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Raising stall speed?

Old 09-12-2009, 10:45 PM
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vertical grimmace
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Default Raising stall speed?

Ok, this is going to sound a little odd. I am a flight instructor with our local club. It seems almost everyone is learning to land right now. I have one student and his father (who are both learning together) that have a Nextstar trainer. Now the problem is, this particular plane of theirs, has to be very slow, I mean really, really slow, before it stalls. I would like to raise the stall speed slightly so it opens up the stall envelope a little. This is mainly an issue when perfectly calm.
My idea is to raise both ailerons slightly. Maybe 1/8" to see if this will have an effect at reducing lift. Another idea maybe to add positive incidence to the wing. The latter would be more difficult. Honestly I am just throwing this out and have not pondered it much (I am being a little lazy).
I have been flying for 25 years and am involved in many types of flying from pylon to scale. This airplane is very hard to get slow enough to stall. I look forward to ideas on this.
Old 09-12-2009, 10:49 PM
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brad59
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Default RE: Raising stall speed?

Just make it heavier...
Old 09-13-2009, 02:48 AM
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Default RE: Raising stall speed?

Adding incidence will just require more down trim to get the trim back with no other changes. Not a good idea.

The reflexing of the ailerons may or may not help. I suspect that any change would be minimal or unnoticable.

Adding weight is the best option but it'll affect a lot of other stuff as well. You may have to live with just flying it nose high until it stalls.

Often times the engine is running at a higher idle setting to ensure positive running. But with our typical grossly over powered trainers these days even a slightly high idle may well have enough power to delay the speed reduction and eventual stall. I'd tend to start on that front first.
Old 09-13-2009, 07:40 AM
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Default RE: Raising stall speed?

As you know, the stall is a function of the AOA, and any model with enough elevator authority to place the wing beyond the critical AOA will stall, flying slow or fast.

Increase the elevator throw enough, and the thing will stall at your will.

Now, slowing the model enough, as stated earlier, may be an engine-propeller problem.
Old 09-13-2009, 08:02 AM
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Rodney
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Default RE: Raising stall speed?

The easiest solution, put on a lower pitched prop. A more elegant but effective is to build in some spoilers on the wing as gliders use. Try some split flaps. Drastic, cut off a few inches off each wing tip.
Old 09-13-2009, 11:16 AM
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vertical grimmace
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Default RE: Raising stall speed?

The engine is idling very nice and slow. I will look at the prop. It is most likely a 10-6. An 11-4 would be a good choice I think. I think adding weight would be the last option for sure. We already have a lot of elevator travel but I can look at that as well.
It just seems that if a perfect final approach is not executed, losing all of the speed is very tough. If the nose is dropped the slightest amount, the wing picks up enough speed to delay the stall.
I will experiment with it at the field, and start with the prop. Thanks for the ideas guys!
Old 09-13-2009, 11:45 AM
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Chad Veich
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Default RE: Raising stall speed?

Not sure why adding weight would be your last option as, IMHO, I think it is the best one for what you are seeking to do. Either that, or clip the wings a bit.
Old 09-13-2009, 12:00 PM
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vertical grimmace
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Default RE: Raising stall speed?

I think this can be achieved aerodynamically. Not just by adding weight. (albeit wing loading is aerodynamics, I know) I have had plenty of very lightly loaded aircraft that did not act this way.
This is also not my plane. They are beginners, and clipping wings, or doing any structural mods is not really an option. Unfortunately, most newbies don't build anymore, so I doubt they have a shop let alone the skills to do any serious modifications.
I was kind of hoping I could turn a clevis or shim the wing mount. There are so many trainers out there and it is too bad that some of the great classics could not be more prevelant at the field, like the Sig Kadets and such that are proven designs that work.
Old 09-13-2009, 02:23 PM
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HighPlains
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Default RE: Raising stall speed?

Just tape a short spoiler on top the high point of the wing on either side of the center section. Start with about 10% of the span and adjust length to taste. Simple blade 3/8" tall should do the trick.
Old 09-13-2009, 02:54 PM
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Jim Thomerson
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Default RE: Raising stall speed?

