Snap Flap. How much and why? - Page 5 - RC Groups
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Jun 02, 2014, 11:07 AM
Detail Freak
target's Avatar
Quote:
Originally Posted by Joe W

(K shows a bit larger difference in my recent experience).
I was going to ask about different soaring disciplines, I'm glad you mentioned this, Joe.
Is this due to the (i am assuming) lower Reynolds numbers of the smaller wing chords and slower flight speeds? Or is it something else?

Thanks.

R,
Target
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Jun 02, 2014, 01:49 PM
Needs to do 52 legs !!
jjmouris's Avatar
Has anybody tried actually measuring the AoA so we know if we are actually anywhere near what we think we are doing?

For instance, banking more and pulling up in a turn to tighten the turn (change the track to center the lift) you will indeed be flying with a higher Cl then normal. You are sacrificing some height to maintain the speed on the promise to get out of the sink faster. So in this case more camber would potentially make a lot of sense.

You think the model is flying to fast, so you pull up to slow it down and convert height into speed or maintain height at the cost of speed. Either way, you have probably just been flying with to much camber. Adding more camber here while you pull up gently may not be an improvement as you already have to much.

You think you are about to slow the model to much (stall) and push down to arrest the reduction in speed. You now have the correct camber for the speed / lift required, just the angle of attack is being adjusted. Reducing the camber here would reduce the capability to produce lift and you could stall or at the very least increase the rate of sink as the wing produces less lift.

What I am saying is, it all depends on the situation. To decide what would suit your flying style you would do well to have some long term measured data.

Potentially short term stick movements should not effect the camber, the longer the stick is displaced, the more the camber should move as the situation is now likely to change from a minor adjustment in AoA to a temporary change in the requirement of the wing to produce lift (more for a tighter turn, less for accelerating through some sink).

A full size glider pilot does just that by moving the flap handle as he sees fit in real time.

My thoughts anyway.
Jun 02, 2014, 02:39 PM
RIP MC
fnnwizard's Avatar
Quote:
Originally Posted by Joe W
Daryl,
... It is important to note that there will be a drag increase when exceeding the lift for the min sink setting. IMO that is not in question (otherwise the "min sink" mode is not aptly named). I'll also note that we are talking about small deltas here. Using SF will make only a small difference in J (K shows a bit larger difference in my recent experience).

Hi Joe, I've been running no added camber to ele mix in true min sink mode for a few years now and looking to learn more if this is correct or not.


My reasoning for that is as you wrote above, but your style of writing always has me needing to find a quite place to concentrate .


Anyhow, my thinking is when I am in min sink mode, the wing is already asked to provide max lift for the speed already. The only time I will add up ele is if there is some added energy to the plane = thermal, updraft ect.
In this scenario, my thoughts are that alpha has changed (decreased) a bit due to the updraft and thus the ele just readjusts the alpha to where it was before for min sink again.


Without any outside added energy source, the addition of up ele in min sink would indeed take the wing to the backside of the polar curve and would cause a steep rise in drag. Of course the rise would be slightly less with camber to ele mixed in, but given the double whammy of added up ele and added camber with reduced speed, while already in min sink, the chance for a steeper rise in drag ( leading to stall) is even more pronounced.


Would the real world best case be to not have any added camber to ele mixed while in true min sink?


This is one of my post to share my findings with the Xplorer 2 thread.
Quote:
Originally Posted by fnnwizard
No, when I am in this mode ( min sink mode) it does not add any more camber with up ele.
My snap flap is constrained between the reflex amount for zoom and this 6mm of camber.

With the X2, I have 2 thermal modes, the other one with 3mm camber. In that one, pulling max ele gives an additional 3mm of camber for a max of 6.
Not sure what is optimum yet though.

With the large stab, that camber needs to kick in at about 1/3 up stick travel for the typical cg/throws , so pretty fast.

My max camber to ele mix absolute throw is when I'm about 1/2 ele stick. IOW, from 0-50% up ele throw the flaps move from neutral setting to 6mm camber. During this range it actually does closely follow Dr. Drela's 1:1: mix. From 50% - 100% up ele throw the flaps don't move down anymore.
Would love to hear more from you.
Add me to the long list of guys that truly have gained from your postings!Thanks!
Tuan
Jun 02, 2014, 02:59 PM
Detail Freak
target's Avatar
Tuan-
It sounds as though you and DP are in the same camp of not adding camber to your min sink setting, instead only adding reflex with down elevator to that fight mode...

JJ's post is very interesting, and I do follow his thinking of delaying the onset of snapflap, but I'm not sure how practical it would be to accomplish this, unless it was done by flipping a switch while flying. I would think that would be impractical for most people. Maybe there is another way.

