Calculate max air speed of a plane?
 

Hi guys, It is an unsual question and I am a Beginner so go easy on me. I'm still at the early stages of learning all about rc planes and ideally I would like to build a F-22 with two turbines in it. I'm having a hard time figuring out the approx. max air speed of the plane assuming each turbine produces about 88lbs of thurst (combined thrust 176 lbs). Can anyone please tell me how I can do this?

The engine I have in mind is Titan UAV (http://www.amtjets.com/specs.html), produces about 88lbs of thrust. On the main page (http://www.amtjets.com/) the website has info about a guy who flew his model F-16 at about 250mph with an engine that produces 51lbs of thrust. Thats fast for just 51lbs of thrust!

I know the airframe & weight matters a lot in determining the max air speed, but the problem is I can't even find a formula in which I can plug my values in and it will give me theoretical max speed. From my last post, I got the idea that there really isn't any formula to calculate max air speed of a plane. Still at this point I'm interested in what sort of speed the plane would achive given 176lbs of thrust and a scaled F-22 airframe...just a rough range i.e. 200-300mph, 300-400mph etc??


@ Moderator: The jets forum only has section for electric jets or asking questions from jet manufacturers. That's why I posted this here, since my post is more about the physics and not the jet engine itself.

RE: Calculate max air speed of a plane?
 

Is the interest in the math, or in a real answer? IE, is this a theoretical question, or are you trying to actually achieve speed?

RE: Calculate max air speed of a plane?
 

I think your projected plane will crash at one end of the field you fly it at.
Speeds over 150 mph get to be problematical with models, as they tend to get real small real fast, unless they're flying small diameter circuits as in dynamic soaring, or large half-pipes, as most jets are limited to at full speed.
Just figure the feet per second of your expected speed, and how long the plane can be visible and controllable in that time.

RE: Calculate max air speed of a plane?
 

Lunarcycle:

The resistance of the air to your model, or drag, will be the only factor determining the limit or final velocity.
That force depends of the projected area perpendicular to the direction of flight, multiplied by a factor smaller than one, that depends on the more or less aerodynamic shape of your model.
The weight of your model will determine the acceleration or time that would take to reach that top speed.
There are books offering long mathematical explanations; but the drag coefficient, or amount of resistance of the air to your model for different velocities is more accurately and easily obtained by experimentation.
Of course, you would need some kind of wind tunnel to do that.

Although I am against the idea of superfast models in inexperienced hands, here is the general formula to start a rough estimate:

Top speed = square root [ (2 * gravity acceleration * Total thrust) / (Drag coefficient * Transversal area *Air density) ]

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RE: Calculate max air speed of a plane?
 

No problem. Theoretical questions are welcome anywhere they might apply.

You did mention you were a beginner. Well, you're designing one awesome trainer. ;)

RE: Calculate max air speed of a plane?
 

haha...I'm definitely not going straight to 200+mph. Actually, I've had that RC jet bug for several years and now I have enough free time to actually start working on it. Still, I will buy a cheap-fast plane just to get some hands-on-experience with rc planes before starting my own project. At this point I just wanted to know the kind of speeds that are achievable with RC planes.

Also, I have a different sort of RC plane in mind:

1) It should be capable of 400+ mph, and a flight time of at least 1 hour.
2) PC interface, so that it's controllable from a PC like a video game - I'm a software developer, so no problem here.
3) Range should be 20-30 miles at least - I'm looking into that as well.
4) Loads of custom onboard electronics & sensors that relay data back to the PC - My brother is a Computer/Electrical Engineer, his expertise will help here a lot.
5) On-board GPS receiver, so that the plane's position is relayed back to a PC and plotted on a mapping software (Google maps maybe).
6) 2D Vector thrust


I know it's a BIG project, and that it will probably take at least a year to complete - unless I outsource some of the work. The plane would definitely be BIG, because each AMT Titan turbine is about 5 inches in diameter, so the fuselage alone will be about a foot wide! That should also give me plenty of room for my custom electronics & loads of fuel.


Do you guys think that two turbines producing a combined 176lbs would be able to take this plane over 300-400mph? or should I plan for more thrust?? At this point, I've no idea about aerodynamics and don't plan to get into this stuff if it's not going to be able to achieve those kinds of speeds. I'm still at the research phase, so if you can give me an intelligent guess about the max airspeed, I'd really appreciate it!

RE: Calculate max air speed of a plane?
 

Ummm.... not to get off the whole speed issue, but where exactly do you think you are going to be able to fly such a creature?

RE: Calculate max air speed of a plane?
 

I live in suburbs and 20-30 miles from where I live, there is miles of farmland. So plenty of room for flying :) besides it's not like I will be using it everyday, maybe everyother weekend or so.

RE: Calculate max air speed of a plane?
 

If I was you I'd make it a point to attend a couple of jet flyin's in your region before you plan on making a model that'll do anywhere near 150 mph let alone 300. As already mentioned they just can't stay in sight long enough even at 150 to 180 mph to fly in a "normal" jet manner. If you were able to reach 300 then it would need to be flying constantly in a turn or loop just to remain in sight. So while it's nice to plan on a future model being a turbine it's really not practical to aim for that sort of speed or even anywhere near close to it.

