I am hoping to sell an old YA A4 DF that's been in mothballs since '93.
It's never flown off grass, but I need to set it up to be able to fly off a small grass field for the buyer.

The A4 has the sport wing, OS91/Dynamax w/Performance Specialty AAC liner/piston, Boca ceramic bearings,
weight=14lbs wet. The power and rpms are there, just doesn't seem to have the thrust for grass.

Currently the thrust tube exit diameter is 4 inches. I would like to change it to 3.5 or 3" to increase the static thrust. Has one done this? What is the optimal diameter for the thrust tube outlet?

Thanks,
Keith

At 14lbs thats gonna be tough, its hard enough for a 14lb byron jet to get off of grass. And how short of a field are we talking? Bigger wheels may help a little, and the only other thing I can suggest is moving the CG back a little or maybe moving the rear wheels forward a little so that the plane can rotate easier, but if it does rotate be careful it will be flying slow and easy to stall, if it does get off you will need a clear and flat area ahead of you, keep it low and level until you pick up speed, if there are any obsticals at the end of the runway don't even try it. 14lbs on a Dynamax fan from grass will be very tough.

Thanks, Mugenkidd

I have changed to larger wheels. I'll try and move the mains forward, but it will not be by much.
The fields is surrounded by trees and bush, so if its not up and climbing in 300 ft there may be trouble.
Shifting the CG is going to be tough due to the placement of the fuel tank and the limited space in the engine/fan compartment.
Any thoughts on increasing the static thrust by reducing the outlet diameter?

300 ft surrounded by trees I would say it probably won't happen, and I'm the optimistic type[]

As far as decreasing the outlet diameter to increase static thrust I'm not sure. I know there is a sweet spot in thrust tube designs, too big and you loose efficiency or too small and you loose efficiency. I don't know any of the equations to justify these statements and not sure how much +and - you can do to increase thrust. But I would think that maybe making the tube a little "larger" may help increase static thrust and I may be totally off, but I know that the larger diameter byron fan units created more thrust than the dynamax, and the dynamax had more top end speed, but I know the actual design of the fan has a lot to do with it too... So I'm really not sure on that part

Only other thing I can suggest is trying to take off on only half a tank of fuel and cut your flight time in half, and maybe try shaving a few ounces off by putting in a smaller reciever pack.

Try that but be careful, don't be in so big a rush to sell it that you do get it off of the grass only to stall it a few seconds after take-off. And don't get me wrong, I'm not trying to tell you what to do I have just done things because I felt it "had to be done" and ended up regretting it.

Hi,

I've always heard that choking down the diameter will give you a better top end, but a lower static thrust. I really don't think that older kit is going to be able to get off of grass unless you have a lot more room than 300' AND the grass is super short. The older inlet design made the plane not want to develop any real thrust until it reached a certain speed on the deck. I had one, and you could plainly see the plane suddenly hit that power band halfway through your takeoff run. I think the grass is going to keep that power-band moment from happening in less than 300'. You might end up in a situation where at the end of the 300', you're yanking back on the stick, and she pops up in ground-effect then stalls onto her back and splats. I hate telling you this, but that's what I think.....

I don't think that'll work either. I've been testing a BVM Viper with an electric system in it. Witht he stock ducting and 3.4 inch exit nozzle and a 5 inch fan I was only getting 5.5 pounds of thrust. I whacked the nozzle off and opened it up to 4" and the thrust went up to 8 pounds +, power input went up too as the fan was partially stalled with the choked down exit...

Hey Guys,
I hear you on the 300' take off. I 'm just trying to get the thing to fly off grass for the buyer and I thought changing the outlet diameter would be the best way to go.

Chris, that's interesting about increasing the diameter. I thought decreasing the diameter would work! I totally agree with mugenkidd, about the sweet spot of the exit velocity. I have a older YA_F18_Single with its dual 2.5" exits, same fan and engine with about 15 degree of flaps, amazing how she halls butt and is up and at'em at 300' on grass.

I thought I'd ask here on RCU to see if someone else has a good viable solution without me having to make any super major changes. Figured the thrust tube would be the best place to start. I was hoping to get this to work out, as I'm getting a sweet price for it that's gonn'a help fund my next turbine.


Thanks For The Suggestions,
Keith

Such an easy solution for this problem!!!!!! Find a new buyer (preferably one that flys off of hard surfaces)!!!!

The only other thing that just popped in my mind is to use a bungee cord like sailplanes guys (and some EDF guys) use attach it to the plane and let her rip (kind of like a carrier take-off), that is the only way I could see it maybe getting off of the grass. But then again, the bungee cord to haul a 14lb plane may need to be longer than 300ft.

BTW thats one sweet looking A-4 [sm=thumbup.gif]

Just another thought, make sure the front strut has a positive angle of attack while sitting on the ground. Normally, this plane will take off fine if all things are set properly. I have owned one of these, but it did not weigh 14 lbs.

where is the CG located on that jet maybe moving it back a tiny bitt will improve take off
happy flying

Thanks to all for the suggestions,

The light bulb went off in my head to try decreasing the exit diameter by creating an adjustable temporary adaptor to
reduce the diameter at various interval and measure the thrust levels. We'll see!

PS. Thanks Ian, I already thought about selling to someone else who flys off hard surfaces and it may end up
going that direction. Unfornunately, I promise the buyer I would sell it to him and I thought for sure I could
get this thing to fly off grass for his use.