gvincent

Hi all,

I'm new to RC aircraft and this forum, but I'm hoping someone can answer my question. I'm the propulsion lead for a University of Washington senior aircraft design course, and have been handed the task of finding a 7.5+ HP gasoline engine that is as light as possible AND has good fuel efficiency. We only need 7.5 horsies during maneuvers, and about 3.5 for cruise. We're attempting to build a UAV that can pull 12 G's (sustained), and has huge winglets for side-force control. Oh, and it's supposed to fly for an hour on 4 lbs of fuel (thank god this is flexible).

Can anyone recommend an engine for this job? We're currently looking at:
• 3W 75iUS
• BME-80
• Brison 4.2ci (are they still in business?)
• Taurus TS 72
• ZDZ 80 RV.
We pulled these out of Model Airplane News (May 2020), and would love for a personal recommendation.

My big question is how much fuel does an engine of this size burn per minute? An ounce? A gallon? I haven't found any numbers on this at all. Can I guesstimate this based on displacement?

I would seriously appreciate ANY input on this at all; all of us in the course are new to RC aircraft.

Thanks for the expert advice!
Graylan Vincent

rc bugman

My Feedback: (30)

First of all, 2 strokes are never known to be miserly on fuel. I have found that a good rule of thumb about fuel consumption at full throttle is the displacement of the engine in cc = to the cc of fuel consumption per min.

data: 3w-24 uses less than 30 cc per min at full throttle and 24 cc is a good estimate.

Similar experience holds up with the other 3W-engines.

Posted Hp ratings in RC are as flakey as corn flakes. ZDZ hp ratings are always overstated and turn less rpms with similar props as 3W engines of similar displacements. Are the stated HP ratings of 3W accurate? Who knows? These engines are always a bit heavier than the competition but produce more torque and propeller rpms.

What is the wing loading of your UAV? That is an important issue when choosing an engine. What is the lift coeficient and is the wing flapped? These are other important issues.

I use RPVs to conduct aerial sampling for biota and I have a bit of experience thinking out of the standard RC box. Perhaps you want to give me a call or email.

Elson
Cornell Univ.

SuperJ

There is a much better way to estimate fuel consumption. The best of two strokes when tuned as lean as possible will have Brake Specific Fuel Consumption numbers in the .42-.54 lb/HPhr range on nonoxygenated unleaded pump gasolines. So figure 7.5 HP is going to take 3.15-4.05 lbs of fuel now the thing is very few of the chainsaw/weedeater/industrial derived cylinders are even going to get close to BSFC numbers like that figure more like .65-.80. I would limit my choice to the few purpose built scale airplane two strokes or consider a multicylinder fourstroke converted to ignition running on gasoline. Those motors should be able to run BSFCs into the .35 lb/HPhr range whether or not any of them can generate the power you require on gasoline would require further inquiries.

Diablo-RCU

My Feedback: (1)

On a power/weight basis, the ZDZ-80 will be your best bet. If you need more power, used a tuned pipe exhaust, not an in-cowl muffler. This will give about 15-25% more power. I haven't seen a BME-80, but the ZDZ-80 is slightly stronger than a BME-100 - from what my tach tells me on the identical prop. The 3W engine will be about a pound heavier and I doubt it has as much power. I haven't seen the 3W-75 single, but I know the ZDZ-80 twin is slightly stronger than the 3W-80 twin (again my tach on identical props).

If you need even more power/weight, there is a rumored BME Extreme 110. No one really knows when it will be available and how strong it will be. Their website claims it will be less than 4 lbs and more power than any 100cc sized engine. We'll have to wait for the product to find out if it's fact or hype.

gvincent

Thank for the fuel consumption info. We've got 4 lbs reserved for fuel, so it sounds like that'll last for at least half an hour (WOT) and probably a lot longer than that if we're cruising at only half power.

One of our aero guys tells me that we're predicting a wing loading of 5 psf, and 1.5 Cl max.

