Ok bentwings threw one out there that put folks in a twist so I thought I would do so too

Lets say the takeoff speed of this plane is 15MPH and the plane takes off with the wind and the wind is blowing at 10MPH
At what speed will the craft lift off

5MPH ground speed 15MPH airspeed. Easy

15mph indicated, 25mph to a ground based observer.
Evan.

Takeoff at 5mph ground speed. The only thing the ground has to do with your plane is ground effect and to mash your plane into a kit again if you make a bad mistake! Look at it this way,if you were flying say ten feet from the ground at 15 mph and you get a sudden gust from the rear at 15 mph,what do you think is going to happen?

He said taking off WITH the wind so that means with a tailwind. So pimmnz wins the gold star. 15 mph airspeed and 25 mph ground speed at liftoff.

Ok Folks think about this a little

I Think ????
A plane taking off with the wind actually would have to take off with even faster airspeed look at the back of your model while it sets on the ground
You will find that the wing does slope down. Now look at the front you will see even stunt planes have an angle.
Have you ever heard that air is fluid? the only difference is Water reacts stronger than that of air.
When a plane takes off with the wind the wind will actually push the plane down and at the time of lift off it will push the tail down increasing the angle this creats a stall situation
This is one of the reasons taking off with the wind is a no no.
A Model plane can get away with it because many are overpowered but you will find scale fliers follow the same rules as full size fliers

So sad, on so many levels.

The thruth of the matter is that the answer would depend on allot of factors
1. Air Temp
2. Groud Temp
3. ground level alt.
4. Cloud cover
5. Surrounding ground cover ie trees slopes ranges and type of runway
6. humidity.
7 barimetric pressure
8 Sun up or Down
and more
All play a part in your lift ratios, for example Air Temp; Air is made up of molacules (sorry for the spelling) the colder the air the closer the molacules become this makes the air
denser and it will produce more lift This is also true with Humidity and Barimetric pressure both effect the density of the air and therefor would effect lift.

Ground temp, Cloud cover, Type of Runway and the Sun all can effect thermals, A Blacktop for example will heat up and create thermals now in a full size plane this effect would

not amount to much but in a light model the effect is more.

Ground cover

As I said before air is considered a fluid therefor it flows around up and down and across objects. This flow can creat all types of currents.

Look at a big wide river running nice and slow but that same river with the same angle just not as wide turns to fast flowing white water.

????
What you talking about

Hey willysims.......is your plane on the runway or the conveyor haha

bentwings

Nope one is on the building board and another is in a UPS truck cost to much for next day air LOL

Was this a trick question? I assumed that you would take off into the wind. Taking off with the wind on your tail ,would be looking for a rekitted model! I overlooked the 'with the wind' part. Also the temp and humidity does have a profound effect on things. I remember one flight lesson I had in a 152 Cesna. It was a hot muggy day ,and we had a problem getting it to climb. If I remember right we could not even to get it to climb 100' per min. In cold dry air 0 degree or so it will climb three times that or more. Models respond the same way to. Of course if you have a little RC rocket you might not notice. By the way ,the flight characteristics efect the prop to.

williesims, you're making it way too hard. Taking off with a tailwind simply means that the first part of the take-off run the plane accelerates to zero airspeed. Thereafter the take-off is normal and the plane handles like normal and will take-off at the same airspeed it usually does. The distance will, of course, be longer as will the time taken, but that wasn't asked.

Where I fly the pit area is at one end of the field, take-offs are almost always from that end 'cos no-one wants to taxi all the way to the other end and take off towards the pits. No one cares about wind direction or speed, goose it and go! No problem. Landings the same, touchdown at the pit end and roll out, taxi back.

Full scale sometimes do take-off with a small tailwind, as long as the required accelerate / stop distance is available, no pproblems.

Cheers from the Barfly.

I agree with Bruce and Evan. Taking off with a tailwind is similar to flying at a higher density altitude; anyway, the higher the altitude the more space between air molecules therefore the groundspeed of an aircraft is faster than the indicated airspeed. The opposite occurs when landing into a 10 mph headwind. Subtract 10 mph from your indicated airspeed and you have a good idea of your ground speed. I suppose that a pilot could whip out the old E6B and figure it out quite precisely if needed. I'm trying to figure out how to hang on to my E6B and my transmitter and fly and calculate at the same time. These forums are really great, they get me to really thinking. Always a student pilot. Chet

Taking off with a tail wind is a little more involved than that it increases the chance your wing will dip on your turn out causing a slide
When you take off into the wind the wind will catch under your wing the underside of the wing is not your lifting surface
When you take off with the wind the wind will catch on top the wing this creats turbulance on your lifting surfaces.
Yes its true that most us will not have to worry about these things but if we are going to fly scale models of an airplane we should at least know what and why a full size reacts the way it does.
Scale is not only how it looks but how it looks in the air.

Really?
Evan.

No, not really. Willie has things a bit wrong.

Willie, I understand you have these explanations to try to explain things you percieve while you are flying, but it just doesn't work the way you say. I'm not going to take you on point - by - point, but I do encourage anyone reading this to just search the aerodynamics forum on the word "downwind". It's all been discussed ad-nausea.

All my take offs are across the runway -If I hook or slice, I have a lot of runway to use ----
This is applied logic.

Your runways are short, but really wide, I guess...
A good tailwheel can straighten out the takeoff, I've noticed.

John I am not a physics teacher so my terminoligy is more than likely wrong
Nor am I saying that you cannot take off with the wind or if you do you will crash
What I am saying is that it increases the chances and why I think so

yup 600ft wide and 50 ft long

Must be in Ireland somewhere :-) (Or really strong crosswinds?)
Evan.

That is a huge runway

Course, when the crosswind blows across the long way, they all tend to miss the runway cos it's so narrow...!
Evan.

Willie, I'm afraid you're making a way more of this than you need to. If you take off downwind you'll take off just fine when it reaches 15 mph of airspeed.

Now if you want to talk about a REAL LIFE downwind takeoff factors that's different. Your model will still lift off just fine at 15 mph. But as you climb up through the first couple of feet you're going through a wind shear zone and the wind at 2 feet is stronger than in the first foot close to the ground. If you try to fly at the same 25 mph ground speed then this shear can bite your model. But if you lift off and accelerate then it's not going to be an issue. This is the same effect you get during landings on windy days. You need to keep the nose down and flying speed up so that as the model drops into the lower velocity air close to the ground it doesn't suddenly fall below the stall speed.

Your wing angles being positive or negative depending on direction don't enter into it at all. That one's a non issue other than how the wind's localized turbulent flow patterns react with the airplane. But that is totally unpredictable and the only defense when it's present is to ensure that you don't lift off until you have more than your minimum controllable flying speed. I've seen windy days when it was like a washing machine up there and other equally windy days when it was very consitent and a non issue to deal with.

But none of that was noted in your original problem. The answer still is 25 mph ground speed if it's a tailwind.