WOT Descents..Who's doing them?

[kmyfm20s]

I have 82 J and have read many of Deakin's and Busch's articles on running WOT all the time. At low altitude cruise I am becoming more comfortable with it but the descents at low RPM I'm not.  I was cruising level last night coming home from the office at 8500 feet, 2500 RPM and 8.7 GPH LOP.  When it came time for my descent I simple pointed the nose down and established a 700 FPM descent and as the airspeed would climb I would reduce the RPM.  Reducing the RPM is effective at slowing the plane down enough to prevent it from going into the yellow arch but by the time I pass through 4500 feet and I'm at 2100 RPM it just doesnt feel smooth in the plane. If I richen it up it doesn't help and it just seems to get worse as the MP gets higher and the RPM stay low. So at this point I reduce the MP and increase the RPM to get where it feels good to me again but leave the mixture alone. I am questioning if the technique is good in the 4 cyclinders vs 6 cyclinders because the 6's will run smoother. For those that fly WOT on their descent can you share your experience with us?

 

Karson

[201er]

Have you considered a more gradual descent and leaving RPM like 2200-2400? Then you don't go too fast and less of a big deal reducing MP until you're over 27"

[Bennett]

One more reason that speed brakes can be a useful tool. I just push over the nose, in cruise settings, and when the IAS approaches "uncomfortable" (to me) numbers, I just pop out the speed brakes, and everything settles down quite nicely  I'm sure some forum members would consider this "unnecessary", or "lazy and sloppy", or "poor flying technique", but they work well in this situation. If i didn't have them, there are other solutions, of course, but I do have them, like them, and use them when I think they are appropriate. I added them to my 261 conversion, where they were even more helpful in coming down from the flight levels in ATC's "slam dunk" approaches.  Frankly, I prefer to slow down well before the airport environment, and get the aircraft set up for landing early. A few minutes lost in block time doing this is not significant to me, especially in light of all things one can do to increase ground speed such as choosing the most favorable altitudes and routes, and as Jose once pointed out, leaning to avoid an unnecessary fuel stop. 

[danb35]

My descent profile is trim nose down to ~500 fpm, WOT, 2500 RPM.  If things are, or look like they could become, bumpy, I'll pull throttle to keep it out of the yellow arc, but otherwise I don't bother.  I also don't generally touch the mixture until shutdown (cruising ~ 9 gph LOP).

[kmyfm20s]

Have you considered a more gradual descent and leaving RPM like 2200-2400? Then you don't go too fast and less of a big deal reducing MP until you're over 27"

My flight profile doesn't allow for to gradual of descent, I fly from the beach (427 ft) to the desert (-25 ft) with the coastal mountian of San Diego in between. Heading east VFR 7500 feet and west 8500 feet. 2200-2300 RPM seems to be the best balance but I read their articles and they take there planes as low as 1900 RPM witch I can't and below 2200 gets uncomfortable.I think I have read other posters stating they get as low as 2000 RPM on this board.

[KSMooniac]

Why are you even reducing RPM?  Our J's cruise nowhere near the yellow arc, and there is enough room in the green arc to nose-over with cruise power settings (esp LOP) and go downhill at 1000 FPM or more.

[chrisk]

One more reason that speed brakes can be a useful tool. I just push over the nose, in cruise settings, and when the IAS approaches "uncomfortable" (to me) numbers, I just pop out the speed brakes, and everything settles down quite nicely  I'm sure some forum members would consider this "unnecessary", or "lazy and sloppy", or "poor flying technique", but they work well in this situation. If i didn't have them, there are other solutions, of course, but I do have them, like them, and use them when I think they are appropriate. I added them to my 261 conversion, where they were even more helpful in coming down from the flight levels in ATC's "slam dunk" approaches.  Frankly, I prefer to slow down well before the airport environment, and get the aircraft set up for landing early. A few minutes lost in block time doing this is not significant to me, especially in light of all things one can do to increase ground speed such as choosing the most favorable altitudes and routes, and as Jose once pointed out, leaning to avoid an unnecessary fuel stop. 

I never said speed breaks were not useful.  I just said they were not necessary.     I have them and use them in a similar way.