Tape a piece of triangle strip to the leading edge of the wing to make the airfoil more pointy and prone to stall.
Old 09-13-2009, 04:08 PM
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da Rock
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Default RE: Raising stall speed?

The spoiler taped to the highpoint of the airfoil can be triangular as well. Easier to create and make uniform.

Place just outside the prop diameter and they seem a bit more efficient. (You can use smaller ones for the effect you want.)
Old 09-13-2009, 04:27 PM
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Default RE: Raising stall speed?

Attach the wing upside-down.

Charles
Old 09-13-2009, 06:57 PM
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vertical grimmace
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Default RE: Raising stall speed?

Now the spoiler idea is a good one. I will try that. I think maybe just some 3/8" tri stock. I want to add, this Nextsar aircraft, I do not like. I do not like the wing droops and flap dealios they added as a marketing ploy. The only good thing about this trainer is the engine. It is basically a .46 AX. Once you get the stupid needle restrictor off it runs great.
It would be nice to just have a simple, practical, tried and true design without auto pilot, simulators, wing droop, spoiler flaps, lean needle restrictor, poor inefficient props included with these trainer packages. Just lower the price! As an instructor, these are just obstacles that have to be overcome every time I greet a new pilot.
Old 09-14-2009, 10:18 AM
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Bax
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Default RE: Raising stall speed?


ORIGINAL: vertical grimmace

Ok, this is going to sound a little odd. I am a flight instructor with our local club. It seems almost everyone is learning to land right now. I have one student and his father (who are both learning together) that have a Nextstar trainer. Now the problem is, this particular plane of theirs, has to be very slow, I mean really, really slow, before it stalls. I would like to raise the stall speed slightly so it opens up the stall envelope a little. This is mainly an issue when perfectly calm.
My idea is to raise both ailerons slightly. Maybe 1/8'' to see if this will have an effect at reducing lift. Another idea maybe to add positive incidence to the wing. The latter would be more difficult. Honestly I am just throwing this out and have not pondered it much (I am being a little lazy).
I have been flying for 25 years and am involved in many types of flying from pylon to scale. This airplane is very hard to get slow enough to stall. I look forward to ideas on this.
If you remove the flaps and wing droops at the tips, the model will fly faster and have a higher stall speed. If you like those accessories, you can raise the stall speed of any model by adding weight, however, a very slow landing speed is not a detriment, in my opinion. Raising the ailerons slightly may help a bit.

We're not sure what you mean when you mention that raising the stall speed will "open up the stall envelope".

Old 09-14-2009, 10:27 AM
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vertical grimmace
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Default RE: Raising stall speed?

Right now the aircraft has to be extremely slow to stall. I know what you mean by this being a good thing. The problem is, it is very difficult to get it this slow without a head wind. The first thing I do is remove the droops and flap/ speed brake things. The wing droops make the airplanes almost unflyable. I have experienced this with 3 of them.
Old 09-14-2009, 11:42 AM
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Default RE: Raising stall speed?

Just some suggestions:

Remove the flaps (added lift), but not the speed brakes (added drag = faster slow down)

The droops are there just to avoid any tip stall.

Also move the CG as far back as possible.

Crabbing the landing approach will also help slowing the beast down, after all, it is a Mustang!
Old 09-14-2009, 12:04 PM
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Default RE: Raising stall speed?

Edited: I chose to remove because it didn't deal with the main issue.
Old 09-14-2009, 02:27 PM
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Villa
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Default RE: Raising stall speed?

Hi vertical grimmace
In your post #6 you pointed out that nosing down the slightest during the landing approach speeds up the plane. During my landing approach I give it up elevator(with low idle) to lose altitude and thus come down, and throttle to stretch the landing. I almost never use down elevator in the approach.
Old 09-14-2009, 02:28 PM
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dignlivn
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Default RE: Raising stall speed?


