Trimming a powered glider?

[Tim Kearsley]

I've just bought an ST Models Blaze and am about ready to give it its maiden flight (as soon as the weather allows).

This is a bit of a noddy question for someone who's been flying for a few years now, but I've never flown a glider before. I always trim my powered models in much the way that Dave outlined in his excellent article this month, i.e. with throttle at a liitle over half I trim the elevator for level flight. How you tackle elevator trim with a glider though?

As I say, apologies for being a bit dim-witted with this one!

[Martin Harris - Moderator]

Unless in a position to adjust the downthrust relatively easily then the way I trim such models is for the normal gliding attitude without power and use a throttle to elevator mix to provide a good climb attitude under power.

Edited By Martin Harris on 02/07/2012 08:13:53

[FunnyFlyer]

Hi Tim,

With "self launched" gliders - ie it's the performance after the motor stops that's really important - I always trim for the highest glide performance. This will be minimum sink (to maximise endurance) or maximum distance (to move away from sink). Most transmitters enable you to assign a trim to do this sort of trimming - plus loads more on the landing phase!

For me, the motor run trim is secondary. I'm quite happy to put in some stick work to control the climb. Most of my motor runs are less than 20 secs to get to around 250 metres.

Having said all that - gliders like the Blaze and Blizzard are "warm-liners" so may spend a lot of their flight with the motor on, burning around and doing aeros. Trimming for these is likely to be more in line with a power model.

I'm sure Andy Ellison will chip in!

Cheers

Edited By FunnyFlyer on 02/07/2012 08:18:06

[PatMc]

I'd trim a model like the Blaze as if it were an aerobatic slope soarer. First the dive test power off to establish the most rearward cg that you are comfortable with. This will give the best glide & minimise the difference between power on/off. Next if the model pitches up with power on decide whether you prefer to control this on the stick as FF does or if you want to mix some down trim with throttle or have a seperate trim "phase" set up if your Tx allows.

I fly an old Protech Unlimited that's been converted to lipo & brushless, it's about the same size but a bit heavier than the Blaze.

With this model I do most aerobatics on the glide & treat the throttle like a switch. It pitches up a bit under power, I've tried using a power trim phase & elevator/throttle mix. Switching phases proved too clumsy with this model so I've have settled on a small amount of E/T mix plus some stick input.

The reason I now don't use full E/T mix is that I once did a fast low level pass after a dive from altitude the banged full throttle & the down elevator trim was effective before the motor picked up luckily it only skimmed the grass & was able to recover in time.

Edited By PatMc on 02/07/2012 12:19:54

[malcolm woodcock 1]

I have a Blaze and it has quite a fierce little motor on it. Trim is done for glide, but expect to put in some down elevator on the climb or it will come over on itself.

[Vecchio Austriaco]

Most powered gliders will raise their nose when the power is on, as mentioned by many above my post you can either play with a trottle elevator mix or hold the thing in the angle preferred yourself as the powered flight is normally not very long (if the thing is still a glider and not weapon....)

VA

[Erfolg]

I think that the comments with respect to stick work with (particularly high) powered gliders is pretty standard.

Within the club I fly with, there is a wide range of models (powered gliders). Moulded 2m, moulded big open types, high powered built up, foamies etc.

On some of the moulded models, rather than do the stick work themselves, it is programmed in with the throttle. Most are set on the assumption that it is full powered, at a max of about 30 degree, climb out to 200m and then cut out.

Some models just go for a vertical launch, for the hell of it. No concern about efficiency of power usage.

All of us seem to accept one way or another, you control the pitch, rather than go for downthrust. I guess on small bodies, it is not easy to build down thrust in, without a banana fuz.

Foam models, often have it built in, partly because it easy to do and that often the models are aimed at a mixed bag of operators, from newcomers to experienced.

[Tim Kearsley]

Thank you very much chaps - all replies much appreciated. I'm happy to control the powered ascent with stick work rather than mix elevator with throttle, partly for the reason you mention PatMc!! It was the way you trim a glider in the glide which I wasn't sure of. I'm looking forward to getting it up into the blue, but there ain't a lot of blue around these parts at the moment....

[fly boy3]

Re Tim's question re trimming, I too am waiting to test a 1st glider. One answer was to trim for min. sink,? could any one elaborate on this. Cheers

[fly boy3]

Re Tim's question re trimming, I too am waiting to test a 1st glider. One answer was to trim for min. sink? could any one elaborate on this please. Cheers

[FunnyFlyer]

Will wait to be shot down by BEB...

Minimum Sink is the state that will give you maximum duration from a given height in still air (no rising or sinking air).