I might try disabling snapflap in my thermal mode, but I normally don't have a separate min sink setup, so I doubt that I run as much camber in my thermal mode as what you guys are talking about.

All very interesting to me.

R,
Target
Jun 02, 2014, 03:00 PM
Registered User
Quote:
Originally Posted by Joe W
Daryl,

I understand your rationale from an efficiency standpoint, but quibble a bit with the details. The "gotcha" is that when you are in min sink mode with the appropriate camber, and then ask for more lift from the wing via up elevator, where is that lift going to come from? With the stock min sink camber, the wing can generate a bit more lift but at the cost of significant drag increase as the CL gets higher than the low drag envelope with the min sink camber setting. With a bit more camber, the drag increase is reduced. It is important to note that there will be a drag increase when exceeding the lift for the min sink setting. IMO that is not in question (otherwise the "min sink" mode is not aptly named). I'll also note that we are talking about small deltas here. Using SF will make only a small difference in J (K shows a bit larger difference in my recent experience).
In 3j and more in 5j i need very fast response from the plane if i am working a small bubble at 20 m from the ground....like a big K! With mix ele> camber i don' t need change the flight phase in and out when small bubbles moves around. I play with stick, mix ele camber active, and a slider for camber.
Jun 02, 2014, 04:46 PM
Daryl Perkins's Avatar
Quote:
Originally Posted by mverardi
In 3j and more in 5j i need very fast response from the plane if i am working a small bubble at 20 m from the ground....like a big K! With mix ele> camber i don' t need change the flight phase in and out when small bubbles moves around. I play with stick, mix ele camber active, and a slider for camber.
Thanks, I'll work on that.

Guys, I'm not saying what I've been doing was right, I just didn't see a noticeable difference in actual performance to bother to mess with the programming. Daryl's don't tinker.

Chris - I'm no expert, just maybe a better than average stick. I will always defer to Joe on aero expertise.

Like Joe, I used to run elevator to camber on all models, all flight modes (except launch.) I had that one experience where it made the TD model quite pitchy, and with that radio being elevator to camber challenged, I got away from it altogether, and never bothered with it again. With my new radio alliance, elevator to camber is a simple add, so it's definitely something I will play when I return from the worlds. I might even add it in this weekend. The amount I would add would be just enough to make me feel better.

B addendum - I don't trim B models the same as I do TD toys, so yes, I've always used elevator to camber in all B flight modes.
Jun 02, 2014, 07:15 PM
Detail Freak
target's Avatar
DP, I never said you did it right; but not too many people can debate the results you have achieved...
And, I think that you are super smart not wanting to change before the worlds.
Why change the recipe now, unless you know for a fact that it will make a huge improvement??
Thanks for hitting my question about the B models.

R,
Target
Last edited by target; Jun 02, 2014 at 08:46 PM.
Jun 02, 2014, 08:31 PM
launch low, fly high
Here is the plot from XFOIL (one of the Maxa airfoils) that summarizes my viewpoint. For example, I use about +6 deg for min sink in calm conditions and between +2 and +4 deg for thermalling in active conditions. When the air is active and turbulent, I find that I need to keep a bit more airspeed margin than in flat conditions. So, the operating CL is a bit lower, and the camber setting is lower.

If I am flying at at trim CL = 1.0 in my min sink mode, and find I have to use a bit of elevator, then the CL changes, and the optimum camber changes. The appropriate assumption for using SF is that the plane is operating somewhat optimally to start with. Without that assumption, well, chaos ensues in terms of defining what is the right thing to do.

I do imagine that SF may be a bit harder to sort out when flying with daryl trim... Maybe now that you are old and slow on the sticks and using wurts trim, SF may be appropriate for you.

BTW: adding SF will increase the elevator effectiveness. When I change my SF %, to get the same pitch response I have to change the elevator authority. More SF has less elevator movement for the same pitch response.
Jun 02, 2014, 09:11 PM
Stable genius
vespa's Avatar
Nice Joe, but something that occurred to me is that if you are operating near min-sink, elevator inputs are really only being made as needed to maintain airspeed, not to increase/reduce lift. For example if you are floating straight in calm air you can tend toward a lower speed but at soon as you want to maneuver you'll want more speed, not more CL.

Also I think that plot might be more interesting if you replace CL with AOA because that's what your eyes see and your thumb controls. As shown it looks like those drag buckets are very narrow and optimum camber mixing is crucial but I think an AOA plot would soften that considerably.
Jun 02, 2014, 09:36 PM
launch low, fly high
For the min sink straight line float case, the elevator is typically used to damp out phugoid oscillations. If the air has enough turbulence as to where one is actively controlling airspeed via frequent elevator applications I suspect that the min sink setting may not be the setting appropriate for minimum sink.

