schematic

Ok i am a aero student engineer who has been given a kit yellow F/A 18 Hornet Single (ducted fan) to modify into a platform for some diagnostic equipment. What i need to determine is am i venturing into the abyss or can i satisfy the following strict design requirments.


Firstly 3kgs of sensitive fragile equipment is being added to the aircaft and it is not compact. Options currently are to attempt to slot the equipment into the cockpit area, however this is going to make a very nose heavy aircraft. If this option is taken i will need to make a new air intake just forward of the cockpit windshield to ventilate the equipment and probably two side outlets just forward of where a standard 24oz fuel tank would be located. (I can already hear people asking am i trying to destroy the aerodynamics and make a brick)

Option two: Is to undersling the equipment beneath the fuselage (ideal for maintaining aerodynamic center). This will virtually add a blended box to the bottom of the aircraft, about 170mm wide *270mm back *80mm thick. This will need a new air inlet again for ventilation and an outlet. Weight still 3kg.


Question 1: How strong are these model aircraft and can it take this amount of weight addition. or is it going to rip itself to pieces on landing. (If we get it off the ground)
Question 2: Aerodynamics, how will the flyability be. (I have only ever flown a trainer configeration but we will be seeking an experienced pilot for this project)
Question 3: If option 2 is selected, how is heat distribution from the ducted fan engine, we do not want it propagating to the equipment underneath.


Take off.
The jet will be running a OS91 ducted fan unit with a precision tuned exhaust. This is the higher specification for the kit aircraft.

Qustion 4:Have i got enough thrust given the extra 3kg????

Landing gear
Well due to the extra weight addition am i going to be punching my retractable landing system through the bottom of the aircraft??. The kit unmodified is around 12 pound so adding another 6.6 pounds due to equipment gives me 18-19 pounds (additional rienforcing structure accounted for)

Question 5: So will the kit specified landing gear be adequate for this extra landing weight???? or is thier other options. In addition a soft landing is definatley required.


I hope there are some experienced RC fliers out there who can answer these issues for me. I have reasonable competance in kit/material modeling but no experience is RC models which need to fly.

Cheers hope you can help
Nick

Craig B.

Hi Nick,

Three words....don't do it!!!

Not with this kit. I have just built one and the thing is going to have a relatively high wing loading as it is. Problem no. 2: you will not have enough thrust with the ducted fan to get off the ground with the extra weight. Space is at a premium in this model. Admittedly I fitted a small turbine but the difference is not that great.

With the ducted fan you will be lucky to get 6-7 pounds of thrust and trying to push an 18 pound model with such a high wing loading.........not me, no way, never.

Heat propagation from the ducted fan unit would not be a problem, lack of thrust and wing loading will be. The wing is not designed to support such a heavy model. I am not saying that it would fail, but the kit is built in anticipation of it being a much lighter model.

My advice, build the kit yourself without your test equipment and use another model as a test platform. At least that way you will be able to fly it!!

Hope this helps.

Kind regards,

Craig.

mdelzer

Build a DV8R....you could fit a small dog in that thing. And that's after all the rest of the turbine equipment is in it.

Mark

(I know that probably isn't an option, but its food for thought)

schematic

Unfortunately i am still being made to push ahead with this project, as it is the platform we have available and it must be made to work. Response to the above information is that we may be able to use a gas turbine which is also available in the lab, however i have not examined this option as yet, and they are not to keen to hand it over for this project. Overnight i drew up a pretty simple truss brace to run down the sides of the aircraft to spread the load around the loading points, IE landing gear, etc to account for the extra loading, but i still need to know just how much weight can these landing gear systems take. Because crashing is not an option. The equipment has previously been carried in a previous project by a trainer configuration fixed wing model, which had a wingspan of about 1.7m and chord of about 0.25m. That kit was prop driven. The wing on the yellow kit seams to be of more sturdy construction than the trainer and should be able to take a higher loading however wing area is probably about the same.

Question still running will we be able to use the yellow specified landing gear what is it rated at.

NickC5FE

My Feedback: (31)

those gear on the f-18 single are weak. The links in em brake when you land semi-hard with a lightly loaded jet. At the weights your talking about, i agree with craig, good luck getting it off the ground, and if you do, you will probably have a difficult time keeping it from falling out of the sky. Thats a small plane

Shaun Evans

Hi,

I looked at this thread a few times earlier. I wasn't sure whether or not to chime in due to the nature of the post. I guess I will, though.