Typically, it is the state just before the stall. Free Flight models are trimmed in this state. As the Angle of Attack is quite high to achieve maximum lift and minimum sink - so is the drag - and the model will not cover much distance for a given drop in altitude. It's all about duration.

With the luxury of RC, we can guide our glider to areas of lift and away from areas of sink. This requires a different trim - where we want to maximise the distance covered for a given drop in altitude. It has a more shallow AoA and an increase in flying speed.

This is what makes flat field glider flying so amazingly enjoyable and tricky at the same time. You have to constantly balance the two states to maximise your flight times and make it back to the landing field.

 

Edited By FunnyFlyer on 03/07/2012 11:51:04

[fly boy3]

Cheers FF, well explained, but I am still stuck with the practicalities of the trim. As a dyed in the wool ic man I am used to trimming for hands off half throttle etc type trimmiming. Once the glider has reached the required height andmotor cut, is the elevator trimmed for slighly nose down attitude ?. Cheers

[malcolm woodcock 1]

This is not easy to describe but I'll have ago. After you've climbed to height and shut your motor down, trim the elevator till the glider starts to stall, then back off a notch or two, that should give you near on hands free gliding.

[FunnyFlyer]

Posted by fly boy3 on 03/07/2012 12:58:44:

Cheers FF, well explained, but I am still stuck with the practicalities of the trim. As a dyed in the wool ic man I am used to trimming for hands off half throttle etc type trimmiming. Once the glider has reached the required height andmotor cut, is the elevator trimmed for slighly nose down attitude ?. Cheers

A glider will always be descending in the air it is flying in. It will climb in thermal lift because the rising air will moving vertically at a greater rate than the glider's rate of descent - if that makes sense. If a glider's sink rate is 1 metre per second and it is in a thermal that is rising at 2 metres per second, it will climb at 1 metre per second.

So, for minimum sink and maximum duration I tend to test on a calm evening or early morning when there is still air - no lift or sink. When you have climbed to height (motor, bungee etc) let things settle down and I then apply up trim until the model is just off the stall. In reality, you get a stall - and then back off the trim a beep or two. In still air you will be close to the minimum sink for that glider. It will probably be over elevated for any other situation and may not have much penetration when the wind increases.

All this must be caveated by the characteristics of the particular aerofoil section the wing uses. Some of the more advanced types are designed for minimum sink at quite high speeds. Also - I'm assuming you do not have camber altering devices such as full length flaps. These can be increase or decrease camber and is a whole new ball game.

From the minimum sink setting - it's a matter of nudging the stick or trim forward and experiment with getting maximum distance for least loss of height. This is not the same as a dive!

Hope this helps as a starting point.

[PatMc]

To achieve min sink it's best to move the cg rearwards in small increments re-trimming the elevator to just below stall as Malcolm advises. If you reach a point where you are not comfortable with the handling because of the rearward cg move it forward slightly.

You can also use the dive test method to determine the best cg position. This is easiest done at a slope soaring site but can be done at a flat field.

A rearward cg also minimises the amount of pitching up that takes place under power & makes thermal detection easier.

Edited By PatMc on 03/07/2012 13:57:22

[FunnyFlyer]

Here's some good advice that echos what PatMc says about CoG shifting.

Trimming Sailplanes

[fly boy3]

Thanks all for the info. Due to the weather I can do a bit more reading of the answers before my maiden. Hope Tim Kearsly( who asked the question originaly) does not think I have hijacked the thread. Apoligies if so. Cheers FB3

[Erfolg]

I do not disagree with any of the descriptions of minimum sink.

Many pilots these days especially competition pilots, will trim for the best lift drag trim, which is a little faster, where the model covers the maximum distance from a given height.

What are they trying to do? They are actively seeking lift or the best air. Once in what they think is the best air that is available they will either slow up, to take stock or thermal type turn, at the same speed. Why thermal turn at the best lift drag speed. It is because they do not want the model to stall, as it may at the lowest sink rate.

Also if some one or a bird is seen to be in lift, this is the speed that gets them there at the minimum height loss. The same goes to heading for any feature where they think there is wave lift, copse of trees, ridge in the landscape.

At the end of the day you need to know your model, really know your model. A very successful model glider pilot once said to me, I know my model is competitive, yet it is knowing the model which is more important, than any slight improvement that another model may purport to have.

The other thing with gliders, it is not luck that finds thermals on a regular basis, nor is it luck that wrings the last seconds out of a model in poor conditions. It is observation and making good decisions.

If you want to win competitions, it is landing that sorts the winners from those that come close.

[Tim Kearsley]

No problem with "hi-jacking" the thread FB3 - your questions were exactly mine also! Thanks again to all for the most interesting and helpful contributions. I still haven't flown the blasted thing, thanks to the weather!!

Cheers,

Tim.