Airspeed Bug

[soarhead]

Does anyone know how to change the limits of the airspeed bug? I can't select anything below 72 kts. I don't see anything in either the install manual or pilot guide and nothing obvious in the setup menus. The only thing I haven't tried is checking to see if the lower limit of the bug is associated with best glide speed.

 

[HFMan]

I know that if you have autopilot, you can't set the airspeed bug any lower than your minimum pitch airspeed in the autopilot setup.

 

[soarhead]

Yeah, I’m trying to bug Vapp without the autopilot.

 

[RV8JD]

There is one Minimum Airspeed defined (must be ≥1.3Vs1). From the Installation Guide, Pages 10-21 & 10-22:

"The minimum airspeed parameter must be set to at least (and defaults to) 30% above VS1, which should be set to the specifications of your aircraft in the Airspeed Limitations Menu (reference page 5-18 for more information) and is specified in the units set in the Measurement Units Menu (i.e., miles per hour, knots, or kilometers per hour; reference page 3-9 for more information regarding configuring the measurement units). As previously mentioned, the minimum value for this parameter is 30% above VS1. This is also the default value.
To set the minimum airspeed:

1. Go to the Pitch Axis Minimum Airspeed Adjust Page (SETUP MENU > AUTOPILOT SETUP > PITCH AXIS > MINIMUM AIRSPEED).
2. Adjust the minimum airspeed.
3. Press ACCEPT to save the value or press CANCEL to return to the Roll Axis Menu."

You could put in an erroneously low value for Vs1 in the Airspeed Limitations Menu to allow the selection of a lower Minimum Airspeed value, but that would also cause an erroneous color band on the IAS speed tape. And if you wanted to use the AP, it would allow the AP to work or engage at a dangerously low airspeed, with little or no maneuver margin. I wouldn't recommend obviating these safety features.

 

[soarhead]

Actually, 1.3 VS1 is exactly what I am trying to achieve but not with the autopilot, just the bug. My VS1 is set as 48 kts yet the bug cannot be set below 72.

i’ll take a look at your reference and try that to see if does anything. Thanks!

 

[RV8JD]

Could there be an airspeed units mixup somewhere, i.e., KTS and MPH? If at some point you were using MPH and put in Vs1 in terms of MPH, and later changed the measurements units to show KTS and did not update Vs1 to KTS, that might explain it.

Using your numbers: 48 KTS = 55.2 MPH. 55.2 MPH x 1.3 = 72 MPH (but the system labels the 72 as KTS). Maybe just a coincidence the numbers work out that way.

From my post above:

"... and is specified in the units set in the Measurement Units Menu (i.e., miles per hour, knots, or kilometers per hour...

 

[soarhead]

Lol….yeah I think that’s a coincidence. I am set up in knots.
I will check into what you both have mentioned regarding the autopilot. It makes sense actually. Could be a safe guard to mitigate someone in IAS mode having the bug set below VS1. Will be a couple of days until I get back to the airplane.

 

[RV8JD]

I checked mine today. My Vs1 is set to 64 MPH. The lowest airspeed bug I can set is 83 MPH (and the listed Minimum Airspeed is 83 MPH). 64 MPH x 1.3 = 83 MPH. So it seems like it's doing what it says it should for my setup.

I'm curious as to what's up with your setup.

 

[soarhead]

I think you are on to it. Just doing the math now, my VS1 should be 55 Kts and X 1.3 = 71.5, the bug is limited to 72 kts. When I get back to the hangar I'll check it out and try putting in a different VS1 value to see if that changes the minimum bug setting. I'll report back but I am now 99% sure that is what's happening and I can see why Dynon would engineer it that way. Coming from an airline background, I hate having instrument bugs that cannot be put where I want them. In this case, Vapp for landing with flap 30. The question now is, do I put an arbitrary value in the Autopilot Pitch setup to allow the use of the bug below it's current value, or just put up with it.

 

[RV8JD]

Yup, we really want a to be able set a bug at 1.3Vs0 (and just inhibit the AP below 1.3Vs1). My Vs0 speed is 58 MPH, so I'd like a bug at 76 MPH.

But in reality, in my RV-8 I normally fly final with full flaps, and in benign conditions fly at about 79 to 83 MPH with a touch of power (to offset the C/S prop drag).

 

[skysailor]

The way I did it was to re-purpose one of the provided bugs. You are given a place to input Vx (max angle climb), Vy (max rate climb) and Va (maneuvering speed) which show up as individual marks on the airspeed tape X, Y and A. While I always reference angle of attack (old habits which kept me alive coming aboard the carrier) I set the "A" bug at my 1.3 Vso for max gross weight as a backup. This now becomes "Approach" speed to me. It is a pilot only reference. The flight director and autopilot ignore it completely (just like Vx,Vy and Va). Perhaps you could consider this as a solution.

 

[soarhead]

So I managed to get to the airplane today and sure enough, in the autopilot pitch setup, was a minimum speed setting. I reduced this to my desired Vapp with flap 20 or 30, which was only a couple knots below what was there and bingo, I can now dial the bug back to what I want to see on approach. Thanks guys.