First of all, none of this is going to work without a talented jet pilot at the helm. That airplane is very easy to fly (as far as jets go), but it is also very easy to crash if the pilot doesn't know exactly what he's doing. I don't think the extra weight is going to be that much of a problem, frankly. I've witnessed this airplane fly on DF power with very heavy takeoff weights. If your model is the newest one (gray fiberglass), then the airframe (if properly built) will be able to hold up to the extra weight without any problems. The landing gear will also support this weight if you're talking about landing the plane (as opposed to crash-landing it). It has been commented that they are weak gear. That's a crock. The pins in some of the gear are shear-pin types, but a simple act of replacing them with music wire will take care of that. I have seen many, many flights on this plane equipped with KJ-66 sized turbines (including my own P-80 powered one) with the stock gear. Again, the possible success of this all depends on the quality of the build and the quality of the pilot.

I respectfully disagree with Craig's measurement of the static thrust you will get from a properly tuned O.S. .91/Dynamax combo. I think you will be able to get more thrust than that, but the amount of static thrust doesn't necessarily play as much of a part in this as does your runway length and your pilot's capability.

The F-18 is a draggy airframe already. If you're talking about slinging the equipment underneath in some kind of aerodynamic pod, that would be much better than having the CG be that far forwardly off.

To me, the bottom line is this: You need a pilot with a fair amount of time on this model, or at least some similar aircraft. If you can find a pilot with successful BVM F-16 time, he'll probably be a good candidate. You need to have a very long, smooth runway to use in a normal density-altitude environment. You'll need a well-built model--not an overbuilt one--with a strong and reliable powerplant. Lose the turbine idea, as it will only complicate what already seems like very precarious adventure.

With all that having been said, I do hope that you don't attempt this. There are platforms I can think of which are far better suited to this. Someone mentioned the Facet, for example. Those kinds of airplanes are relatively inexpensive and less complicated to operate. Best of luck, however.

NickC5FE

My Feedback: (31)

Shaun, the shear pin type was what i was alking about......I have NO beefs with any yellow products, they are top notch. Nick

mr_matt

My Feedback: (10)

Why not just pony up 120 bucks for an ARF prop plane or something (it can lift 5 pounds with no issue), the kit you have been given will cost you 2-3X that just to get it flying....

Sorry if I missed something.

schematic

Shaun, thanks for your reply. experienced information is extreamly useful.

In regards to to experienced pilots that area should not be a problem as we plan to hire someone who has a long history of experience in flying these aircraft. As you have probably already realised we have no expertise in flying jets. Without having looked into it extensivly yet i have been told that moorabin airport has a dedicated model jet aircraft strip which is used by a club in that area. I am assuming that these clubs will know what they are doing in regards to landing surfaces.

The OS 91 engine has has been modified for this project in a previous thesis so i can give the exact static thrust we were able to produce of the rig.
of the top of my head without going back to the other side of the uni to grab the data
we were getting about 17newtons at 17000rpm (location where fuel efficiency was maximised) and about 25-27 Newtons at around 25000rpm. However at the higher rpm it is stated that the readings may not have been accurate due to the vibration developing in the engine. I did not take the data a previous undergraduate did but from a simple conversion this is putting craig right on in terms of thrust readings of about 6-7 pounds max. However i will be trying to verify this in the next few days, on the rig. Just have to get down to the hobby store and get some fuel and a glow plug.

The kit we have is not the grey fibreglass, it is of a green colour, i think it has been sitting around the lab for about 5 years and was brought as a damaged kit which has since been repaired by the workroom. Is this kit going to be strong enough, or are there know areas of weakness that need to be addressed. We have access to highly experienced modelers who can do any fibreglass modifications . We still have a small crack in the fuselage in the right hand corner of the engine bay door. Is this likely to propergate or should we apply a small patch underneath? Do you have any knowleged if the pilot seat base pan and seat back is performing any structural function as we were considering discarding it and replacing it with a couple of simple ply braces. This space could then be used to house the lighter components of the equipment.

As for loading i did a simple design last night in regards to the undersling option, i figured it shouldnt be a problem in terms of aero characteristics as i was looking at some of the built F18's kits online with full spreads of missiles and drop tanks and that would have to have equaly as much form drag increase as what we propose.

