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Nov 03, 2017, 09:41 AM
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randall_l's Avatar
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Control surface throw measurement

Hi folks.

I've been giving a lot of thought about control surface throws since I started migrating towards 1m DLGs and reorganizing my Getting into DLG thread.

Almost every model thread and even Gerald's (G_T) Initial programming of a DLG thread uses ruler measurements. Obviously Gerald's thread is aimed at 1.5m DLGs, but even for the ~1m specific threads, most of them use the same measurements for throws. For example, the GO mini, the throws are close to the same (~12mm, ~1/2" for ailerons--what Gerald calls the "tree hugger" deflection).

As studious glider guiders, we know that control surface deflections >15 degrees start to have more drag than lift. With this in mind, I started calculating the deflection angles for my DLGs. The two 1.5m ships I have are ~13.4 degrees (using Gerald's 1/2" "tree-hugger" deflection for ailerons) and the Prcek I'm setting up has the settings from the GO mini (~12mm, ~1/2"), but that makes the aileron deflection ~17.5 degrees--seems that's going to have to change for the sake of efficiency.

So what I'm trying to get at (and doing a very poor job) is the following...

How are manufacturers determining the throws for their planes?

Shouldn't control deflections be in degrees (which will be relative for each model) instead of ruler measurements (mm/inch)?

I know that grabbing a ruler and measuring is easier than calculating what the linear distance for the throw should be based on the chord of the aileron, but it seems to me that having a set of efficient deflections in degrees would be more universal--and more helpful to folks who do their own scratch builds at the same time.

Thanks in advance for steering me in the right direction.

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Nov 03, 2017, 03:42 PM
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I printed a gauge that works pretty well but I use that primarily to make sure they are the same from side to side. When my customers ask what I fly I try to answer in mm because most will have a ruler handy but probably not a gauge. Keep in mind that any of these numbers we see posted are a rough idea. If i am flying with a more conservative CG I may need more throw to get the same response someone else gets who is flying a more neutral CG.

There are also personal flying styles. My buddy Mike has one plane with throws very much like a 3-D plane and another with very small throws for very precise thermal flying.

How wide the flaperon is will also effect the angle you have for a given mm of throw.

I really cannot answer for other designers but I always recommend less and then dial in more as needed. The numbers I recommend are also based on the experience of other pilots. People fly my planes and give me feed back, I then share that with the community. Real world flying is often quite different from the drawing board.
Nov 04, 2017, 10:08 AM
Jens Hoffmann
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The easiest way is to fly them in, rarely the deflections are more than a couple of mm in flight.
Nov 04, 2017, 10:01 PM
Registered User

I think you are mixing up some terms here. As I read it you are trying to reconcile a flap angle limit for lift production vs drag with Gerald's stated aileron throws. Aileron throw is about roll rate not lift production. You would rarely run more than 15* of camber/flap flying in a straight line, as this would effectively be using the brakes. However if you are putting your wingtips into the gaps between the tree tops stretching out a flight you may need 15*+ of aileron throw to maneuver and keep the model out of the trees.

Like many questions in RC soaring such as "How much camber is thermal camber?" "What angle should the airplane climb out at on launch?" or "What is the best CG?" The answer is that it depends on the situation. Some thermals are big and smooth allowing 12mm camber some are small and violent and will kick you out if you are carrying any camber at all. Launching in dead calm you want the glider to climb out at 90* to the ground, if it is blowing you want it more in the 70* range to take advantage of wind shear. The best CG is the one you are comfortable with but if it is blowing like crazy an aft CG will be harder to fly.

Always in soaring the least control movement possible is the best. Any time you move a surface you create drag. Some guys run huge throws with a lot of expo so they can make small adjustments. Some run low throws and no expo for the same result. Some run large throws and no expo but use very slight stick movements. Some set up dual rate switches or flight mode depended throws to accommodate varying conditions. I have had my aircraft flipped upside down in the blink of an eye by violent air, sending me down behind a treeline and needed essentially 3D flying throws to right myself and save the aircraft. Thus I run big throws and I don't like expo so I don't run it. Whatever is comfortable for you make it happen. Just remember if you run big throws, don't bang the sticks if you don't need it.

This brings me to your question about measurement strategies. Would it be more accurate to use a meter or gauge to measure angle? Absolutely! But it really isn't necessary. Who wants to keep track of a meter or gauge anyway? Just use a straight edge perpendicular to the wing at the TE of the wing and set at the manufactures specs. If you don't feel like the aircraft rolls fast enough add more aileron if it is twitchy take some out. I don't even measure my throws anymore I just eyeball em and adjust to get the performance I want.

Gerald's setup guide is an exhaustive and an excellent resource. Know that it took me years to internalize what the whole document means. There are still portions that I choose to skip as I am simply too lazy to show up to the field at dawn with a timing partner and fly straight lines around the sky mapping average flight times with a range of camber settings.

