Hangar Flying => Hangar Flying => Topic started by: marc22181 on January 01, 2008, 11:14:35 AM
Title: Lowering Stall Speed
Post by: marc22181 on January 01, 2008, 11:14:35 AM
I'm looking for ways to reduce approach speed and runway requirements. Has anybody had success with lowering the stall speed of a longez? Micro vortex generators claim a 9 knot reduction, but if that's real, I'd think lots more folks would have them installed. It would be interesting to hear if there are any ideas or better yet history with devices or airframe changes to improve low end performance.
Marc 770EZ
Title: Lowering Stall Speed
Post by: Waiter on January 01, 2008, 12:18:06 PM
What is the target speeds are you looking for??
Waiter
Title: Lowering Stall Speed
Post by: Drew on January 01, 2008, 12:36:33 PM
There are all kind of articles in CSA about installing trailing edge fences. You need to build them bigger than on a Varieze. They will allow you to maintain roll response at lower speeds. In my longez, the first thing to notice about going too slow was a dangerous lack of roll response---while maintaining good pitch authority.
Title: Lowering Stall Speed
Post by: marc22181 on January 01, 2008, 06:08:13 PM
RE: how much reduction am I looking for...I'd be happy with 5 kts, very happy with 10. Marc 770EZ
Title: Lowering Stall Speed
Post by: Waiter on January 01, 2008, 07:26:37 PM
What Speed are you looking for;
i.e. my speeds are typically; Approach at 100 KIAS, cross fence at 85 KIAS, touch down at 75 KIAS.
On rare occations I can lower the speed and touch down at 60 KIAS. HOWEVER, This places the plane in the very lower left corner of the envelope and leaves absolutly no margins for error.
Waiter
Title: Re: Lowering Stall Speed
Post by: Jack on January 01, 2008, 08:49:28 PM
Quote from: marc22181
I'm looking for ways to reduce approach speed and runway requirements.
A good approach starts on downwind and being on speed. In my Long I use 75KIAS and bring it around to short final and touch down at 60-65 KIAS. Stall is about 54 KIAS. Main problem with a slick bird like the EZ series is keeping control of your airspeed. You drop the nose a tad and it's an extra 10 knots.
IMO, before you start adding devices, ensure your airmanship is up to par. Less speed on touchdown will translate to less rollout.
This is actually the first bird I have flown where I use the brakes to stop on the runway. The other birds I have flown range from single seat jets up to 4 motor birds.
Good Luck!
Jack
Title: Lowering stall speed
Post by: Bruce Hughes on January 02, 2008, 02:34:20 AM
Those who have replied to your question are ASSUMING the following:
Your CG is exactly in the right place.
If that is not true, you have a very big problem at slow speeds.
Maybe you should stay away from slow speeds.
Bruce Hughes
Title: Lowering Stall Speed
Post by: marc22181 on January 02, 2008, 08:09:08 AM
I very much appreciate the comments from all. My rationale for posting this was primarily safety and secondarily making smaller fields available. In the first case, touching down at 50 Kts vs. 60Kts represents a decrease in energy of 31% as the energy goes as the square. In an off field landing thats pretty significant. In a short field landing, thats a lot less pressure on the brakes. I completely agree with the need for proficiency, but for a given level of proficiency, lower stall speed (AOA) is always good.
The vortex generators and trailing fences sound good. Is there a web site for the vortex gens? I've seen the fences on the verieze, does anyone have dimensions and locations for the long? Do the fences increase lift or just improve aileron performance?
Has anybody tried high lift devices like leading edge cuffs or drooping ailerons?
Marc 770EZ
Title: Re: Lowering stall speed
Post by: Jack on January 04, 2008, 02:33:17 AM
Quote from: Bruce Hughes
Those who have replied to your question are ASSUMING the following:
Your CG is exactly in the right place.
If that is not true, you have a very big problem at slow speeds.
CG in limits is the limitation and you can check it fairly easy by doing a check in flight. When you hit pitch buck and it's your target speed your CG is inside of limits.
If your out of limits you could end up in a deep stall in which case your not having a good deal.
If you hit the databases and just watch folks in the pattern. Most like to add knots for the kids, mom, and apple pie. If you want to be a precise pilot for landing, you have to control the variables and the biggest is speed.
Then I cheat, I was taught how to fly in the Navy and being on speed for landing was priority one.
Jack
Title: Lowering Stall Speed
Post by: Bill James on January 22, 2008, 12:19:34 AM
Howdy Marc-
A few more thoughts on your reduced approach speeds,
http://www.ezchronicles.com/blogger.html
Title: Lowering Stall Speed
Post by: marc22181 on January 23, 2008, 02:12:29 AM
Bill, Many thanks for your well thought out response. I agree that the stall angle of attack is pretty much fixed for a given wing(s) configuration, as you point out, increasing gross weight adversely affects stall speed. The problem is most, mine included, 0320 equipped, IFR longeze's weigh in at over 1000 lbs. With two on board and reasonable fuel, 1600lbs is pretty much the norm. That means the stall speed is well above the original longeze design. Bottom line, other than making it a one seat aircraft, lots of kinetic energy is dissipated in the roll out. Techniqe can make shorter fields available, but in a real off field emergency, you want a little extra energy and the brakes are not going to do much good in slowing you down. In that situation, the airframe is the brakes.
That's basically what prompted the question. The Mike Price VGs sound interesting. Are the Price vortex generators the same as the micro VGs?
I wonder if anybody has thought about a drooped leading edge like all Cessna 172s went to a few decades ago, or for that matter some kind of flap or drooping aileron configuration.
