Jenny JN4D and Downthrust

[JohnReid]

What is the purpose of the 15deg Downthrust of the engine in the JN4D Jenny?Any ideas? Cheers! John.

here's what I think....

Downthrust compensates for positive incidence. There has to be some + incidence, or, at least + AOA, to maintain flight, downthrust is good because you don't have to hang on the stick at low throttle settings.

The Jenny is a low and slow plane, as well as a trainer, so, its part throttle/low speed performance is of primary importance, downthrust helps make low speed handling easier, and since the old goat is too draggy to see any negative effects at higher speeds, it's all good.

F=MA

[JohnReid]

Thank you F=MA. I think I understand.(math was never my best subject)However,after calling her an old goat,I wouldnt accept any rides in the jump seat.Cheers! John.

[wolfenbill]

Well, you either climb in a Jenny or you turn in a Jenny, but it's not wise to do both at the same time.

[EricGoedkoop]

Originally Posted by wolfenbill

Well, you either climb in a Jenny or you turn in a Jenny, but it's not wise to do both at the same time.

Where was it I saw the Jenny flight procedures? It started out with:

Step 1: Inspection

Do not inspect this ship. If you do, you'll never fly it.

[jumpinjan]

I would think down thrust would eliminate some P-factor, wouldn't it?
We use to put down thrust in our free flight models to help eliminate "looping". They were trimmed well for gliding, but under power there was a tendency of the model to go into a loop. Must have been the increase of aerodynamic forces when the speed increased.

[EricGoedkoop]

Originally Posted by jumpinjan

We use to put down thrust in our free flight models to help eliminate "looping". They were trimmed well for gliding, but under power there was a tendency of the model to go into a loop. Must have been the increase of aerodynamic forces when the speed increased.

It's called "power stalling." The model needs to be trimmed to glide nice and flat. Under power, however, you're travelling a lot faster than you are in the glide and therefore generating a lot more lift. Sometimes you'll loop, but on a rubber-powered model you have a quick burst of thrust at the beginning of the motor run that usually doesn't last long enough to pull you all the way over the top. Power dies off with your nose pointing to the sky, you lose speed, you lose lift and next thing you know you're looking through the grass for missing parts. Downthrust is one way to avoid this by pulling the nose down while the power's on, thereby counteracting the excessive lift you're generating.

I assume the principle would hold true for a full size aeroplane, but since you have the luxury of a living, thinking pilot behind the stick it seems unneccesary.

[Brad]

Originally Posted by EricGoedkoop

It's called "power stalling." The model needs to be trimmed to glide nice and flat. Under power, however, you're travelling a lot faster than you are in the glide and therefore generating a lot more lift. Sometimes you'll loop, but on a rubber-powered model you have a quick burst of thrust at the beginning of the motor run that usually doesn't last long enough to pull you all the way over the top. Power dies off with your nose pointing to the sky, you lose speed, you lose lift and next thing you know you're looking through the grass for missing parts. Downthrust is one way to avoid this by pulling the nose down while the power's on, thereby counteracting the excessive lift you're generating.

I assume the principle would hold true for a full size aeroplane, but since you have the luxury of a living, thinking pilot behind the stick it seems unneccesary.

Downthrust prevents the tendacy to increase pitch (nose up) when throttle is added on a high wing plane, or on a biplane with the thrust line closer to the lower wing. It helps the aircraft handle more "neutral" with throttle changes.

Brad

[wolfenbill]

It was a hell of a training aircraft with a stall speed of about 45 and a top speed of 75 or so, depending on how healthy the engine was. Not a whole lot of room for error. Pretty simple. You either learned how to fly really well or you died. The ultimate training machine.

Well, given some of what passed for training a/c in the period before it...the MF Short and Longhorn for example, it looks like an armoured safety cage, with sextuple airbags in comparison.

I think the only reason people flew these things is they didn't know any better or knew how to follow orders.

F=MA