly concerns the two-blade "Hartzell" propellers, sensitive to a "brineling" of the blade feet below 2350 rpm. This is a well-known fatigue problem. Tri-blades whose harmonic rank H3 is out of resonance with the motor H3 are not affected. The "MC Cauley" two-blades have a slightly lower H3 level than the "Hartzell".

The Hartzell top-prop has no rpm restrictions on the C, and only at higher MPs on the E&F.  

[Brian E.]

58 minutes ago, 0TreeLemur said:

The Hartzell top-prop has no rpm restrictions on the C, and only at higher MPs on the E&F.  

So I get a new prop since this one is "defective"?  I'll tell the CFO ASAP and get one on order

[Tcraft938]

You are in an interesting position, one I have been in.  Just remember when you do your CFI checkride, some examiners are testing you as much on knowing regs and the POH as they are airmanship.  So for example, he/she says "lets do slow flight at 1.3 Vso, and you do a marvelous job but he/she frowns at you as they say tell me about this thing on the tach and the story goes on and dominos fall.  You are also demonstrating to them that as a CFI (candidate) you are PIC and you know the plane well and won't do manuvers you shouldn't.  Here's how I handled it, not saying it's the correct way, just saying I did it and the particular examiner was good with it......   At the end of the oral portion of the checkout just prior to going to the plane, I simply showed the examiner what the POH said about limitations and said, "I obviously don't know at this time what you intend to have me do up there, but I feel it important to cover with you any limitations the aircraft might have so that it does not appear while flying that I am disagreeing with you or refusing to do something.  For example, this prop limitation and my experimenting has revealed that 1.3 Vso puts me right in that arc, so if you don't mind, I propose that if we do slow flight we do the following......  For our estimated weight and conditions at 4000' or above if we fly around 1.1 Vso that will likely put the rpm on the edge or even into the green".  The examiner looked perplexed, read the POH then seemed please.  "I did a checkride in a Mooney J a few months ago and didn't know the older ones have this limitation, thank you for bringing to my attention.  I agree fully with your plan and I recommend we add that other than landing/approach we limit any demonstration that places us in the red to less than one minute".  He was a real gentleman and professional about it.  We did slow flight for about 30 seconds which acutally is pretty long and then he turned it into vertical S's where at 30 seconds, "okay maintain this speed and climb 100'", which meant power  and out of the red.  Then he modified his protocol where he had me add or decrease power to get it out of the red but maintain airspeed.  If that meant we were climbing or descending say 50 fpm then he called that "our demonstrated level", so as long as you don't wander more than 50 fpm +/- from that I'm calling that stabilized.  Our goal is not to necessarily fly the manuver exactly per book, but fly to the airplane and within the prescribed parameters and tolerances".  

I passed the checkride and as we walked from the plane he turned around and stared at the plane and he made me laugh with, "that little bird is a good girl and very angry with us right now.  She was made to somehow have tame stall characteristics or go fast and efficient, not this slow and dirty crap, but she complied like a lady.  Partially because you thought about her needs and took care of her, but she's still angry so I suggest on the way home, take her high and WOT and reward her with stretching her wings as intended by Mr. Mooney".  He made a photocopy of the POH page and put in his notes to check on all future Mooney commercial and CFI checkrides.  

So it worked okay for me.  My suggestion is just be upfront and show documentation to the examiner.  Remember they can get in a lot of trouble (especially if they are FAA employee) for trying to get you to do something the POH says or recommends not to.  

[PT20J]

2 hours ago, Tcraft938 said:

So it worked okay for me.  My suggestion is just be upfront and show documentation to the examiner.  Remember they can get in a lot of trouble (especially if they are FAA employee) for trying to get you to do something the POH says or recommends not to.  

That's how I would handle it.

The red arc is likely considered an operating limitation (the TCDS calls it a restriction). It is a violation of FAR 91.9 to operate in violation of a operating limitation, so operating there continuously during a practical test should be a bust.

The DPE cannot ask you do something that the airplane is not equipped to do. But, you are supposed to supply an airplane that will allow you to perform all the required operations. Since this is a bit of a gray area, discussing it with the DPE before the test would be my approach.

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