Some years ago, California Microwave and another outfit (can't remember their name) built up some LEs for use as UAVs One of them had extended wings for high altitude flight. That would be a pretty extensive retrofit. That is unless there was some value in extensions beyond the rudders.
Any further thoughts?
Title: Lowering Stall Speed
Post by: Juan Rivera on January 24, 2008, 09:42:20 PM
I am suprised that no one has said that if you just fly a 70 knot approach and landing speed, you can land at a much shorter airport than you can take off from...
Title: Lowering Stall Speed
Post by: Jack on January 25, 2008, 03:12:54 AM
Quote from: Juan Rivera
I am suprised that no one has said that if you just fly a 70 knot approach and landing speed, you can land at a much shorter airport than you can take off from...
Actually I did note a slower approach speed. Tis something I see everytime I hit the airport. Folks come smoking into the runway and wonder why they eat up so much tarmac on the rollout.
I hit my approach speed and hold it while using power for glide slope. Control your variables and you will end up with a better landing. This is a slick bird and any extra knots on rollout will cost you in more burned out brakes and higher tire usage.
I knew some pilots that just kept it above pitch buck speed and landed their Longs.
Jack
Title: Lowering Stall Speed
Post by: marc22181 on January 25, 2008, 05:39:42 PM
If I approach at 60 I won't need any runway.
Title: Lowering stall speed
Post by: Bruce Hughes on January 25, 2008, 08:24:54 PM
Yes, some people who were flying Ercoupes found that out too.
They will not stall but they glide like a brick when the approach speed is too low. :(
Bruce Hughes :D
Title: Re: Lowering stall speed
Post by: Jack on January 25, 2008, 09:42:12 PM
Quote from: Bruce Hughes
Yes, some people who were flying Ercoupes found that out too.
They will not stall but they glide like a brick when the approach speed is too low. :(
Bruce Hughes :D
In all aircraft you need to get down and dirty in slow flight and see how the beast performs up at a safe altitude.
Practice it first before you find out what nasty habits have devoloped in your bird and your airmanship. Or trying to park your bird in a short field and finding out a tad late it's a bit short.
When your flying a slick bird, trying to lose excess speed is very challenging.
Jack
Title: Lowering Stall Speed
Post by: go ez on January 26, 2008, 09:56:14 AM
Interestingly, after recently fitting trailing edge flow straighteners / fences to my Vari Eze, I found during testing at different approach speeds (at a good height) with LB & Gear down that the aircraft needed less power (1500 rpm) at 65 kts to maintain 200-300 ft/min descent rate than at 70 kts (1550 rpm) or 80kts (1650-1700 rpm). This was at full gross and aft CoG.
Visibility over the nose during actual approaches at these speeds was better speed for speed than without the fences.
Take off run was reduced by about 25 -50 m
They also lowered my minimum speed from about 53kts to 48 kts and the a/c felt even more docile & controllable in roll at min speed and we saw a clear increased climb rate between 80 kts and 120 kts of between 8 -10%. Landing ground roll was less to by about 50-75m and flare at 60 kts was again very docile & controllable.
Overall the a/c felt much more solid & stable at low speeds.
Regards Steve
Title: Lowering Stall Speed
Post by: Bill James on February 22, 2008, 04:42:26 PM
Marc- On 1 January you asked this question- >>I'm looking for ways to reduce approach speed and runway requirements. Has anybody had success with lowering the stall speed of a longez?
Since then, working through a response has resulted in my coming to several forks in this road. And one mental reversal.
The fork, or roadblock, that all of us recognize has to do with the critical relationship between the wing and canard. So for fun, I am now asking you Marc, and anyone else, the simple, basic question on this landing speed issue. The words are carefully chosen:
- For acceptable landing characteristics, if the main wing is modified to provide improved characteristics and would reduce landing speed significantly, say ten mph, is a corresponding and equal modification required to the canard?
Title: Lowering Stall Speed
Post by: marc22181 on February 22, 2008, 09:34:34 PM
if the main wing is modified to provide improved characteristics and would reduce landing speed significantly, say ten mph, is a corresponding and equal modification required to the canard?
Bill, It depends. Lets talk stall speed, but its really angle of attack that makes the difference.
As a thought experiment, lets say you make the wing stall at 10 kts. The canard still stalls first-at 50 kts or so, so the possibility of a deep stall is avoided. Now in effect, the aircraft stall speed remains limited by the canard, but a wing with substatially reduced stall speed should produce greater lift at slower speed than the same untreated wing. Therefore the approach speed can be reduced a bit (still 50 + reasonable margin) without experiencing the sink you might currently get at very slow approach speed. On the other hand, to truly take advantage of whats done, the canard stall speed should also be reduced, but never below that of the wing. That can be done in several ways. The two easiest are changing the canard angle of incidence or adding lift enhancing devices ie. vortex gens, flaps, slats etc. The micro vortex generators treat both the wing and canard. I'd be interested in any first hand experience with them.
Quite frankly, I'm very reluctant to play with the canard angle of incidence. I think one could get into very serious trouble with that.
Title: Lowering Stall Speed
Post by: Bill James on February 27, 2008, 09:34:52 AM
Interestingly, the answer to your initial question about lowering landing speed is yes. This has been done.
A main wing has been modified to provide slower landing speed, successfully, with no change to the canard.
So the answer to my question, >>If the main wing is modified to provide improved characteristics and would reduce landing speed significantly, say ten mph, is a corresponding and equal modification required to the canard?<< in at least one example,
-- is no.
Title: Lowering Stall Speed
Post by: Bill James on March 06, 2008, 09:14:28 PM
Marc- Another low pass over your lowering stall speeds question, thinking about slats and flaps...