[ncdmtb]

What is "WOT"?

[chrisk]

Wide Open Throttle.  Throttle to the fire wall.

[kmyfm20s]

I'm am more curious about technique of slowing down using RPM as described in their articles. It just doesn't seem that the 4 cylinders are very good for this technique below 2200-2300 RPM or you need to shallow the rate of descent if available. I am not able to shallow my descent because of the short distance and high climb I need to do for my routine flights, 72 nm and 8500 ft of course unless I do cicles. Since I do fly over the coastal mountians I do experience short period of light to moderate turbulence which is why I prefer to stay out of the yellow arch.

[kmyfm20s]

What is "WOT"?

Wide Open Throttle

[jetdriven]

Why are you even reducing RPM?  Our J's cruise nowhere near the yellow arc, and there is enough room in the green arc to nose-over with cruise power settings (esp LOP) and go downhill at 1000 FPM or more.

That's my standard descent profile. WOT, 2300-2500 RPM, and mixture at LOP cruise setting, if the FF rises at lower altitudes, say below 4000', lean a bit more to maintain 8.5-9 GPH.  You can descend at 160 KIAS this way, around 700 FPM. As you get lower, below 4000',  the speed approaches 175 KIAS, top of the green, if you are uncomfortable with that, lean it down a bit more. 

[Hank]

I cruise at (WOT-a tad) to create turbulent mixing in the carburetor. For descent, I nose over for 500 fpm and trim for hands off. As I come down, I walk throttle back and mixture forward to maintain cruise MP & EGT. This usually puts me at the top of the green arc.

First time I did this in an F from 4500 msl, it was at 190+ mph indicated. After that, I started reducing throttle to descend. But in my C, just nose her over and keep everything green.

[chrisk]

I'm am more curious about technique of slowing down using RPM as described in their articles. It just doesn't seem that the 4 cylinders are very good for this technique below 2200-2300 RPM or you need to shallow the rate of descent if available. I am not able to shallow my descent because of the short distance and high climb I need to do for my routine flights, 72 nm and 8500 ft of course unless I do cicles. Since I do fly over the coastal mountians I do experience short period of light to moderate turbulence which is why I prefer to stay out of the yellow arch.

I have not read the article, and would like to, but I don't know where it is.   Having said that, I would start with a a simple equation where is Power = Torque * RPM.  Slowing the RPM reduces the power, assuming Torque is constant.  And I would think this is slightly more efficient than using a throttle plate to limit the air / fuel intake, which also limits the power.  (Efficiency gain mostly due to air resistance around the throttle plate).     --But at some point, its probably better for the engine to run smooth.

 

Just a guess though

[Shadrach]

My flight profile doesn't allow for to gradual of descent, I fly from the beach (427 ft) to the desert (-25 ft) with the coastal mountian of San Diego in between. Heading east VFR 7500 feet and west 8500 feet. 2200-2300 RPM seems to be the best balance but I read their articles and they take there planes as low as 1900 RPM witch I can't and below 2200 gets uncomfortable.I think I have read other posters stating they get as low as 2000 RPM on this board.

 

hmmmm, My POH shows 1850RPM as an endurance setting and has performance numbers for said setting up to 17,500 IIRC.  I have not experimented with it in quite some time, but I have had the engine at 1850RPM in the past and it was not harsh at all as I recall but it was way too slow, ~110KIAS at 3500 IIRC.

[kmyfm20s]

I have not read the article, and would like to, but I don't know where it is.   Having said that, I would start with a a simple equation where is Power = Torque * RPM.  Slowing the RPM reduces the power, assuming Torque is constant.  And I would think this is slightly more efficient than using a throttle plate to limit the air / fuel intake, which also limits the power.  (Efficiency gain mostly due to air resistance around the throttle plate).     --But at some point, its probably better for the engine to run smooth.

 

Just a guess though

This is Deakin's article 4 of 4. Good reading.

 

http://www.avweb.com/news/pelican/183094-1.html?redirected=1

[John Pleisse]

Well, you can buy speed brakes. Or use your propeller governor for a speed brake. I guess those unbelievable fuel savings on decent are worth it..I just read the entire article. Interesting.