Ok, so it sounds like Brison is out of business, so I'll scratch them. I heard about the BME 110 (10+ HP at only 3.8 lbs!) which sounds absolutely incredible, but is almost twice the cost of what we're expecting to spend. That leaves:

• 3W
• Taurus
• ZDZ

I appreciate your input on these. Thanks,

Graylan Vincent

rc bugman

My Feedback: (30)

Is 5 psf , 5 pounds per sq ft? = 80 oz/sq ft wing loading? What is the wing area?

Elson

Flyfalcons

Brison is still in business. Their website is temporarily down. Don't scratch them yet.

SuperJ

Reread my post none of the motors using Mahle's Nikasil chainsaw cylinder castings are going to get anywhere near the optimum BSFC numbers I posted. Figure more like 6 lbs per hour for a motor making 7.5 hp. BSFC for two strokes operating without an expansion chamber is a non linear function of throttle position IE halving the power output does not halve the fuel consumption figure more like 65-75 % of WOT.

tkg

My Feedback: (1)

OK, you have to understand there are a lot of opinions running around here and not a lot of facts.
So another opinion, A piston design has a sweet spot for best performance above or below that rpm they are a mess.
A reed valve design has a wider sweet spot because the reed is self regulating to a point.
A rotary intake can be adjusted to make
power or get better MPG, but not both.
A single will make more power than a twin of the same size and type of design.
A tuned exhaust can work wonders at the tuned RPM, out of the tuned spot and again its a mess
A big slow turning prop is more efficent.
Check out prop drive reducers
Look for used engines the old BME 100
are not to pricey now. Also maybe a Quadra 75/100 single.

gvincent

Wow, a lot of info to digest.

To answer bugman's question, the vehicle is expected to weigh 20 lbs, and have a wing area of 4 square feet (1 ft chord, 4 ft span). It's a recycled wing mold--we used the same wing last year for a hand launched UAV. I believe we're planning on having flaperons.

So, based on weight, power, and cost, we've narrowed the search down to:

• 3W-75 US (7.5 HP, 5.3 lbs, $555)
• ZDZ 80 (8.5 HP, 4.3 lbs, $615)
• Taurus 72 (8.1 HP, 4.9 lbs, $700)

I like the ZDZ, but I think it's going to come down to cost in the end. Our prof will probably try to get an educational discount if possible, and I'd even go so far as to ask to borrow the engine for the next 4 months.

Now, we just have to choose a propeller (smaller=better, because of the gear length) and muffler. The tuned pipe idea sounds interesting.

Any suggestions on any of this? I'm all ears. I really appreciate all of your input on this. It's nice to be able ot run my ideas by some experts!

Thanks again,
Graylan Vincent

rc bugman

My Feedback: (30)

80 oz per sq ft wing loading is a lot of airplane to take off, fly and land. Stall speed must be in excess of 35 mph. There will be no forgiveness with that amt of wing loading.

Someone on your team has experience flying this heavily loaded of a RC plane? What was the wing loading of last years' hand launch?

I believe you may want to give serious consideration to reducing wing loading a significant amt.

Elson

tkg

My Feedback: (1)

20 lbs with 576sq in, your going to need about a 16 pitch prop to fast enough for this thing to fly

Flyfalcons

gvincent,
Try mating a three-bladed prop to your engine. You'll be able to run a smaller diameter than a two blade, with a minimum of performance loss. I agree that the wingload will be high, but if you have a stretch to get it up to speed then you should be okay.

SuperJ

Use some large diameter wheels and carefully plan the linkages if you have any steerable wheels or the ROG is going to be very exciting.

gvincent

Yeah, once again I give thanks that I'm not doing the aero/structure design of this beast, just the propulsion. Oh, one more thing to make it more confusing--it's a box wing design (I'm told this is different from a biplane).

I don't remember exactly where they calculated the stall speed being at, but I know that with a 7.5 HP engine, we're considering vertical takeoff. Kidding. But, we are actually worried that this engine will be too powerful to land with--that is, it may be putting out too much power at idle to get it down. The current thought is that we'll optimize the prop for high speed/RPM flight, and at the lower RPM, the inefficiencies will slow it down enough to land. Or at least that's the plan. Think this would work with the ZDZ-80, for instance?

