What is winch height?

[shaneyee]

I often read something like "hang time is about eight minutes after a launch to winch height..." How high is winch height? I fly electric sailplanes and I want to gauge how well my plane my plane glides...so how much is winch height and how long should my plane be able to stay airborne from that height in dead air?

Thanks in advance.

Shane

[MDM]

From http://www.hsa.lr.tudelft.nl/~frits/F3B.html

[SoCalGliderFlyr]

Zooming off of the end of the launch is an amazing thing to watch and almost nirvana when you first do it yourself.

Some flying fields can hardly manage 300 feet to the turn around. Some could go a 1/4 mile.

[shaneyee]

Wow, thats a huge zoom to go up another 50-80m! 260m is pretty high.....about as high as I want to go even with an electric! So how does the zoom work? The plane is being pulled down by the winch when it gets above the turnaround pulley? How does it get off the tow hook then??

I'm impressed.

Shane

[fprintf]

Here is what an F3B launch looks like from my perspective:

The plane is going up the winch line and gaining altitude quickly. At the same time the winch is pulling in the line and stretching it at the same time. About the time that the plane starts trading altitude for line (the launch is leveling out) there is a tremendous amount of stretch in the line. You use the elevator to tip the nose over and head for the turnaound and the potential energy stored in the stretched line is translated very quickly into speed. This kinetic energy (speed) gain is greater than the altitude lost in the dive. At the bottom of the short dive you pull up and rocket up to amazing altitudes. As the plane loses speed you push the nose over to normal thermaling speed. Done well the launch is an amazing thing to watch and the plane loses just enough speed and goes right into thermal mode. Done poorly (like me) with an inappropriate plane you can a) shred the plane to pieces (see the Marauder thread for the results) or b) try to climb too high, lose energy and stall the plane at the top of the zoom.

Note, the above describes a launch on Mono which is very stretchy and stores a lot of energy. Launches on braided line/twine are not as spectacular because the line is stiffer and not as stretchy, but still follows approximately the same routine. When you see a plane go from one size in the sky to twice as small (e.g. almost a speck) you know it was a good launch!!! This is why winch launching is so cool (and occasionally destructive).

[aeajr]

The zoom I have seen off winches is controlled by the pilot. When ready, they hold the switch down, dive then climb very fast. It can almost double the height of the launch.

I am not adept at this yet, but hope to become better soon.

[SoCalGliderFlyr]

The "trick" is taking your foot off of the pedal BEFORE you pull up. As noted in MTM's diagram you have some droop in the trailing edge (four servo wing) during the initial launch. However you want to take the wing back to undrooped "normal" flying configuration for the zoom part. As you loose speed toward the apex of the zoom you can bring in some camber to get that extra footage. But as mentioned above don't stall or you can loose half the height you just gained.

In some cases I've traded off altitude for distance when I've noted others in a good thermal off the end of the launch lines. A bit of reflex helps to get the distance.

[shaneyee]

Is there any way to get anything close to this type of launch with a histart? How high can a Hi start bring you up to? Is there some sort of ballpark formula perhaps?

Thanks

Shane

[Jose E Bruzual]

Is there any way to get anything close to this type of launch with a histart? How high can a Hi start bring you up to? Is there some sort of ballpark formula perhaps?

Thanks

Shane

Read this article and it will help you answer your question...

http://www.charlesriverrc.org/articles/design/dickwilliamson_histartphysics.htm

A couple of things, you will not get the same launch or even close with a hi-start as a winch... and if you are going to compare hang time between electrics an sailplanes you'll never get an idea of how well your ship performs.

Jeb

[aeajr]

Is there any way to get anything close to this type of launch with a histart? How high can a Hi start bring you up to? Is there some sort of ballpark formula perhaps?

Thanks

Shane

How high you go with a hi-start is dependant on the rubber and the line first of all. My hi-start is VERY strong with 100' of 1/2" rubber and 450 feet of line. I get very high launches on my 2M and 3M planes. Sometimes I out launch the winch, except for the super zooms that that hot shots do.

With a little head wind, I can get the line and rubber completely off the groudn and almost verticle, so call it 450-500 feet. Add a 100 foot bounce and we are up in the 600 foot range off a hi-start, under optimum conditions.

You can zoom off a hi-start and gain some extra height, but it is not as effective as it is off a winch. On the other hand you are less likely to rip the wings off the plane with a hi-start than a winch.

I get a mini-zoom off my hi-start launches. Let's call it a 50-150 foot bounce rather than the 250+ zooms I see the real super flyers doing off the winch. However you need a very strong glass/carbon plane to do it or you break the wings in the attempt.

You don't see too many people flying built up balsa gliders with big zooms at the end of the winch launch. Some, but not many.

[fprintf]

Trust me, zooms with a balsa plane are not effective in gaining launch height. :D

[SoCalGliderFlyr]

Trust me, zooms with a balsa plane are not effective in gaining launch height. :D


Realy??? You've never launched and flown a carbon reinforced 3 meter Gnome.

