HO-229

What should my aircraft speed be aprox. for an all wing aircraft with a wing area of 250-300 sq in, powered by a .074 norvel with a 6X4 prop, with a 20oz total weight?
I've used an algorithm that I have found on the web but it seems aweful low.

Tall Paul

50 mph maybe.
The way to find out is take it to a slope. When it hovers in a high wind, and can't penetrate, that's its maximum speed.
What's the algorithm, and what did it say for speed?

BMatthews

For that prop turning at 18,000 RPM and not allowing for any prop slip your max would be 68 mph.

I'm using...

(rpm x pitch x 60min/hr) to give inches per hour then divide by (12in/ft x 5280ft/mi)

Your actual RPM will change that of course as required.

Realistically the speed will be much less due to prop slippage and model drag. Typically our model props give us about 80% of what the pitch is rated at and in this case a flying wing will be quite clean so figure 68x .7 to .8 to give 47 to 54 mph roughly. In a dive a clean model can overrun the prop and engine and you hear this as a wind up that actually over revs the engine. Some of it is doppler but a lot of it is the engine being sped UP by the model diving. At that point the engine and prop is acting as a very efficient airbrake.

This charactaristic was rammed home a few years back when the absolute RC model FAI speed record was actually held by a GLIDER rather than the usual power model. It was the old Dassel team that set this using a glider in the Swiss alps. The record being near or just a bit over 200 mph. Yes they dived the model but the runs through the traps were within the height change requirements of the course and they did the return run within the time frame as well. There was "some" slope lift to help as well.

HO-229

formula used was
Weight of model (grams) divided by wing area (sq cm) multiply by 160 & then taking the square root of the results (meters per sec) then convert to MPH by multiplying by 2.24

HO-229

Is your 80% of prop pitch a good number to ball park speed or does it apply only to all wing?
How does weight factor into this?
The prop driving the engine is not a new concept to me but thank you for the insight.

We have just finished flight test of an eight bladed prop (NP2000)
Electronic controlled prop "E2 & C2" The aircraft has a negative torque sensor on the gearbox this senses the condition of the prop driving the engine. If a negative torque condition is sensed the electronic prop control will increase pitch. If the negative torque is extreme & the electronic prop control cannot change pitch fast enough the prop will decouple & then recouple after the airspeed is reduced.

Tall Paul

Bruce has pretty much laid it out mathematically. No one really knows how efficient props in our flight regime are, so 80% is a good ball-park top limit.
With an inflight rpm measurement, and a reliable speed measurement, then the number could be refined.
It's been my experience with ZAGIs that they are less tolerant of high winds, i.e. their maximum speed is lower, than the equivalent fuselage/tail airplane, unless the plane is ballasted. Mine, both glider and powered seem to have lower speed capabilities than the normal slopers I fly.
.
on the inflight de-coupling and re-coupling, I'm presuming you're talking full-scale Grummans. If the 8-blade props are quieter than what they have now. GO FOR It! Those planes are the WORST noise polluters around! They fly around here a lot, I guess from Mugu and Miramar, and at 8 miles away they're bad!
When I was back at Pax on the ES-3A, they would run up the motors on the E-2s outside the Force Warfare hangar, I'd go as far away as I could to get away from that noise. Bone-shattering!
The P3 has a de-coupler, but it doesn't re-couple. Mainly it's to prevent a prop which has failed into low pitch from dragging the airplane down. There's enough asymetric drag to overwhelm the control system. I understand when a prop does de-couple, it goes supersonic and can be heard for miles!

HO-229

It sounds like a bee hive

BMatthews

OK, now I'm confused. Did you want the stall speed or the top speed provided by the engine and prop?

Your formula basically gives you the wing loading and then manipulates it in some form I've never heard of. What is the 160 constant?

What I've been doing for minimum speed calculations is using FoilSim. Load in the size of the wing. For a flying wing this should probably be 2/3 the true chord since the rear 1/3 of the wing's area is basically given to stability rather than lift. Add in the airfoils camber value or a reasonable flying wing to conventional guess. Then up the angle of attack to about 7 degrees and lower the speed until the lift matches the weight of your model. I like 7 degrees since that's about where most of our models stall for the most part. Probably 5 or 6 would be more realistic for a slow flying safe AoA. Years ago Frank Zaic showed that free flight models glide at about a 6 degree AoA for minimum descent speed. Not much has changed since then.

You can get FoilSim2 for either online testing or download the applet at.....

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

For 2 sq feet and 1.25 lbs at +6 degrees AoA I get 15 mph for a low speed. Allowing for the 2/3-1/3 split I get closer to 19 mph.

Ed Smith

Forget all of the theory BS you will be fed. Forward speed is a function of RPM x Pitch, minus all the inefficiances and drags and nothing else!

Ed S

HO-229

I'll be honest, I don't know where the 160 number comes from but it was in the equasion.

BMatthews

Gee Ed, that was helpful Go back and re-read my earlier post about the high speed end of things. We got the RPM's and the pitch and then we put in the inefficiencies and tried to allow a little for the drag. So what's missing or inaccurate about it?

HO, did you try the FoilSim? It's generic but then so are the factors it plays with. The best part I like about it is that you can set up the test model for level flight at some airspeed or AoA and then switch to the Lift Coefficient. This let's you see just where on the lift curves the model is working at.

It was a real eye opener for me at the high speed end of things. I'd always believed that a lifting airfoil is best even for a speed oriented model. As in low camber for sure but at least a little. But looking at the extremely low Cl's generated by most models at full speed I soon realized that camber is often superfluous unless it has to pull high G loads like a racing model.

You got a link to that equation?

Strat2003

My Feedback: (1)

Here's a formula from RCM (I think) to give an estimate of the speed of a "reasonably clean" model based on pitch and ground RPM:

Pitch (in inches) X RPM (in thousands) X 1.056 = MPH

So, a reasonably clean airplane turning a 6 inch pitch prop at 15,000 rpm would be flying about 95 MPH.

Lots of assumptions going on here, but it at least SEEMS reasonable

SCALECRAFT

My Feedback: (13)

is there a general formula for estimating the horse power needed for turning blades with certain configurations.

I ask because I'm working on running a scale BF 109 3 blade carbon system I made up.

22" prop arc varible pitch. 80' span, aprox 20lbs.

Considering 1.08 geared down.

Going from the set of my pants right now.

Thanks

Steve

Tall Paul

Working backwards, knowing the airspeed via car-pacing and the pitch, on one of my Kadets running an OS 70 Surpass, at 65 mph with a 5 pitch prop, I get 12311 rpm. On my Baker Racer, OS 26 running a 4 pitch prop, I get 13021 inflight rpm. Both seem high. The OS 26 ground tached at 9100, and the 70 about the same.