Maximum Flap angle for a P51

[Mystar]

I have been flying for many years but recently got into fly scale warbirds. I have a 72inch P51 with flaps and retracts. It is a lovely model to fly but i do find that the landings tend to be a bit fast. With the flaps down it just seems to float on for ever The flap angle i am using is about 40 degrees. I would like to have a good ballance between lift and drag. So my question is do i need more flap angle, if so how far can i push it

[Keith Simmons]

I think you can push it till you are happy with the flap angle. The hurricane's flaps is 90 degs so you can experiment. Good luck. Keith

[Mystar]

Thanks for that Keith, I am not a brave pilot but i will have a play the next time i take it out for a spin.

[Danny Fenton]

Hi Mystar depends if you want to be scale. Up to 45 degrees tends to slow the aircraft and increase lift, any more than that and they act more like brakes and less lift is generated.

The markings speak for themselves, the flaps are hydraulic as far as I know, and drop when the engine stops running certainly the inner gear the gear doors work that way.

Cheers

Danny

[Keith Simmons]

Do you want the landings to be slower and scale like? You can have the flap angle to scale but may result in a higher landing speed which is not to scale, why? You cannot "scale" air itself so it is a trade off. Having flaps greater than 40 or 45 degs increases drag and less lift so if you want it to look right may not be scale like and you would be watching the model land and would you or anyone notice the flap angle?

[Mystar]

My aim is to get the landing slower, but i don't want to compromise lift to much and risk it stalling

[Martin Harris - Moderator]

Many people throttle right back and glide in to try to land as slowly as possible. In order to set up a nice scale approach, you need to come in with the nose a little higher than the normal trimmed glide attitude. A little more throttle will be required to balance the drag caused by the higher angle of attack and the flaps but there's a sweet spot where there is sufficient airspeed but lower lift (known as being behind the drag curve) and the net result is a steady descent at a nice airspeed - corrections to the glide path (either way) can then be made with the throttle.

Practice at a good height until you're familiar with the model and its stalling characteristics and be ready to apply power and go around again if you're at all uncomfortable when doing it for real.

The application of flap really helps here as it effectively provides washout which helps tame any tip stalling tendencies.

[liam bennett]

Ok,

Here is a simple formula.

Below 40° the lift is higher than the drag. The added drag will slow the model down a tad but it'll glide for longer.

Above 40° The higher the degree the more drag and less lift. Above 45° the drag is greater than the lift

You need to test and find that happy, safe medium.

[Mark Powell 2]

The idea of flaps is to reduce the stalling speed, so you can come in slower. This gives a steeper descent. I have even seen it said in a model plane magazine that flaps 'increase the lift', which is the last thing you want when trying to come down. Flaps do increase the lift, but nor by themselves.  They allow the speed to be lowered, because they allow a high angle of attack, but they are 'part of the wing'. You have to open the throttle and raise the nose to keep the speed low. Always remember "The elevator is for control of speed, the throttle is for control of height'

You are coming in too fast. Be brave in small steps!  With the flaps down it is very unlikely to tip stall, as Martin said.

And whatever you do, if you have to do a 'go around', don't pull the flaps up until your reduction of elevator from your 'nose high' approach has considerably increased the airspeed. and your opening of the throttle has made you climb..

Edited By Mark Powell 2 on 30/08/2012 09:05:18

[Simon Chaddock]

Mark

"......beacuase they allow a high angle of attack"

Surely a wing stalls at a particular angle of attack regardless of what the flaps are doing at the trailing edge.

Flaps work by effectively increasing the camber of the wing so creating extra lift (and drag) to enable the wing to continue to support the plane at the same angle but at a slower speed.

As far as I am aware the only way to increase the angle at which a wing stalls is to modify the wing leading edge profile in some way (droops, slots or slats).