ORIGINAL: vertical grimmace

Now the spoiler idea is a good one. I will try that. I think maybe just some 3/8'' tri stock. I want to add, this Nextsar aircraft, I do not like. I do not like the wing droops and flap dealios they added as a marketing ploy. The only good thing about this trainer is the engine. It is basically a .46 AX. Once you get the stupid needle restrictor off it runs great.
It would be nice to just have a simple, practical, tried and true design without auto pilot, simulators, wing droop, spoiler flaps, lean needle restrictor, poor inefficient props included with these trainer packages. Just lower the price! As an instructor, these are just obstacles that have to be overcome every time I greet a new pilot.

A tried and true design = A Stick.

I made that Suggestion at my Club, for
a Club Trainer.


Bob
Old 09-15-2009, 12:10 AM
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Doc.316
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Default RE: Raising stall speed?

Well if you want the wing to stall at a higher speed add stall strips to the leading edge, basically these are triangle stock on the leading edge of the wing to promote seperation. They use them on full size aircraft to taylor stall characteristics, and are generally found at the root of the wing.

I don't think ya really want this. Generally trainer are designed to have a slow stall speed. What you really want is the aircraft to slow down faster for better flight path control. Fixed spoilers or a different pitch prop would do this. A fixed speedbrake (maybe a piece of tape between the landing gear) might also help to give it some more drag for a steeper approach. This might be something to try. Of course I would add a little tape at a time and see if it helps.

Steve
Old 09-15-2009, 09:43 AM
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Bax
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Default RE: Raising stall speed?

We're not sure what you mean when you say the plane stalls at a very slow speed, but you need a headwind to get it to stall at that speed. Wind has nothing to do with it. If an airplane stalls at X mph airspeed, then it always stalls at X mph airspeed. A headwind makes the groundspeed lower, but airspeed is not at all affected.

If you cannot make the model stall when there's no wind, then there's something wrong with the flying technique. An aircraft in flight does not notice the direction of the wind, it only responds to changes, so if the wind is 10-15 mph, the model will only see changes of 0-5 mph, and only respond to the direction and magnitude of the wind changes. It doesn't recognize that there's a base 10 mph wind speed.

The problem with most model flyers is that they tend to land too fast. They don't take the model up high and play with airspeeds to find the best speed for the model on approach. You need to determine the proper throttle setting and fuselage attitude for the correct approach. That means you will have to find the proper elevator setting to give you the correct approach speed. Shoot for the end of the runway, and then you'll land a bit farther down when you round out and flare. If the model floats down the runway, you've made the approach too fast.
Old 09-15-2009, 10:28 AM
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Default RE: Raising stall speed?

Well done, Bax. Someone had to say it!
Old 09-15-2009, 01:22 PM
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vertical grimmace
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Default RE: Raising stall speed?

Obviously there is flaw in flying technique. These are students. It is thier airplane (not the clubs). I personally, can land it perfectly regaurdless of conditions but I am trying to find a way to make this design fly better. If it were me, (and it is not), I would not have bought the hunk of garbage in the first place. Too bad prospective pilots do not talk to actual experienced pilots before being led astray by hobby shops.
Bax, you are very correct in that most RC pilots land too fast. Almost everyone at my club is guilty of this. That is one of the things I am trying to teach. You are also very correct in learning about the planes slow speed characteristics up high. Especially with high performance aircraft. I am going to try some spoiler\stall strips just to see what happens, but I am not going to go too far. If they want a better flying airplane they will have to spring for a better design like a Kadet.
Old 09-15-2009, 03:56 PM
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Default RE: Raising stall speed?

My question is are you looking for a stall break or are you trying to increase the landing speed and kill the float? If you want it to stop floating I would add weight. The plane should act the same but come down easer. Adding stuff will change the stall chariteristics, but the speeds will be too close to the same unless you get evil with the mods. I dont think you will get a light flat bottomed wing pane to plop on a runway. My sons Kadet MK2 likes to be slightly fast and flat to maintain control. I like the way Sticks can plop down or flown flat.
Old 09-15-2009, 04:44 PM
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Default RE: Raising stall speed?

VG,

Put all of this on the shelf until after you show up to the Arvada race this Saturday! TT40 class is seeing great turnout - but your viper is still one of the fastest. Maybe its because a c/l combat geek knows how to tune a motor?

Kurt

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