I don't know what the plan is about having steerable gear, but we are concerned with the prop size and the required stilts...er, gear to go with it. I know we're trying to avoid retractable gear.

The 3-blade prop came up in our discussions today, and someone even mentioned a 4-blade prop. I haven't seen any 4-bladed props yet, and only a few 3-blade props. What's the thought on these? Also, what's a typical prop efficiency for RC aircraft props?

Again, many thanks for all the advice.
Graylan Vincent

rmh

I just gotta ask-- does anyone in your class have even a basic understanding of how an aircraft works?
Not making fun- just that the parameters you describe for the model are not at all what I would expect from anyone with even a basic understanding of small aircraft.
Ask the "prof" , to get together with any long time builder/ modeler -and get some more input on what makes models fly -
A local friend Dave Stuart, works with the guys at Utah State , on national competition on models -
this is payload , power etc., trials stuff.
The have won or done very well , simply because they adopt very basic low speed airframe rules .

Antique

My Feedback: (4)

About the power thing...There is no correlation between the power of an engine and the idle speed...ANY engine will idle down to 1500 rpm or less, slow enough to not move the plane.....

tkg

My Feedback: (1)

What ever happens, keep us posted and when you can add pictures.
It looks like your team is pushing the envelop a bit.
Maybe the rest of us could learn something new

gvincent

Ok, we're working on acquiring an engine, so in the mean time, does anyone know what some typical prop efficiences would be for RC props? Would you expect them to be comparable to a full size (~0.7 efficiency) prop? Any suggestions on where to buy 22 to 24 triple bladed props?

Second, to reply to the question of whether or not our class has any background in this area; this is the senior year aeronautical engineering design course. This same course helped to produce the Aerosonde a couple of years back (first UAV to fly across the Atlantic, www.aerosonde.com). It also produced a trans-pacific UAV, a backpackable UAV, and a hand launched UAV.

http://www.aa.washington.edu/courses/aa410/

We'll get some photos up when we start construction, probably in a couple of weeks.

Also, tuned pipes--are they designed for specific engines and mounts, or are they generic and just based on engine size? And the obvious question, where's a good place to look for them?

Thanks again.

Cheers,
Graylan Vincent

PS-Stall speed is at 32 mph for you who asked.

tkg

My Feedback: (1)

Tuned pipes are sized by engine CID, the headers are the special parts. Try
www.macspro.com

3&4 bladed large props. Try
www.zingerpropeller.com

gvincent

Ok, so here's the initial CAD drawings:

http://students.washington.edu/drwat...ane_design.htm

Question: The ZDZ 80RV looks like it's got a fuel pump and carburetor built in the back. Do these built-in pumps work pretty well during maneuvers or is there any reason we should be shopping for a seperate pump?

Thanks,
Graylan Vincent

rmh

pumper carb is a unit -just supply a tank of gasoline -mixed with oil (about 50 -1)
the craft looks fine -but it will have to fly fast if the wing loading is high.
"fast" is relative .

Diablo-RCU

My Feedback: (1)

Perhaps you should get someone like Chip Hyde to fly that thing.
I'd want a pilot with a fast connection between the brain and fingers.

gvincent

We just received a prop and it seems like it may not be the best choice for our engine (I let someone else find it). It's a Zinger 3 bladed prop, but it's 3 separate blades with a huge spinner and mounting plate.

The ZDZ has a 6-hole mounting pattern, whereas this prop does not. It's not hard for us to take it down to the shop and drill the holes, but I wonder if this is wise; we'd be drilling through metal and wood, and I don't think the was ever intended by the prop mfg (am I wrong?). Also, this prop (with plates) is thicker than the 6 bolts that came with the engine.

I guess I just have a bad feeling about using this prop with this engine, and I'd like an opinion on whether or not we've got a good prop for this engine/plane. Any suggestions?

Thanks,
Graylan

I'll insert some (large) photos of the prop in the next message.

gvincent