[BigTex]

General question for the group... With the exception for Hank, it looks like everyone else posting is flying behind a fuel injected engine.  Do you all see any reason the letdown procedures would be different between a fuel injected engine and an engine with a carburetor?

[jetdriven]

Not really, leave cruise mixture set, and lean to the edge of rough. At low altitudes you may need to pull the MP back to 27"

[pinerunner]

Hmm..

 

Thanks for including the link to Deakins article.  As I read it I noticed he doesn't actually necessarily insist you stay WOT the entire descent. After the initial RPM drop and allowing to CHT's to settle down, presumably just under 300 degrees he reccomends you reduce the throttle too, if you wish, to get the speed you want. This for the "cruise descent". For the "crowbar descent" he reccomends RPM down, gear down for drag, and then .. big throttle pull. And down you come ..fast.

 

The point being, as I read it, to get the descent but not drop the CHT's as quickly. No need t go rich when dropping power. That actually seems absurd. These are low power settings so detonation is not an issue.  Even though neither Deakins or Busch seem to be big believers in shock cooling, this methodology is aimed at minimizing sudden changes in CHT. It really makes most instructors eyebrows go up. I talked with my instructor about it when getting checked out in my M20E and he couldn't believe there was any sense in low RPM's for descent. Ditto with another instructor friend who said I should just drap full flaps and use more power in the winter.

 

My biggest worry if I go all the way through landing with low RPMs and leaned is what if I need to make that rare go-around. Busch points out no problem just make a big wide hand and push all the levers forward at once which works on his 310 but I've got two verniers which have to be pinched and then the throttle advanced. My internal jury is still out on that issue. I tried it at altitude and it wasn't hard to do but I'm still concerned that, in a pinch, I'll revert to old habits. Deakins says here that with the leaning suggested it won't be as lean as aggresive leaning for taxi and so it shouldn't sputter and fail just when you want power in the event you forget the first two knobs and go straight for full throttle. Hmmm. I think I'll find out; at altitude.

 

Dave

[jetdriven]

At the power reduction abeam the numbers, setting the prop full and the mixture slightly lean from full rich takes care of the go-around. Simply go full throttle and execute the go-around. Mixture and prop are already set.  I do this and I have the levers too.

[N201MKTurbo]

My flight profile doesn't allow for to gradual of descent, I fly from the beach (427 ft) to the desert (-25 ft) with the coastal mountian of San Diego in between. Heading east VFR 7500 feet and west 8500 feet. 2200-2300 RPM seems to be the best balance but I read their articles and they take there planes as low as 1900 RPM witch I can't and below 2200 gets uncomfortable.I think I have read other posters stating they get as low as 2000 RPM on this board.

Just fly along the border fence until you are east of the mountains. You can do it at 4500.

[stevesm20b]

Sounds great on paper. How do you descend at 500 feet per min with any kind of power in a "B" model or early "C" model without going way into the yellow. I'm in the yellow at cruise power when flying level below 6,000ft. The yellow arc starts at 150 miles per hour in the B model and early C models.

[BigTex]

Sounds great on paper. How do you descend at 500 feet per min with any kind of power in a "B" model or early "C" model without going way into the yellow. I'm in the yellow at cruise power when flying level below 6,000ft. The yellow arc starts at 150 miles per hour in the B model and early C models.

You should know that most of the folks that have posted on this thread have aircraft (mostly J Models) that have a much higher green arc. So even though it is interesting reading, their technique doesn't always translate to the C model. In reality you'll need to back off on the throttle to keep it in the green.

[jetdriven]

Its the same wing as a J, and the yellow arc is not a limitation. In fact, I cant find any evidence of an accident (outside a thunderstorm) which resulted from flying in the yellow arc, below redline airspeed.  My friend's M20E cruises in the yellow too. And I descend in it at ~180 MPH IAS as well.

EDIT, I just read the TCDS, and yes the M20B has a V_NE of 183 MPH IAS. Interesting, the M20E is 200 MPH.  You would probably have to limit power to 23" or so to keep the descent going and keep a reasonable margin away from redline IAS.