Hope this helps.
Last edited by austinhlg; Nov 04, 2017 at 11:03 PM.
Nov 05, 2017, 12:46 PM
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randall_l's Avatar
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Thanks for your response, austinglh.

I had purposefully left flaps out to focus on the parts of the control throws that invoke maneuvering/steering. I really did do a poor job of explaining what I'm asking about, thanks for calling my attention to that.

What I'm referring to--and trying to reconcile--is this:

On both of my 1.5m DLGs, the 3/8" throws equate to 9.88 (55.5mm aileron-chord) and 10.15 (54mm aileron-chord) deflection. Using the 1/2" aggressive throws, those same aileron-chords are at 13.22 and 13.6 deflection respectively.

Without knowing the aileron-chord of the GO mini and using those throws for the Prcek, the 12mm (~1/2") throw on ailerons at 40mm aileron-chord is 17.46 deflection. That isn't inline with what I believe Gerald is referring to as "efficient".

I'm just trying to wrap my head around what is "efficient". Looking at lift vs drag graphs for control surface deflections (on google), less than 14 seems to be where it's at. I know there's a difference between lift vs. drag as in camber and as in control movements, but it's the lift vs. drag as in control movements that I'm trying to wrap my head around at the moment.

I know there isn't going to be one-answer-to-rule-them-all, but it seems that using degrees as base deflections (and converting to ruler measurements for the control surface chords of whatever model) and adjusting from there will work for me.

Thanks for humouring me.

Nov 05, 2017, 02:15 PM
Registered User

Unless I am reading this wrong you are still trying to compare apples and oranges. Yes large control surface throws create big drag and are inefficient but we are not looking for efficiency when we are banging the sticks against the stops, and we rarely leave them at these extremes for more than the fraction of a second it takes to right the aircraft . When we use the extremes we are trying to maintain aircraft flight attitude in wild air or perhaps flying loops, Cuban 8s, or rolling circuits. For efficiency the goal is to apply the minimum amount of surface deflection to attain the desired aircraft heading/attitude.

Again degrees are more accurate but really are unnecessary. Ideally you will never use full throws unless you are rolling the aircraft around the sky (which is inefficient but fun). Manufacturers give guidelines for surface deflections in distance instead of degrees because it is easy and it will stop people from running 90* of aileron.

You are also recognizing different aileron cord without considering the effectiveness (not efficiency) of these differing ailerons. Some aircraft will roll fast with small aileron throws some will require much larger throws to achieve similar roll rates. Tune full throw for max roll rate that you want to be able to achieve then use very small stick movement to work light lift.

Remember that there are many distinct flight phases in DLG flying that need to be addressed differently. Launching, Cruising, and Coming home all require different control strategies. A prime example of this is down elevator throw. You want a LOT available to push over but in no other flight phase will you have cause to use it so it is not a big deal to program it in. Conversely a lot of up elevator will loop the aircraft fast but can be extremely inefficient and is very easy to over control as you are almost always pulling on the stick to circle, thus one might chose to limit the up elevator deflection to eliminate stupid pilot inputs. With ailerons many people choose to have fast roll (big "inefficient" throws) available for aerobatics knowing that if they use them when trying to circle light morning air they will be penalized. Dual/flight mode dependent rates can be utilized to reduce throws if you are prone to over controlling the aircraft.

Perhaps an anecdote will help. Tom Siler and I have been flying the same aircraft for about 5 years now. This "aircraft" being the entire Fr3aK progression from hollow Fr3aK to Super to NXT. Thus we know these aircraft well. We also time for each other often as we are a good team and we both constantly talk smack so it is a good time. One reason we are a good team is we can read each others aircraft for lift at extreme distance because we are familiar with its habits. The fun (and relevant) part is, I hate flying his gear because it feels dead and he hates the nearly un-flyable twitchy garbage he experiences when he flies my gear. Both of us regularly perform in the top third at contest, meaning we can both achieve efficient flight.

The take away from Lift vs Drag being inefficient @>15* deflection is that if you are trying to fly efficiently don't use more than that, not that there is no reason to have +15* available for certain flight situations.

Nov 05, 2017, 03:23 PM
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randall_l's Avatar
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So I'm getting hung up on the outer-limits that happen rarely. It's not the first time--and it's why I count on the folks at RCGroups to give me a smack upside the head.

I think I understand now. At those extremes, you're not concerned about losing energy, you're trying to not have to take your plane home in a plastic bag.

I shouldn't be setting the maximum throws to be efficient, I should be setting my normal (inner 1/3 of the stick) movement for most-of-the-time flying (which itself is fairly efficient) and have more aggressive maximum throws for the holy-spit moments. Roughly correct now?

Nov 05, 2017, 03:32 PM
Registered User
Yep! How much you want for Holy [email protected]?"!! moments is up to you.

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