Shaun could you also send me a quote for the full landing gear retracts, brakes, wheels etc. my email is [email protected]. I will also need to know shipping times and cost, for both express and standard post. Please also state if you have credit card facilities for the purchase as this is the only means by which the university will provide funds.

Shaun Evans

Hi,

Tell me more about your engine. What mods did you guys do? What type of fan? What type of exhaust system? What type of ducting? Any chance of seeing some pics of your kit? I will send you the quote to your email, but I think Matt had a great idea with the trainer setup. I've seen 60-sized trainers for sale at around $120. Add another couple hundred for radio gear and a cheap engine, and you've got yourself a safer platform that an average pilot can handle, with a MUCH higher probability of success.

FalconWings

My Feedback: (57)

Hey Nick, where you in the Aerodesign team that came to Fort Worth earlier this year??

My honest opinion.........you've got the equipment, now it's time for you to start brainstroming ideas on how to make it work.

That said, go back to your CAD, design a cargo pod with a low Cd, and if allowable, increase wing area. Also, if this is a one flight deal, work on how to get 30K RPM on the OS .91, try higher nitro fuel, larger tuned pipes...etc....of course, the engine will not last a lifetime at this setting, but you should be able to get 10-12lbs peak.

Like Shaun said, your key ingredient is a good pilot. If it helps, I did fly a TGA F-15 at 18.5 lbs with an OS .91 and lots of drag. I was able to even fly aerobatics.

Good luck,

David

schematic

OK ive got the report in front of me now

The fan we are using is a ramtec fan, from trim aircraft in victoria. 135mm diameter, 9 blade rotor, 10 element stator. Made from high impact glass and carbon filled nylon and aircraft grade aluminium. The engine then utilises a tuned pipe for an exhaust system which has been custom built and tested. I have been told that noise reduction was excellent, but there was only small performance improvments at the high end however at low rpm there was noticable performance increases. however have not witnessed it yet as ive just put the engine into a fuel bath to get rid of the gummed up residue. For those who keep saying go get a trainer, it has already been done, so no point doing it again. We need to show that the systems can be adapted to a new aircraft, and we already have a f18 kit lying around so we might aswell give it a shot


Rig readings
At 14061 Rpm 17 N
At 23500 RPM 25 N

Craig B.

Hi Nick,

I have the bifurcated Tom Cook pipe that was designed for use with that kit if you are interested. It is brand new in bag (never even opened) as I was going to do mine as a ducted fan when I first purchased it. Let me know if you are interested.

Please understand that I have not flown this particular model yet but I have flown several others and built others again.

Best of luck,

Kind regards,

Craig.

travinski

I have this model almost ready to go with a Dynamax fan andOS91. My question is that the springs in the main gear seem to be very weak. The struts bottom out when the plane sits on the gear without the wings or the engine and fan installed. Shouldn't there be at least enough spring tension to extend the strut about half way up? Otherwise, the plane will hit the ground hard as soon as it touches down.

ravill

My Feedback: (11)

Could you post the video when you finally do it?

Sounds like a fun experiment. If the equipment can transmit real time, then it won't matter how you land it! [&:]

I flew a ducted fan BVM F-86 (ie trainer) and moved right along after 9 flights. Ducted fan will be much lighter but with a corresponding decrease in thrust. When the airplane gets in trouble(bleeding too much airspeed in the sky, missed approach, etc..) it will drop like a brick and you will want thrust ASAP!

I think you will have a hard time getting it off the ground and it will be a handful in the sky, but you are learning, is this correct? Then go learn and have fun! [sm=thumbup.gif]

Raf

joeflyer

My Feedback: (48)

You're not putting out enough thrust to get the plane off the ground without the extra equipment. There is something wrong with your engine/fan/pipe. You should be able to get 10-11 lbs. (45-50 N) of thrust. I suggest that you have one of your experienced modeller contacts check it out.

Joe

ianober

My Feedback: (12)

It will never get off the ground with this little thrust, you will need 10 pounds to even think about it.

ravill

My Feedback: (11)

I bet it will fly like the little kyosho F-86. Many will try, few will succeed. The one's I have seen fly just barely make the first turn, but after that they're fine.

But what do I know, I've never flown either! (Kyosho F-86 or Yellow's little 18)

Come to think about it, I don't know anything at all, hey what's this I'm doing? Typing? Oh, see, there, I just now, know 1 thing!

Raf