Anyhow, getting back to the topic. First off you'd need a way to determine the overall drag coefficient for this model. From that you can plug it into a formula to find the drag at speed. Then just keep raising the speed until it matches the available thrust and you've got your answer for level flight. A dive would be a whole other issue. And there's one more item that likely won't matter. If the efflux of the turbine is only able to reach a speed of 1 to 2 times the planned speed then there'll be a thrust degradation as the model flies faster and faster. For the thrust to work it needs to be thrown back at a significantly higher speed than the forward speed of the plane. The thrust is mass x acceleration after all. And if the efflux reduces this value as the plane goes faster then there's not as much thrust.

RE: Calculate max air speed of a plane?
 

Any jet that can handle TWO 88lb thrust turbines will be able to be seen beyond radio contact IMO. I have heard of 2.4 being tested to one mile, I have not done that. But I have considerable experience with turbines, and the AMT 88lb thrust turbine is something like 10 inches diameter. Two of them would be huge, you are talking about a model that will be somewhere in the 150 lb range. For an F-22, I would guestimate the airplane would be 150 inches long or so, just to be big enough in scale to handle the engines. The landing gear for a model of that size have yet to be considered.

If the goal was speed, it would probably do something in the 300 mph range, but that has been busted with a BVM Bandit, and even 350 has been busted by a GLIDER.

Exhaust velocity of a single stage turbine is in the 400 to 500 mph range. Depending on temperature and RPM.

This project is too large to be taken on but by a very few select modelers. Much less someone who's first question about the project would be "how fast can it go".

RE: Calculate max air speed of a plane?
 

It's not only a big project, but one that faces some serious government challenges.

What you propose is actually something that needs permits and qualifications. Our Federal government restricts what you porpose to do. The most recent distance record flight by a citizen was restricted to over the ocean "out of sight" operation. The gummit refused to allow out of sight operation over the land. As near as I can tell, there have been no more recent attempts. Also, the operation of RPVs is regulated. And is very much of interest to a less that "recreationally minded" agency.

Also, have you looked into what is required to fly jets at club fields in the US? There are pilot licensing requirements. Noone is permitted to operate jets until they've qualified for a license. So not only do you have the Federal gummit to please, you've got a private organization to qualify with and they're actually working for all of us to HELP us succeed at modeling.

It is really good advice to take this project a step at a time. Ducted fans are not restricted like jets. They also provide a great learning experience for the thrust problems etc associated with "push propulsion". They would be an excellent entry into the modeling world and one that would lead directly into your chosen path. And you don't have to pass any tests or accumulate any approvals.

All of the above is for the benefit of every newbie that stumbles on this thread, it's actually not directed just at you. There are a lot of first timers who plan to build B-52s or F-16s and such. The ones who see a B-17 as their 2nd or 3rd model have an easier row to hoe. They won't have to be licensed to fly a "prop job" nor probably have to have the airplane certified, or get permission to fly it. All they've got to worry about is the final weight of the airplane. OK, the only legal restriction they face might be the weight of the model. They certainly are going to have lot's to worry about.

BTW, one reason you're not getting the answer you've asked for, is that it's not readily available. There aren't too many modeling publications that cover your thrust problem, and most nowadays are more focused on reviews of foreign made ARFs. Scratch building and model design are slowly vanishing from our publications. Few modelers nowadays are designing their own, and fewer still who design jets.

RE: Calculate max air speed of a plane?
 

its not just about power to weight + drag ect its also got to do with the airframe strength some airframes might only be rated to say 150 mph and others to 200mph ect or they will moast likley flutter and break up in flight
good luck with this project let us all know how it goes
happy flyin

simmo

RE: Calculate max air speed of a plane?
 

175 lbs of thrust -
I may be wrong - but I bet that would fly me in a very light ,small craft. I don't think a Cri Cri has that much static thrust from the two tiny two stroker engines on pipes .
and - they are very aerobatic
here is a pic of one with jet power

RE: Calculate max air speed of a plane?
 

LOL i was just thinking that to myself then

RE: Calculate max air speed of a plane?
 

That CRI CRI is flying on AMT's Olympus engines, 50 lbs thrust each. Hence my comments that a radio controlled jet with twin 88's will fly beyond radio contact.

I am a custom home built RC turbine jet guy. My jet is designed around 50lbs of thrust (single). Believe me, there is no beginner that can take this project on. I have spent $25K developing my airplane. CNC Mills, CNC Routers, Lathes, more wood and aluminum than I care to think about, molds, glass, balsa, 4 axis cnc foam cutter, you name it. This guy can't build the airplane he is speaking of.

RE: Calculate max air speed of a plane?
 

If you know the frontal face area, the wetted area, the coificient of frontal and parisitic drag, the coificient of drag of the wing, the expected angle of attack, and probably a few factors I forgot, yes you can calculate it. Getting the information other than the frontal and wetted area will probably be the hard part as it would take a wind tunnel test and most home builts have not been extensively tested in a wind tunnel, although the airfoil may have been tested. But I suppose an experianced aerodynamic engineer could give you an educated guess.

