Downthrust on Merlin powered racers and fighters !

[First time Juke]

I want to ask and please someone explain in layman terms what is gained by it. Is this because of the thrust axis in Spits was higher up ?

It is more obvious in Griffon powered Spitfires, but I recall Tsunami was also added more downthrust at the developement phase.

German fighters like FW-190 and Me-109 did not have it.

I recall also Yak-3 fighter had it.


rgds,

Juke

[First time Juke]

Re: Downthrust on Merlin powered racers and fighters !

I ask this because I added also 2 degrees downthrust on my racer model. Is that 2 degrees too much ?

See the pics at DESIGNER SECTION ! I think it needs it ( downthrust ) too.

[AirDOGGe]

Re: Downthrust on Merlin powered racers and fighters !

I know that with model aircraft, downthrust is set so that the aircraft doesn't climb too much when power is increased. I assume it's the same reason for full-size aircraft, but if this is incorrect please say so.

How much downthrust you need on a model can vary....Try it out with 2 degrees in level flight, and if adding power results in the plane heading towards the ground, you have too much downthrust. I've had a few high-lift models that required more than 2 degrees.

[First time Juke]

Re: Downthrust on Merlin powered racers and fighters !

This was a simple good advice. I liked it. Thank you AIRDOGGE.

[Wild Bill Kelso]

Re: Downthrust on Merlin powered racers and fighters !

On a model 1 degree down and 2 degree right is my base. Seems to work 9/10 times.

[First time Juke]

Re: Downthrust on Merlin powered racers and fighters !

Thanks FNG,

I think I am going to re-engine the little kite and try once more.

rgds,

Juke

[Hawkeye]

Re: Downthrust on Merlin powered racers and fighters !

There are a lot of variables that determine what the thrust line incidence should be.
First is the incidence of the wing. A lot of the WW2 fighters had already a little positive incidence designed in. Most airfoil's on these type are a semi-symmetrical airfoil that develops lift at 0 degrees incidence. That's why you put a little down thrust in the engine. As the faster you go the airfoil will develop more lift. the down thrust will counteract that. A Cub type model will require more down thrust due to the Clark Y airfoil it employs compared to a fully aerobatic design with a symmetrical airfoil. Remember, models have a much higher power to weight ratio which require less down thrust than full size aircraft. You will find that most high performance models with a symetricial airfoil don't have any down thrust designed in them. That way there is no trim change when you fly inverted. Also the horizontal Stabilizer incidence has an effect on it also. It is a balancing act between the thrust, wing and tail incidence to achieve the required performance and handing that is desired.

[First time Juke]

Re: Downthrust on Merlin powered racers and fighters !

Roger that Hawkeye !

[AirDOGGe]

Re: Downthrust on Merlin powered racers and fighters !

This opened up an interesting thought.

If a plane with a significant amount of downthrust is flying at or near full speed under power, does cutting the power result in a climb?

Counter-intuitive to what one might expect when cutting the engine power, but it seems like this would be the case (at least until the speed bleeds off).

[Hawkeye]

Re: Downthrust on Merlin powered racers and fighters !

Typically, no. It is a balancing act between the airfoil, powerplant down thrust, wing and stab incidence. If the design is balanced correctly, there will be little trim change when the power is reduced.

[First time Juke]

Re: Downthrust on Merlin powered racers and fighters !

I think we are nearing the real issue here.

Why does the nose not go right up when the power is turned down ?

I think the down thrust causes the plane to fly in a different posture and possibly the propwash when " shooting " the air behind toward the wing causes more lift in the affected area in the wing.

My little racer without this downthrust did not have this needed new posture nor the added lift. That is why the plane did not fly with its thin foiled small wing.

My other question would be then...how much dihedral is enough and where does it affect ?

Several german fihters like Me 109 G had a lot of dihedral so did the Me 309. Mustang on the otherside is equipped with far less dihedral. Any good explanation for this phenomena ?

best rgds,

Juke T

[AirDOGGe]

Re: Downthrust on Merlin powered racers and fighters !

More dihedral will make the plane fly stable and wings-level, but you want zero to very little dihedral for aircraft that will be flying stunts as perfectly as possible. It all depends on how much maneuveribility you want to sacrifice for the sake of stable flight.

if you want to fly perfect stunt patterns or just have greater maneuverability, then use a flat or near-flat wing, but for more relaxing stable flying and easier control of the model I'd suggest 5-7 degrees of dihedral. Advanced sport flyers who want a little bit more stunt ability may split the diff and shoot for 3 degrees (I do).

With a flat wing you will have great inverted flight control, but you have to maintain control of the aircraft pretty much every second, so it's more for advanced RC pilots. A model plane with 7 degrees of diherdral or more like a high-wing trainer will keep flying in wings-level flight even if you take your eyes off it for several seconds.