As to the phugoid oscillations, this is where the plane is doing gentle increases/decreases in airspeed. Of interest is that these oscillations in airspeed are at a constant lift coefficient (the trimmed lift coefficient), and obviously at constant alpha. The ground based pilot sees the plane doing mild and periodic climbs and descents, trading altitude for airspeed and vice versa. The tuned in pilot will use the elevator to momentarily increase or decrease the lift midway through the oscillation to damp out the phugoid motion. For this momentary input I like the camber setting to be appropriate to the lift being generated by the wing. This requires SF.

Plotting the coefficients vs alpha will only confuse the issue. What is important is the lift being generated by the wing, and whether the camber setting is appropriate for the lift.

BTW, the straight line min sink part of the envelope is not where elevator is used much. It follows that SF doesn't matter much in the absolute min sink case. Where it matters and provides real benefits is in the active air case where the elevator does get used to respond to thermals/gusts/turbulence.
Jun 03, 2014, 02:04 PM
RIP MC
fnnwizard's Avatar
@ Joe, Thanks for those polars . Now I actually have some data to compare to!!!
Did you have to export the polars into Excel to produce that graph or did XFOIL do that for you? I can't get Xfoil to make nice graphs like that .


Anyhow to make full use of your graph to help the newer guys to understand...


If we take a look at the area "under" each curve ( in this case to the right of each curve), that area gets smaller and smaller as we add camber. With extreme camber that curve becomes more of a "point"


What this means is that the efficient range gets smaller and smaller so that speed (and thus angle of attack) becomes very critical.


We control speed and angle of attack via the ele. So as camber increases, an overly large stab and or an overly aggressive cg setting makes the plane progressively harder to fly efficiently/cleanly.


Turbulence/gusts/ etc can change our speed and AoA without us giving input so that's why as the conditions become choppy, we revert back to less camber. It makes the plane have a wider "efficiency range" and thus is easier to fly cleanly.
That's not to say that JW or DP can not fly them cleanly. Just us twitchy finger types (me) that cannot.


@Imageek, when I first started back into r/c I didn't know a thing and had no one to mentor me. I felt like an idiot asking so many questions but getting so many conflicting advice at the local club level ended up having my planes fly like @#$%. Moral of the story? It's prudent to ask why too instead of just how .
Jun 03, 2014, 05:28 PM
Needs to do 52 legs !!
jjmouris's Avatar
Well this is exactly the point I am making. Without knowing the Cl or the AoA how can we say what the right theoretical camber setting should be?! We are relying on people's judgement and rumors rather then hard data.

To much camber can actually lead to way more drag then if you left the wing well alone. Just have a look at Cl 0.8 in JW's graph and you will realise that in fact anything more then 2 degrees just increases the drag (dramatically). Flying at Cl 0.8 or 1.0 is only a very small change in AoA that we would be very hard pushed to judge visually. You have to depend on that your camber is matched well to the elevator input as this more or less directly sets the AoA.

IMHO, how we have been flying around without an active on board system that sets the right camber for the current Cl / AoA is beyond me.
Jun 03, 2014, 06:07 PM
launch low, fly high
jj,

It is clear that we have arrived at different conclusions from similar input data. Yes, there is some judgment. There is also some judgment validation that one can do. Is there enough extra lift available such that I can pull excess g at the current airspeed? If yes, then the current CL has a max. limitation, and if no then the current CL has a min limitation. There are other validation methods available via telemetry. Once one does some form of validation for the airplanes trim state one can define the appropriate camber setting. After a while a good pilot will intuit the flight regime via the planes control response. That is, a pilot will know that the plane is flying like the airspeed is in the "min sink" airspeed, cruise airspeed, etc. That is the kind of stuff that pilots do... once there are active onboard closed loop feedback systems that remove the piloty things, soaring will definitely change. I'm not looking forward to measuring my programming prowess at a contest rather than my soaring prowess.

Add: It is clear that there are conflicting opinions as to the utility and appropriate usage of an elevator > camber mix. I suggest that y'all go forth and experiment as you see fit rather than reading some internet commando pontification about their version of reality.
Last edited by Joe W; Jun 03, 2014 at 06:33 PM. Reason: appeal to "learn by doing"
Jun 03, 2014, 10:52 PM
Registered User
Avaldes's Avatar
I tend to have a similar assessment as Joe wrt the appropriate use of camber as a function of CL command. The biggest assumption you need to make is that we will operate the aircraft at the approprate condition in order to take advantage of these design features. This is most likely where piloting style comes in.

I especially like the description of the pilot inputs damping the phugoid mode. It is very difficult to describe the technical merits of a good pilot.
Last edited by Avaldes; Jun 04, 2014 at 12:54 AM.


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