RE: Calculate max air speed of a plane?
 

Before you start construction on this money pit, you'd be wise to invest in a $70 box trainer.

RE: Calculate max air speed of a plane?
 

This is just tooo much fantasy-
Before all this aerodynamic mumbo jumbo goes anywhere - SOMEONE, who understands materials and strengths and structures, must provide a LOT of input
lets face it - the reason planes are what they are is first a design which is structurally sound and capable of a payload of sometype.
You can't design a wheelbarrow, much less an airplane, without knowing the task.
Once you get that nailed down you can see if it will lend itself to the shape you want then the airfoils etc..
This entire idea is the cart before the horse.

RE: Calculate max air speed of a plane?
 

I have a similar problem/project:
I sell pet gerbils and need a better way to deliver them to my customers. I have an electric foamy “Easy Glider” by Multiplex. I need to upgrade it to transport pet gerbils to my customers homes, some of which are 55-miles away. There is not much air inside the cargo compartment so the gerbil cannot stay in their long without suffocating. To make the 55-mile trip quickly it must fly my foamy faster the sound barrier, most likely mach 3 or mach 4, so that it arrives at my customer’s home before the gerbil suffocates. How many LiPo cells do I need to power my foamy mach 3 for the 55-miles?

Forgot to mention this will not be remote control, because the gerbil will be expected to pilot the foamy. What brand of miniature control yoke and throttle stick do you think would be best for a brown gerbils that have never flown before and have an average weight of 4-oz?

RE: Calculate max air speed of a plane?
 

We have those controls in stock, at our on line store .
For west coast buyers, Fantasies R Us .com
Our east coast store is "We be Stuff @ dude.get the net.

RE: Calculate max air speed of a plane?
 

"How many LiPo cells do I need to power my foamy mach 3 for the 55-miles? " - Depends... do you plan on detonating the lipo, or using it as a battery?

RE: Calculate max air speed of a plane?
 

Edited by BMatthews-

The last few posts to this thread have been removed becuase they are ridiculling the original poster. He's a newcomer that doesn't know and has asked a question even if it is a bit on the outside of what we consider normal for our hobby. Either post practical and helpful replies or don't post at all.

RE: Calculate max air speed of a plane?
 

Okaaay, well obviously some of you didn't even bother reading my posts entirely, in which I clearly mentioned that I just wanted to know if those kinds of speeds were possible or not and I also mentioned that I Don't have any aeronautics knowledge and Don't want to spend my time learning it, IF the speed I want is Not possible. I also mentioned that I Wasn't going to jump straight into this project, I would definitely buy a learner plane and get hands-on experience before starting this much bigger project.

On the other hand, some people did provide very valuable input and I really appreciate that! Especially daRock for pointing out the government regulations, I was completely unaware of those.

As of the project, I have the will, skills & knowledge to take care of both software & the hardware. The only thing I need to learn is aeronautics/aerodynamics etc, however, I am getting the feeling that it's quite complicated so when it comes to airframe etc, I've decided to outsource the work. I came across some members who have CAD models of F-22, and I'm sure they'll be willing to sell the design for a reasonable price. After that it's about building the plane, I still need to research if there are any freelance developers but I am sure there are, so that shouldn't be much of any issue either.

Please remember, this isn't a temporary fantasy for me. I have always been interested in model airplanes but didn't have the money to buy one. Now, after about 10 years, I have the necessary resources & financing to build something not only as an engineering excercise but also to fullfill one of my oldest dreams.

Cheers!

RE: Calculate max air speed of a plane?
 

If flying a RC jet is a goal of yours, I’d suggest getting a trainer and start the process of learning how to fly. Most who fly model turbines took years of practice across multiple models of increasing performance before they were willing/able to take on a high speed turbine powered model. My recommendation is to contact a flying club in your area and checking it out.

http://www.modelaircraft.org/clubsearch.aspx

RE: Calculate max air speed of a plane?
 

It's kewl to just ask just to get an answer. It's a shame the written word is so lousy at communicating feeling and meaning and such. But this ain't the best way for people who don't know each other to sit and chat.

It's a problem all the time, that answer posts very often really aren't exactly on target. Lots of times guys address their answer posts to the whole population they expect to be wandering by. These are open forums after all. And lots of members have some teacher in them, and want to answer the question so the whole class benefits. Sometimes those answers spin pretty far off the topic in an effort to cover everything the "teacher" wants the class to know about, now that subject has come up.

RC jets very often trigger emotions and way off topic "warning" posts. Ever been over in the jet forums???? It ain't pretty there sometimes.

Truth is, I'm not entirely sure there actually has been a straight answer to the question. And personally, I too would like to know what the present equipment we have available today would probably give us for speed. I'm pretty sure from experience with pylon racers and observations of "normal" jets today, that the speeds would probably be impractical with our other equipment and gummit limitations, but it would be nice to know. If just for the fun of knowing it.

What equipment do we have today that'd be practical to try, and in what configuration? And to what theoretical result?