Proper shortfield-softfield technique

A place to relax and discuss flying topics.

Moderators: GAHorn, Karl Towle, Bruce Fenstermacher

Post Reply
flyboy122
Posts: 324
Joined: Mon Apr 21, 2014 2:30 am

Proper shortfield-softfield technique

Post by flyboy122 »

Hi All,

My neighbor has a grass strip that is relatively soft with some trees at the end. As I took off the other day, it got me thinking about proper short field and soft field technique for the 170. My airplane has the Avcon conversion, so it's kind of hard to get in trouble with either....but that just leads to complacency which tends to bite. Hence I've been really trying to hone my technique.

What I was always taught for soft field was flaps (I used 20 in my B model), horse it off, let it build to normal rotation speed in ground effect, then fly away normal. For short field it's roll till Vx, then rotate and climb at Vx until obstacles are cleared and transition to Vy. For a short/soft field, I've used flaps 20, horsed it off, let it accelerate in ground effect to Vx, then climb at Vx until obstacles cleared, dump flaps, and accelerate to Vy. Questions:

1. With flaps 20 in ground effect, my plane does not seem to accelerate. It takes forever to hit Vx. Then I get short zoom and that's it. I played around and it seems to be better if I just let the plane gently climb while accelerating, however that's not book technique, so I wonder if I'm missing something?
2. Should I be dumping the flaps sooner? I tried dumping the flaps in ground effect. At flaps 10 it seemed to help, though at flaps 0 I got a healthy drop and it wasn't very comfortable.
3. Should I try using flaps for a pure short field (not soft) take off? If so, how much? Vx is lower with flaps, though I'm not sure that would result in getting me over the trees sooner.
4. If I try to hold the plane on until Vx, it becomes really, really squirrelly. Like hard to control squirrely. Is this normal, or do I need my to check my gear alignment? Usually I just pop it off and let accelerate in ground effect, but should I gut it out?

I don't have a ton of time in my 170, and unfortunately never got any type specific training. I'm comfortable in the plane, and feel like I'm getting a lot out of it, but I don't feel I'm getting everything out of it.

Thanks,
DEM
User avatar
daedaluscan
Posts: 497
Joined: Fri Apr 20, 2012 7:03 pm

Re: Proper shortfield-softfield technique

Post by daedaluscan »

I always start with zero flaps, then pull in more than 20 with back yoke to get off the ground as soon as I see a little over 40mph, then accelerate in ground effect with 20 flaps locked. It seems to go straight up, but it is not really flying yet. I just installed a Sportsman STOL cuff and this seems to work at even lower speeds, really just when the airspeed starts to bounce off the stop. I have an o-300.

Whether I am really gaining anything I do not know, but I do this almost all the time if I am on my own and do feel that it is the fastest way to get the wheels off the ground and then use low ground effect to accelerate. Its also really fun.
Charlie

1956 170B C-GDRG #27019
counsellj
Posts: 420
Joined: Sat Apr 30, 2005 2:58 pm

Re: Proper shortfield-softfield technique

Post by counsellj »

Realize this is just my technique, but for my stock O-300 and wing it gives me my best performance. I start my takeoff with flaps 10, when she is ready to fly I rapidly pull 30-35 flaps to break the ground. Once you get airborne, milk the flaps down to 10 degrees while staying in low ground effect to aid in both acceleration and climb. Practice, practice, practice.

Here is the link to a video showing this technique.

https://vimeo.com/251941123

https://vimeo.com/248242153
User avatar
Joe Moilanen
Posts: 596
Joined: Sun Dec 28, 2003 5:45 am

Re: Proper shortfield-softfield technique

Post by Joe Moilanen »

I've got an 0-300 and a Sportsman as well. When I take off from my 650' strip with trees at the end, I start out with no flaps, fairly neutral elevator, and when I reach VX I grab 20 degrees of flaps and start breathing again. I've tried it different ways and this seems to work best. My strip is real firm and I keep the grass cut short. When I take off from soft strips in similar conditions, I keep the tail low to ensure a "light footprint" and therefore less resistance from the surface. Keeping the control surfaces (flaps and elevator) streamlined in your take off roll allows the prop blast to do its thing without any blockage.

Joe
4518C
flyboy122
Posts: 324
Joined: Mon Apr 21, 2014 2:30 am

Re: Proper shortfield-softfield technique

Post by flyboy122 »

Guys,

Thanks much for the advice! With the 180hp and constant speed, getting off the ground is generally not a problem. The issue seems to be in ground effect, where the plane just lags.

DEM
User avatar
GAHorn
Posts: 20967
Joined: Fri Apr 12, 2002 8:45 pm

Re: Proper shortfield-softfield technique

Post by GAHorn »

A couple things to keep in mind:
Flaps hurt your climb... but reducing flaps prior to the obstacle will hurt even more. So whatever flap setting is chosen for takeoff should be left alone until the obstacle is cleared.
Cessna recommends a "tail low" takeoff technique for rough, short, and soft fields. 20 degrees flaps are recommended for short/soft fields ...UNLESS density altitude reaches/exceeds 4K' ...where flap will increase takeoff distance. (See the performance chart in the Owner's Manual and AFM.)

