Pitch Vertical Speed Relationship

[mzwijacz]

Has any one documented the relationship between pitch angles and vertical speed in decent at various speeds? Our attitude indicators are measured in increments of 2.5 degrees, I believe. I've been lax in verifying this relationship. Getting tired of chasing the VS needle.

Thanks,

[Kent Ashton]

a one degree pitch change will lose 100 ft in a nautical mile.    So if you are 50 miles from a sea level airport at 15,000 feet and want to arrive at 1000 agl pattern altitude, pitch down (14000/100)/50 = 140/50 = 2.8 deg.

But I think it’s more useful to use your estimated ground speed to compute a vertical speed.   For example 150 KGS = 2.5 nm/min.  So to lose 14000 ft at 500 fpm vertical speed (a nice rate) it’d be 14000/500 or 140/5= 28 mins.     28 mins X 2.5 nm/min is 70.    Start down 70 miles out and hold 500 fpm. 

Or just use the “descend now” feature on ForeFlight. :-)

[mzwijacz]

Kent,

That is exactly the quantitative explanation I need. I was thinking about approach descents. I'm down right now replacing a fuel pump. BTW I don't use ForeFlight, I use Avare. It is less capable, but doesn't require a yearly subscription or Apple hardware. 

Truth be known, I'm having a hard time finding a use for a tablet. Most of what people use a tablet for, is in my Dynon Skyview. I flight plan with my laptop or workstation in my office at the hangar. It frequently seems in the way, even on a knee board. I don't have a right seat in my Long eze. Right now, I'm seeing it as a battery powered emergency backup.

Thanks for the help.

 

[Marc Zeitlin]

3 hours ago, mzwijacz said:

That is exactly the quantitative explanation I need. I was thinking about approach descents.

So I think that you and Kent are talking about two different things. Your original question is ambiguous. Are you asking about AOA? Are you asking about longeron angle with respect to the horizon? Something else?

The relationship between any of those things and descent speed/angle will be completely dependent upon power settings as well as drag devices being used and IAS.

With power at idle, the AOA vs. descent rate will NOT be a linear function, as the AOA is a function of V^2 and on either side of the minimum sink rate speed, the sink rate will be higher.

The fact that your Attitude Indicator is demarcated in degrees shows your angle with respect to the horizon, which is NOT AOA. Slow way down to barely above stall speed, and your deck angle will be very high, and your descent rate will be very high. Now push the nose over and go faster than minimum sink speed and get to the same decent rate that you saw just above stall speed - your deck angle will be a LOT lower, but your decent rate will be the same.

I'd argue that your question is not useful - the relationship between deck angle and descent rate is not something that means anything. What is it you're trying to determine?

If you're trying to determine how to pitch for a constant descent rate in an approach at idle throttle, I've set my zero on the EFIS horizon for 90 KIAS with the throttle at idle - this is a good speed for our planes on an instrument approach, as it allows for easy timing calculations, while giving margin on either side to slow down or speed up as required by ATC or wind, and allows for lowering the descent rate by slowing to best L/D or min sink speed as required.

[mzwijacz]

Marc,

<If you're trying to determine how to pitch for a constant descent rate in an approach>

This is what I am trying to do. I'm trying to create a map of airspeed/pitch/RPM. I was hoping to get some starting points. It's still a bit cool up here in the mountains, so I have a shorter window than most to do testing. I'm getting too old to be cold in the airplane 😃 all the time.

 

[Marc Zeitlin]

4 hours ago, mzwijacz said:

This is what I am trying to do. I'm trying to create a map of airspeed/pitch/RPM. I was hoping to get some starting points.

There's no reason to concern yourself with "pitch", in this case, particularly when what the EFIS displays is the relationship to the horizon, not the AOA. The object in an approach is to be on a 3 degree (approximately) glideslope at a reasonable IAS. For our planes, that IAS is about 85 - 90 KIAS, and with power pulled to idle, nose gear extended, LB retracted, rudders in trail, and no wind, you'll be just about there. What's the pitch angle? Who cares? Just set your EFIS pitch adjustment so that with those conditions, the pip is on the horizon.

See my commentary above on why you're not going to be able to create any sort of useful three dimensional map that'll give you any more utility than this simple rule of thumb.

The problem is that it's non-linear in all axes...

[Quinton O]

Do you have a VSI? I assume that a Dynon Skyview would have such a feature.

A 3 degree glide slope (typical for ILS approach) is 304 ft/nm, and 60 kts is 1 nm/min. Using that you can calculate that at 90 kts your descent rate should be 456 ft/min on your VSI, which I think is pretty comfortable. If you're feeling the need for speed, or you're cruise descending a bit further out from the airport you could do 120 kts and descend 608 ft/min. Or just round it to 600 😄

You can also use that 304 ft/nm figure to calculate where you should start your descent, and/or to crosscheck what altitude you should be at given your distance from the airport. ~3000' above aerodrome elevation at 10 miles, ~6000' at 20 etc.

I used to have to sketch this out for my ATC students all the time as they would try to vector aircraft too high and too close to the airport for the ILS, forcing interception of the glide slope from above. A fighter pilot might not mind so much, but grandma is gonna toss her cookies on that passenger flight because her pilot got vectors to intercept at 10 miles at 6000 feet...

[mzwijacz]

Thank you all.