[aeajr]

I would not say that you can't zoom a built up plane, you just have to be a little more careful unless it has been specifically prepared for this. SoCalGliderFlyr obviously has one that was prepared with the big winch zoom in mind.

If I recall correctly, I believe I have seen Mark Drela zoom his 3 meter 31 ounce polyhedral RES bubble dancer of the winch at last year's ESL contest in Syosset, NY. But again, that is a plane that was specifically built for the big winch launch.

[fprintf]

Sorry guys, I really meant to edit this and that is what I get from trying to be funny while at work. ;)

My edits were going to be exactly what you have posted. *My* experience on mono was no so much that the wing cannot take the bending force of the zoom, but that my wing fluttered, couldn't take the speed, and thus my zoom made for a rather rapid decrease in launch height. :D

I have seen plenty of purpose built balsa planes take a really good zoom. AVA and Bubble Dancer are two that I have seen, and I have read plenty about the Houston Hawk as well. I wonder how they do in comparison to moldies when on monofilament (which I'd guess adds quite a bit of stress and speed to the whole affair).

[shaneyee]

This is really educational for someone like me who hasn't seen a winch launch. Its given me some real respect for winch launching and the pilots and planes that can zoom! Hmmm maybe I could go to my friend's workshop...get a used car starter motor and make a winch.....

Shane

[SoCalGliderFlyr]

Early Ford starter motor. Has about six inches of shaft sticking out of the motor housing. Made a basic winch out of a rolling pin and thin ply sides in the late 60's. Had to use the side of your shoe to brake the drum.

[ICTHRMLS]

Assuming we can all agree on an approximate "winch launch" height of 500 - 700 feet, lets re-examine the initial post of 8 minute "hang time". That is a very doubtful figure in neutral / dead air. My experience has been on the order of 2-1/2 to 3 minutes max. Anything over that means you have found some rising air. If there indeed is a sailplane with 8 minute dead air hang time we will be soon flying 20 minute tasks in competition. :eek:

I can still vividly remember an accomplished pilot stating "it's only a 3 minute task... anyone can fly 3 minutes" - and landing at 2:15. Ah, the elusive thermal!! :cool:

[Ollie]

The Allegro 2-meter class has Dr. Drela's calculated as a min sink 0.95 ft/s. It has a launch from foot 500 altitude of 500/0.95=526 sec. or 8:46 minutes.

2-1/2 to 3 minute time of plane ft/s plus down of 2 to 3 down ft/s air for short launch time.

[ICTHRMLS]

I will bet those 8 minutes don't include any turns.

[Ollie]

At dawn to 8 AM and no wind, and smallest controls and at small trim speeds for min sink. For your practice?

[ICTHRMLS]

Okay... I concede...... calculated minimum sink in no lift/no sink air can produce an eight minute flight under ideal conditions including expert piloting skills. However in the "real" world (times and conditions we mainly fly in), I do not believe this should be a realistic expectation to gauge one plane's performance versus another's as those conditions are rare indeed.

[aeajr]

Okay... I concede...... calculated minimum sink in no lift/no sink air can produce an eight minute flight under ideal conditions including expert piloting skills. However in the "real" world (times and conditions we mainly fly in), I do not believe this should be a realistic expectation to gauge one plane's performance versus another's as those conditions are rare indeed.

I agree-2.5-3.5 minutes is the realistic range one should expect from a winch hight launch.

[schrederman]

Houston Hawks zoom quite effectively. I also have a Legionair that gets some ooooohhs and aaaahhhhhs when she goes up. Of course, the carbon in the wing spar isn't wood, so I really have to qualify that statement.....hehe.....those carbon sparred woodys can really be bad boys!

Jack Womack

[SoCalGliderFlyr]

Houston Hawks zoom quite effectively. I also have a Legionair that gets some ooooohhs and aaaahhhhhs when she goes up. Of course, the carbon in the wing spar isn't wood, so I really have to qualify that statement.....hehe.....those carbon sparred woodys can really be bad boys!

Jack Womack


Don't think the wing joiner or the fuse is wood either.

[shaneyee]

I agree-2.5-3.5 minutes is the realistic range one should expect from a winch hight launch.

This is what I get but now I know what I should reach for ...

The Allegro 2-meter class has Dr. Drela's calculated as a min sink 0.95 ft/s. It has a launch from foot 500 altitude of 500/0.95=526 sec. or 8:46 minutes.

[fprintf]

Not knowing the qualities of your electric sailplane (e.g. motor and battery type and wingloading), I would say it is a good bet that you cannot approach the performance of Dr. Drela's sink rate. #1 on a pure glider there is no extra weight to carry around as the launch apparatus remains on the ground. #2 Dr. Drela's planes are highly optimized and therefore nothing goes into the air that is not functional - they are very lightweight for their size and have amazing sink rates.

I think the best bet is to use what most e-sailplaners use -- one minute motor run and then compare yourself vs. other electric sailplanes since you are carrying a motor and batteries.