My personal technique for "tail low" is: Neutralize elevator. (This is an easy thing to do....just look back at the tail and level the elevator with the horiz. stab. I've found this places my yoke directly beside the doorpost.) Keep this elevator postition until lift-off. (When the elevator/stab generate sufficient lift it will raise the tailwheel off the ground...just a few inches. Now the tailwheel is out of the grass/mud and that drag is dispensed.) This also provides a postitive angle-of-attack for the wing to generate max lift. (Raising the tail further results in a reduction of lift and lengthens ground-run.) When the mains come off the ground, remaining in ground effect until 67 (Vx) then allowing the airplane to climb at Vx until obstacles are cleared and then retracting the flaps. (BTW, 67 is Vx clean. I've come to the conclusion that with 20 flaps deployed that approx 62 mph achieves Vx with flaps 20.)
This all works in my B-model. Although I have a 10-degree flap selection available...Cessna claims that 20 flaps and 10 flaps have equivalent performance results. This makes me wonder why they added the 10-degree option in later airplanes.
However, I've come to the conclusion that short ground rolls are better done with flaps 20...and short total takeoff distance (obstacles involvefd) the airplane has less climb penalty with flaps 10.
Again these comments are relative to my experiences in a B model.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
User avatar
gfeher
Posts: 571
Joined: Fri Sep 26, 2014 9:19 pm

Re: Proper shortfield-softfield technique

Post by gfeher »

To add to the debate, here's an interesting AOPA video on the subject that came out about two years ago. You may have seen it already. A C-180 is used, but I think there's enough similarity to make us think.
https://m.youtube.com/watch?v=M7J-Trxb0CA
Gene Feher
Argyle (1C3), NY
'52 170B N2315D s/n 20467 C-145-2
Experimental J3 Cub Copy N7GW O-200
counsellj
Posts: 420
Joined: Sat Apr 30, 2005 2:58 pm

Re: Proper shortfield-softfield technique

Post by counsellj »

With all due respect that AOPA video is complete BS if you really want to fly to the max of your plane. That video is the safe 80% answer for those that just occasionally want to operate their aircraft in a STOL type of environment or operation. However, go talk to or watch those of us that fly off really short stuff, sub 500 foot surfaces and/or compete in STOL competitions, and you won’t see those techniques used.

Also realize, the techniques in the POH are not necessarily the best or most effective technique. They are a compromise of the best technique that the company lawyers feel comfortable publishing for the average pilot to be able to safely and consistently reproduce. My .02


Sounds like we need a 170 STOL competion/demo at Cody.

Jughead.
User avatar
GAHorn
Posts: 20967
Joined: Fri Apr 12, 2002 8:45 pm

Re: Proper shortfield-softfield technique

Post by GAHorn »

I'm not convinced it's "complete" BS.... but it does not qualify as a fair comparative because they change flap settings in a comparison of "tail low" vs "3-point". If they'd kept the same flap setting in that demo, I suspect the "tail low" would win. (I.E., in one takeoff they use Flaps-10.... then they use Flaps-20 which they state made the shorter ground-roll.... DOH! They invalidate the test by changing techniques in that comparison.)

I would personally expect flaps 20 to result in shorter ground runs than flaps 10... but the flaps 20 would also penalize any climb to an obstacle more than would flaps 10.

One of the most complicated performance charts I've ever witnessed is one which compares TORA with TODA ....and compares both to ASDA and that is the one produced by the British. (They have a knack for complicating matters anyway.) :lol:
TORA is "Take Off Run Available" while TODA is "Take Off Distance Available" while ASDA is "Accelerate Stop Distance Available." The former is basically considering only the ground-run while the latter also considers an obstacle clearance problem. They spend considerable effort in trading ground-run against obstacle-clearance within a prescribed runway/stopway/clearway length available. (The use of ground-effect to build speed before the climb past an obstacle is basically doing the same thing... the ground-effect portion is comparable to a "clearway" and/or a "stopway".)
Despite the complicated British graphs,... it's a good way to evaluate aircraft field performance.
But the KEY to every comparison is consistent technique and any change in technique adversely affects predictable performance.
All that is a long-winded way of saying that unless you use the factory technique...then factory performance charts become fiction.... and the factory has already spent the time with perfect airplanes flown by professional test-pilots to achieve the best performance they can print. Any local pilot's attempts to improve on it by using odd techniques ("popping-in" flaps, etc) therefore invalidates the comparison.
At risk of irritating those who recommend that technique, I'll point out that each time that technique is used ...is a unique event... and therefore not comparable for prediction purposes. (And the short-field videos that are so often quoted as "proof" of such-and-such technique are invalid also, because invariably extremely light weights, special powerplants, tires, STOL-kits, etc are involved that cannot be relied-upon by the average guy trying to get his personal airplane out of a hole somewhere. What that average guy needs is a prediciton-chart he can rely upon ...and that is factory techniques and factory comparisons.... which involve standardized variables such atmospherics, weights, powerplants and airframes... Something the average guy can actually use day in/day out.

