espalier support system necessary?

Hi, I have  played at grafting some apples and medlars and want to grow espaliers (can you espalier medlars?), on fairly tall stems, along the length of my polytunnel , on the out side. It is then a protected south facing face for the trees. I am wondering please can they support them selves or is a wire or fence system better? I was thinking of a clear stem of about 3-4' as it is a veg bed below. I guess a support  frame work could be added after if i try to make the branches at similar heights.
and what are the requirements of a plant to be able to espalier it? Am I daft to think I can train medlars( Mespilus)? quince?(Cydonia) I am fairly new to this much pruning intervention/manipulation so apologies if these are silly questions.
thanks

Espalier trees have to have support so the branches will not fail under the weight of the fruits.
Since the branches are horizontal from the main trunk, any fruit on a branch will exert quite a bit of strain on the joint, the more fruits on a branch, the more strain on the joint.

How many years do the branches need support? Or perhaps, for apples, how thick do the branches need to be if the espalier is 6 feet wide, in total.

Thanks Redhawk,
Yes I can see there will be a lot of strain on unusally horizontal branches.
I shall plant my apples that are sick of being in pots in this heat and aim to put up post & wire support this winter.

thanks

Not sure about medlars, but quince can be espaliered. Quince on pear rootstock will have more vigor than onquince rootstock.

If possible, it is best for you to know the characteristics of the variety before training the branches, since spur-type fruit trees are trained in a different manner than tip-bearers.

If you think of it in terms of a "Y-shaped joint, each limb will fail the soonest when it is pulled back against itself.  I do this all the time when breaking green limbs off of fallen, or cut branches.  Trees are amazing, but that's asking a lot of them to grow out farther and farther, and then hold the extra weight of fruit.

Thanks Nick,
Looked up my apple varieties & they are tip and partial tip bearers! So I need to do a lot more reading and get a better understanding before doing anything.

One possibility is to use the trunks of the next tree as support.  I use bailing twine to train the growth in the direction I want to provide shade for my raspberries. When possible I bend the leader towards an adjoining tree so that I have an arch rather than a limb crotch. Some T posts and twine can get the training started and eventually you can build a self supporting ladder like I have seen pictures of. I am using plums and peaches and they are inside the winter coverso they blossom and fruit earlier than the ones outside.

Joylynn Hardesty wrote:How many years do the branches need support? Or perhaps, for apples, how thick do the branches need to be if the espalier is 6 feet wide, in total.

The 200 year old pear trees in France still have supports in place.  
The problem most people have with Formal techniques is that they fail to understand how much support or how much maintenance is required.
These techniques were developed in as far back as the Roman Empire, there were plenty of people whose job was to take care of the grounds.
Formal gardens usually have around 20 gardeners or more that continually tend the grounds.

Espalier was developed to be able to place trees against walls for maxing out the available orchard space. Trees pruned this way are not intended to be standing out in the open, they are supposed to be up against a wall or a fence.
Pollarding was developed because apple trees produce from first year branches and when you cut them all off you can force a growth called a crown, where all new  branches originate.

yes I appreciate that it is a high maintenance method but I thought it would focus my attention to pruning different fruits and help my understanding.
Just playing really!
thanks for your advice