But it IS fun to watch, heh?
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
User avatar
KG
Posts: 493
Joined: Thu Feb 09, 2006 5:14 pm

Re: Proper shortfield-softfield technique

Post by KG »

I remember that video from a couple of years ago. I thought then, and still do, that there is a reason ALL of the participants in the short field contests raise the tail. For some of those guys it's like the Superbowl.... they practice all year for their chance to shine. I suspect they know what they are doing and, again, they ALL raise the tail. Every time.
53 170B
counsellj
Posts: 420
Joined: Sat Apr 30, 2005 2:58 pm

Re: Proper shortfield-softfield technique

Post by counsellj »

The tail is raised, because there is a very slight amount of forward kinetic energy obtained as the tail is lowered and the aircraft rotated around the cg. There is also a slight increase in the AOA that can be obtained as the tail strikes the ground with enough energy to compress the tail springs. Both are very small gains, but guys believe in them.
counsellj
Posts: 420
Joined: Sat Apr 30, 2005 2:58 pm

Re: Proper shortfield-softfield technique

Post by counsellj »

gahorn wrote:I

One of the most complicated performance charts I've ever witnessed is one which compares TORA with TODA ....and compares both to ASDA and that is the one produced by the British. (They have a knack for complicating matters anyway.) :lol:
TORA is "Take Off Run Available" while TODA is "Take Off Distance Available" while ASDA is "Accelerate Stop Distance Available." The former is basically considering only the ground-run while the latter also considers an obstacle clearance problem. They spend considerable effort in trading ground-run against obstacle-clearance within a prescribed runway/stopway/clearway length available. (The use of ground-effect to build speed before the climb past an obstacle is basically doing the same thing... the ground-effect portion is comparable to a "clearway" and/or a "stopway".)
Despite the complicated British graphs,... it's a good way to evaluate aircraft field performance.

George's discussion of TORA and TODA is a good argument for determining performance on a nice smooth, consistent surface. And as he correctly stated, when operating off-airport and from non-paved strips, the surface is most often varied in its surface condition, drag etc.

Once one has the experience and skill necessary to operate out of the very short and unique conditions of off-airport, short surfaces, the ability to accurately predict one's aircraft performance is key to safely operating from them. I learned by operating off of longer landing surfaces. And I still routinely pass up landing areas because I'm not 110% sure of my planes performance margins operating from them.

Conducting off-airport ops, the biggest threat is often the surface condition and its associated concerns; ie soft spots, wood, throwing rocks, etc. Therefore, we want to get airborne, even if only a few inches, as quickly as possible.

I'll agree with George, that the book answers and techniques are the best place to go for most pilots and most operations.

Don't worry George. You wont offend me with a fun, education debate of flying techniques.
User avatar
TFA170
Posts: 132
Joined: Tue Jan 26, 2016 3:18 am

Re: Proper shortfield-softfield technique

Post by TFA170 »

Understand the difference between AoA and airspeed. Know your plane. Practice your plane. If you've got one place you operate into/out-of that's challenging, don't assume what works there is best for all situations. Every takeoff is unique. Every landing is unique. Yes, you can standardize a lot of it, but fly the plane...always.

There are some good videos here:

https://backcountrypilot.org/photo-and-video/stol-tips

The POH is where you learn the basics. After that, every modification of your plane or deviation from stock will affect your outcome...be as consistent as you can.

For short field, I like to set 10 degrees flaps, get the tail up, once airspeed is alive and over 40, pop flaps and increase AoA with elevator (because my tail is up), push over to remain in ground effect as I milk flaps down to 10 and climb out at Vx. My non-factory ASI goes down to 20 (and is accurate), so it's nice to see movement before 40...

For soft field, I like to set 20 degrees flaps, neutral elevator, and fly off.

For soft & short - it depends - which is worse, the softness or the shortness! :lol:
User avatar
ghostflyer
Posts: 1390
Joined: Mon Apr 21, 2008 3:06 am

Re: Proper shortfield-softfield technique

Post by ghostflyer »

I have read and reread what every body has said in the last lot of posts and I think you are all right in some way . Why??? Because every aircraft is different and it’s a practice and practice situation and you must get to know your own limitations and the aircrafts limitations . But stay with in the rules . I often take off in swamps and found the technic different as to taking off in soft sand . All great fun but has many dangers. I have flown a friends 170a and it flys totally different to mine . Ok we have different engines but aerodynamically it’s different . Both the same animal but different stripes.
User avatar
GAHorn
Posts: 20967
Joined: Fri Apr 12, 2002 8:45 pm

Re: Proper shortfield-softfield technique

Post by GAHorn »

counsellj wrote:...Don't worry George. You wont offend me with a fun, education debate of flying techniques.
WhatMeWorry.jpg
WhatMeWorry.jpg (9.88 KiB) Viewed 21305 times
I'm always surprised when some people are offended when it is never intended so, ... why worry? :lol:
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
Post Reply